Instruction Manual VGC301A Controller
InstruTech, Inc.
Page 18
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
Gas Correction Chart
The Y- Axis is actual pressure as 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 convection gauge (CVG) and Granville-
Phillips
®
Convectron
®
gauge to illustrate that the response of both these type of gauges is virtually
indistinguishable.
CAUTION !
Do not assume this data applies to other convection gauges which may or may not be the same.
Refer to the table on the next page and note the following examples:
Ex A: If the gas is nitrogen (N
2
), when the true total pressure is 500 Torr, the gauge will read 500 Torr.
Ex B: If the gas is argon (Ar), when the true pressure is 100 Torr, the gauge will read about 9 Torr.
If you are backfilling your vacuum system with Ar, when your system reaches a pressure of 760 Torr
true pressure your gauge will be reading about 23 Torr. Continuing to backfill your system, attempting to
increase the reading up to 760 Torr, you will over pressurize your chamber which may present a hazard.
Ex C: If the gas is helium (He), the gauge will read 999 Torr when pressure reaches about 10 Torr true pressure
and opening the chamber to atmosphere prematurely may present other hazards for both people and product.
CAUTION!
What these examples illustrate is that using gases other than nitrogen (N
2
) without using accurate gas
conversion data and other proper precautions could 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 that states "Do Not Exceed ____ Torr Indicated Pressure" (fill in the blank for
maximum indicated pressure for the gas you use) so that an operator using the gauge will not exceed a safe
pressure.