It is believed that a Cole Parmer Polystat 12002 immersion heater in a University of California, Santa Cruz (UCSC) microbiology laboratory did not shut off as the water level in the plastic bath dropped, allowing the temperature to rise to the point that the bath material ignited. The fire caused extensive damage to the building and shut down research for several months.
What was the cause?
Immersion water heaters/circulators used in plastic tubs (baths) have been associated with laboratory fires at several universities in recent years. When the heater temperature rises above 65 degrees Celsius (65ºC), acrylic or plastic baths can lose rigidity, allowing the heating element to come into contact with the container material. Prolonged contact with an active heating element can cause the dry surface of such synthetic materials to ignite. In the incident at UCSC, failure of the thermostat safety cutoff may have permitted temperatures to reach hazardous levels.
It is also important to note that some facilities may still be using older immersion heaters designed without safety cutoff switches. These older units pose the same fire risk as models with thermostats that fail.
It is generally believed that water bath immersion heaters manufactured after 2003 offer excellent safety features. These newer models incorporate low-water shutoff devices to interrupt power to the heating element if the water drops below a safe level. Optimally, the unit utilizes a fluid-bulb thermostat, hardwired to the heating element, which shuts off power when safe temperatures are exceeded. When purchasing or working with immersion heaters, the following precautions should be observed.
Look for the following features in immersion heaters:
- Safety cut-off thermostat
- Fluid-bulb thermostat strapped directly to the heater
- Bulb located lower than the temperature sensor
- Thermostat contacts wired in series with the heater
- Mechanical reset capability
- Thermal breaker or fuse in the control box
- Control box grounded with grounding wire
Look for the following features in baths:
- Non-flammable material; metal, if possible
- Dimensions matched to the circulator head
- Circulator head-mounted securely
Perform these safety checks each time unit is set up:
- Verify that the thermostat is set below 65°C. This is essential when plastic baths are used.
- Test the safety cutoff thermostat by deliberately allowing the bath to run dry. Be sure the cutoff thermostat trips under these conditions.
- Verify that the impeller is not obstructed.
Additional safety recommendations:
- Cover the bath to reduce evaporation.
- Use GFCI–protected power strips on all heating equipment.
- Make every effort not to leave heating devices unattended. If you must leave the device temporarily, use a shutoff timer.
- Unplug heating equipment when they are not in use.
- Be familiar with the specifications for each model of the bath.
For more information, contact the Office of Environment, Health & Safety (EH&S), or see your department equipment technician. If necessary, an EH&S representative can inspect the water heaters in your laboratory.