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Pressure Measurement & Pressure Measuring Instruments
Pressure is probably the most common measurement made by the instrument engineer. Pressure measuring instruments are used not only for pressure measurement, but also to infer other industrial measurements e.g.
Flow - measuring the pressure dropped across a restriction e.g. across a flow measuring orifice plate,
Liquid Level - measuring the pressure created by a vertical liquid column,
Liquid Density - measuring the pressure diﬀerence across a ﬁxed-height liquid column,
Weight - as used in a hydraulic load cell.
Atmospheric pressure, also known as barometric pressure, is the pressure within the atmosphere of Earth. So when measuring pressure the instrument engineer can either measure pressure above atmospheric pressure, this is called gauge pressure, or measure pressure above an absolute vacuum, so called absolute pressure.
See our page on Gauge and Absolute Pressure measurement and conversion for more details of why this is important, and how to easily convert between gauge and absolute pressures.
Common Units of Pressure
The SI unit of pressure is the pascal (Pa), defined as a force of one Newton per square meter. However in industry it is very common for other units to be used e.g. psi, bar, torr etc. Converting between units is relatively straight forward, and something that instrument engineers and process engineers have to do daily.
See our page on Pressure units and pressure unit conversion for more details of the more common units of pressure and a handy pressure unit conversion table.
Pressure Measuring Instruments
There are numerous types of pressure instrument, ranging from the simple manometer through the mechanical pressure-sensing elements like the bordon tube, to the more complex electronic differential pressure transmitter.
A manometer is a ﬂuid-ﬁlled tube where an applied gas pressure causes the ﬂuid height to shift proportionately. This is why pressure is often measured in units of liquid height (e.g. inches of water, inches of mercury).
Mechanical Pressure Elements
Mechanical pressure-sensing elements include the bellows, the diaphragm, and the bourdon tube. Each of these devices converts a ﬂuid pressure into a force. If unrestrained, the natural elastic properties of the element will produce a motion proportional to the applied pressure. Mechanical pressure-sensing elements are at the heart of most pressure gauges. See our page on Mechanical Pressure Gauges for an expalnation of the different types, and how to select the correct type for your application.
Electronic Pressure Instruments
Several diﬀerent technologies exist for the conversion of ﬂuid pressure into an electrical signal. These technologies form the basis of electronic pressure transmitters: devices designed to measure ﬂuid pressure and transmit that information via electrical signals such as the 4-20 mA analogue standard, or in digital form such as HART or FOUNDATION Fieldbus.
The links below lead to a wide variety of pressure measurement related reference data, tools, resources and look up tables as used by instrument engineers when specifying pressure instruments, calibrating transmitters and purchasing transmitters.
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For those who want to read further about the theory of pressure measurement and the differing types of pressure instrument, then the following books from Amazon will be of interest: