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Instruction Manual
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Principal of Operation
Thermal conductivity gauges measure pressure indirectly, by sensing the loss of heat from a sensor to the
surrounding gases. The higher the pressure of the surrounding gas, the more heat is conducted away from the
sensor. Pirani thermal conductivity gauges maintain a sensor (usually a wire) at some constant temperature, and
measure the current or power required to maintain that temperature. A standard Pirani gauge has a useful
measuring range of about 10
above 1 Torr, convection-enhanced Pirani gauges increase the measuring range to just above atmosphere.
Output Signals and Displays
A vacuum gauge sensor, by itself, does not generate any output signal or readout. Electronic signal conditioning
is required to convert heat loss into a useable signal and/or readout of pressure. Note that the output signal
voltage is not the same as the displayed pressure readings. Table 3 below shows the displayed readings at
various pressures for selected gases.
Non-Linear Output
Granville-Phillips' first Convectron
produced a non-linear output signal of 0.375 to 5.659 VDC
for 0 to 1000 Torr of N
, roughly in the shape of an "S" curve,
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as shown at right. GP adopted the same output curve for
most of their Mini-Convectron
non-linear output (though in recent years, some GP
controllers produced a different S-curve).
The non-linear output from InstruTech convection gauges,
modules, and controllers duplicates exactly the original S-
curve of 0.375 to 5.659 VDC for 0 to 1000 Torr.
Table 1 below contains the lookup data for converting the
non-linear output voltage into pressure values.
Log-Linear Output
Many InstruTech modules and controllers also provide a log-
linear output signal, as an alternative to the non-linear signal
described above. This output, shown at right, is a 1 Volt per
decade signal that may be easier to use for data logging or
control.
Table 2 below contains the lookup data and provides the
formulas for converting the log-linear output voltage into
pressure values.
InstruTech, Inc
InstruTech VGC-301 Convection Vacuum Gauge Controller
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Torr to 10 Torr. By taking advantage of convection currents that are generated
®
gauge controllers
®
modules and controllers with
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Output - Volts
Output - Volts
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Non-Linear Analog Output
Non-Linear Analog Output
Output - Volts
Output - Volts
Output - Volts
Pressure - Torr
Pressure - Torr
Pressure - Torr
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Log-Linear Analog Output
Log-Linear Analog Output
Pressure - Torr
Pressure - Torr
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page 16
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