Laboratory Equipment and Engineering Controls

Research laboratories are filled with a variety of experiment.  Knowledge of this equipment, maintenance, and regular inspection of equipment are all important parts of running a laboratory.  This section will highlight a few common groups of laboratory equipment and safe work practices and procedures for using this equipment.  Engineering controls and laboratory ventilation systems are discussed in the fume hoods and other laboratory exhaust sections of this site.

Common types of equipment include:


Only trained personel should operate centrifuges. They must be properly installed according to manufacturer recommendations.  It is important that the load is balanced each time the centrifuge is used and that the lid be closed while the rotor is in motion. The disconnect switch must be working properly to shut off the equipment when the top is opened, and the manufacturer’s instructions for safe operating speeds must be followed.

For flammable and/or hazardous materials, ensure the centrifuge is under negative pressure and connected to a suitable exhaust system. 

Take the following precautions when operating and inspecting centrifuge rotors:

  • Balance the load each time the centrifuge is used. The disconnect switch should automatically shut off the equipment when the top is opened.
  • Do not overfill the centrifuge tubes. Ensure that they are hung properly.
  • Ensure that the lid is closed before starting the centrifuge.
  • Do not overload a rotor beyond the rotor’s maximum mass without reducing the rated rotor speed.
  • Follow the manufacturer’s instructions for safe operating speeds. Do not run a rotor beyond its maximum rated speed.
  • Check O-rings and grease the seals routinely with vacuum grease.
  • Do not use harsh detergents to clean the rotors, especially aluminum rotors. Use a mild detergent and rinse with deionized water, if possible.
  • Follow the manufacturer’s guidelines for when to retire a rotor.
  • For flammable and/or hazardous materials, keep the centrifuge under negative pressure to a suitable exhaust system.
  • Keep a usage and maintenance log.
  • Always use the rotor specified by the manufacturer.
  • Inspect the components of the centrifuge each time it is used:
    1. Look for signs of corrosion of the rotors. Metal fatigue will eventually cause any rotor to fail.
    2. Ensure that the coating on the rotor is not damaged.
    3. Check the cone area for cracks, because this area is highly stressed during rotation.
    4. Look for corrosion or cracks in the tube cavity.

Fume Hoods

One of the primary safety devices in a laboratory is a chemical fume hood. A well-designed hood, when properly installed and maintained, can offer a substantial degree of protection to the user, provided that it is used appropriately and its limitations are understood.

picture of bio safety cabinet and chemical fume hood

bio safety cabinet filters the potentially contaminated air through high efficiency particulate (HEPA) filters and then vents that air back into the room and therefore should not be used when working with hazardous chemicals. 

All fume hoods and other capture devices must be installed in consultation with University Facilities and EHS. All new installations or relocation of fume hoods must pass the American Society of Heating, Refrigerating and Air-Conditioning Engeneers (ASHRAE) Method of Testing Performance of Laboratory Fume Hoods (ASHRAE 110-2016), and be commissioned by EHS prior to use. To request that a new or relocated fume hood be commissioned, contact EHS.

If you know your fume hood is not working properly, contact your building's Special Facilities staff or submit a Facilities work order (8-8000 or online).

If you are not sure if your hood is working properly, contact EHS to request a hood evaluation.

When is a Fume Hood Necessary?

The determination that a fume hood is necessary for a particular experiment should be based on a hazard analysis of the planned work. Such an analysis should include:

  • A review of the physical characteristics, quantity and toxicity of the materials to be used;
  • the experimental procedure;
  • the volatility of the materials present during the experiment; 
  • the probability of release; and
  • the number and sophistication of manipulations. 

How a Fume Hood Works

This section covers the basic design and functioning components of a fume hood and the difference between constant volume and variable air volume (VAV) hoods.

Performance Indicators

This section covers the various flow and performance indicators and the survey sticker.

Proper Work Practices

This section covers a number of topics aimed at helping laboratory workers understand conditions and proper work practices for using fume hoods safely

Common Misuses and Limitations

This section covers a number of topics aimed at helping laboratory workers understand the limitations and proper use of the fume hood

Keeping Track of Your Fume Hood

This section covers how to keep track of your fume hood in SHIELD.

Changes or additions to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor is prohibited. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device. Additionally some components of older hoods may contain asbestos and therefore should not be damaged.

How a Fume Hood Works

How a Fume Hood Works

A fume hood is a ventilated enclosure in which gases, vapors and fumes are captured and removed from the work area. An exhaust fan situated on the top of the laboratory building pulls air and airborne contaminants through connected ductwork and exhausts them to the atmosphere.

The typical fume hood found in Princeton University laboratories is equipped with a movable front sash and an interior baffle. Depending on its design, the sash may move vertically, horizontally or a combination of the two and provides some protection to the hood user by acting as a barrier between the worker and the experiment.

The slots and baffles within the hood direct the air and, in many hoods, can be adjusted to allow the most even flow. It is important to prevent the baffles from becoming blocked, by excessive material storage or equipment, since this significantly affects the exhaust path within the hood and as a result, the efficiency of hood capture. 

The beveled frame around the hood face, called the airfoil, allows for even air flow into the hood by eliminating sharp curves to reduce turbulence.

how fume hood works diagram

There are two basic types of fume hoods. They are:

Constant volume – where the exhaust flowrate or quantity of air pulled through the hood is constant. In this configuration, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of airflow (face velocity) through the hood increases proportionally. Thus, the velocity of air at the hood face is increased with the lowering of the sash.

