McLeod Gauge for low pressure measurement

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McLeod Vacuum Gauge

A McLeod gauge is an instrument for measurement of very low pressures , down to 10^−6 Torr. Similar to mercury-column manometer in design, it is filled with mercury.
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Basic Principle of McLeod Vacuum Gauge:

Working on Boyle’s law i.e the product of pressure and volume for a given quantity of gas remains constant if a constant temperature is maintained.

Operation of McLeod Vacuum gauge:

A sample volume of gas is taken from a vacuum chamber, then compressed by tilting and infilling with mercury.

To take a sample of gas, the mercury level in the McLeod gauge is raised which seals off the gas from the connected system. When the level of mercury is raised further, the gas is compressed.

The pressure of this smaller volume is then measured by a mercury manometer, and from the predetermined compression ratio (the ratio of the initial and final volumes), the pressure of the original vacuum is arrived by using Boyle’s Law.

The difference in mercury level between the trapped volume and the system being evacuated is directly proportional to the pressure in the trapped volume.

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Precaution using McLeod Vacuum Gauge:

Incorrect usage can let the mercury to escape and the vacuum system attached to the gauge is compromised.

For gases that are condensible i.e those that are in gaseous form at low pressure of the vacuum chamber and that condense when compressed by the McLeod gauge, cold trap is used. Else a reading less than the actual pressure is read.

cold trap

a device that condenses all vapours except the permanent gases into a liquid or solid, to prevent vapours being evacuated from an experiment from entering a vacuum pump where they would condense and contaminate it.

Cold traps usually consist of two parts: The bottom is a large, thick round tube with ground-glass joints , and the second is a cap, also with ground-glass connections. The length of the tube is usually selected so that, when assembled, the total reached is about half the length of the tube.

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Arrangement

Cold traps should be assembled in a way that the down tube B is connected to the source of gas whilst the cap A is connected to the source of vacuum. When connected in reverse with the down tube B to the source of vacuum, the inlet of the vacuum is placed directly above the condensate, and the chances of vapour phase condensate moving up the (uncooled) down tube (towards the pump) is increased or, if the trap begins to fill to an appreciable volume, liquid phase condensate being pulled into the pump is increased.

Symbol of the McLeod Gauge:

Symbol of McLeod Gauge according to ISO 3753-1977 (E)

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Applications

McLeod Gauge is an absolute gauge as it depends on directly measured parameters like initial volume trapped, the final compressed volume, and the pressure in this final volume. Hence they are stable and can be used as calibrating standard for electronic gauges.

Advantages of the McLeod Gauge:

  • It is independent of the gas composition.

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

  • A linear relationship exists between the applied pressure and h

  • There is no need to apply corrections to the McLeod Gauge readings.

Limitations of McLeod Gauge:

  • The gas whose pressure is to be measured should obey the Boyle’s law (Real gases)

  • Chance of contamination due to moisture.

  • Since It measures on a sampling basis, It cannot give a continuous output.

Automating a McLeod Gauge:

  1. a small electric motor can be used to periodically rotate the assembly for sample collection.

  2. a fine platinum wire in the capillary tube, indicates the height of the mercury column around it by virtue of its resistance.

How to use a McLeod Gauge