HELIX GRANVILLE-PHILLIPS Convectron ATM, Instruction Manual

Page: 89 / 98

Theory of Operation
Convectron
®
ATM Vacuum Gauge Module Instruction Manual
81
Bridge voltage is a non-linear function of pressure. This relationship is
illustrated in Figure B-2. If the ambient temperature does not change, R
1
remains constant.
Figure B-2 Wheatstone bridge block diagram
As vacuum chamber pressure decreases, the number of molecules in the
vacuum chamber and the resulting heat loss from the sensing wire also
decrease. Temperature and R
1
resistance therefore increase.
The increased resistance through R
1
causes the bridge to become
unbalanced and a voltage to develop across the null terminals. The bridge
controller senses the null voltage and decreases the voltage across the
bridge until the null voltage again equals zero. When the bridge voltage
decreases, the power dissipation in the sensing wire decreases, causing R
1
resistance to decrease to its previous value.
A pressure increase causes an opposing series of occurrences, during which
the bridge controller increases the bridge voltage to maintain a zero null
voltage.
Vacuum
and ATM
adjust
Process
control
Vacuum
output
Amplifier
Buffer
Bridge Control
R1 R3
R4 R2