Pressure is defined as the “force per unit area”. Such forces always produce a deflection, distortion or a change in volume or dimension, no matter how small or large the force is. The “force per unit area” can be measured in many different engineering units. Those commonly used in the UK are, bar, Pascal, pounds per square inch (PSI) and height of water column.
The “deflection” is connected to a sensing element that provides the basic measurement and controlling movement for any pressure instrumentation. The proportional deflection of the sensing element can also be used in pressure control, for example to trip an electrical switch at a given pressure. This is known as a pressure switch.
The deflection that changes the value of the sensing element can be used as a linear measurement sensor for measuring pressure over a given range. The linear output signal can be processed by electronic circuitry and displayed or controlled by slowing down or speeding up the process depending on the linear output. This comprises a basic pressure element.
Normally, the next step with the basic pressure element is to add circuitry to compensate for both zero and span as well as temperature in order to provide a specified accuracy. This includes linearity, hysteresis, repeatability, thermal effects, interchange ability and stability. (Each of these is discussed in a separate Engineering Note).
Having added the compensation circuitry you now have to package the unit. This comprises the mechanical port, housing, electrical output etc, which gives the sensing element and circuitry the optimum performance. This also covers issues such as vibration, shock, RFI protection etc.
The result is a fully calibrated pressure sensor with a known set of specifications, which is easy to install and guaranteed to meet a set specification for each manufactured batch.
However, be warned: there are sensors on the market with good-looking but incomplete specifications. Be careful when comparing devices, and ensure that all the relevant specs are available, such as zero, span offset and temperature effect.
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