MKS 275420-1-GQ-T-00011, Инструкция по эксплуатации

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34 Series 275 Convectron Module with DeviceNet
Instruction Manual - 275502
6 Calibration
6.4 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 controller converts this
measurement into gas pressure 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.
In order 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
designed for Convectron Gauges to translate pressure readings for gauges made by other
manufacturers. Their geometry is very likely different and dangerously high pressures may be
produced even at relatively low pressure indications.
NOTE: You must ensure that the atmosphere adjustments for the Convectron Gauge are
correctly set. See Section 6.5.