HELIX GRANVILLE-PHILLIPS Convectron ATM, Instruction Manual

Page: 88 / 98

Appendix B
80
Convectron
®
ATM Vacuum Gauge Module Instruction Manual
B.2 Convectron heat-loss
Pirani gauge
The Convectron gauge is a convection-enhanced Pirani gauge. It operates
like a standard Pirani gauge, which employs the principle of a Wheatstone
bridge to convert pressure to voltage, but uses convection cooling to enable
accurate pressure measurement, when properly calibrated, from 10
–4 to
1000 Torr.
The sensing wire is an ultra-fine strand of gold-plated tungsten, which is
corrosion resistant and has exceptionally stable heat transfer characteristics.
The heated sensing wire loses more heat as the ambient gas pressure
increases. The more molecules contact the sensing wire, the more power is
required to keep the sensing wire at a constant temperature. So, as pressure
increases, the voltage across the Wheatstone bridge also increases.
The large interior volume of the Convectron gauge enables convection
currents to develop, enabling greater measurement resolution at higher
pressures. The Convectron gauge has a temperature compensator, which
causes bridge voltage to remain unaffected by changes in ambient
temperature.
B.3 Wheatstone bridge circuit
description
Figure B-2 is a block diagram of the module controller. The Convectron
gauge sensing wire is designated R
1
in the Wheatstone bridge circuit. The
temperature compensator is designated R
2
. At bridge null, the following
equation applies:
R
1
R
2
R
3
+
R
4
------------------ =