4 Operation
23
Series 275 Convectron Gauge Bridge Amp PC Board
Instruction Manual - 275331
4.4 Preparing For Convectron
Gauge Operation
Install pressure limiting devices calibrated to a level that the vacuum system can safely withstand. In
addition, install pressure relief valves or rupture disks that will release pressure at a level
considerably below the maximum safe pressure level of the system.
Suppliers of pressure relief valves and pressure relief disks can be located via an internet search, and
are listed in the
Thomas Register
under “Valves, Relief” and “Discs, Rupture.” Confirm that these
safety devices are properly installed before operating the Series 275 Mini-Convectron Module. In
addition, check that (1) the proper gas cylinders are installed, (2) gas cylinder valve positions are
correct on manual systems, and (3) the automation settings are correct on automated gas delivery
systems.
NOTE: Vacuum gauges with compression fittings may be forcefully ejected if the vacuum system
is pressurized.
4.5 Understanding Convectron Gauge Pressure Measurement In Gases Other Than
Nitrogen or Air
Convectron Gauges are Pirani type thermal conductivity gauges. These gauges measure the heat loss
from a heated sensor wire maintained at constant temperature. The bridge amplifier PC board
converts this measurement into gas pressure voltage readings. For gases other than nitrogen or air the
heat loss varies at any given true pressure and can result in inaccurate pressure readings.
It is important to understand that the pressure indicated by a Convectron Gauge depends on the type
of gas, the orientation of the gauge axis, and on the gas density in the gauge. Convectron Gauges are
normally factory calibrated for N
2
(air has approximately the same calibration). With proper
precautions, the Convectron Gauge may be used for pressure measurement of certain other gases.
NOTE: The information in this section applies only when the Convectron Gauge is calibrated
for N
2
and the Convectron Gauge is mounted with its axis horizontal.
At pressures below a few Torr, there is no danger in measuring pressure of gases other than N
2
and
air, merely inaccurate readings. A danger arises if the N
2
calibration is used without correction to
measure higher pressure levels of some other gases. For example, N
2
at 24 Torr causes the same heat
loss from the Convectron sensor as argon will at atmospheric pressure. If the pressure indication of
the Convectron Gauge is not properly corrected for argon, an operator attempting to fill a vacuum
system with 1/2 atmosphere of argon would observe a pressure reading of only 12 Torr when the
actual pressure had risen to the desired 380 Torr. Continuing to fill the system with argon to 760 Torr
would result in a 24 Torr pressure reading.
Depending on the pressure of the argon gas source, the chamber could be dangerously pressurized
while the display continued to read about 30 Torr of N
2
equivalent pressure.
NOTE: This type of danger is not unique to the Convectron Gauge and likely exists with other
thermal conductivity gauges using convection to extend the range to high pressures.
To measure the pressure of gases other than air, or N
2
with a Convectron Gauge calibrated for N
2
,
you must use the conversion curves listed specifically for Convectron Gauges to translate between
indicated pressure and true pressure. Do not use other data.
Never
use the conversion curves
Using the N
2
calibration to pressurize a vacuum system above about 1 Torr with
certain other gases can cause dangerously high pressures which may cause
explosion of the system. See Section 4.5 before using with other gases.