Seven factors affecting the sensitivity of vacuum gauges

In terms of engineering and science, it is hard to over emphasise the importance of measurements.

They are the very essence of these two disciplines, which we use to explain the otherwise unexplainable with equations, tables, graphs and figures. In turn, this allows us to compare, contrast, repeat and define the apparent chaos which defines our world.

However, in appreciating the importance of measurement we must also accept that accuracy must go hand-in-hand with the ability to compare, contrast, repeat and define, and likewise we must acknowledge that when using vacuum gauges, that these are sensitive to a range of external influences which vacuum engineers need to work with, and around. (N.B. for the purposes of this blog: accuracy, sensitivity and resistance to gauge damage, are all part of the same Venn Diagram “overlap”).

There are 7 factors that impact the sensitivity, accuracy and resistance to damage of vacuum gauges:

In order of sensitivity, there are two key types of pressure gauges:

• Ionisation
• Thermal.

Both of these types are known as “indirect”, but vary in the way in which they measure the pressure associated with residual gas molecules.

• Ionisation gauges are most commonly used for low-pressure measurement where high accuracy is not critical. They work by ionising gas molecules, which are then accelerated to a detector, which measures any current caused by molecular impact.
• There are several types of thermal gauges, but each is based upon thermal “transport”, with the Pirani gauge being the most common. Thermal gauges work on the principle that when gas molecules come into contact with a hot surface, there will be a transfer of energy from the surface to the gas. The rate of energy loss will depend on the number of collisions with gas molecules, and therefore the pressure of the gas.

Vacuum gauges usually come from the manufacturers who calibrate for nitrogen (which means that when measuring nitrogen, there is a correction factor of 1).

If the gas is different to nitrogen, then the true pressure P_i is expressed as:
P_i = ((S_N2÷S_i) x P_N2)

where:

• S_i and S_N2 are the relative sensitivity of the gauge to gas i (with nitrogen’s S_N2 being equal, by definition, to 1.0), 
• P_N2 is the indicated (measured) pressure.

If the gas being measured contains a mixture of gases, then Dalton’s law for total or partial pressure can be used to “extend” this correction:

where r_i is the relative proportion (partial pressure) of gas species i compared with nitrogen, so r_i = P_i ÷P_N2.

For the calibration of ionization gauges, where the collector and emission currents are known, the following equation applies:

P = [I_c ÷ (S_g x I_e)]

where:

• I_c is the ion collector current in amps, 
• I_e is the electron emission current in amps, 
• and S_g is the sensitivity factor for gas g in units of mbar^-1, and where
  • S_g = S_N2 x R_G 
  • and where R_G is the gas correction sensitivity factor (as shown in graph below).

Vacuum gauge selection depends on an understanding of the working principles of the gauge, and the range of pressures it can measure, as well as its accuracy over the required range.

These factors have been determined by experiments and verified by experience.

• For low (rough) vacuum ranges between 10 mbar and atmospheric, Bourdon tubes, bellows, active strain gauges and capacitance sensors are suitable. 
• Between 10¹ and 10^-3, the capacitance manometer, the thermocouple or Pirani type gauges are suitable. 
• Between 10^-3 and 10^-9 mbar, cold cathode, and Bayard-Alpert hot cathode gauges are suitable (but both require frequent “wiping” of spent molecules/electrons and then re-calibration).

It has been shown that higher mass molecules tend to require larger correction factors.

In the case of thermal transfer gauges, this can be attributed to larger molecules generally having higher (heat) conductivity. There are a number of factors which contribute to the conductivity of a particular gas type, including interaction effects, specific heats and accommodation coefficients.

A second temperature issue is of a more general kind: even though pressure gauges are designed to be employed at various temperatures, they may give false readings if the temperature is extreme.

If the ambient conditions are at the extremes (i.e. too hot or too cold), then there can be a loss of “containment” which can cause the components to erode or break-down. Furthermore, if a gauge is employed around water, it may burst if exposed to frost/ice or become foggy due to condensation.

The accuracy of a vacuum gauge will depend on many factors but in general, a gauge will arrive from a manufacturer with only a “rough” calibration (which with no correction factor applied) and can have between 20 and 50% uncertainty within the stated range.

Using a constant gas correction factor can improve this to between 10 and 20%.

However, where higher accuracy is required, gauges should be calibrated over the whole pressure range.
For a high-quality gauge, individually calibrated for each gas type over the full range against a primary standard, accuracy can be improved to between 2 and 5%.

Regular (and often repetitive) overpressure spikes, can cause accuracy issues and lead to gauge damage.

Vibrations have a much unappreciated impact on pressure gauge readings.

In fact, vibrations (due to motors, heavy machinery, pumps and other rotating equipment) may result in excess wear-and-tear on pressure gauges, resulting in inaccurate readings, as well as frequently causing pointer mechanism damage as they are constantly being moved off zero. Even when working properly, vibrations can make an accurate reading difficult to take.

Exposure to continuous vibrations can lead to gauge failure.

As you can see, there are a number of factors vacuum scientists need to consider to ensure their vacuum gauge is working accurately.