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Pressure Measurement Glossary

Find an explanation here for pressure measurement terms listed in alphabetical order. By clicking on the pressure measuring terms highlighted in blue you will link to pages with pressure measurement definitions explained in more detail or extra information related to that pressure measurement glossary description.

4-20mA Current Loop Output - is a type of electrical signal that is used in a series circuit to provide a robust measurement signal.

Absolute Pressure - is one which is measured relative to a perfect vacuum.

Barometric Pressure or Atmospheric Pressure - is the total outside air pressure measured with reference to absolute vacuum. The pressure varies depending on geographical location, altitude and local weather conditions. For weather reporting purposes the barometric pressure is normally adjusted to a sea level value so that all locations can be compared independent of the altitude at each location

Bar - is a widely used metric unit of measure for pressure and 1 bar converts precisely to 100,000 Pascals.

Best Straight Line or BSL - is a mathematically derived straight line which runs through a set of recorded pressure measurement readings in a way that on average every point is at its closest to that line.

Best Straight Line Accuracy - defines the limit that any pressure measurement reading will deviate from the Best Straight Line.

Bonded Foil Strain Gauges - are metal based foil elements that are incorporated into a flexible electrically insulating material which can then be bonded to a diaphragm with adhesives. As pressure is applied to the diaphragm the strain gauge will deform and its resistance will change.

Burst Pressure - is a design safety limit which should not be exceeded. If this pressure is exceeded it may lead to mechanical breach and permanent loss of pressure containment.

Centimetres of H2O (cmH2O) , Centimetres of Water Gauge (cmWG) or Centimetres of Water Column (cmWC) - is a manometric unit used to relate a pressure reading to the height of fresh water at a temperature of 4 degC. 1 Centimeter of Water Gauge equals 98.0665 Pascals.

Centimetres of Mercury (cmHg) - is a small pressure unit which represents the pressure pushing down due to gravity of any volume of liquid mercury which is 1cm high. 1 Centimeter of Mercury at zero degrees Celsius equals 1333.22 Pascals.

Ceramic Pressure Sensors - typically use diaphragms made from Alumina (Al2O3) with a screen printed thick film strain gauge circuit. Ceramic diaphragms are very inert and provide a high level of protection from aggressive chemicals.

Compensated Temperature Range - refers to limits that Temperature Errors have been tested and are not necessarily related to the operating temperature range. A Pressure Measurement Device may continue to function outside the compensated temperature range but the accuracy of any measurement is not defined.

Compound Pressure Ranges - are ones that combine positive and negative pressure ranges into one pressure measurement device.

Differential Pressure (DP) - is the difference in pressure between two separate points.

Digital Compensation - is the process of collecting many measured points at different pressures and temperatures and using this data to correct the output of a pressure transducer so that the pressure accuracy and thermal errors can be improved beyond what is practically achieved with analogue compensation.

Dyne per Square Centimetre (dyn/cm²) - is one of the smallest pressure units and is derived from the CGS unit for force where 1 Dyne equals 1 gram accelerated by 1 centimeter per second per second. 1 Dyne per Square Centimetre equals 0.1 Pascals.

Feet of H2O (ftH2O), Feet of Water Gauge (ftWG) or Feet of Water Column (ftWC) - is an American and English low value pressure unit, now used mostly in the USA where feet and inches are the standard units for measuring distance. 1 Foot of Water Column at 4 degC (39.2 degF) equals 2989.07 Pascals.

Full Range Output or FRO - is the maximum electrical output signal for an analogue output pressure sensor.

Full Scale or FS - is the maximum measurable pressure for a particular measurement instrument.

Full Scale Output or FSO - is the maximum electrical output signal for an analogue output pressure sensor.

Gauge Reference Pressure or G - is a pressure measured relative to atmospheric or barometric pressure.

Grams per Square Centimetre (g/cm²) or Grams Force per Square Centimetre (gf/cm²) - is a cgs system (centimetre, gram, second) pressure unit which is now largely obsolete and officially superceded by SI system of units. 1 Gram per Square Centimeter is 98.0665 Pascals.

Hectopascal hPa - is a 100x multiple of the Pascal which is the SI unit for pressure. The Hectopascal is the international unit for measuring atmospheric or barometric pressure. 1 Hectopascal equals 100 Pascals.

Hydrostatic Pressure - is what is exerted by a liquid when it is at rest. The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure.