Variable air volume (VAV) - where the exhaust flowrate or quantity of air pulled through the hood varies as the sash is raised or lowered in order to maintain a constant face velocity. Therefore, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of air flow (face velocity) through the hood remains constant, reducing the total air volume exhausted.

Illustration of Fume Hood With Horizontal Sash

Fume Hood With Horizontal and Vertical Sash

Fume Hood Performance Indicators

This section pertains to all buildings except Frick Chemistry building – see separate section below.

Hood Survey Sticker

Every chemical fume hood on campus is performance tested annually by EHS as indicated by a survey sticker affixed to the front of the hood. Do not use a hood that has not been tested within a year; e.g., is not labeled or the survey date exceeds one year. The sticker contains basic information about hood performance as of the survey date and should be consulted each time the hood is used.

The EHS Hood Number is a unique identifier for the particular hood. Refer to this number when communicating issues/problems about a particular hood.

survey sticker

The Inspection Sticker is aligned on the hood so the arrow indicates the proper location for the maximum safe sash position.

Static Pressure Gauge (Magnehelic)

magnehelic gauge

Most fume hoods are equipped with static pressure gauges that measure the difference in static pressure across an orifice in the duct, or between the laboratory and the fume hood exhaust duct. Most of the devices are aneroid pressure gauges, such as magnehelics, that are typically mounted on the front of the hood above the sash.

The gauge reads in units of pressure (e.g. mm Hg), rather than velocity. Changes in the magnehelic reading are not linearly proportional to changes in face velocity; therefore it should only be used as an index of hood performance.

The magnehelic gauge reading at the time of the most recent hood survey is shown on each fume hood survey sticker. A difference of 20% or more in the magnehelic reading from that shown on the sticker is an indication that the flow rate in the duct, and thus the face velocity, may have changed significantly since the last survey. If the user notices such a change, or has any other reason to suspect that the hood is not operating properly, contact EHS at 258-5294 for an evaluation of the hood. Please refer to the hood number when calling.

Color Coded and Digital Flow Indicators

Digital Flow Monitors

Some hoods are equipped with color indicating devices, or digital flow rate displays rather than or in addition to magnehelic gauges. These devices constantly measure the face velocity of the hood and point to green (for good) or red, or give the actual digital flow rate value.

Frick Chemistry Laboratory

Waldner hoods are measured at the upper sash stop (though this higher opening should only be used for loading and unloading). A PASS/FAIL sticker with date is affixed near the alarm lights. The magnehelic is marked to indicate proper reading.

Fisher hoods in the teaching labs are measured at the normal operating sash height stop and a PASS/FAIL sticker with date is affixed.

The number to use when referencing a hood in Frick is the Maximo number on the yellow sticker in the upper left hand corner of the hood sash.

Frick Fume Hood Labels

If hood performance is judged to be unsuitable for use with hazardous chemicals, a sticker with this information is placed on the hood instead of the survey sticker.

Hood do not use sticker

Hoods are routinely inspected at least annually. If a hood fails during a routine annual survey EHS will arrange for its repair. If it has been more than a year since the last inspection, contact EHS.

Note: Do not use a hood that has no survey sticker!


Fume Hood Proper Work Practices

Proper Work Practices

The level of protection provided by a fume hood is affected by the manner in which the fume hood is used. No fume hood, however well designed, can provide adequate containment unless good laboratory practices are used, as follow:

The hood user should know the normal operating configuration (NOC) of the hood and should design experiments so that this configuration can be maintained whenever hazardous materials are at risk. The NOC refers to the position of the sash established when the hood was installed and certified (i.e. how far open is the maximum safe sash position). This is where the survey sticker with arrow is located.

Before using the hood, the user should check the hood survey sticker to determine where the sash should be positioned for optimum containment for that particular unit.

The hood user should also check the magnehelic gauge or other hood performance indicator and compare its reading to the reading indicated on the hood survey sticker. If the reading differs significantly (20% or more for a magnehelic gauge) from that on the sticker the hood may not be operating properly. Contact EHS with hood location and contact information.

Never use a hood to control exposure to hazardous substances without first verifying that it is operating properly.

To optimize the performance of the fume hood, follow the practices listed below:

Mark a line with tape 6 inches behind the sash and keep all chemicals and equipment behind that line during experiments. This will help to keep materials from escaping the hood when disturbances like air currents from people walking past the hood, etc., interfere with airflow at the face of the hood.

Poor Good Best
Poor placement of materials in a fume hood. Good placement of materials in a fume hood. Best placement of materials in a fume hood.
Poor placement of materials. Good placement of materials. Best placement of materials.
Images from Kewaunee Fume Hoods  

Keep the sash completely lowered any time an experiment is in progress and the hood is unattended. Note: Lowering the sash provides additional personal protection from projectiles and can also result in significant energy conservation. Keep the hood sash clean and clear.

Check area around the hood for sources of cross drafts, such as open windows, supply air grilles, fans and doors. Cross drafts may cause turbulence that can allow leaks from the hood into the lab. Also, avoid placing fans or equipment with fans in the hood in an orientation that causes the fan to blow out of the hood as the inward flow is unlikely to be strong enough to keep air and possible contaminants from escaping out of the hood. 

Equipment with fans in a fume hood

Avoid placing fans or equipment with fans in the hood. 

Extend only hands and arms into the hood and avoid leaning against it. If the hood user stands up against the face of the hood, air currents produced by turbulent airflow may transport contaminants into the experimenter's breathing zone.

