4 Operation
35
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
4.3 Convectron Gauge Operation
4.3.1 Reading Pressure
The Convectron Gauge (CG) Module provides pressure measurement from 1.0 x 10
-3
Torr (1.3 x
10
-3
mbar or 1.3 x 10
-1
Pa) to 1000 Torr, and one meaningful digit pressure indication down to 1 x
10
-4
Torr, air equivalent. Two transducers are displayed simultaneously in the second and third
display lines and are denoted by "CGA" and "CGB", respectively.
Analog output is also provided. The Convectron can also be used to automatically turn ON or OFF
the ion gauge and the process relay.
4.3.2 Special Considerations For Use Below 10
-3
Torr
During a fast pumpdown from atmosphere, thermal effects will prevent the Convectron from
tracking pressure rapidly below 10
-3
Torr. After about 15 minutes, readings in the 10
-4
range will be
valid and response will be rapid. Calibration at vacuum may be performed at this time, or sooner if
readings in the 10
-4
range are not needed.
The 10
-4
Torr range is accurate to about 0.1 milliTorr provided the instrument has been carefully
zeroed at vacuum. See Section 6.6 on page 53 for vacuum and atmosphere calibration procedures.
For accurate use in the 10
-4
Torr range, zeroing should be repeated frequently.
Pressure readings in the 10
-4
Torr range may differ from those measured by ion gauges, since ion
gauges usually lose sensitivity near their upper pressure limits.
4.3.3 Use With Gases Other Than N
2
and Air
Before using the Convectron gauge to measure the pressure of other gases make certain the ATM
adjustment is correctly set for air. Refer to Section 6.6 on page 53.
It is important to understand that the indicated pressure on a Convectron gauge depends on the type
of gas in the gauge, and on the orientation of the gauge axis as well as on the gas pressure in the
gauge. Convectron gauges are supplied calibrated for N
2
within the accuracy of the instrument.
With certain safety precautions, the Convectron gauge may be used to measure pressure of other
gases.
Convectron gauges are thermal conductivity gauges of the Pirani type. These gauges transduce gas
pressure by measuring the heat loss from a heated sensor wire maintained at constant temperature.
For gases other than N
2
and Air the heat loss is different at any given true pressure and thus the
indicated reading will be different.
4.3.4 Indicated vs. True Pressure Curves
Figures 4-5 to 4-10 show the true pressure vs. indicated pressure of a Convectron gauge for eleven
commonly used gases. The following list will help to locate the proper graph for a specific
application:
If used without proper calibration or without reference to proper calibration tables,
Convectron gauges can supply misleading pressure indications. This may result in
dangerous overpressure conditions within the system. As supplied from the factory,
the controller is designed to read pressure for nitrogen. For use with any other gases,
consult the gas type correction charts found later in this chapter.
