McLeod gauge condensable vapors

The McLeod gauge is not suitable for the determination of pressures of easily condensed gases, such as water vapor, and it has the additional disadvantage of being slow and sometimes clumsy to operate. Because of this naturally slow response, the electronic vacuum gauges are superior for tracing leaks. 

The McLeod gauge has no ability to compensate for condensable vapors. 

A liquid trap can be placed between the McLeod gauge and the rest of the system to prevent mercury from accidentally spraying throughout your system. If you do not want condensable vapors affecting the McLeod gauge readings or do not want mercury vapors to enter your system, a cold trap can be placed between the liquid trap (shown in Fig. 7.41) and the main vacuum line. 

Fig. 7.45 By making linear measurements as the mercury rises within a McLeod gauge, it is possible to determine whether there are condensable vapors within the McLeod gauge.

Cold traps are used in instrumentation and elsewhere to prevent the introduction of vapors or hquids into a measuring instrument from a system, or from a measuring insfrument (such as a Mcleod gauge) into the system. A cold trap provides a very low-temperature surface on which such molecules can condense and improves pump-down (the achievable vacuum) by one or two magnitudes. 

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