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Pressure Measurement GlossaryFind 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 that is proportional to applied pressure. 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 100000 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. 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. 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. 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 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. 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. 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. 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. 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. Negative Gauge Pressure - is the difference in pressure between any vacuum pressure and atmospheric pressure. 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. 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. 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 - 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 Kilogram per Metre Squared (kg/m2). 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. Pounds per Square Inch (PSI) or Pound Force per Square Inch (PFSI) - is a widely used British and American unit of measure for pressure. 1 psi converts to 6,894.76 Pascals. 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 or ATM - is a universally adopted standardised pressure reading for barometric pressure at sea level and equals precisely 1013.25mb absolute. Standard atmosphere is a particularly useful datum point in altitude measurement since all altimeters are calibrated to this pressure. 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 the difference in pressure between any vacuum pressure and atmospheric pressure. 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. 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. |
Site Last Updated: 09/02/10
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