Gauge sensitivity thermal conductivity gauges

The fact that the pressure reading at vacuum gauges (see Section 3.3) is sensitive to the type of gas involved can, to a certain extent, be utilized for leak detection purposes. Thus it is possible to brush or spray suspected leaks with alcohol. The alcohol vapors which flow into the device - the thermal conductivity and ionizablity of which will vary greatly from the same properties for air - will affect and change pressure indication to a greater or lesser extent. The availability of more precise, easy-to-use helium leak detectors has, however, rendered this method almost completely obsolete. 

Analysis of hydrogen mixtures with a thermal conductivity cell is well established. The most accurate measurements are obtained by use of a thermal conductivity gauge with the walls immersed in liquid nitrogen and the wire heated to 160° K. This is the teniperature when the difference in the rotational specific heats of orthpara-hydrogen is a maximum . Various modifications of thermal conductivity gauges have been made to improve their convenience in use . A room temperature flow analyser based on a thermal conductivity cell has been developed by Weitzel and White which is claimed to be as sensitive as low temperature units. Bridge current and temperature must be controlled very carefully, but the unit is relatively insensitive to changes in pressure and flow rate. 

---