Instruction Manual IGM402 Hornet
InstruTech
Page 49
WARNING!
Using a thermal conductivity gauge with gases other than that for which it is calibrated could result
in death or serious injury. Be sure to use gas correction data in this manual when measuring pressures of gases
other than N
2
/ air.
For N
2
the calibration shows excellent agreement between indicated and true pressure throughout the range
from 10
-4
to 1000 Torr. At pressures below 1 Torr, the calibration curves for the different gases are similar. The
difference in readings at these low pressures is a constant, a function of the difference between thermal
conductivities of the gases.
At pressures above 1 Torr, indicated pressure readings may diverge significantly. At these higher pressures,
convection currents in the gauge become the predominant cause of heat loss from the sensor, and calibration
depends on gauge tube geometry and mounting position, as well as gas properties.
Generally, air and N
2
are considered the same with respect to thermal conductivity, but even N
2
and air will
exhibit slight differences in readings at higher pressures. For example, when venting a system to atmosphere
using N
2
, you may see readings change by 30 to 40 Torr after the chamber is opened and air gradually displaces
the N
2
in the gauge. For most other gases, the effect is much more significant, and may result in a hazardous
condition if the appropriate gas conversion data is not applied.
The Y- Axis is actual pressure, measured by a capacitance manometer, a diaphragm gauge that measures true
total pressure, independent of gas composition. The X-Axis is the pressure reading indicated by the convection
gauge under test. The chart above shows readings for an InstruTech’s CVG101 convection gauges as well as for
a Granville-Phillips® Convectron® gauge, to illustrate that the response of both these type of gauges is virtually
the same.
Convection Vacuum Gauge
Indicated vs. True Total Pressure
Test Gases - N2, Ar, He
0.0001
0.001
0.01
0.1
1
10
100
1000
0.0001
0.001
0.01
0.1
1
10
100
1000
Indicated Pressure (Torr)
(convection gauge)
T
ru
e
T
o
ta
l
Pr
e
s
s
u
re
(T
o
rr
)
(c
a
p
a
c
ita
n
c
e
m
a
n
o
m
e
te
r)
CVG N2
Convectron N2
CVG Ar
Convectron Ar
Convectron He
CVG He
Ar
N2
He
