MKS HPS 903 Series, Operation And Maintenance Manual

Page: 22 / 36

16
Cold Cathode Ionization Vacuum Sensor
Using the Series 903 (IMT)
with Other Gases
Before using the (IMT) to measure pressure of gases other than
air or nitrogen, you should read and understand this section. To
answer further questions, contact Applications Engineering at
HPS
TM Products at 1-303-449-9861 or 1-800-345-1967.
In a cold cathode ionization sensor, the degree of ionization, hence the
indicated pressure, is gas-type dependent. (see page A.1, Theory of a Cold
Cathode Ionization Sensor.)
The Series 903 (IMT) is calibrated to give a voltage output for air or nitrogen
according to the graph on page 12 or the equation on page 13. If used with
another gas, the (IMT) will give an output which corresponds to the ionization
from the sensor for nitrogen. This is the
nitrogen equivalent pressure, or
indicated pressure of the gas, which may be higher or lower than its true
pressure. True-versus-indicated-pressure curves for some common gases are
shown below.
10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2
Indicated Pressure (Torr)
10 -1
10 -2
10 -3
10 -4
10
-5
10 -6
10 -7
10 -8
10 -9
10
-10
Ar
N
2
H
He
True Pressure (Torr)