Visually inspect the baffles (openings at the top and rear of the hood) to be sure that the slots are open and unobstructed. Keep baffles and other ventilation openings clean, free of accumulating dust, and unobstructed by collections of numerous containers. The work surface of a fume hood is not the place to store materials. It is meant to keep hazardous materials from being inhaled during manipulation.

Keep vents and baffles unobstructed.

For optimum performance, adjust the baffles when working with high temperature equipment and/or heavy gases or vapors. See figure below for suggested baffle positions.

Normal Hot Work Heavy Gases
Baffle in normal position. Baffle for hot work. Baffle for heavy gases.
Normal baffle positioning Baffle position to use for hot work Baffle position for heavy gases


Normal baffle position all slots are evenly opened.

High temperature work such as using hot plates; lower slots are minimized since heated vapors tend to rise

Heavy gasses and vapors are better captured when upper slots are minimized

Do not block slots. If large equipment must be placed in the hood, put it on blocks to raise it approximately 2 inches above the surface so that air may pass beneath it. See figure below.

Poor Placement Good Placement
poor large equipment placement good large equipment placement
Poor large equipment placement. Good large equipment placement.


Place large or bulky equipment near the rear of the fume hood. Large items near the face of the hood may cause excessive air turbulence and variations in face velocity.

Do not use the hood as a storage device. Keep only the materials necessary for the experiment inside of the hood. If chemicals must be stored in the hood for a period of time, install shelves on the sides of the hood, away from the baffles.

Provide secondary containment for containers that could break or spill, to minimize the spread of spilled liquids. Dishpan type containers are provided by EHS upon request. Contact EHS or use the Safety Supply Order Form (login required).

Items contaminated with odorous or hazardous materials should be removed from the hood only after decontamination or if placed in a closed outer container to avoid releasing contaminants into the laboratory air.

When using cylinders containing highly toxic or extremely odorous gases, purchase only the minimal practical quantity. Consider using a flow-restricting orifice to limit the rate of release in the event of equipment failure. In some circumstances, exhaust system control devices or emission monitoring in the exhaust stack may be appropriate.

All electrical devices should be connected to receptacles located outside the hood to eliminate the potential for electrical arcing that could ignite a flammable or reactive chemical.

DO NOT USE A HOOD FOR ANY FUNCTION FOR WHICH IT WAS NOT INTENDED. Certain chemicals or reactions require specially constructed hoods. Examples are perchloric acid or high-pressure reactions. Most special use hoods are labeled with the uses for which they are designed.

Fume Hood Common Misuses & Limitations

Common Misuses & Limitations​

Used appropriately, a fume hood can be a very effective device for containment of hazardous materials, as well as providing some protection from splashes and minor over pressurizations. Even when used as designed, the average fume hood does have several limitations.

  • Projectiles: A fume hood is not designed to contain high velocity releases of particulate contaminants unless the sash is fully closed.
  • Pressurized systems: Gases or vapors escaping from pressurized systems may move at sufficient velocity to overcome the capture velocity and escape from the fume hood.
  • Explosions: The hood is not capable of containing explosions, even when the sash is fully closed. If an explosion hazard exists, the user should provide anchored barriers, shields or enclosures of sufficient strength to deflect or contain it. Such barriers can significantly affect the airflow in the hood.
  • Perchloric Acid: A conventional fume hood must not be used for perchloric acid. Perchloric acid vapors can settle on ductwork, resulting in the deposition of perchlorate crystals. Shock-sensitive perchlorates can accumulate on surfaces and have been known to detonate on contact, causing serious injury to researchers and maintenance personnel. Specialized perchloric acid hoods, made of stainless steel and equipped with a wash-down system must be used for such work.
  • Air Foil Sills: Many fume hoods are equipped with flat or rounded sills or air foils which have been specifically designed to direct the flow of air smoothly across the work surface. Sills should not be removed or modified by the hood user. Objects should never be placed on these sills. Materials released from containers placed on the sills may not be adequately captured. In addition, an object on the sill may prevent the quick and complete closure of the sash in an emergency.
  • Spill Containment Lips: Most modern fume hoods have recessed work surfaces or spill containment lips to help contain minor liquid spills. In many cases, these lips are several inches wide. Containers of liquids should not be placed on the hood lip.
  • Tubing for Exhaust: Tubing is frequently used to channel exhaust to the hood from equipment located some distance away. This is not an effective control method.
  • Connections to the Exhaust System: Occasionally, a researcher may need local exhaust ventilation other than that provided by an existing fume hood. A new device may not be connected to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device.
  • Microorganisms: Work involving harmful microorganisms should be done in a biosafety cabinet, rather than a chemical fume hood. See the Biosafety Manual for more information.
  • Highly Hazardous Substances: A well designed fume hood will contain 99.999 – 99.9999% of the contaminants released within it when used properly. When working with highly dangerous substances needing more containment than a fume hood offers, consider using a glove box.
  • Pollution Control: An unfiltered fume hood is not a pollution control device. All contaminants that are removed by the ventilating system are released directly into the atmosphere. Apparatus used in hoods should be fitted with condensers, traps or scrubbers to contain and collect waste solvents or toxic vapors or dusts as appropriate.
  • Waste Disposal: A fume hood should not be used for waste disposal. It is a violation of environmental regulations to intentionally allow volatile waste to evaporate and escape up the hood stack. As described above, the hood is not a pollution control device.

