Heat a 100-mL flask in a 100-mL THERMOWELL, a 250-mL flask in a 250-mL THERMOWELL, etc. This is the most efficient method with the fastest heating and the greatest control.
Heating undersized flasks in a THERMOWELL works very well. For example, a 50-mL flask in a 100-mL unit, or a 25-mL flask in a 50-mL unit. The open space between the flask and the cavity wall should not be filled with sand or anything else, but this space can be covered loosely with a piece of household aluminum foil (C-shaped piece) when distilling high-boilers. The flask and contents are heated by radiated heat.
A sand bath is good for microscale procedures only, because sand is a paradox — it's a better insulator than it is a heat transfer material. The unique thing about a sand bath is the big temperature gradient that it can generate. For example, fill a 100-mL THERMOWELL (use no other size heater) with quality sand, operate it for at least 20 minutes at the maximum 40% power (75v or 145v) and the temperature of the sand's surface is about 100°C , and the bottom of the sand bath is almost 400°C. Touch a small test tube containing 1 mL of water to the sand surface and you get gentle heating, but push the test tube down to the bottom of the sand bath and the water boils in a few seconds.
Never use sand with THERMOWELLs larger than 100 mL (they superheat too easily). Don't use round-bottom flasks larger than 10 mL in a sand bath. The maximum power level is 40%, equivalent to 75v (120v units) or 145v (230v units). A 100-mL THERMOWELL sand bath can be operated directly on a stone bench-top if it sits on an aluminum heat sink (6"x6" x .040 sheet) at a maximum of 40% power.
We sell sand—a high quality type for sand baths. It's 16-30 mesh, packed in a carton that can be placed directly in the lab. Ten pounds per carton, enough to fill 40 THERMOWELLs.
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