MKS Granville-Phillips Mini-Convectron 275 series, Instruction Manual

Page: 41 / 54

Calibration
Mini-Convectron Module with RS-485
41
Instruction Manual - 275545
Saf
e
ty Ins
tructions
Introduction
Installatio
n
O
p
e
ra
tion
M
a
inten
ance
Calibration
Figure 4-1, Figure 4-2, and Figure 4-3 show the true pressure vs. indicated
pressure for eleven commonly used gases. The following list will help to
locate the proper graph:
Note that 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, for example, 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 4-1). 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 Mini-Convectron gauge tube utilizes 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
Mini-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
Mini-Convectron nor is it safe to use curves for the Mini-Convectron gauge
with gauges supplied by other manufacturers.
If you must measure the pressure of gases other than N
2
or air, use
Figure 4-1, Figure 4-2, or Figure 4-3 to determine the maximum safe
indicated pressure for the other gas as explained below.
Table 4-2 Pressure vs. Indicated N
2
Pressure Curve
Figure Pressure Range and Units Gases
Figure 4-1 10
–4
to 10
–1
Torr All
Figure 4-2 10
–1
to 1000 Torr Ar, CO
2
, CH
4
, Freon 12, He
Figure 4-3 10
–1
to 1000 Torr D
2
, Freon 22, Kr, Ne, O
2