The Fume Hood as a Storage Device

Fume hoods are designed specifically to provide ventilation for the protection of lab occupants during chemical manipulations. The airflow they provide is greatly in excess of that needed for storage of closed containers of even the most toxic of volatile materials. Storing materials in this way is, therefore, a misuse of an expensive piece of equipment.

In general, the storage of chemicals in fume hoods is strongly discouraged. The realities of available space and equipment in some laboratories may make it difficult or impossible to completely prohibit the use of hood workspaces for storage. In such a case, the following general policy is recommended:

Hoods Actively in Use for Experimentation

Storage of materials should be minimized or eliminated altogether. Materials stored in the hood can adversely affect containment. In addition, the hood is frequently the focus of the most hazardous activities conducted in the laboratory. The presence of stored flammable or volatile, highly toxic materials can only exacerbate the problems resulting from an explosion or fire in the hood. Even if they are not directly involved in such an event, attempts to control or extinguish a fire may result in the spilling of stored materials.

Hoods Not in Active Use

Materials requiring ventilated storage (e.g., volatile and highly toxic, or odorous substances) may be stored in a hood if they are properly segregated and the hood is posted to prohibit its use for experimental work.

Alterations to Fume Hood

Changes or additions to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor is prohibited. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device. Additionally, some components of older hoods may contain asbestos and therefore should not be damaged.

Keeping Track of Your Fume Hood

All fume hoods are assigned to a principal investigator and lab or shop group in SHIELD. They can be found under the ‘Equipment’ tab on the main lab/shop page.

If you have questions or do not see your hood listed, contact

Fume Hood in SHIELD Screenshot

Click on the “View” link to open the equipment detail page to review important information corresponding to the hood. The current and all previous annual performance check reports for the hood are available to be reviewed here. Documents or other service records can also be uploaded and shared/viewed as you wish.

Fume Hood in SHIELD Profile

Glove boxes

A glove box is a sealed container used to manipulate materials where a separate atmosphere is desired.  They are commonly used to protect workers from hazardous materials or to protect chemicals and materials that may be sensitive to air or water vapor. 

Glove boxes may be used under either positive or negative pressure.  Glove boxes operated under positive pressure usually contain materials sensitive to outside contaminates such as air or water vapor.  Exposure to outside contaminates can lead to degredation or a violent reaction with these compounds.  Negative pressure glove boxes are used to protect workers and are used for hazardous materials such as toxic gases or pathogens. 

Daily Inspections

When using glove boxes, perform daily inspections prior to use.   As part of your daily checklist, perform the following:

  • Check the condition of the gloves.  Look for holes, areas of discoloration representing a compromised integrity, and the connection to the exterior.
  • Inspect the condition of the window, paying special attention to the area where the window is connected to the rest of the box.
  • Perform a vacuum pump inspection and ensure that all lines are in good condition and that the oil (if applicable) has been changed recently.
  • Inspect vacuum pump exhaust oil-mist filter and ensure it is still within operating parameters.
  • If your box is equipped with a solvent scrubber and solvent delivery system, ensure that the scrubber cartridges are within operating parameters.
  • All pressure gauges and indicators are functioning and are within acceptable ranges.

Other Considerations:

  • If it is a shared glove box, assign 1 or 2 senior people in the lab to ensure that all maintenance on the box and components are up-to-date. 
  • Maintain service contracts with the manufacturer and have them perform routine maintenance on the system.
  • Avoid abruptly extending gloves into the box, this can severely stress the system and cause an overpressurization.
  • Use nitrile gloves on the glove box gloves.  This extends the life of the glove box gloves and helps to avoid cross contamination and makes cleanup easier
  • Train all individuals working in the box.  Document this training in a laboratory specific training file.
  • Ensure proper backup measures are in place for a loss of power or loss of facility nitrogen. 

Heating Devices

Most labs use at least one type of heating device, such as ovens, hot plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, hot-air guns and microwave ovens. Steam-heated devices are generally preferred whenever temperatures of 100o C or less are required because they do not present shock or spark risks and can be left unattended with assurance that their temperature will never exceed 100o C. Ensure the supply of water for steam generation is sufficient prior to leaving the reaction for any extended period of time.

General Precautions

When working with heating devices, consider the following: 

  • The actual heating element in any laboratory heating device should be enclosed in such a fashion as to prevent a laboratory worker or any metallic conductor from accidentally touching the wire carrying the electric current. 
  • If a heating device becomes so worn or damaged that its heating element is exposed, repair the device before it is used again or discard of the device. 
  • Use a variable autotransformer on a laboratory heating device to control the input voltage by supplying some fraction of the total line voltage, typically 110 V.
  • Locate the external cases of all variable autotransformers where water and other chemicals cannot be spilled onto them and where they will not be exposed to flammable liquids or vapors. 

Fail-safe devices can prevent fires or explosions that may arise if the temperature of a reaction increases significantly because of a change in line voltage, the accidental loss of reaction solvent or loss of cooling. Some devices will turn off the electric power if the temperature of the heating device exceeds some preset limit or if the flow of cooling water through a condenser is stopped owing to the loss of water pressure or loosening of the water supply hose to a condenser.


Electrically heated ovens are commonly used in the laboratory to remove water or other solvents from chemical samples and to dry laboratory glassware. Never use laboratory ovens for human food preparation.

