Page 300 - Electromechanical Devices and Components Illustrated Sourcebook

262 Electromechanical Devices & Components Illustrated Sourcebook

Voltmeter Heater Compensator Gauge Element
Envelope
Envelope

Gas Gas
Voltmeter

Junction
Heater Power Supply
Figure 15-54 Thermocouple Vacuum Gauge Schematic
Battery
Figure 15-56 Pirani Vacuum Gauge Schematic

As pressure decreases, the effectiveness of mechanical
gauges diminishes. To improve accuracy, thermal elements
are employed. Figure 15-54 shows a schematic representation
of a thermocouple vacuum gauge. In this case a heating ele-
and, therefore, change the resistance. This change is reflected
ment is fed a constant power. At the center of the element a
by the voltmeter and, thereby, the pressure of the system can
thermocouple is attached. When the element is exposed to
be gauged.
gas, a certain amount of heat is drawn off and the output of
Figure 15-57 shows a typical commercial Pirani vacuum
the thermocouple junction changes in direct proportion to the
transducer. These units are generally supplied with a quick
gas pressure.
disconnect vacuum flange, cable, and DIN connector. The cal-
Figure 15-55 shows a typical commercial thermocouple ibration controls are usually placed directly on the gauge
1
gauge. These units are generally supplied with a / -inch NPT head. Like the thermocouple gauge, these transducers also
8
(national pipe thread) and octal base. The transducer will also
require a power supply and readout calibrated in pressure
require a power supply and readout calibrated in pressure
units.
units.
Gas Input Vacuum Flange
Calibration Controls
NPT Thread
A

B

Element Housing

Octal Base
Connector
Figure 15-55 Commercial
Thermocouple Vacuum Sensor Figure 15-57 Commercial Pirani Vacuum Gauge

To provide even lower pressure accuracy, a Pirani gauge is High Vacuum Sensors
typically specified. This particular gauge element will normally
bridge the vacuum and high vacuum regimes. Figure 15-56 The second vacuum regime is high vacuum, or random
shows a schematic representation of a Pirani gauge. The molecular motion. In this regime the gas molecules are not in
gauge element makes up one resistor in a Wheatstone bridge constant contact with one another and are moving at random
and is exposed to the pressure to be monitored. A compen- in space. When gases are added or removed there is a cause
sator resistor is placed into a vacuum envelope opposite the and effect that is statistically linked. Because the gas
element. The element and compensator are matched at high molecules are not in constant contact with one another,
vacuum pressures. A battery is used to heat the gauge ele- mechanical measuring methods are ineffective; therefore, the
ment. Any gas coming in contact with the element will cool it gas content must be measured statically.