Inch of H2O (inH2O), Inch of Water Gauge (inWG) or Inches of Water Column (inWC) - is an English and American unit for measuring liquid level. 1 inch of water column at 4 degrees celsius equals 249.089 Pascals.

Inch of Mercury (inHg) - is a British and American unit of measure for pressure. 1 inch of Mercury at 0 degrees Celsius (32 deg F) equals 3386.39 Pascals.

IP Ratings or Ingress Protection Rating - is an accepted engineering standard for defining the protection of electrical equipment enclosures from the ingress of solids and liquids such as dust and humidity.

Kilogram per Square Centimeter (kg/cm²) or Kilogram Force per Square Centimeter (kgf/cm²) - is a pressure unit that has been largely superceded by the SI unit system of pascal units. It is the metric equivalent of pounds per square inch (psi). 1 kg/cm2 equals 98,066.5 Pascals.

Kilopascal (kPa) - is a 1000x multiple of the Pascal unit which is the SI unit for pressure. 1 Kilopascal equals 1,000 Pascals.

Long Term Stability or Long Term Drift - is the amount of change of a measured reading at exactly the same pressure and ambient conditions over a given period of time which is typically quoted as an annual figure.

Maximum Pressure Hysteresis - is the largest difference between measurements recorded during an increase in pressure from zero to full scale pressure and a decrease in pressure from full scale to zero pressure.

Mean Time Between Failures (MTBF) - is a theoretically calculated value which considers all the components used in the construction of a product. It may also be a statistically calculated value derived from real operational data collected from a sufficient number of the same product.

Measurement Precision - is used to describe how closely grouped a set of readings are to each other.

Megapascal (MPa) - is a 1000000x multiple of the Pascal unit which is the SI unit for pressure. 1 Megapascal equals 1,000,000 Pascals.

Metres of H2O (mH2O) , Metres of Water Gauge (mWG) or Metres of Water Column (mWC) - is a metric unit for measuring liquid level. 1 metre of water column at 4 degrees Celsius equals 9806.65 Pascals.

Microns of Mercury (µHg) or Micrometres of Mercury (µmHg) - is a very small pressure unit which is derived from the pressure due to gravity that is generated by a 1 micrometer (1/1000mm) column of liquid mercury. 1 Micron of Mercury of Mercury at zero degrees Celsius equals 0.133322 Pascals.

Millibar (mbar, mb or mbr) - is a metric unit of pressure mainly used in European countries and is derived directly from the Bar pressure unit which equals 1,000 mbar. In SI units 1 mbar equals 100 Pascals.

Millimetres of H2O (mmH2O) , Millimetres of Water Gauge (mmWG) or Millimetres of Water Column (mmWC) - is a low range metric pressure unit derived from the hydrostatic pressure of 1mm head of water at 4 degC. 1 Millimeter of Water Gauge equals 9.80665 Pascals.

Millimetres of Mercury (mmHg) - is a small Metric pressure unit which is derived from the hydrostatic pressure generated by a 1 mm tall column of mercury liquid. 1 Millimeter of Mercury equals 133.322 Pascals.

Millitorr (mTorr) - Millitorr is a very small pressure unit used for high vacuum measurements and is a 1/1000x multiple of the Torr pressure unit. 1 mTorr equals 0.133322 Pascals.

Negative Gauge Pressure - is the difference in pressure between any vacuum pressure and atmospheric pressure. The maximum possible negative gauge pressure is always limited by the current ambient atmospheric pressure, which constantly varies, but is typically around 1 bar absolute.

Newton per Square Metre (N/m2) - is a unit that shows how the Pascal unit is derived from other SI units. Pressure is defined as Force/Area and the SI unit for Force is Newtons (N) and the SI unit for Area is Sq Meters (m^2). 1 Newton per Square Metre equals 1 Pascal.

Non-Linearity - is how much a set of recorded pressures deviate from the best straight line and it is expressed as a percentage of full scale.

Non-Linearity, Hysteresis and Repeatability or NLHR - is often used to describe the room temperature precision of a pressure sensor and excludes all zero & span offsets, temperature errors and long term stability.

Non-Referred Thermal or Temperature Error Band - is derived from two measurement points that exhibit the maximum deviation in the positive and negative direction from the measurements at room temperature and adding the deviation of each together and expressing it as a percentage of full scale e.g. 2%FS. This is often expressed as a ±figure by dividing the Total Error Band by two e.g. ±1%FS.