  • Laboratory ovens are constructed such that their heating elements and their temperature controls are physically separated from their interior atmospheres.
  • Laboratory ovens rarely have a provision for preventing the discharge of the substances volatilized in them. Connecting the oven vent directly to an exhaust system can reduce the possibility of substances escaping into the lab or an explosive concentration developing within the oven. 
  • Do not use ovens to dry any chemical sample that might pose a hazard because of acute or chronic toxicity unless special precautions have been taken to ensure continuous venting of the atmosphere inside the oven. 
  • To avoid explosion, rinse glassware with distilled water after rinsing with organic solvents before being dried in an oven.
  • Do not dry glassware contianing organic compounds in an unvented oven.
  • Bimetallic strip thermometers are preferred for monitoring oven temperatures. Do not mount mercury thermometers through holes in the top of ovens so that the bulb hangs into the oven. If a mercury thermometer is broken in an oven of any type, turn off and close the oven immediately.  Keep it closed until cool. Remove all mercury from the cold oven with the use of appropriate cleaning equipment and procedures in order to avoid mercury exposure.

Hot Plates

Laboratory hot plates are normally used for heating solutions to 100o C or above when inherently safer steam baths cannot be used. Ensure any newly purchased hot plates are designed in a way that avoids electrical sparks. Older hot plates pose an electrical spark hazard arising from either the on-off switch located on the hot plate, the bimetallic thermostat used to regulate the temperature or both.

In addition to the spark hazard, old and corroded bimetallic thermostats in these devices can eventually fuse shut and deliver full, continuous current to a hot plate. 

  • Do not store volatile flammable materials near a hot plate 
  • Limit use of older hot plates for flammable materials. 
  • Check for corrosion of thermostats. Corroded bimetallic thermostats can be repaired or reconfigured to avoid spark hazards. Contact EHS for more info. 

Heating Mantles

Heating mantles are commonly used for heating round-bottomed flasks, reaction kettles and related reaction vessels. These mantles enclose a heating element in a series of layers of fiberglass cloth. As long as the fiberglass coating is not worn or broken, and as long as no water or other chemicals are spilled into the mantle, heating mantles pose no shock hazard.

  • Always use a heating mantle with a variable autotransformer to control the input voltage. Never plug them directly into a 110-V line.
  • Be careful not to exceed the input voltage recommended by the mantle manufacturer. Higher voltages will cause it to overheat, melt the fiberglass insulation and expose the bare heating element. 
  • If the heating mantle has an outer metal case that provides physical protection against damage to the fiberglass, it is good practice to ground the outer metal case to protect against an electric shock if the heating element inside the mantle shorts against the metal case. 
  • Some older equipment might have asbestos insulation rather than fiberglass. Contact EHS to replace the insulation and for proper disposal of the asbestos. 

Oil, Salt and Sand Baths

Electrically heated oil baths are often used to heat small or irregularly shaped vessels or when a stable heat source that can be maintained at a constant temperature is desired. For temperatures below 200 °C, a saturated paraffin oil is often used; for temperatures up to 300 °C, a silicone oil should be used. Care must be taken with hot oil baths not to generate smoke or have the oil burst into flames from overheating.  Molten salt baths, like hot oil baths, offer the advantages of good heat transfer, but have a higher operating range (e.g., 200 to 425oC) and may have a high thermal stability (e.g., 540oC).There are several precautions to take when working with these types of heating devices:

  • When using oil, salt, or sand baths, do not spill water or volatile substances into the baths. Such an accident can splatter hot material over a wide area and cause serious injuries.
  • Take care with hot oil baths not to generate smoke or have the oil burst into flames from overheating.
  • Always monitor oil baths by using a thermometer or other thermal sensing devices to ensure that its temperature does not exceed the flash point of the oil being used. 
  • Fit oil baths left unattended with thermal sensing devices that will turn off the electric power if the bath overheats. 
  • Mix oil baths well to ensure that there are no “hot spots” around the elements that take the surrounding oil to unacceptable temperatures. 
  • Contain heated oil in a vessel that can withstand an accidental strike by a hard object. 
  • Mount baths carefully on a stable horizontal support such as a laboratory jack that can be raised or lowered without danger of the bath tipping over. Iron rings are not acceptable supports for hot baths. 
  • Clamp equipment high enough above a hot bath that if the reaction begins to overheat, the bath can be lowered immediately and replaced with a cooling bath without having to readjust the equipment setup. 
  • Provide secondary containment in the event of a spill of hot oil. 
  • Wear heat-resistant gloves when handling a hot bath. 
  • The reaction container used in a molten salt bath must be able to withstand a very rapid heat-up to a temperature above the melting point of salt. 
  • Take care to keep salt baths dry since they are hygroscopic, which can cause hazardous popping and splattering if the absorbed water vaporizes during heat-up.

Hot Air Baths and Tube Furnaces

Hot air baths are used in the lab as heating devices. Nitrogen is preferred for reactions involving flammable materials. Electrically heated air baths are frequently used to heat small or irregularly shaped vessels. One drawback of the hot air bath is that they have a low heat capacity. As a result, these baths normally have to be heated to 100oC or more above the target temperature. Tube furnaces are often used for high-temperature reactions under pressure. Consider the following when working with either apparatus:

  • Ensure that the heating element is completely enclosed. 
  • For air baths constructed of glass, wrap the vessel with heat resistant tape to contain the glass if it should break. 
  • Sand baths are generally preferable to air baths. 
  • For tube furnaces, carefully select glassware and metal tubes and joints to ensure they are able to withstand the pressure. 
  • Follow safe practices outlined for both electrical safety and pressure and vacuum systems.

