A SIMPLE PRESSURE REGULATOR FOR VACUUM DISTILLATIONS
The pressure regulators used in vacuum distillations are usually quite complicated. The device to be described here is thought to be much more simple than the ordinary types, and at the same time fully as dependable. A distillation is started with the mercury in the manometer touching the contact C and the armature A closing the capillary leak L. Stopcock S is left open until the pressure in the apparatus is reduced to the desired value and then closed, thus cutting off a volume of air in the bulb B which acts as a pressure standard. Should the pressure in the apparatus become lower than that in B, the mercury in the manometer is forced down on the side
joined to B, thus breaking the connection between the contact C and the mercury, and stopping the current in the magnets M. This allows the armature A to uncover the end of the capillary leak which lets air into the apparatus. When the pressure in the apparatus becomes greater than that in the bulb, the mercury is forced up on the side connected to B until it touches the contact C again, thus completing the electrical circuit and letting the current flow through the magnets, which then pull the armature back against the capillary leak, thus closing it. This cycle is repeated several times each minute when the apparatus is correctly adjusted. The dimensions of the apparatus may vary. A convenient size for the bulb B is about 75 cc. The manometer can be made from 8-mm. tubing. The capillary leak should be a piece of 0.5-mm. capillary tubing with the end ground to a cone so that it will seat well in the soft rubber pad fastened 1275
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JOURNAL OF CHEMICAL EDUCATION
JULY.
1932
on to the armature with shellac. Relay magnets with a resistance of 60 ohms allow the use of a single dry cell to operate them and keep the current through the manometer so low that fouling of the mercury surface is negligible. The spring which pulls the armature away from the capillary leak must be made strong enough to pull it against atmospheric pressure when the regulator is working a t the lowest pressure to be used. It should be noted that the apparatus can be used with a vacuum pump which is not electrically operated, such as an aspirator.
