5 Electrometer Module
48
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
5.3.5 Relative Gas Sensitivities
Sensitivity depends on the gas being measured as well as the type of ion gauge being used. Table
5-1 lists the relative gauge sensitivities for common gases.
NOTE: The values in Table 5−1 are from NASA Technical Note TND 5285,
"Ionization Gauge Sensitivities as Reported in the Literature", by Robert L.
Summer, Lewis Research Center, National Aeronautics and Space
Administration. Refer to this technical note for further definition of these
average values and for the gauge sensitivities of other gases.
To adjust the 340 VGC to be direct reading for gases other than air or N
2
, calculate the sensitivity S
x
for gas type x as follows:
where SN
2
is the gauge sensitivity for N
2
and R
x
is found from Table 5-1.
For example: if the normal sensitivity for the ion gauge you are using is 25/Torr, and you want to
measure Argon, the sensitivity should be set to 1.29 x 25/Torr, or, 32.25 or 3.2+1.
5.3.6 Analog Output
The Analog Voltage Output (Figure 5-4) is proportional to the logarithm of the pressure, scaled to 1
volt per decade with 0 volts at 1 x 10
-12
Torr at 10 mA emission current. Emission range setting
affects the scaling of the analog output, refer to Figure 5-4. When the ion gauge is turned OFF, the
output will switch to slightly over +10 V.
Table 5-1 Relative Gas Sensitivities
Gas
R
X
Gas
R
X
He
0.18
H
2
0
1.12
Ne
0.30
N0
1.16
D
2
0.35
Ar
1.29
H
2
0.46
C0
2
1.42
N
2
1.00
Kr
1.94
Air
1.00
SF
6
2.5
O
2
1.01
Xe
2.87
S
x
= (R
x
) (SN
2
)