Heat Guns

Laboratory heat guns are constructed with a motor-driven fan that blows air over an electrically heated filament. They are frequently used to dry glassware or to heat the upper parts of a distillation apparatus during distillation of high-boiling materials.

Read the Heat Gun Advisory for more information on proper selection and use of a heat gun for research operations.

Microwave Ovens

Use microwave ovens specifically designed for laboratory use. Domestic microwave ovens are not appropriate. Microwave heating presents several potential hazards not commonly encountered with other heating methods: extremely rapid temperature and pressure rise, liquid superheating, arcing, and microwave leakage. Microwave ovens designed for the laboratory have built-in safety features and operation procedures to mitigate or eliminate these hazards. Microwave ovens used in the laboratory may pose several different types of hazards.

  • As with most electrical apparatus, there is the risk of generating sparks that can ignite flammable vapors.
  • Metals placed inside the microwave oven may produce an arc that can ignite flammable materials.
  • Materials placed inside the oven may overheat and ignite.
  • Sealed containers, even if loosely sealed, can build pressure upon expansion during heating, creating a risk of container rupture.

To minimize the risk of these hazards, 

  • Never operate microwave ovens with doors open in order to avoid exposure to microwaves.
  • Do not place wires and other objects between the sealing surface and the door on the oven’s front face. The sealing surfaces must be kept absolutely clean.
  • Never use a microwave oven for both laboratory use and food preparation.
  • Electrically ground the microwave. If use of an extension cord is necessary, only a three-wire cord with a rating equal to or greater than that for the oven should be used.
  • Do not use metal containers and metal-containing objects (e.g., stir bars) in the microwave. They can cause arcing.
  • Do not heat sealed containers in the microwave oven. Even heating a container with a loosened cap or lid poses a significant risk since microwave ovens can heat material so quickly that the lid can seat upward against the threads and containers can explode.
  • Remove screw caps from containers being microwaved. If the sterility of the contents must be preserved, use cotton or foam plugs. Otherwise plug the container with kimwipes to reduce splash potential.
  • Do not modify a microwave for experiemental use. 

Other Laboratory Exhaust Systems

Other Laboratory Exhaust Systems

Many laboratories use equipment and apparatus that can generate airborne contaminants, but cannot be used within a fume hood. Examples include gas chromatographs, ovens, and vacuum pumps.

Other types of local exhaust ventilation systems may be required to control contaminants generated by these operations. Such systems must not be installed without explicit approval of the building facility manager, Facilities Engineering and/or maintenance personnel.

Elephant Trunks

An elephant trunk is a flexible duct or hose connected to an exhaust system. It can only capture contaminants that are very close to the inlet of the hose, typically less than a distance equal to one half of the diameter of the duct.

Elephant trunks can be effective for capturing discharges from gas chromatographs, pipe nipples or the end of tubing. However, the effectiveness of the elephant trunk should be carefully evaluated before they are used to control releases of hazardous substances.

Canopy Hoods

A canopy hood in a laboratory is constructed in a similar fashion to the overhead canopy hoods seen in kitchens. In order for the canopy hood to be able to capture contaminants, the hood requires a relatively large volume of air movement, making them somewhat costly to operate. The canopy hood works best when the thermal or buoyant forces exist to move the contaminant up to the hood capture zone.

One of the biggest problems with canopy hoods is that, in most cases, they are designed such that the contaminated air passes through the individual's breathing zone. The airflow is easily disrupted by cross currents of air.

For the most part, canopy hoods should only be used for exhaust of non-hazardous substances.

Slot Hoods

There are many types of slot hoods, each suited for different types of operations. In general, a slot hood requires less airflow than a canopy hood and is much more effective than an elephant trunk or canopy hood, when installed properly.

Slot hoods are best used for operations that require more working room than a fume hood and where a limited number of low toxicity chemicals are used. The placement of the opening(s) and the velocity of airflow are based on the application, particularly dependent upon the vapor density of the chemical(s).

Examples of good uses for slot hoods are darkrooms and acid dipping operations.

Downdraft Tables

Downdraft hoods or necropsy tables are specially designed work areas with ventilation slots on the sides of the work area. This type of system is useful for animal perfusions and other uses of chemicals with vapor densities heavier than air.

Toxic Gas Cabinets

Highly toxic or odorous gases should be used and stored in gas cabinets. In the event of a leak or rupture, a gas cabinet will prevent the gas from contaminating the laboratory.

Gas cabinets should be connected to laboratory exhaust ventilation using hard duct, rather than elephant tubing, since such tubing is more likely to develop leaks. Coaxial tubing should be used for delivering gas from the cylinder to the apparatus. Coaxial tubing consists of an internal tube containing the toxic gas, inside another tube. In between the two sets of tubing is nitrogen, which is maintained at a pressure higher than the delivery pressure of the toxic gas. This ensures that, in the event of a leak in the inner tubing, the gas will not leak into the room.

Biosafety Cabinets

Please see the Biosafety Cabinet page for more information.

Ductless Fume Hoods

Use of a "ductless fume hood" is strongly discouraged. These devices work by using a fan to draw air into a chamber, through one or more filters, and back into the laboratory. EHS and several professional safety and engineering organizations do not recommend the use of ductless fume hoods for several reasons. First, it is difficult to determine whether the filters are functioning adequately or need to be changed; thus, the potential for recirculating toxic materials into the laboratory is significant. In the event of a chemical spill, the hood is usually not able to contain the spilled material or the potentially high concentrations of chemical vapors.

