Teledyne Marine Instruments 2002 User Manual download pdf (Page 22)

149-072011 Model 2002 Vacuum Gauge Page 22 of 38

Ta = ambient temperature

P = pressure

Ag = surface area of the heated portion of the membrane

For nitrogen at a pressure of 760 Torr and a temperature of 20°C the mean free path (l) is less than 1 x 10-7

meters and is inversely proportional to pressure. Since the thermal transfer distance (Dx) is a few

micrometers, this sensor will remain in the molecular flow regime at a much higher pressure (10 Torr) than

is typical for a thermal vacuum gauge. This extends the linear response part of the output curve up into the

1 Torr range. The nonlinear transition region will extend up to 1000 Torr.

5.3. Dual Sensor Operation

The microprocessor in the control unit continuously monitors the outputs of both the piezoresistive sensor

and the Pirani sensor. Figure 5.4 shows representations of the sensors output over the pressure range from

10-5 Torr to 10+3 Torr. The microprocessor uses the output of the piezoresistive sensor at high pressures

(>32 Torr) and uses the output of the Pirani sensor at low pressures (<8 Torr). In the crossover region, a

software averaging algorithm ensures a smooth transition between the two sensors.

Fig. 5.4

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Teledyne Marine Instruments 2002 User Manual Page 22