HELIX GRANVILLE-PHILLIPS Convectron ATM, Instruction Manual

Page: 26 / 98

Chapter 3
18
Convectron
®
ATM Vacuum Gauge Module Instruction Manual
3.2 Analog output
The module contains a convection-enhanced Pirani thermal conductivity
gauge. The gauge measures the heat loss from a heated, gold-plated
tungsten sensing wire that is maintained at a constant temperature.
The vacuum chamber pressure indicated by the gauge depends on the gas
type, gas density (pressure), and the module orientation. The module is
factory calibrated for N
2
. (Air has approximately the same calibration.) For
gases other than N
2
or air, heat loss varies at any given pressure, and you
must apply an appropriate correction factor.
N
2
or air
Refer to Table 3-4 on page 19 to calculate pressure in Torr (y) as a function
of output voltage (x). Figure 3-2 and Figure 3-3 on pages 20–21 are graphs
that represent true pressure for N
2
or air (y axis) versus voltage (x axis).
• Output impedance is 100
Ω
.
• The output is normalized to 0.375 VDC at vacuum chamber pressure and
to 5.659 VDC at 1000 Torr for N
2
or air.
Commonly used gases
other than N
2
or air
Refer to Table 3-5 on page 22 for pressure versus output voltage for 10
commonly used process gases other than N
2
or air.
Refer to Figure 3-4, Figure 3-5, or Figure 3-6 on pages 23–25 to determine
true pressure versus indicated pressure for the gas that is being used.
Other gases
If the gas being used is not included in Table 3-5, or for a gas mixture, you
will need to generate a calibration curve using a gas-independent transfer
standard such as a capacitance manometer.