Second, the face velocity of the hood is normally below 80 feet per minute. The hood is normally designed such that the air does not flow smoothly and evenly through the hood. Both of these characteristics make it likely for disruption of airflow or turbulence, causing unfiltered air to leak into the laboratory.

Clean Benches

Clean benches are similar to appearance as a fume hood however do not exhaust air from the laboratory. A clean bench is a device that draws air from the lab through a HEPA filter and vents the filtered air downwards onto a work surface to keep the materials within free from particulate contamination. These devices are not to be used with hazardous materials as they provide no personal protection. Do not store materials on top of this hood as this will block the filter, overload the motor, and provide poor product protection.



Pressure and Vacuum Systems

Working with hazardous chemicals at high or low pressures requires planning and special precautions. Procedures should be implemented to protect against explosion or implosion through appropriate equipment selection and the use of safety shields. Care should be taken to select glass apparatus that can safely withstand designated pressure extremes.

High Pressure Vessels

  • Only perform high-pressure operations in pressure vessels appropriately selected for the operation, properly labeled and installed, and protected by pressure-relief and necessary control devices.
  • Vessels must be strong enough to withstand the stresses encountered at the intended operating temperatures and pressures and must not corrode or otherwise react when in contact with the materials it contains.
  • Equip systems designed for use at elevated temperatures with a positive temperature controller. Avoid using a manual temperature control, such as a Variac. Use use of a back-up temperature controller capable of shutting the system down.
  • Inspect and test all pressure equipment at intervals determined by the severity of the equipment's usage. Perform a visual inspection before each use.
  • Perform hydrostatic testing before equipment is placed in initial service. Perform hydrostatic testing every ten years thereafter, after significant repair or modification, or if the vessel experiences overpressure or overtemperature. Contact the EHS at 609-258-5294 for more information about hydrostatic testing.

Vacuum Apparatus

Vacuum work can result in an implosion and the possible hazards of flying glass, splattering chemicals and fire. All vacuum operations must be set up and operated with careful consideration of the potential risks. Equipment at reduced pressure is especially prone to rapid pressure. Such conditions can force liquids through an apparatus, sometimes with undesirable consequences.

  • Use personal protective equipment, such as safety glasses or chemical goggles, face shields, and/or an explosion shield to protect against the hazards of vacuum procedures, and the procedure should be carried out inside a hood.
  • Do not allow water, solvents and corrosive gases to be drawn into vacuum systems. Protect pumps with cold traps and vent their exhaust into an exhaust hood.
  • Assemble vacuum apparatus in a manner that avoids strain, particularly to the neck of the flask.
  • Avoid putting pressure on a vacuum line to prevent stopcocks from popping out or glass apparatus from exploding.
  • Place vacuum apparatus in such a way that the possibility of being accidentally hit is minimized. If necessary, place transparent plastic around it to prevent injury from flying glass in case of an explosion.

Vacuum Trapping

When using a vacuum source, place a trap between the experimental apparatus and the vacuum source.  The vacuum trap

  • protects the pump and the piping from the potentially damaging effects of the material
  • protects people who must work on the vacuum lines or system, and
  • prevents vapors and related odors from being emitted back into the laboratory or system exhaust.

There have been incidents at Princeton where improper trapping caused vapor to be emitted from the exhaust of the house vacuum system, resulting in either re-entry into the
building or potential exposure to maintenance workers.  Unfortunately, this type of incident is not the worst that can happen.  In 2001, at the University of California -
Davis, two plumbers were injured when a house vacuum line burst after one of the plumbers attempted to solder a fitting on the copper line.  Results of analysis found
evidence of copper perchlorate (an oxidizer) and acetate, which created an explosive mixture upon heating by the torch.

Proper Trapping Techniques
To prevent contamination, all lines leading from experimental apparatus to the vacuum source should be equipped with filtration or other trapping as appropriate.
For particulates, use filtration capable of efficiently trapping the particles in the size range being generated

  • For most aqueous or non-volatile liquids, a filter flask at room temperature is adequate to prevent liquids from getting to the vacuum source.
  • For solvents and other volatile liquids, use a cold trap of sufficient size and cold enough to condense vapors generated, followed by a filter flask capable of collecting fluid that could be aspirated out of the cold trap.
  • For highly reactive, corrosive or toxic gases, use a sorbent canister or scrubbing device capable of trapping the gas.

Cold Traps
For most volatile liquids, a cold trap using a slush of dry ice and either isopropanol or ethanol is sufficient (to -78 deg. C).  Avoid using acetone.  Ethanol and isopropanol
are cheaper and less likely to foam.

Liquid nitrogen may only be used with sealed or evacuated equipment, and then only with extreme caution.  If the system is opened while the cooling bath is still in contact
with the trap, oxygen may condense from the atmosphere and react vigorously with any organic material present.

Glass Vessels

Although glass vessels are frequently used in pressure and vacuum systems, they can explode or implode violently, either spontaneously from stress failure or from an accidental blow.

