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:
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.
A 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.
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:
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.
This section covers the various flow and performance indicators and the survey sticker.
This section covers a number of topics aimed at helping laboratory workers understand conditions and proper work practices for using fume hoods safely
This section covers a number of topics aimed at helping laboratory workers understand the limitations and proper use of the 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.
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.
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.
This section pertains to all buildings except Frick Chemistry building – see separate section below.
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.
The Inspection Sticker is aligned on the hood so the arrow indicates the proper location for the maximum safe sash position.
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.
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.
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.
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.
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.
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.
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 |
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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.
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.
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 |
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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 |
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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.
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.
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:
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.
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.
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.
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 ehs@princeton.edu.
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.
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.
When using glove boxes, perform daily inspections prior to use. As part of your daily checklist, perform the following:
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.
When working with heating devices, consider the following:
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 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.
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.
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:
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:
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.
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.
To minimize the risk of these hazards,
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.
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.
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.
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 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.
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.
Please see the Biosafety Cabinet page for more information.
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 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.
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.
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.
Vacuum Trapping
When using a vacuum source, place a trap between the experimental apparatus and the vacuum source. The vacuum trap
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
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.
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.
Dewar flasks are under vacuum to provide insulation and can collapse from thermal shock or slight mechanical shock.
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.
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 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.
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.
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.
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).
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:
All monitored parameters are trended and available in real time. Please contact EHS for more information.
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.