Pressure Measurement

One of the most common measurements on any machine or process is pressure. Air, gas and liquids are used extensively on many types of machines as the medium used to operate moving parts.

Processes which involve gases or liquids, to produce a substance from a mixture of ingredients, or a change of state, will generate a pressure at various stages which provide important indicators of process performance and status.

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Instrumentation Types

The physical measurement of pressure is carried out by utilising a strain sensing device incorporated into an elastic material with a set surface area such as a flat diaphragm, or a specially shaped capsule made from an elastic material such as a bourdon tube.

The range of pressurement can be varied by changing the size or thickness of the elastic material to tune the stiffness so suit a higher or lower levels of pressure.

There are a range of instruments used to measure pressure which can be divided into the following device categories, although there is some overlap between types due to inconsistent interpretations and language translations.

Pressure Sensors

The literal meaning is a device that detects, indicates or measures pressure. However when requesting a pressure sensor, most suppliers would assume you want a pressure measuring device which provides a pressure reading within a defined range via an analogue or a digital signal.

Pressure sensor products

Pressure Transducers

This is a device which converts changes in pressure into an electrical signal. There is a pressure transducer of some kind in nearly every electronic pressure measurement device.

Fundamentally a pressure transducer will produce an output when a suitable power supply is applied to it, and due to manufacturing variations, pressure transducers will exhibit widely varying output characteristics.

In order to utilise a pressure transducer it is therefore necessary to collect data on the characteristics by testing each transducer or a representative sample so that users can process the transducer output to provide a compensated and conditioned pressure reading or signal.

To distinguish the type of electrical outputs, conditioned pressure transducers will often include a prefix such as millivolt output, voltage output or current output.

Pressure Transmitters

These are a particular kind of pressure sensor or conditioned pressure transducer, and as the name implies they are designed to transmit a signal over a great distance, which in the world of sensors is a matter of feet and metres rather than kilometres and miles.

The most widespread definition of a pressure transmitter is a pressure sensor with 4 to 20 mA current loop signal output. This a robust and lossless signal which is used extensively throughout the process industry, and since the introduction of more compact devices, for industrial automation and test applications.

Although it’s a less common usage, a pressure transmitter may also be used to describe other outputs such amplified voltage signal, which can be transmitted over relatively large distances, but not as far as a current loop signal without signal losses and interference.

Pressure transmitter products

Pressure Switches

This is a device which provides an open or closed electrical contact when a set pressure has been reached. The electrical contact can be a simple physical open and closing of a circuit breaker to prevent or allow current to flow, or a solid state based device which turns a fixed voltage on and off.

Pressure switches typically consist of a relatively simple technology consisting of a diaphragm connected to an electromechanical device such as a microswitch, which opens or closes without the need for a separate external power supply, when the movement of the diaphragm due to pressure change, reaches a preset calibrated position.

To achieve a greater accuracy and offer more features such as a digital display, multiple switch outputs, hysteresis adjustment, more complex technology is utilised such as pressure transducers and solid state microelectronics which unlike the simple pressure switch require an external power supply to function.

Pressure switch products

Pressure Gauges

There are many pressure measurement instruments which are called pressure gauges, purely because they all show a visual display of the pressure reading.

The instrument that is closest to the original definition of a pressure gauge is one which has a vertical process connection for mounting directly onto a pressure line, and a circular calibrated dial and needle linked to a mechanism that moves in response to pressure changes.

In order to facilitate installing in a test panel, many pressure gauges provide the option for a rear mounted process connection along with a front panel mounting plate with fixing holes around the bezel.

The pressure reading components are encased in a metal or plastic housing with a clear glass or plastic viewing window. These pressure gauges are distinguished from other types by adding analogue or dial as the prefix.

Analogue dial pressure gauges are still used in large quantities today, due to the affordability, reliability, no need for batteries or external power, and always on pressure reading.

The introduction of digital electronics, LCD & LED displays has led to replacement of the glass, dial, needle & mechanism with user interface push buttons, a digital readout and a pressure transducer.

Digital pressure gauges allow users to measure with a higher precision and accuracy, from a greater distance, at broader viewing angles, and reduces the risk of damage when the pressure gauge is dropped accidentally.

Despite the digital innovations the majority of digital pressure gauges have retained the round shape and vertical process connection stem of the analogue dial type.

In addition to reading pressure, digital pressure gauges often provide other features such as a choice of pressure units in which to display the pressure readings, max/min reading storage, data-logging function, leak testing mode, digital interface connectivity, computer software, configurable contact switches and much more.

Pressure gauge products

Pressure Indicators

This is a broad class of pressure measurement instruments, that describes any device which provides a visual indication of a pressure reading.

