Granville-Phillips Mini-Convectron 275 Series, Installation, Operation And Service Instructions, Page: 34 / 52

26 Mini-Convectron
®
Module with DeviceNet
®
Digital Interface
Chapter 4
Use with Gases
other than N
2
or Air
Make sure the 275 Mini-Convectron Vacuum Gauge Module Gain
adjustment is correctly set for measuring Air.
The indicated pressure on a 275 Mini-Convectron Vacuum Gauge depends
upon the following parameters:
• Type of gas in the tube
• Tube axis orientation
• Gas pressure within the tube
275 Mini-Convectron Vacuum Gauges are calibrated for N
2
within the
accuracy of the instrument. By observing certain safety precautions, the
gauge tube may be used to measure the pressure of other gases.
Mini-Convectron Gauge tubes are thermal conductivity gauges. these
gauges transduce gas pressure by measuring the heat loss from a heated
sensor wire maintained at a constant temperature. For gases other than N
2
and air, the heat loss is different at any given true pressure and the indicated
pressure will be different.
Indicated versus
True Pressure
Curves The graphs in Figure 4-2, Figure 4-3, and Figure 4-4 show the true pressure
versus indicated pressure for 11 commonly used gases. Table 4-14 will help
to locate the proper graph for a specific application.
1 mbar = 100 Pa = 1.33 Torr, so the charts may be used for pascal and mbar
units.
A useful interpretation of these curves is, for example, that at a true pressure
of 2 x 10
-2
Torr of CH4 the heat loss from the sensor is the same as at a true
pressure of 3 x 10
-2
of N
2
as shown in Figure 4-2. The curves at higher
pressure vary widely from gas to gas because the thermal losses at higher
pressures are greatly different for different gases. If you must measure the
pressure of gases other than N
2
or air, use Figure 4-3 or Figure 4-4 to
determine the maximum safe indicated pressure for the other gas as
explained below.
WARNING
Do not use the gauge tube to measure the pressure of combustible gas
mixtures. The sensing element normally operates at only 125º C but it is
possible that momentary transients or controller malfunction can raise the
sensor above the ignition temperature of combustible mixtures which may
explode causing injury to personnel or equipment damage.
Table 4-14 Indicated Versus True Pressure Information
Range and Units Gases Figure Reference
1 - 100 mTorr All 2-2
0.1 - 1000 Torr Ar, CO
2
, CH
4
, Freon 12, He 2-3
0.1 - 1000 Torr D
2
, Freon 22, Kr, Ne, O
2
2-4