Page 297 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 15 Sensors 259

have zero electrical resistance. The section above the fluid
level retains its noncooled resistance. By measuring the over-
Stand Pipe all resistance of the loop, the amount of cooled versus non-
cooled wire can be calculated and the level of the fluid can be
determined.
Fluid Level
Pressure Sensors

Detecting pressure is an important function in all manner of
consumer, commercial, and industrial equipment. The basic
Fluid Column pressure switch, as shown in Figure 15-43, consists of a pres-
sure housing with a diaphragm. A plunger style limit switch is
arranged, so the diaphragm button will activate the unit when
a preset pressure is reached. The switch is equipped with a
through actuator, which is countered with a preload spring and
adjustment screw. The pressure actuation, or set point, can be
adjusted by turning the screw until the switch actuates.
Pressure
Transducer
Diaphragm Button
E F Switch
0- to 10-Volt Output Pressure Terminals
Housing
Through Plunger
Pressure Preload
Port Adjustment
Figure 15-41 Pressure Fluid Level Schematic Preload Spring
Diaphragm
Figure 15-43 Pressure Switch with Adjustable Set Point
fluids are liquefied oxygen, nitrogen, helium, and argon. The
natural temperatures at which these gases exist in a liquid
state, is well below the zero resistance level of the super con- Figure 15-44 shows a commercial pressure switch with
ductor. Therefore, if a loop of high-temperature super con- adjustable set point. These units are generally equipped with
ducting wire is immersed into a quantity of cryogenic fluid, as an NPT thread, two terminals, and a top mounted adjustment
shown in Figure 15-42, the section below the fluid level will screw.

Set Point Adjustment

High-Impedance Ohm Meter Terminals

SB
Switch
1 OFF 1 Housing
AC VOLTS 10 10 DC VOLTS
100 100
1000 1000
.1 .1
AC AMPS AC AMPS
1 1
10 100K
100 1K 10K
OHMS Diaphragm
Housing
10A A COM V/
High-Temperature NPT Thread
Super Conductor
Figure 15-44 Commercial
Pressure Switch with Adjustable Set
Fluid Level Point
Diaphragm pressure transducers are similar in construction
Cryogenic to their switch counterparts. The switch assembly is replaced
Fluid
with a potentiometer assembly, which will produce a contin-
Figure 15-42 Low-Temperature Super Conductor Fluid Level uously variable output. Figure 15-45 shows a diaphragm pres-
Schematic sure transducer layout.