MKS Granville-Phillips 358 series, Instruction Manual

Page: 102 / 122

Chapter 7
102
Series 358 Micro-Ion Controller Instruction Manual - 358013 - Rev. B
7.2 Convectron Gauge Theory
of Operation
The Convectron Gauge transducer is represented in Figure 7-2 as R1, R2,
R3, and R4. These four resistances form the legs of a bridge circuit, with R1
designating the sensor wire of the transducer. R2 is a resistive network in the
tube that compensates for changes in the ambient temperature. At bridge
null, R1=R2xR3/R4. If there are no changes in ambient temperature, the
value of R1 is a constant and the bridge is balanced.
Figure 7-2 Convectron Gauge Schematic
As the vacuum system pressure is decreased, there are fewer molecules in
the system to conduct the heat away from the sensor wire causing the
temperature and resistance of R1 to increase. The increased resistance of R1
causes the bridge to unbalance and a voltage is developed across the null
terminals. The bridge control circuit senses the null voltage and decreases
the voltage across the bridge until the null voltage is again zero. When the
bridge voltage is decreased, the power dissipated in the sensor wire is
decreased causing the resistance of R1 to decrease to its previous value. The
opposite events happen for a pressure increase. The bridge voltage is a
nonlinear function of pressure.
All materials have been chosen for ultra high vacuum service, corrosion
resistance and bakeability to 150 °C. The gauge tube envelope is type 304
stainless steel. All metallic joints in the envelope are TIG welded. No solder
is used within the envelope. The following materials are exposed to the
vacuum. Type 304 stainless steel, Carpenter Alloy 52, Kovar
®
, Kapton
®
,
gold-plated tungsten, borosilicate glass and Dow-Corning
®
9015 glass. The
blue trim cover is molded of Ultem
®
polyetherimide resin suitable for
service to 150 °C.
NULL
BRIDGE
CONTROL
AMP
–
+
CONVECTRON
GAUGE
TRANSDUCER
R2
R1
R4
R3
V
BR