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Laboratory Hot Plate Fails

The Incident

A Laboratory hot plate was in use, the chemist became aware that although both the external controller and the internal controller were set to a low temperature, the hot plate was, if fact, becoming very hot, with no sign of turning off the heat.

The power was removed to prevent an accident

The Investigation

This kind of hot plate is very common in laboratories, typically they have two adjustable features.  One is the stirrer speed, and the other is the temperature.   In addition, the units can have a second temperature probe, that plugs in, which will then control the temperature of the flask/beaker contents, rather than the temperature of the hot plate.  This second probe may be electronic or mechanical (e.g. contact mercury type).  When the external probe is used then the normal internal one is still active, so is set at a higher temperature than the desired temperature.

The problem now lies in that the chemist thinks he has two working thermostats, so regards the internal one as a "safety thermostat".  Sadly he is very wrong.   Both controls work the same power relay, in fact with the electronic controls, the relay is probably worked harder with much shorter pulses of power.  Should the relay fail "on" (i.e the contacts weld together - a not uncommon failure), then the hot plate will heat to destruction

The problem occurred with an old IKA hot plate, but since then the same failure has been reported with early Heildolph hot plates.  It is not known if later models of these hot plates can have the same failure mode - it is understood that some models now have a solid state relay, instead of the mechanical one

Conclusions

Simple hot plates and oil baths are best protected by a separate secondary controller, which will turn off the mains power to the equipment in case of over heat, this does have the disadvantage of losing the stirring as well, so a sensible over temperature figure should be used, so that the protection unit only operates at failure time

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