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Instrumentation and Process Control

McLeod Gauge : Principle, Construction, Working, Advantages and Disadvantages

  • Vacuum pressure are those which are below atmospheric. 
  • Two methods for vacuum measurement are 

1. Direct Method: 

Resulting in a displacement caused by the action of the force. For example, 

  1. Bourdon tube 
  2. Manometers 
  3. Diaphragm and capsules. 

2. Indirect Method: 

In which pressure is determined through the measurement of certain other pressure controlled properties such as volume, thermal conductivity etc. 

For example, 

  1.  McLeod Gauge 
  2.  Pirani Gauge 
  3.  Thermocouple Gauge.
  4.  Ionization Gauge.

Already we studied Bourdon tube, bellows, diaphragm and manometers in previous section. 

Let us see Indirect Methods here. 

McLeod Gauge

  • McLeod vaccum gauge is used for measuring very low pressure down to one-hundred thousandth of inch of the mercury. 
  • The unit of vacuum measurement are Torr or micron. 
  • This is used between 0.01 to 1000 microns and is a modified mercury manometer. 

Principle of Operation: 

It operates on the principle of compressing a known volume of low pressure gas to a high pressure and measuring the resulting volume change.  

Construction and Working

McLeod Gauge

A → Unknown pressure source 
B → Bulb 
C → Capillary 
D → Reservoir 
E → Reference column 
F → Cut-off point a known volume of gas 

Working: 

Operation of the McLeod vaccum gauge is based on Boyles law for the isothermal compression of gas.  

P1 = P2V2/ V1 
where, 
P1 and P2 = Pressures at initial and final conditions 
V11 and V2 = Volumes at corresponding conditions

  • By compressing a known volume of the low pressure gas to a higher pressure and measuring the resulting volume and pressure, one can calculate initial pressure.
  • Fig. illustrate the basic construction and operation of McLeod vaccum gauge.  
  • The unknown pressure source is connected at point A and the mercury level is adjusted to fill the volume represented by the darker shading. 
  • Under these conditions the unknown pressure fills the bulb B and capillary C. 
  • Mercury is then forced out of the reservoir D, up into the bulb and reference column E. 
  • When the mercury level reaches the cut-off point F, a known volume of gas is trapped in the bulb and capillary. 
  •  A mercury level is then further raised until it reaches a zero reference point in E. 
  • Under these conditions the volume remaining in the capillary is read directly from the scale and the difference in heights of the two columns in the measure of the trapped pressure. 
  • The initial pressure is then calculated using Boyle’s law.  

Measuring Range: 

  • The pressure of gases containing vapour cannot normally measured with a McLeod gauge, for the reason that the compression will cause condensation. 
  • A total pressure range of from 0.01 Pa to 10 kPa may be measured with this type of gauge. 

Range: 

  • McLeod vaccume gauge used for pressure range of 10−1 to 10−5 torr. 

Advantages: 

  1. High Accuracy: McLeod gauges are renowned for their exceptional accuracy, making them suitable for calibrating other low-pressure gauges. They can provide precise measurements, especially in the ultra-high vacuum range.
  2. Wide Pressure Range: These gauges can be designed to measure pressures ranging from atmospheric down to extremely low levels, such as 0.05 micron (0.00005 torr). This versatility makes them valuable in various applications.
  3. Stability: McLeod gauges tend to be stable and reliable over extended periods. They are less prone to drift or errors compared to some other pressure-measuring devices.
  4. Robustness: They are typically robust and durable, able to withstand harsh conditions, which is essential for many industrial and scientific environments.
  5. Minimal Contamination: McLeod gauges are less susceptible to contamination by gases, making them suitable for clean vacuum systems where maintaining purity is critical.
  6. Direct Measurement: They offer a direct measurement of pressure without the need for complex conversions or calculations, simplifying pressure monitoring.
  7. Compatibility: McLeod gauges can be integrated into various vacuum systems and setups, making them versatile tools for researchers and engineers.

 Disadvantages: 

  1. Accurate operation assumes that no condensable gas (for example water vapour) is present. It results in lower than actual pressure. 
  2. A fairly large error will occur at very low vacuum. 
  3. Lack of continuous output reading.  

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