MANUAL P/N 316005 REV: B
25
Indicated vs. True Pressure Curves
Figures 2.3, 2.4, 2.5, 2.6, 2.7 and 2.8 show the true pressure vs. indicated pressure of Convectron Gauge for
eleven commonly used gases. The following list will help to locate the proper graph for a specific application:
Figure
Range and Units
Gases
2.3
0.1 to 100 millitorr
All
2.4
0.1 to 1000 Torr
Ar, C0
2
, CH
4
, Freon 12, He
2.5
0.1 to 1000 Torr
D
2
, Freon 22, Kr, Ne, 0
2
2.6
1 x 10
-4
to 0.1 mbar
All
2.7
0.1 to 1000 mbar
Ar, C0
2
, CH
4
, Freon 12, He
2.8
0.1 to 1000 mbar
D
2
, Freon 22, Kr, Ne, 0
2
Note that 1 mbar = 100 pascal, so the mbar charts can be used for pascal units by multiplying the values on
the axes by 100.
A useful interpretation of these curves is, that 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
(see Figure 2-3). 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 tube 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 tube are likely to be
much different from curves for heat loss tubes made by others. Therefore,
it is not safe to attempt to use
calibration curves supplied by other manufacturers for their gauges with the Convectron Gauge, 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 2-3 through 2-8 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 2-4, 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.
For safety, it is prudent to place a warning label on the instrument face which under the assumed conditions
would read "DO NOT EXCEED 24 TORR FOR ARGON."
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 2-4, 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 220 Torr. Thus 220 Torr argon pressure produces an indication of
10 Torr (N
2
equivalent).
Example 3: True to indicated pressure conversion.
Assume you want to set a process control setpoint at a true pressure of 20 Torr of CO
2
. On Figure 2-4, locate
20 Torr on the true pressure scale, travel horizontally to the right to the CO
2
curve and then down to an
indicated pressure of 6.5 Torr (N
2
equivalent). Thus, the correct process control setting for 20 Torr of C0
2
is
6.5 Torr (N
2
equivalent).
