What is Pressure Measurement?
Measuring pressure is very common throughout industrial processes and it must be understood what reference any particular reading is made from as this can drastically affect the accuracy of the measurement.
Gauge Pressure means that the pressure measured is relative to the atmospheric pressure at the location of the measurement.
Absolute Pressure is the pressure measured with reference to full vacuum or no atmospheric pressure.
For example, 1 bar gauge pressure is approximately 2 bar absolute and 1bar absolute is approximately 0 bar gauge or roughly normal atmospheric pressure.
Since atmospheric pressure changes constantly, truly accurate measurements are usually based on absolute pressure but for most processes, gauge pressure is sufficient as the atmospheric pressure variations are not critical to the process.
Units of Pressure Measurement
There are a wide variety of Imperial, S.I. and other units commonly used in measurement and these can include: bar, Pa, kg/cm2 & psi amongst many others. For measuring very low or negative (below atmospheric) pressures, other units are more common such as mbar, inWC, kPa, mmHg & mHd.
It is not uncommon that a working pressure range may be above and below atmospheric pressure during the working cycle and rather than going to the expense of absolute pressure instruments, it is common to use Compound ranges referenced to gauge pressure such that the scale range will go from less than 0bar Gauge (negative pressure) to a positive gauge pressure.
Differential Pressure Measurement
Combines inputs from 2 pressure inlets through a mechanical or electronic sensing assembly to determine the difference between the two inlet pressures. This can be useful for determining flow through orifice plates or pitot tubes for very low or very high pressure systems. Differential pressure changes can be used to indicate fliter clogging to inform when maintenance is due or as a means of measuring accurate liquid levels in tanks.
Type of Pressure Measurement Products
Mechanical pressure gauges are commonly used as indicators but highly accurate versions are used as calibration references with anti parallax mirrored scales for test and calibration labs. Pressure gauges can be supplied with electrical contacts for alarm indication. Electronic pressure gauges offer excellent accuracy and can combine relay outputs and data loggers for additional functionality.
Pressure switches provide an electrical contact for triggering alarm or control conditions and often are based on a mechanical diaphragm or piston sensing element.
Pressure transmitters use electronic sensors to provide accurate proportional output of the measured fluid, commonly using 4-20mA current loop for the output signal.
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