Pressure measurement and regulation

It is very important to be able to measure the pressure in a vacuum system, particularly when carrying out a distillation. For low vacuum measurement a simple manometer, such as that shown in Fig. 8.3a, is commonly used and the pressure is taken by subtracting the heights of the mercury levels. Dial gauges are also useful for in-line measurements and they are particularly valuable when used with rotary evaporators. For high vacuum 

Quite a common requirement is for a distillation pressure somewhat higher than that delivered by the high vacuum pump. Achieving this is no simple matter because a very accurate leak must be provided in order to maintain a constant vacuum. There are various devices available, one of the simplest being an accurate needle valve (such as an Edwards LV5) which will be suitable for most distillation purposes. 

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