In both stages of distillation it is necessary to know the temperature of the vapour stream inside the system in order to know what's going on, since temperature and composition are closely related, and the simplest method for introducing a thermometer would be to use a cork. This is the method we used for many years but it leaves much to be desired and there is a better method. Hot alcohol/water vapour is very aggressive and corks rather quickly turn into a gnarled object closely resembling the withered core of a rotten apple. Rubber is unacceptable because it gives a flavour to the alcohol. Nowadays we use a brass compression fitting and teflon seal for all thermometer inserts into metal columns.
The construction of the thermometer adapter is shown in Figure 4. Use a 3/8" x V4" compression fitting. There is a shoulder inside these fittings at the mid-point and you will need to drill away this shoulder to let the glass thermometer pass right through. Use a 17/64" bit and drill from the large end, trying to avoid going right through and damaging the seat for the ferule at the V4" end. If you use a digital thermometer, which usually has a 1/8-inch probe, the V8-inch compression fitting is large enough as is without removing the internal shoulder.
Solder a short length of 3/8" copper tubing vertically to the elbow at the top of the column and attach the compression fitting. Use teflon plumber's tape to make the seal, winding several turns around the ther mometer so that when the nut is tightened the teflon is compressed between the thermometer stem and the brass fitting. There is no pressure in the apparatus and no leakage. The bulb of the thermometer should be at the midpoint of the elbow so that it is in the main stream of vapour flow.
Note 1. Some thermometers have stems which are slightly too large in diameter to go through a 17/64" hole. Be careful, therefore, to choose a thermometer which will go through. Or, drill a slightly larger hole.
Note 2. A glass thermometer in such a rigid set-up is very vulnerable to breakage. The slightest touch and
! It is prudent, therefore, to remove it while working round the still.
Note 3. Some of you may wish to use an electronic digital thermometer. They usually have V8" diameter probes. They can be sealed into the system in the same way as a glass thermometer, using either a small teflon plug with a V8" hole drilled through it or simply by winding more teflon tape around the stem.
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