Instruction Manual
InstruTech CVM-211 "
Stinger
" Convection Vacuum Gauge Module
InstruTech, Inc
page 12
Effect of Different Gases on Signals and Displayed Pressure
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 gauges) are
normally calibrated using nitrogen. When a gas other than N
2
is used, correction must be made for the difference
in thermal conductivity, between N
2
and the gas in use. The charts and tables below indicate how different gases
affect the display and output from an InstruTech convection gauge.
For N
2
the calibration shows excellent agreement between indicated and true pressure throughout the range from
10
-4
to 1000 Torr. (As it
should
, of course, since they're calibrated for N
2
.) 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 tube geometry and mounting position, as well as gas properties.
Generally, air and N
2
are considered the same as far as thermal conductivity goes, but even these two gases will
exhibit slight differences in readings at higher pressures. For example, when venting a system to atmosphere
using N
2
, you may see readings change by 30 to 40 Torr after the chamber is opened and air gradually displaces
the N
2
in the gauge.
For most 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
2
Flammable or explosive gases
:
InstruTech CV gauges and modules are not intrinsically safe or explosion proof, and are not intended for the
measurement, or use in the presence, of flammable or explosive gases or vapors.
Under normal conditions, the voltages and currents in InstruTech CV Gauges and Modules 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 gauges like the InstruTech CV gauges and modules 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 "atmosphere" settings), these changes are small
enough to be ignored. If you cannot set zero and atmosphere, the gauge should be replaced or cleaned. (See
the Appendix C on cleaning for more information.)
