Chapter IX – Measuring Pressure and Temperature

CHAPTERIX

Measuring Pressure and Temperature

9.1. Pressure transducers

Pressure is usually measured using a diaphragm strain-gage pressure transducer. The test pressure is applied to one side of the diaphragm and reference pressure is applied on the other side. The difference in pressure will lead the diaphragm to deflect. This deflection is measured using a strain-gage. The reference pressure can be the atmospheric pressure (to get the gage pressure) or almost a vacuum (to get the absolute pressure).

The diaphragm is usually made of a semiconductor material (usually silicon + corrosion-resistant metal) with semiconductor strain-gages.

Figure 9.1. Schematic of a strain-gage pressure transducer.

If the objective if to measure very small pressures (order of 0.1 Pa) a capacitive pressure transducer is probably the optimal choice. This is because capacitive sensors can detect very small deflections.

Figure 9.2. Capacitive pressure transducer.

The technical challenge regarding measuring pressure appears when the objective is to measure time-varying pressures. This is because the frequency has to be taken into account and the rapid variations in the pressure have to be detected by the transducer. In this case, a piezoelectric pressure is probably the optimal choice.

Figure 9.3. Piezoelectric pressure transducer.

These transducers usually use transverse-effect piezoelectric sensing elements. Such transducer are usable up to about 30 kHz.

9.2. Thermocouples

Let us take two metals and connect them together, to create a junction. If now you start heating up the junction, you will end up with a voltage. This effect is called the Seebeck effect (1821). This is the basic idea behind thermocouples: the voltage induced at a junction is a function of the temperature.

Figure 9.4. Thermocouple.

Thermocouples are very widely used in experiments; however there are several problems in their use:

1. The voltage measurement must be made with no current flow. This issue is solved by using very high input impedance in the acquisition system.

2. Connections to voltage-measuring devices result in additional junctions. This problem is solved by using another junction as a reference junction. This reference junction is held at a known constant temperature usually (0C) (An electronic device simulates this temperature). Furthermore, the terminals of the acquisition system must be placed into the same thermally insulated box.

3. Voltage depends on the composition of the metals used in the wires. This problem has been solved by restricting the materials used for thermocouples.

There are also several factors that have to be considered when selecting a thermocouple:

-Sensitivity

-Linearity of the output

-Stability and corrosion resistance

-Temperature range

-Cost

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Instrumentation and Measurements \ LK\ 2009