Mcleod gauge: Working, Diagram, Principle, Advantages, Formula

What is Mcleod gauge?

Mcleod Gauge also known as a compression gauge is a device used to measure pressure at a very low range of up to 10-6 torr.

It is similar to a mercury manometer. It uses mercury as measuring fluid and the height of the mercury column determines the pressure difference.

In this gauge, a known volume of gas is compressed at a constant temperature. This gauge gives the final volume & pressure difference. From this data, we can calculate the unknown pressure of the gas.

mcleod gauge setup

Mcleod gauge was invented by British scientist Herbert McLeod in the year 1874. It is also used to measure vacuum pressure.

Working principle:

This gauge works on the principle of Boyle’s law.

Boyle's law:- For a gas of a certain quantity at constant temperature, the pressure of a gas varies inversely with respect to the volume of gas.  


PV = constant

P1V1= P2V2

Now in this case gas of known volume with unknown pressure is compressed in measuring capillary at a constant temperature.

By knowing the final volume & change in pressure, we can calculate the initial pressure of gas or vacuum.


This gauge consists of the following parts:-

1) Working Fluid or Measuring Fluid:- Here mercury is used as a working fluid for compression of the gas at unknown pressure. The level of mercury should be below the cut-off point at the start of the procedure.

2) Mercury (Hg) Reservoir:- The purpose of the reservoir is to store mercury. It contains a piston or plunger that pushes the mercury into the reference column, bulb, and measuring capillary for compression of gas trapped in the bulb & capillary.

3) Reference Column:- One end of the reference column is attached at the cut-off point & another end is used as an entrance for gas whose pressure is to be measured.

The gas first enters into the Reference column & then enters into the bulb from a cut-off point. The diameter of the Reference column is greater than the diameter of the Reference capillary.

4) Reference Capillary:- Both ends of the reference capillary are connected to the reference column. It is constructed in such a way that it should be closer to measuring capillary.

The purpose of the reference capillary is to measure the difference in levels of both tubes. The diameters of the measuring capillary and reference capillary are the same and less than the diameter of the reference column to reduce capillary errors.

5) Bulb:- The bulb is connected to the reference column at the cut-off point. Another end is connected to the measuring capillary.

The volume of the bulb is considerably higher than the volume of the measuring capillary to take the maximum volume of gas and to increase accuracy during pressure measurement.

6) Measuring Capillary: The top end of the measuring capillary is sealed & another end is connected to the bulb.

The measuring capillary is constructed closer to the reference capillary & scale is fixed in between both capillaries to measure the difference in levels of mercury.

Mcleod gauge working:

Following are the working steps to measure the pressure using this gauge.

First, lower the mercury level in the bulb and reference column below the cut-off point so that gas can enter into the bulb & capillary. This can be done by moving the piston in the upward direction.

mercury level before compression

Connect the reference column to the source of the gas or vacuum whose pressure is to be measured.

After filling the gas into the bulb & measuring the capillary, start pushing mercury into the bulb and reference column by pressing the piston downward.

When mercury crosses the cut-off point, the gas trapped in the bulb and measuring capillary starts compressing.

Fill the mercury till it reaches zero reference point on the reference capillary.

Mercury level after compression

After reaching mercury up to the zero reference point in the capillary, measure the difference in levels between the measuring capillary & reference capillary. It is denoted by ‘h’.

By knowing this height ‘h’, we can calculate the final volume of gas.

It also gives the difference between initial and final pressure.

Now by using Boyle’s law (use the below formula), we can find the unknown pressure of the gas.

Formula for McLeod gauge:

After finding the value for ‘h’ from the McLeod gauge, the unknown pressure P1 can be found by using the following formula.

`P_{1}= frac{ah^{2}}{(V_{1})}` mm of Hg

P1 = Unknown pressure that is to be measured

V1 = Known initial volume = It is the sum of bulb volume and measuring capillary volume.

h= Difference in levels of reference capillary & measuring capillary

a = Area of cross-section of measuring capillary

Advantages of Mcleod Gauge:

The Mcleod gauge has the following advantages:

  1. It ranges up to 10-6 torr.
  2. It can be used for the calibration of gauges.
  3. No effect of gas composition.

Disadvantages of Mcleod Gauge:

The Mcleod gauge has the following disadvantages:

  1. Capillary effect in measuring capillary can affect the measurement.
  2. Gas should follow Boyle’s law.
  3. The mercury can be contaminated.
  4. No continuous output. We need to complete the procedure again and again for pressure measurement.
  5. The presence of moisture in gas can affect accuracy.


  1. How you will use McLeod gauge to measure vacuum pressure?

    Steps to measure vacuum on McLeod gauge:
    a] Lower the mercury to the cut-off point
    b] Connect vacuum source to reference column
    c] Raise the mercury level to zero level of reference capillary
    d] Measure the difference between the levels of measuring capillary and reference capillary.

  2. How do you read a McLeod gauge?

    When the mercury level in the reference capillary reaches the zero point, measure the difference in levels of the reference capillary and measuring capillary.

  3. Where McLeod gauge is used?

    Mcleod Gauge is used to measure very low-range pressure or vacuum.

Read also:

Pratik is a Graduated Mechanical engineer. He enjoys sharing the engineering knowledge learned by him with people.

Leave a Comment