54
937A Multi-Sensor System
Calibrating the Convection
Pirani Sensor for Atmosphere
Since the Series 317 Convection Pirani sensor gives numerical pressure
readings at atmospheric pressure, the local atmospheric pressure must be
determined in order to correctly calibrate the sensor. Atmospheric pressure
depends on altitude and to a lesser degree, weather systems and climate
control (HVAC systems).
There are two methods to calibrate the Convection Pirani sensor at
atmosphere with the Series 937A Controller.
A considerable atmospheric calibration change by either method
may activate or deactivate a sensor's set points and shift
various pressure output readings.
Method 1
If the local atmospheric pressure is 760±2 Torr, or the system pressure can
be controlled to 760±2 Torr, then the user calibration procedure in the previous
section may be used to calibrate the Convection Pirani sensor.
After pressing and releasing the
User Cal push-button in step 4, the display
will read 7.6x10
+
2 Torr and the indicator U will appear next to the display of
the channel.
This method assumes the sensor is exposed to 760 Torr of air/nitrogen. If the
Convection Pirani sensor is calibrated using this method at pressures other
than 760 Torr, errors will result. These errors are largest between 30 and 200
Torr, where the sensor has the least amount of sensitivity. Below 10 Torr, the
differences are less than 10 precent.
Method 2
If local atmospheric pressure is not 760 Torr, atmosphere calibration at other
pressures can be achieved by adjusting the potentiometer on the rear panel of
the Convection Pirani module. A reference sensor such as a capacitance
manometer is required to determine local atmospheric pressure.
Once the potentiometer has been adjusted, the original
calibration of the channel is lost. You must continue to use this
method until the channel is recalibrated at 760 Torr using the
procedure below. Using the normal factory and user calibration
procedures after adjusting the potentiometer will result in slight
errors in the pressure reading.