  • Conduct pressure and vacuum operations in glass vessels behind adequate shielding.
  • Ensure the glass vessel is designed for the intended operation.
  • Carefully check glass vessels for star cracks, scratches or etching marks before each use. Cracks can increase the likelihood of breakage or may allow chemicals to leak into the vessel.
  • Seal glass centrifuge tubes with rubber stoppers clamped in place. Wrap the vessel with friction tape and shield with a metal screen. Alternatively, wrap with friction tape and surround the vessel with multiple layers of loose cloth, then clamp behind a safety shield.
  • Glass tubes with high-pressure sealers should be no more than 3/4 full.
  • Sealed bottles and tubes of flammable materials should be wrapped in cloth, placed behind a safety shield, then cooled slowly, first with an ice bath, then with dry ice.
  • Never rely on corks, rubber stoppers or plastic tubing as pressure-relief devices.
  • Glass vacuum dessicators should be made of Pyrex or similar glass and wrapped partially with friction tape to guard against flying glass. Plastic dessicators are a good alternative to glass, but still require shielding.
  • Never carry or move an evacuated dessicator.

Dewar Flasks

Dewar flasks are under vacuum to provide insulation and can collapse from thermal shock or slight mechanical shock.

  • Shield flasks with friction tape or enclose in a wooden or metal container to reduce the risk of flying glass.
  • Use metal flasks if there is a significant possibility of breakage.
  • Styrofoam buckets offer a short-term alternative to dewar flasks.

Rotory Evaporators

Rotory evaporators can implode under certain conditions. Since some rotovaps contain components made of glass, this can be a serious hazard. Glass components of the rotary evaporator should be made of Pyrex or similar glass. Glass vessels should be completely enclosed in a shield to guard against flying glass should the components implode. Increase in rotation speed and application of vacuum to the flask whose solvent is to be evaporated should be gradual.

Refrigerators and Freezers

The potential hazards posed by laboratory refrigerators and freezers include release of vapors from the contents, the possible presence of incompatible chemicals, and spillage. Only refrigerators that have been specified for laboratory use should be utilized to store chemicals.


General Purpose:

General laboratory refrigerators and freezers are domestic use units that are traditionally used to store food and beverages. While not usually suitable for a laboratory environment, they may be used for storing aqueous solutions. No flammable materials should be stored in these units. Food and drink are not allowed and the units should be labeled as such. 

Not for flammable materials storage


Flammable material refrigerators and freezers are designed for the storage of flammable solids and liquids. There is no internal switching or wiring that can arc, spark, or generate a source of ignition. The compressor and other circuits usually are located at the top of the unit to reduce the potential for ignition of flammable vapors. These refrigerators also incorporate features such as thresholds, self-closing doors, and magnetic door gaskets. Special inner shell materials limit damage should an exothermic reaction occur within the storage compartment. Be sure to observe flammable storage units, which should be listed on the label of the unit. 

Flammable material refrigerator label

Explosion Proof:

These units are designed to be operated in areas where the atmosphere outside of the unit could become explosive. These are not usually necessary in a typical laboratory setting. Please contact EHS if you feel the need for one of these units.

explosion proof refrigerator

Food and Drink:

Absolutely no food or drinks are allowed to be stored in laboratory refrigerators containing reagents, samples, and any other research materials.  All refrigerators and freezers must be labeled "No Food or Drink to be Stored in this Refrigerator".  Conversely, if food and drinks are stored in a unit near a laboratory, it must be labeled as "Refrigerator for Food Only".  Both of these labels are available from EHS by calling 8-5294 or using the Safety Supply Order Form (login required).

no food to be stored in this refrigerator    refrigerator for food only


Minus80 Monitoring is an environmental monitoring system used to monitor of critical refrigerator and freezer storage, incubators and other temperature sensitive equipment.  The system provides researchers with real-time access to cloud-based data accessible via web portal, mobile phone or tablet.  The system can deliver fully configurable alerts and alarms directly to multiple email, text, or telephone contacts.  

The system can monitor:

  • Internal (storage) temperature
  • Ambient temperature & humidity
  • Door open/close status (including frequency)
  • Unit power status

All monitored parameters are trended and available in real time. Please contact EHS for more information.

Safe Handling and Operating Procedures

  • Label all materials with the contents, owner, date of aquisition, and any associated hazards.  Readily identifiable coding to a reference document (laboratory notebook, posted inventory, etc.) may be used.
  • Follow all chemical compatibility storage guidelines.
  • All materials must be properly capped and sealed.  Avoid to use of foil or parafilm as a primary method for sealing the container.
  • Shelves must be compatible with the materials stored and secondary containment should be used when storing liquids.
  • Remember that power outages will cause a rise in termperature within the unit. This may lead to energetic decomposition.  Please keep this in mind and use emergency power outlets where available. 
  • Avoid using frost-free refrigerators and freezers.

Stirring and Mixing Devices

The stirring and mixing devices commonly found in laboratories include stirring motors, magnetic stirrers, shakers, small pumps for fluids and rotary evaporators for solvent removal. These devices are typically used in laboratory operations that are performed in a hood, and it is important that they be operated in a way that precludes the generation of electrical sparks.

Use only spark-free induction motors in power stirring and mixing devices or any other rotating equipment used for laboratory operations. While the motors in most of the currently marketed stirring and mixing devices meet this criterion, their on-off switches and rheostat-type speed controls can produce an electrical spark because they have exposed electrical conductors. Do not control speed of an induction motor operating under a load should with a variable autotransformer. 

Consider the consequences of stirrer failure, electrical overload or blockage of the motion of the stirring impeller. Stirring and mixing devices, especially stirring motors and magnetic stirrers, are often operated for fairly long periods without constant attention.