Ounce per Square Inch (oz/in²) or Ounce Force per Square Inch (ozf/in²) - is a low pressure unit which is part of the British and American system of units. 1 Ounce per Square Inch equals 430.922 Pascals.

Overpressure or Proof Pressure - is the maximum pressure that can be applied to a pressure instrument without significantly effecting its reading accuracy. Some manufacturers will state a maximum calibration shift following an over-pressure condition where others will state that the specification is unaffected.

Pascal (Pa) - is the SI unit for pressure and is derived from other SI units using the following relationships: Pa = (kg.m/s²)/m² = kg/m.s² = N/m². Since 1 pascal is a very low pressure being 1/100 of a millibar it's use is limited to ultra low pressure applications such as measuring the pressure differences in ventilation systems.

Pounds per Square Inch (psf) or Pound Force per Square Inch (pfsf) - is an British Imperial and American pressure unit which is directly related to the psi pressure unit by a factor of 144 (1 sq ft = 12 in x 12 in = 144 sq in). 1 Pound per Square Foot equals 47.8803 Pascals.

Pounds per Square Inch (psi or lb/in^2) or Pound Force per Square Inch (pfsi or lbf/in^2) - is a widely used British and American unit of measure for pressure. 1 psi converts to 6,894.76 Pascals.

Pressure - is the amount of force or load applied to a unit area; Force divided by Area = Pressure. The SI unit for pressure is the Pascal (Pa) which is derived from Newton per metre (N/m2).

Pressure Accuracy - defines the limit that any pressure measurement reading will deviate from the ideal point.

Pressure Hysteresis - is the difference in measurement taken at exactly the same pressure but recorded directly after an increased change in pressure and a decreased change in pressure. This is caused by the natural reluctance of a pressure sensing material such as a diaphragm to return to its original position, shape or form after being stressed.

Pressure Transducer - is any device that can convert a change in pressure into a varying electrical signal.

Pressure Transmitter - is a pressure transducer which has been combined with an amplification circuit to allow the electrical signal to be sent over longer distances without degradation.

Primary Pressure Standard - are instruments that can measure pressure very accurately by deriving a pressure reading from the fundamental components of mass, acceleration and area.

Reference Pressure - is the pressure present on the reverse or negative side of a sensing diaphragm. For example the pressure reading will be zero when the total pressure measured on the positive side of the diaphragm equals the reference pressure.

Referred Thermal or Temperature Error Band - is derived from the maximum deviation expressed as a percentage of full scale in either the positive or the negative direction from the measurements at a defined temperature which is typically room temperature. This maximum deviation is then expressed as ±figure e.g. ±2.5%FS.

Repeatability - is the amount of change of a measured reading at exactly the same pressure and ambient conditions over a series of pressure cycles from zero to full scale pressure and back to zero again. To ensure no Pressure Hysteresis is introduced into Repeatability measurement, readings are always taken during an increase in pressure or a decrease in pressure but never a mixture of the two.

Sealed Gauge Reference Pressure (SG) - is a pressure measured relative to an atmospheric or barometric pressure which has been sealed in a fixed closed volume.

Secondary Pressure Standards - are instruments that cannot be used to measure Force and Area directly to derive a measurement of pressure but are still used as a traceable standard for calibrating other pressure instruments, e.g. an Electronic Pressure Calibrator that uses a Pressure Transducer inside would be classed as a Secondary Pressure Standard.

Semiconductor Strain Gauges - are typically made from Silicon and have very high gauge factors compared to metal foil strain gauges. Ions are implanted into the n type Silicon substrate to create p type regions which are significantly more conductive than the surrounding n type Silicon. A p-n junction is created which provides electrical isolation for the p-type region when a reverse bias voltage is applied, thus creating an isolated resistive region that will change with applied stress.

Silicon on Insulator or SOI - pressure sensing technology enables sensing diaphragms to be used at much higher temperatures. Unlike ion implanted silicon strain gauge diaphragms SOI does not have P-N junctions that begin to breakdown at high temperatures producing leakage currents which cause unstable performance.

Silicon Pressure Sensors - bring together the benefits of the high gauge factors of semiconductor strain gauge technology with the high elasticity of a Silicon sensing diaphragm. Silicon pressure transducers have high mV/V output signal sensitivity, high overpressure and very low non-linearity, hysteresis and repeatability.

