Mcleod gauge: What it is, Working, Principle, Diagram, Formula, Advantages and Disadvantages [with Pdf]

What is Mcleod gauge?

1] Mcleod Gauge is a device used to measure pressure at a very lower range up to 10-6 torr.  

2] It is also used to measure vacuum pressure.

3] Mcleod Gauge is also known as compression gauge.

4] It is similar to a mercury manometer. As it uses mercury as measuring fluid & height of the mercury column determines pressure difference.

5] Mcleod gauge is invented by British scientist Herbert McLeod in the year 1874.

6] In this gauge known volume of gas is compressed at a constant temperature.

7] This gauge gives the final volume & pressure difference. From this data, we can calculate the unknown pressure of the gas.

Mcleod gauge working principle

Mcleod 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.  

`Palphafrac{1}{V}`

PV = Constant

`P_{1}V_{1} = P_{2}V_{2}`

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.

Mcleod gauge diagram

Before Compression:-

McLeod Gauge

After Compression:-

Mcleod Gauge diagram

Construction of Mcleod Gauge

The McLeod 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 starting of the procedure.

2) Mercury (Hg) Reservoir:- The purpose of the reservoir is to store mercury. It contains a piston or plunger which pushes the mercury into the reference column & bulb & 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 measure.

The gas firstly 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 & reference capillary are the same & 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 & to increase accuracy while pressure measurement.

6) Measurement Capillary:- Top end of the measuring capillary is created is sealed & another end is connected to the bulb.

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

Mcleod gauge working

1) First, Remove the mercury in bulb & reference the column below the cut-off point so that gas can enter into bulb & capillary. the piston moves in the upper direction therefore it causes the suction, so mercury comes into mercury reservoir.

2) Connect the reference column to the source of gas or vacuum whose pressure is to be measure.

3) After filling of gas into the bulb & measuring capillary, start the filling of mercury into bulb & reference column by moving piston downward.

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

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

6) After reaching of mercury upto the zero reference point in capillary, measure difference in levels between measuring capillary & reference capillary. It is denoted by ‘H’.

7) By knowing this height ‘H’, We can calculate final volume of gas.   8) It also gives difference between initial & final pressure.

9) Now by using Boyle’s law we can find unknown pressure of the gas.

McLeod gauge derivation

`V_{1}` = Known initial volume = It is sum of bulb volume & measuring capillary volume.

h= Difference in levels of reference capillary & measuring capillary

a = Area of cross-section of measuring capillary

`P_{1}` =  Unknown pressure of gas

`P_{2}` =  Final Pressure

`V_{2}` =  Final volume atter compression

`gamma_{mercury}` = Weight Density of Mercury

Now by Boyle’s law,

`P_{1}V_{1} = P_{2}V_{2}`

But `V_{2}` is given by,

`V_{2}` = a × h

& Pressure difference is given by

`P_{2}-P_{1}=gamma_{mercury}times h `

`P_{2}=P_{1}+gamma_{mercury}h`

Now putting value in Boyle’s law equation

`P_{1}V_{1}=(P_{1}+gamma_{m}h) X (a X h)`

`P_{1}V_{1}=P_{1}ah+gamma_{m}h^{2}`

`P_{1}V_{1}-P_{1}ah=gamma_{m}h^{2}`

`P_{1}(V_{1}-ah)= agamma_{m}h^{2}`

`P_{1}= frac{agamma_{m}h^{2}}{(V_{1}-ah)}`

As we know,

`V_{1}`>>ah

`therefore ah approx 0`

`thereforeP_{1}= frac{agamma_{m}h^{2}}{(V_{1})}`

Now to calculate in mm of Hg,

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

Mcleod Gauge Formula :- `P_{1}= frac{ah^{2}}{(V_{1})}` mm of Hg

Advantages of Mcleod Gauge

The Mcleod gauge has the following advantages:

  • Mcleod Gauge range up to `10^{-6}` torr.
  • It can use for the calibration of gauges.
  • 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 be followed Boyle’s law.
  3. The mercury can be contaminated.
  4. No continuous output. We need to complete the procedure again & again for pressure measurement.
  5. The presence of moisture in gas can affect accuracy.

FAQ’s:-

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

    To measure vacuum pressure using McLeod gauge, First, remove the mercury in the bulb & reference column below the cut-off point so that gas can enter into bulb & capillary.

  2. How do you read a McLeod gauge?

    To read a McLeod gauge, First, remove the mercury in the bulb & reference the column below the cut-off point so that gas can enter into the bulb & capillary.

  3. Where McLeod gauge is used?

    Mcleod Gauge is device used to measure pressure at very lower range upto 10-6 torr.

  4. What is the principle of McLeod gauge?

    Mcleod 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.

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

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