4 Operation
36
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
NOTE: 1 mbar = 100 pascal, so the mbar charts may be used for pascal units
by multiplying the values on the axes by 100.
A useful interpretation of these curves is: at a true pressure of 2 x 10
-2
Torr of CH
4
the heat loss from
the sensor is the same as at a pressure of 3 x 10
-2
Torr of N
2
(refer to Figure 4-6). The curves at
higher pressure vary widely from gas to gas because the thermal losses at higher pressures are
greatly different for different gases.
The Convectron gauge uses convection cooling to provide resolution superior to any other thermal
conductivity gauge near atmospheric pressure of N
2
and Air. Because convection effects are
geometry dependent, the true pressure vs. indicated pressure curves for the Convectron gauge are
likely to be much different from curves for heat loss tubes made by others.
It is not safe to attempt
to use calibration curves supplied by other manufacturers for their gauges with the Convectron,
nor is it safe to use curves for the Convectron gauge with gauges supplied by other manufacturers.
If you must measure the pressure of gases other than N
2
or Air, use Figures 4-5 through 4-10 to
determine the maximum safe indicated pressure for the other gas as explained below.
Example 1: Maximum Safe Indicated Pressure
Assume a certain system will withstand an internal pressure of 2000 Torr or 38.7 psia. For safety you
want to limit the maximum internal pressure to 760 Torr during backfilling. Assume you want to
measure the pressure of argon. On Figure 4-6, locate 760 Torr on the left hand scale, travel to the
right to the intersection with the argon (Ar) curve and then down to an indicated pressure of 24 Torr
(N
2
equivalent). Thus in this hypothetical situation the maximum safe indicated pressure for argon is
24 Torr.
Example 2: Indicated to True Pressure Conversion
Assume you want to determine the true pressure of argon in a system when the Convectron is
indicating 10 Torr. On Figure 4-6, read up from 10 Torr (N
2
equivalent) indicated pressure to the
argon curve and then horizontally to the left to a true pressure of 250 Torr. Thus 250 Torr argon
pressure produces an indication of 10 Torr (N
2
equivalent).
Table 4-2 Indicated vs. True Pressure Curves
Figure
Range and Units
Gases
4-5
1 to 100 mTorr
All
4-6
0.1 to 1000 Torr
Ar, C0
2
, CH
4
, Freon 12, He
4-7
0.1 to 1000 Torr
D
2
, Freon 22, Kr, Ne, 02
4-8
10
-3
to 10-1 mbar
All
4-9
0.1 to 1000 mbar
Ar, C0
2
, CH
4
, Freon 12, He
4-10
0.1 to 1000 mbar
D
2
, Freon 22, Kr, Ne, 02
For gases other than those listed, the user must provide accurate conversion data for
safe operation. The Convectron gauge is not intended for use above 999 Torr true
pressure.
