Page 299 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 15 Sensors 261

Moving Head
Plunger Rod
Bellows
Linear Potentiometer
Input Head

Input
P 1 P 2 Input

Bellows Weld

− +

R 1
+/− Voltmeter

Zero Adjustment

Battery
Figure 15-50 Differential Bellows Transducer with Slide Wire Bridge Output

Spring Return
Cylinder Rods Vacuum Sensors
Plunger Rod Cylinders
Linear Potentiometer
Vacuum Sensors generally fall into two different categories—
negative pressure, or vacuum, and random molecular motion,
or high vacuum. The negative pressure regime is usually con-
Terminals
sidered to be between 14.7 pounds per square inch absolute
P 1 P 2
Connector Nuts
Jam Nuts (PSIA), or atmosphere, and 0 PSIA (0 to 30 inch Hg.). In this
Figure 15-51 Cylinder Differential Pressure Transducer pressure range the molecules are in constant contact with one
another and when gases are added or removed there is a cause
and effect that is mechanically linked. Because the gas
molecules are in contact with one another, the pressure can be
cylinder pressure transducer, the sensitivity of this cylinder gauged using mechanical methods.
arrangement will not be as sensitive to minute changes as a The most common vacuum gauge is the Borden tube
bellows unit because the pistons and cylinders will exhibit a gauge. These units use the same basic mechanism as their
certain amount of stiction. pressure counterparts, except that they are set up to read vac-
For the most sensitive differential pressure applications, a uum. Figure 15-53 shows a typical Borden tube vacuum
capacitance manometer is generally specified. These trans- gauge with a potentiometer output.
ducers are extremely sensitive to minute changes in pressure
and are often found on vacuum systems to monitor and con-
trol the blending of partial pressure gases. Figure 15-52 shows
a schematic representation of a capacitance manometer.
Potentiometer
Gauge Housing Terminals
High-Voltage/Frequency P
Transformer Envelope 1 Bezel 15
Fixed Plates 20 10
Amplifier
25 5
10 kHz Output 30 "Hg 0 A B C
Supply Needle
USA
Diaphragm NPT Thread
P Vacuum Port
2
Figure 15-52 Capacitance Manometer Differential Pressure Figure 15-53 Commercial Vacuum Gauge with 0- to 10-Volt
Gauge Schematic Output