•McLeod Gauge is used for measurement of vaccum (low pressure) which uses the same principle as that of a manometer.

•By using the pressure dividing technique, its range can be extended from a value of 〖10〗^(-4) Torr.

•It can measure the pressure of gas which obeys Boyle’s law i.e. P1V1 = P2V2

•The basic principle in the multiple compression technique.

•It there are two bulbs A and B connected with the McLeod and test gauges through capillary tubing, the pressure on the right hand side of the test gauge is very small and the capillary connection between sealed end and bulb is very long.

Working

•The gauge is used to compress a small quantity of low pressure gas to produce a readable large pressure.

•Bulb of the gauge is attached to capillary.

•First the plunger is withdrawn lowering the level of mercury below the opening and hence the gas at low pressure is admitted in the tube as shown in the figure (a).

•Let this volume of gas admitted be V.

•The bulb and capillary tube are at same pressure P now. When then plunger is pushed in, the mercury level increases and seals off the gas sample of known volume V as shown in figure (b)

•Further motion of plunger causes the compression of this gas.

•The motion is continued till the mercury reaches capillary reference.

•Assuming the compression of gas as isothermal compression, the unknown pressure is calculated by using Boyle’s law as follows, PV = Pc Al h Where, Pc = Pressure of gas in capillary tube, Pascal Al = Area of cross-section of capillary tube, m^2 Where, h = Height of mercury column in reference capillary ρm ρm = Density of mercury; kg/m^3.

The above equation becomes,

P = (ρm h^2 AlPc)/(V-(h Al) ) (h^2 Al)/V

•The McLeod gauge is independent of gas composition.

•If, however the gas containing condensable material and during compression it condenses, the reading of the gauge becomes faulty.

•The gauge is not capable of continuous reading and the scale is of square law type.

•For linearizing the scale at comparatively higher pressures, a second volume is introduced as shown in the figure below, where the scale shown is linear.

Advantages

•The gauge reading is independent of gas.

•It serves as a reference standard to calibrate other low pressure gauges

•There is no need of any zero error corrections.

Disadvantages

•The gas should obey the Boyle’s law.

•It does not give a continuous output.