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Instruction Manual
4 Using the gauge with different gases
A thermal conductivity gauge senses heat loss which depends on the thermal conductivity of the gas surrounding the
sensor. Since different gases, and mixtures, have different thermal conductivities, the indicated pressure readings and
outputs will also be different. InstruTech convection gauges (and most other thermal conductivity gauges) are calibrated
using nitrogen (N
). When a gas other than N
2
conductivity between nitrogen (N
(such as the InstruTech VGC301 controller) User Manual indicates how different gases affect the display and output from an
InstruTech convection gauge.
WARNING! Using a thermal conductivity gauge with gases other than that for which it is calibrated could result in
death or serious injury. Be sure to use the correction factor data listed in the controller User Manual operating this device.
For N
the calibration shows excellent agreement between indicated and true pressure throughout the range from 10
2
1000 Torr. At pressures below 1 Torr, the calibration curves for the different gases are similar. The difference in readings
at these low pressures is a constant, a function of the difference between thermal conductivities of the gases.
At pressures above 1 Torr, indicated pressure readings may diverge significantly. At these higher pressures convection
currents in the gauge become the predominant cause of heat loss from the sensor and calibration depends on gauge sensor
geometry and mounting position as well as gas properties.
Generally, air and N
are considered the same with respect to thermal conductivity, but even N
2
differences in readings at higher pressures. For example, when venting a system to atmosphere using N
readings change by 30 to 40 Torr after the chamber is opened and air gradually displaces the N
other gases the effect is much more significant and may result in a hazardous condition as described below.
Other considerations when using gases other than N
Flammable or explosive gases
WARNING! InstruTech convection gauges are neither intrinsically safe nor explosion proof and are not intended for
use in the presence of flammable or explosive gases or vapors.
Under normal conditions the voltages and currents in InstruTech convection gauges are too low to cause ignition of
flammable gases. However, under certain failure conditions, sufficient energy could be generated to cause flammable
vapors or gases to ignite or explode. Thermal conductivity gauges like the InstruTech convection gauges are not
recommended for use with flammable or explosive gases.
Moisture / water vapor
In some processes (lyophilization, for example) the gas composition may not change significantly, except for moisture
content. Water vapor can significantly change the response of a thermal gauge and correction should be made, as you
would for any other gas.
Other contaminants
If your gases condense, coat, or corrode the sensor, the gauge calibration and response to different gases will change.
Generally, if the gauge can be "calibrated" ("zero" and "span" settings), these changes are small enough to be ignored. If
you can't set zero and span, the gauge should be replaced or return to factory for evaluation and possible cleaning.
InstruTech, Inc.
/ air is used, correction must be made for the difference in thermal
2
) and the gas in use. The gas correction data, charts and tables listed in your controller
2
/ air
2
CVG101 Worker Bee
and air will exhibit slight
2
, you may see
2
in the gauge. For most
2
-4
to
Page 7
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