Page 330 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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322 CHAPTER 12 Sensor Theory
Metal A
Hot junction
Termination block
FIGURE 12.18
Cold junction
Cold junction
Volt meter
Current
Metal B
Metal C
Simple diagram of a thermocoupler.
differential between the metals. Heated junction thermocouplers, however, amplify this phenomenon by placing a thermocoupler next to a heat source and another farther away.
The thermocoupler next to the heat source will measure a much higher reading while in gas (or steam), versus while in fluid, due to gas conducting heat much more efficiently than fluid. Thus, the two thermocouplers are compared and sense the level (or degree thereof) by comparing the degree of in-fluid temperature/conductance on the primary versus the reference secondary.
12.2.7 Force/load/weight
For the purposes of this section, the term “force” measures the interaction between bodies, “load” measures force exertion, and “weight” measures gravitational acceleration forces. Sensors in this category most commonly measure these parameters based upon either piezoelectric quartz crystal or strain gauge sensing elements. While piezoelectric sensors are best for a dynamically changing load situation, the strain gauge is best suited for a static load environment.
12.2.7.1 Quartz sensors
Quartz crystals have the unique characteristic of generating an electrostatic charge when force is applied (or removed). This is quite useful in a host of force/load/strain measurement functions such as depth gauges where vehicle depth is constantly changing. The quartz sensor uses the piezoelec- tric effect to measure the pressure strain upon the transducer. This technology is typically used for the measurement of highly dynamic pressures (such as those encountered in deepwater applications).
12.2.7.2 Strain gauge sensors
With strain gauge sensors, the transducer forms a variable resistor based upon the load applied to the sensor (typically a foil-type transducer). The load applied is directly proportional to the
