HydroCarbon
Hazard to Self
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Location: Anytown, USA
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Using a sand bath on a hot plate
I've got a hot plate (Nuova Thermolyne) that has a warning tab which states to not use a sand bath with it. I'm guessing to prevent overheating of
the plate. It also has a warning to not use any flammable materials near or on the hot plate.
I was under the impression that these were two things that hot plates were normally used with; mostly in the process of distillations. Are these
warnings over-exaggerated?
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densest
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No, and somewhat. The reasons:
The heating element is a (probably nichrome) wire inside the top plate.
The top plate is a bad conductor of heat.
The temperature control turns the power full on and full off with the ratio controlling the output. These pulses are multiple seconds long. The top
plate gets very hot near the element during the "on" pulse.
A sand bath is a reasonably good insulator - one uses it to distribute heat gently over an area.
The heating element under a sand bath can get hot enough to melt or crack the top plate.
The local hot spots can get hot enough to ignite some kinds of flammable vapors.
The temperature control has electrical contacts open to the air which make sparks every time they open and close. This is sufficient to ignite almost
any flammable vapors in the right concentration in air.
So, if one understands what's happening, one might put a (say) copper plate on the hot plate and then put the sand bath on that. No guarantees -
monitor how hot the plate gets!
Heating a volatile flammable liquid in an open container would be a very bad idea.
Distilling such a liquid in a closed system which is carefully clamped and doesn't have scratches on the glass, overtightened clamps, loose water
fittings, etc. might be OK if one already has planned how to put out any fire if glass breaks and monitors the distillation to be sure nothing leaks.
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BromicAcid
International Hazard
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| Quote: | Originally posted by HydroCarbon
It also has a warning to not use any flammable materials near or on the hot plate. |
This is because the hotplate is not explosion proof. Turning the variac on the plate can cause tiny spark which could ignite solvent vapors.
Hotplates that are XP rated for hazardous situations are significantly more expensive. The likelyhood of flashback us usually limited so often people
don't give using solvents and hotplates together much of a second thought.
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smuv
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Considering the nuova has a thick, coated aluminum heating surface, it will not crack under the types of temperature deltas we are dealing with in
laboratory settings. Also the nuova is controlled by a thermostat, so it will not overheat.
I have a nuova and before you posted, I wouldn't have thought twice using a sand bath; although I have always avoided using a sandbath with my
hotplate because, I find it to be kind of weak for heating up a sand bath and therefore I usually use a dimmer controlled griddle (1.2Kw w/ NO
thermostat  ) for my sand baths.
I guess I am saying, I have no idea why you can't do it, although it doesn't really matter because it would take forever to heat up and would have a
low max temp; might as well use an oil bath or get yourself a griddle and a dimmer switch 
[Edited on 3-9-2009 by smuv]
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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