Instruction Manual
InstruTech CVM-211 "
Stinger
" Convection Vacuum Gauge Module
InstruTech, Inc
page 13
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 P
ressu
re (
T
o
rr)
(cap
aci
ta
n
ce man
o
met
er)
CVG N2
Convectron N2
CVG Ar
Convectron Ar
Convectron He
CVG He
Ar
N2
He
Gas Correction Chart
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 shows readings for an InstruTech CVG as well as for a GP Convectron
®
gauge, to illustrate that the
response of the gauges to these different gases is virtually indistinguishable. (Caution: Do not assume this data
applies to other convection gauges, which may or may not be the same.)
If the gas is N
2
, when the true total pressure is 100 Torr, the gauge will read 100 Torr
If the gas is Ar, when the true pressure is 100 Torr, the gauge will read only 10 Torr.
If you are backfilling your vacuum system with Ar, when your system reaches atmospheric pressure of 760 Torr
true pressure, your gauge will be reading about 20 Torr. If you continue to backfill your system, attempting to
increase the reading up to 760, you will overpressure your chamber and probably blow up your vacuum system !
If the gas is He, the gauge will read 1000 Torr when pressure reaches about 10 Torr actual pressure.
You probably won't blow up the vacuum system, but opening the chamber to atmosphere prematurely may
present other hazards for both people and product.
What these examples illustrate is that, without proper precautions,
use of different gases can result in injury to personal and/or damage to equipment
.
Suggested precautions when using gases other than N
2
:
Install a pressure relief valve, or burst disk, on your chamber, to protect it from overpressure.
Post a warning label on your gauge readout "Do Not Exceed ____ Torr Indicated Pressure" (fill in the blank for
your gas) so that an operator using the gauge will not exceed a safe pressure.
