937B Controller Instruction Manual
pg. 101
Figure 11-8 Natural Convection Heater Transfer in Horizontal (left) and Vertical (right) Sensor
Tubes.
The size of gas molecules has significant impact on the Pirani sensor. Since smaller molecules (such as
helium) move faster in the gas, this gas can transfer more heat energy under the same pressure as
compared with a gas composed of heavy molecules (e.g. Argon). This explains the gas sensitivity of
these sensors shown in Figures 8-5 and 8-6.
The standard Pirani sensor will read continuously between 5 x 10
-4
to 100 Torr, and, with lower resolution,
up to atmospheric pressure.
The Convection Pirani and Convectron sensor design enhances heat transfer at higher pressures through
convection. This sensor will read continuously with full resolution from 1.0 x 10
-3
to 1000 Torr.
11.5.1 Cleaning the Series 345 Sensor
Roughing pump oils and other fluids condensing or decomposing on the heated filament can contaminate
the sensor. This changes the emissivity of the filament, which in turn can cause the calibration to change,
especially at low pressure.
It is not advisable to clean the sensor. Attempts to clean it may either deform or
break the filament. The deformed filament can
further shift the sensor’s output, increasing
the errors in reading.
Replace the sensor if it becomes contaminated.
11.5.2 Testing the Series 345 Sensor
This procedure tests function only. Lower levels of sensor damage that are due to
contamination or rough handling can affect calibration, but the tube may still be
functional.
The most common cause of sensor failure is a broken filament.
Test the sensor using an ohmmeter with less than 5 mA of current. The resistance readings of a normal
Series 345 Sensor measured at atmospheric pressure and at room temperature (20°C) are shown in
Table 11-2.