Page 295 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 15 Sensors 257

expands, the bellows extends and the lever arm actuates the Fahrenheit / Centigrade Key Pad
potentiometer. The output of the potentiometer can drive
either a voltage divider or a Wheatstone bridge.
When two dissimilar metals, such as iron and copper, are Readout 1 2 3
placed in contact with one another and heated they will pro- 4 5 6
Case
duce a voltage. This junction is referred to as a thermocouple. CLR 7 8 9
Panel
The characteristics of thermocouple junctions are well under- F/C . 0 +/−
ON/OFF A B C D
stood and predictable, making them ideal as temperature
transducers. A simple temperature transducer can be con-
structed by simply twisting two wires of dissimilar metals Power
together and connecting them to a voltmeter, as shown in Set Point Controls
Figure 15-33. The output of a thermocouple is very low, so an Figure 15-35 Thermocouple Temperature Controller
amplifier must be deployed to drive controls of any conse-
quence.
Figure 15-34 shows a digital thermocouple readout. These
units generally consist of a high-impedance digital voltmeter
that has been calibrated to match the characteristics of the
specific thermocouple used.
Figure 15-35 shows a commercial thermocouple tempera-
ture controller. These units normally provide a readout, serial
port, and one or more adjustable set points. Many units also
provide programming functions that allow time/function/
.3 .4 .5 .6 .7
.1 .2 .8 .9
0 1 temperature curves to be programmed.
DC VOLTS
Voltmeter
Level Sensors

The simplest types of fluid level indicators are float switches.
− + Principally, there two different types, as shown in Figure 15-36.
Top mount switches have a cylindrical float that carries a
magnet. The float is fitted around a central core containing a
Thermocouple magnetic reed switch. The normal condition of the switch can
Junction
be changed by removing the float, flipping it, and reinstalling it.

Heat Source

NPT Thread
NPT Thread
Float Pivot
Switch Housing Switch
Housing
Float
Float
Figure 15-33 Thermocouple Temperature Sensor Side Mount
Snap Ring
Top Mount
Figure 15-36 Fluid Level Switches

Readout Side mount switches have a hinged float, which carries a
Case magnet. When the float is aligned with the end of the body,
Panel
the switch activates. Orienting the float either up or down can
ON/OFF °F/°C
change the normal condition of the switch.
Level indication can be provided by installing a float
Fahrenheit / Centigrade
Power switch array, as shown in Figure 15-37. In this case the fluid
Figure 15-34 Thermocouple Temperature Readout level controls a simple string of panel lamps. It should be