Span Output - is the difference between the minium and maximum output signals of a pressure sensor.

Span Offset - is the amount of deviation in maximum span output signal of a pressure sensor from the ideal value at full scale pressure. The span offset is normally expressed as percentage of full scale pressure or electrical units, e.g. ±0.5% FS or ±3mV.

Span Sensitivity - defines the output signal characteristic of a pressure sensor and is expressed as a ratio of maximum span output signal divided by the pressure range, e.g. 10mV/bar. For ratiometric outputs that are proportional to supply voltage, the span sensitivity would be expressed as 10mV/V/bar.

Standard Atmosphere (atm) - is mainly used as a reference value for the average barometric pressure at sea level. It is often used to indicate the depth rating for a water resistant watch, but otherwise is rarely used as a unit for measuring pressure. 1 Standard Atmosphere is defined as being exactly equal to 101,325 Pascals.

Static Line Pressure - is the total pressure present at a particular point along a pressurised pipe. It is often quoted on the specifications for differential pressure sensors as an indicator of the maximum pressure that can be applied to both the high and the low side pressure ports at the same time. This should not be confused with the over-pressure limit which is related to the differential pressure range rather than the static line pressure.

Suction Pressure - is what is generated when a negative difference in pressure is generated between two points which draws a gas or a liquid from a higher to a lower pressure region.

Technical Atmosphere (at) - is similar although not the same value as the Standard Atmosphere pressure unit (atm) and both have been used historically to relate a pressure value to multiples of atmospheric pressure. 1 Technical Atmosphere is defined as being exactly equal to 98,066.5 Pascals.

Terminal Straight Line Accuracy - defines the limit that any pressure measurement reading will deviate from the Terminal Straight Line.

Terminal Straight Line - is a straignt line drawn between the output signal or reading at zero and full scale pressure.

Thermal Hysteresis - is the measured change in output or reading at a specific pressure and temperature taken during a sequence of increasing temperatures and then during a sequence of decreasing pressures.

Thermal or Temperature Errors - define the limit of change of any pressure output or reading over a given compensated temperature range.

Thermal Span Error - is the maximum amount the span output or reading could change at any point within the compensated temperature range. This error is normally expressed either as a percentage of full scale output or reading. It can also be expressed as a percentage of full scale per degC, degF or K e.g. ±0.01%FS/DegF.

Thermal Zero Error - is the maximum amount the output or reading at zero pressure might deviate over the compensated temperature range. This error is typically expressed as a percentage of full scale output or reading. It can also be expressed as percentage of full scale per degC, degF or K e.g. ±0.02%FS/DegC.

Tons per Square Foot (tsf), USA, Short or Tons Force per Square Foot (tfsf), USA, Short - is an American pressure unit comparable in magnitude to atmospheric pressure. 1 Ton per Square Foot (USA, Short) equals 95760.5 Pascals.

Tons per Square Inch (tsi), UK, Long or Tons Force per Square Inch (tfsi), UK, Long - is a very large pressure unit used historically for measuring extremely high pressures. 1 Ton per Square Inch equals 15,444,300 Pascals.

Tons per Square Inch (tsi), USA, Short or Tons Force per Square Inch (tfsi), USA, Short - is one of the largest pressure units and is used in the measurement of ultra high pressures. 1 Ton per Square Inch (USA, Short) equals 13,789,500 Pascals.

Torr - is a pressure unit which is defined as 1 Standard Atmosphere divided by 760 (1 atm/760 or 101325 Pa/760). Used mostly for measuring high vacuum, the Torr has largely been superceded by the hPa (mbar) pressure unit. 1 Torr equals 133.322 Pascals.

Traceable Pressure Calibration - means that the equipment which was used to calibrate a pressure measuring device has been regularly calibrated by another more accurate instrument for which its own calibration can be traced back either indirectly or directly to a National Standards Laboratory e.g. National Physical Laboratory in the UK.

Transducer - is a device that converts a non-electrical change into an electrical change.

Vented Gauge Reference Pressure (VG) - is a pressure measured relative to "actual" atmospheric or barometric pressure.

Wheatstone Bridge Strain Gauge - circuits are used extensively inside pressure transducers to convert mechanical stress into an electrical output signal.

Zero Offset - is the deviation in output or reading from the ideal point at zero pressure.

Zero Tare - is the operation of removing any Zero Offset to obtain the optimum measurement at zero pressure.

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Site Last Updated:  04/02/12