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Measurements of pressure less than 1 atm are most conveniently made using purely electronic methods. There are three common methods of electronic pressure measurements. The first two devices are useful for pressure less than 1 atm, down to about 0.001 atm. They are both based on the rate at which heat is conducted and radiated away from a heated filament placed in the low-pressure environment. The heat loss is proportional to the number of gas molecules per unit volume, and thus, under constant filament current, the filament temperature is proportional to gas pressure. We have thus transduced a pressure measurement to a temperature measurement.
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Pirani Gauge
This gauge determines the filament temperature through a measure of filament resistance in accordance with the principles established in Section 4.3. Filament excitation and resistance measurement are both performed with a bridge circuit. The response of resistance versus pressure is highly nonlinear.
FIGURE 5.34 The lonization gauge is used to measure very low pressures, down to about 10-13 atm.
Thermocouple
Ionization Gauge
This device is useful for the measurement of very low pressures from about 10-3 atm to 10-13 atm. This gauge employs electrons, usually from a heated filament, to ionize the gas whose pressure is to be measured, and then measures the current flowing between two electrodes in the ionized environment, as shown in Figure 5.34. The number of ions per unit volume depends on the gas pressure, and hence the current also depends on gas pressure. This current is then monitored as an approximately linear indication of pressure.
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Pressure Principles
Pressure Sensors (p > one atmosphere)
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