It is often used to describe instruments which do not belong to another distinct pressure instrument category such as ones which are designed for use on a benchtop or for sliding into a rack as part of a test panel.

These types of pressure indicators are often powered by external means, and the pressure sensing device may be integral or external to the indicator connected to it by a length of cable.

Manometers

This is a particular form of pressure measurement instrument that historically describes a liquid filled U tube that was typically filled with water or mercury.

One end of the tube is connected to a reference pressure or sealed with a vacuum. The other end is connected to the pressure which is to be measured.

The tube is mounted onto a board with a rule, which is used to measure the height difference between the surface of liquid on either side.

To make it easier to use some manometers they features such as a larger well on one side, or a vertically sliding rule or tube column so that the reference side can be set to zero.

The use of water and mercury as the liquid fill led to the widespread use of water and mercury column pressure units e.g. inHg and inH2O.

Due to the practical height constraints, liquid column manometers were mostly used in low pressure applications such as ventilation and air handling in large buildings, laboratories and factories to manage environmental conditions, remove dust particles and exhaust gases.

Liquid column manometers have today largely been replaced by electronic devices, but it is still used as a simple, low cost, reliable and repeatable form of pressure measurement.

When an electronic device is used to replace a u tube manometer to measure low pressures it is often called a digital manometer. As with all digital instruments, a number of display features and instrument functions such as a choice reading in different pressure units, peak hold, data storage and zero tare are often provided.

Although liquid column manometers were mainly used for low pressure measurement due to the height limitations, the digital versions are not limited, and therefore some manufacturers also include high pressure ranges in their range of digital manometers.

Pressure Meters

Although strictly it can be used to describe any type of device for measuring and displaying a pressure reading, pressure meter is mostly used to describe portable handheld instruments for testing and checking pressures.

These instruments are typically battery powered and have LCD digital displays for indicating the pressure in a specific pressure unit.

A pressure meter will normally include buttons or soft keys to display the pressure in additional units, typically bar and psi or mbar, hPa, Pa and inH2O for lower pressure ranges, a zero button for nulling any offset on the display, and a high/low reading memory recall button.

Higher spec pressure meters will include a logging and leak test mode, and other software enhancements useful to the intended users industry.

Reference Pressure

Pressure readings are not all measured from the same starting point, but the most common is ambient or atmospheric air pressure. Since most machines are vented when stopped so that pressure inside equals the outside air pressure, only the additional amount of pressure generated by the machine when operational is important for monitoring and control.

The pressure reference can be deducted from the pressure reading in two main ways; either by physically applying it to the sensing device so that it self-compensates, or that it is measured separately and incorporated into the pressure reading manually.

Gauge Pressure

Since the majority of environments where pressure measurement equipment is used are at atmospheric pressure, it is usual to measure pressures relative to this pressure because most processes commence at the local air pressure. This is called measuring gauge pressure or to a lesser extent relative pressure.

When the pressure is higher than atmospheric pressure this is called positive pressure, and when it is lower, it is called negative (gauge) pressure, suction or vacuum.

Gauge pressure measurement products

Absolute Pressure

Although measuring pressure relative to a perfect vacuum is the most primary and fundamental form of pressure measurement, it is used in much fewer applications than gauge reference pressure. This form of measuring pressure is called absolute pressure measurement.

Leak checking is one example of where using absolute pressure is important, since you want to ensure the measurement is independent of atmospheric pressure changes.

Another example, and one which is very common, is the measurement of ambient or barometric air pressure for weather monitoring and altimetry.

Absolute pressure measurement products

Differential Pressure

In any complex pressurised system the pressure will vary at different points, for example due to process stages, chemical reactions, temperature variations, flow restrictions from filters and narrow lengths of pipework.

It is useful to monitor the effect that these variations in pressure have on the process, by measuring the difference in pressure between two different pressures on a system. This is called differential pressure measurement or delta pressure measurement.

Differential pressure graphical explanation

The differential pressure can be measured directly by applying each of the two pressures to either side of one measurement device, or by using two measurement devices and using electronics or software to subtract the two readings to determine the pressure difference.

Differential pressure measurement products

Engineering Units

Since pressure is derived from the combination of a force and an area, there are a complex mix of pressure units available made from force units: e.g. Newtons (N) kilogram force (kgf), pounds force (lbf, pf), etc…and area units: e.g. square centimetres, square metres, square inches, etc… to create units such as N/m2, lb/in2, kg/cm2, etc…

There are precisely defined pressure units such as the pascal (Pa), bar, torr, etc… and units which can vary in their defined value due to differing reference values for temperature and density, such as metres of water column (mWC), inches of water column (inH2O) or inches of mercury column (inHg).