Mcleod Gauge – Principle, Parts, Working, Advantages, and Disadvantages

Mcleod Gauge is an instrument used to measure very low pressures nearly 10-6 Torr. This device is named after its inventor Herbert Mcleod. It was invented by Herbert Mcleod in the year 1874.
The shape of the Mcleod Gauge is similar to the mercury manometer which is the most common instrument used for the measurement of pressure.

This Mcleod Gauge also has mercury inside it and the pressure measurement is done by seeing the changes in the mercury level.

In today’s world, these gauges are replaced largely by electronic vacuum gauges.

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Principle of Mcleod Gauge:

A known volume of gas is compressed to a smaller volume whose final value provides an indication of the applied pressure.

Mcleod Gauge works on the principle of Boyle’s Law.
Boyle’s Law states that if the temperature and amount of gas remain unchanged, the absolute pressure exerted by a given mass of gas is inversely proportional to the volume it occupies.

According to Boyle’s Law:
P1V1 = P2V2

Parts Of Mcleod Gauge:

1 Mercury Reservoir:
Mercury reservoir is used to store mercury which can be used for measurement of pressure in the Mcleod Gauge.

2 Piton:
A piston is used to raise or lower the level of mercury in the measuring capillary, bulb, reference column and reference capillary.

3 Reference column with reference capillary:
The pressure which is to be measured is applied to this reference column from top.
This reference column is attached with reference capillary which has a scale and the zero reading of this reference capillary is called zero reference point.

4 Bulb and measuring capillary:
This bulb and measuring capillary is connected with the reference column. Mcleod gauge is designed in such a way that there will be some space left in the measuring capillary when the mercury will reach the zero reference point.
The point where the measuring capillary and reference column meets is called the cut off point. It is called cut off point because when the mercury level is raised above this point, the entry of the applied pressure from the reference column to the measuring capillary and bulb will be cut off.

5 Scale:
There is a common scale for measuring capillary and reference capillary.

Working Of Mcleod Gauge:

Mcleod gauge works by taking a sample of gas from the vacuum chamber and then compressing it by tilting and infilling with mercury.

As in this Mcleod Gauge, the pressure is calculated using Boyle’s Law. So, we have to first apply pressure to a known volume of gas.

So the pressure that we want to measure or the initial pressure (P1) is applied to the top of the reference column.

The mercury level is raised by pressing the piston down. As the piston is pressed down, the mercury level is mercury reservoir decreases and mercury level in the measuring capillary increases.
The mercury level in the measuring capillary is brought just below the cut off point and at this point the applied pressure fills the bulb and the capillary.

After the bulb and the capillary is filled, the piston is again operated and the mercury level in the gauge increase and reaches the cut off point.
When the mercury level reaches the cut off point, a known volume of gas is trapped in the bulb and the measuring capillary.

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When we were increasing the level of mercury to cut off point, at that time we have noted the volume of the gas in the bulb and measuring capillary. This volume is called the initial volume.

After that, the piston is again operated and the mercury level is further raised so that the trapped gas in the bulb and measuring capillary are compressed. The mercury level is increased to a point so that it reaches the zero reference point.

After the compression, the volume of the gas is again noted and this volume will be called the final volume of the gas. This volume is read directly by the scale.

The difference in the height of the measuring capillary and the reference capillary which is denoted by “h” is a measure of volume and pressure of the trapped gas.

After this we know the final volume (V2), final pressure (P2) and we already know initial volume (V1). From these values we will find initial pressure (P1) using Boyle’s Law:

Now according to Boyle’s Law:
P1 V1 = P2 V2

Now let us suppose,

Volume of the bulb from the cut off point to the beginning of the measuring capillary = V
Area of cross-section of measuring capillary = a
Measuring capillary height = hc

Initial volume of gas which is trapped in bulb and measuring capillary ( V1 ) = V + ahc

Intial or applied pressure of the gas ( P1 ) = Unknown

When the mercury reaches the zero reference point in the reference capillary, final volume (V2) = ah

Final pressure ( P2 ) = P1 + h

Now we have the required values of the known pressure and volumes. Now we will put these values in equation of Boyle’s Law.

P1 V1 = P2 V2
P1 V1 = (P1 + h) ah
P1 V1 = P1ah + ah²
P1 V1 – P1ah = ah²
P1 (V1 – ah) = ah²
P1 = ah² / V1 – ah

(Since, ‘a’ i.e cross-section of measuring capillary is very small and ‘h’ which is the difference in the height of the measuring capillary and the reference capillary is also ver small. So, the product ‘ah’ will also be very small and can be neglected. )

P1 = ah² / V1

So in this way P1 i.e the initial pressure can be measured using Mcleod Gauge.

Advantages of Mcleod Gauge:

1) It is independent of the composition of the gas whose pressure is to be measured.
2) This Mcleod Gauge is also used as a reference standard to calibrate other pressure gauges as the value of the pressure measured by this gauge is very accurate.
3) There is no need to correct the readings of this gauge.
4) Linear relationship exists between the applied pressure and ‘h’.
5) This gauge is not influenced by gas composition.

Disadvantages of Mcleod Gauge:

1) The gas whose pressure is to be measured must obey Boyle’s Law. The pressure of the gas can not be measured which does obeys Boyle’s Law.
2) Moisture trap is must to prevent any vapor from entering into the gauge otherwise the vapor will cause an error in reading of the gauge.
3) It measures only on a sampling basis.
4) Mcleod Gauge cannot give a continuous output.
5) There are chance that it can get contaminated.

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