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Definitions of Pressure Sensor Specification Parameters
Definitions of technical terms used to describe the features, functions, construction and operation of pressure sensors. Related explanations are grouped under the headings of pressure range, accuracy, output signal, electrical connectors and cables, pressure connections, media compatibility and approvals. Select sensor parameter to view associated topics Pressure Range Absolute (ABS, A) – is a pressure referenced to zero pressure by evacuating the reverse or negative side of a sensing element. Commonly used in measuring vacuum, environmental chambers and leak testing where measurements need to have an unchanging reference point independent of surrounding atmospheric pressure. Burst Pressure - the mechanical integrity of the sensor may fail if this pressure is exceeded. Differential Pressure (DP, Diff) - is the pressure measured between two points. Differential sensors have 2 process connections and precautions should be taken in ensuring that the sensor is able to withstand the line pressure. If the line pressure is much higher than the differential range it should only be applied to both sides at the same time unless the sensor includes diaphragm mechanical protection stops. Gauge (G) – is a pressure referenced to ambient atmospheric pressure and is the most common form of pressure reference. Hydrostatic Pressure - is the pressure produced by a column of liquid and is a convenient method for measuring liquid level or depth since it is independent of size or shape of the volume being measured. Line Pressure - mostly used in specifying differential pressure sensors and refers to the overall mechanical integrity of the sensor in withstanding the total pressure of the system. The line pressure is not normally accompanied by the pressure reference but is typically a gauge pressure. Overpressure - this is the maximum pressure that can be applied to a sensor without affecting its performance. Pressure Units - Since all pressure units, apart from altitude and non-linear flow conversions, are directly proportional to each other a pressure sensor is not pressure unit dependent and its pressure range can be expressed in any units without the need to change any settings. There are many types of pressure units used depending on country, application and what level of pressure is being measured, e.g. Pa and inH2O are used for defining low air pressures such as used in ventilation ducts in HVAC or building management systems, whereas mbar (hPa) and inHg are the preferred units used in barometric measurements. Sealed Gauge (SG) – is a pressure referenced to atmospheric pressure by offsetting an absolute reference sensor by 1 bar or sealing in atmospheric pressure. Typically only used for higher ranges above 35 bar, where atmospheric pressure changes are relatively small and not greater than the stated accuracy of the sensor or instrument. Suction Pressure - is like a vacuum pressure but it is measured in the negative direction referenced to atmospheric or zero gauge pressure. Vented Gauge (VG) – is a pressure referenced to atmospheric pressure via a vent path on the reverse side of the pressure sensing diaphragm. Measurement performance maybe effected if vent path is not installed correctly; moisture ingress or air tight seals will prevent vent from breathing or may cause corrosion to internal components of measurement device. Performance Accuracy - how close the measured reading is to the true value. Best Straight Line (BSL) - is an average line drawn through all the measured points. Typically a mathematical least squares fit method is used. Compensated Temperature Range - defines the limits over which the thermal errors have been tested and verified. Full Scale (FS) - refers to the maximum measurable pressure or output signal. When the sensor has a plus and minus pressure range or output signal the full scale is taken from the lowest point to the highest, but positive pressures ranges or output signals are always referenced to zero. Full Span (FS) - refers to the difference between the lowest and highest measured pressure or output signal, e.g. the full span of a -200 to +200 mbar range would be a 400 mbar span or a +100 to +200 mbar range would be 100 mbar span. (Non) Linearity, Hysteresis and Repeatability (NLHR) - rolls together all the room temperature uncertainties into one statement of precision (accuracy is often used instead of precision even though the latter is more technically correct). Precision - how close the measured reading is to the mean or average of all the measured values. Span Offset - The maximum deviation of the span output signal that should be expected across a number of sensors of the same specification. This is typically specified as a percentage of span output. Stability or Long Term Drift - an indication of the maximum change in accuracy over a period of time in a benign environment, typically quoted as annual percentage e.g. ±0.1% FS/year. This is not normally a guaranteed figure since so many factors related to the sensors use can accelerate the life of the sensor causing it to drift by a greater degree. Terminal Straight Line (TSL) - is a reference line drawn between the lowest and highest measured value. This type of reference tends to produce a wider accuracy value because many sensing technologies have a natural bow in the middle which is approximately halved using the best straight line method. Thermal Span Shift (TSS) - the maximum deviation of output signal at a given span over the compensated temperature range. Typically expressed as a percentage e.g. ±3% Span/Deg C. Thermal Zero Shift (TZS) - the maximum deviation of output signal at zero pressure over the compensated temperature range. Typically expressed as a percentage e.g. ±2% FS/Deg C. Zero Offset - The maximum deviation of the lowest output value that should be expected across a number of sensors of the same specification. Typically expressed as a fixed value such as ±2mV or a percentage of full scale or span such as ±2% FS. Output Signal Amplified Voltage (V) - convenient measurement signal for most applications since most analogue to digital converters (ADC) will accept 5 or 10 full scale inputs. Used extensively in the automotive and aerospace industry in 3 (common negative connection) or 4 wire (isolated input and output) configurations the former being the most widely used. The amplified voltage uses less power than the more robust current output but is less susceptible to losses than the mV output signal and is compatible with most ADC. Current Loop (mA) - by far the most popular current output is the 4-20mA 2 wire type which only requires 2 wires for installation and can be used over great distances without significant signal losses. Traditionally used in process plants it is now found throughout industry with the introduction in recent years of more compact sensor construction. 3 wire 4-20mA and 0-20mA are rarer and generally not specified unless required for replacement on legacy installations. HART (Highway Addressable Remote Transducer) - is a type of digital communications protocol for configuring and reading instrumentation via the 4 to 20mA current loop. HART data is communicated via a low level AC digital signal which is superimposed over the 4-20mA analogue signal without any disruption and therefore can be used at the same time as the analogue signal. Millivolt (mV) - predominantly an un-amplified ratiometric output signal produced by a wheatstone bridge strain gauge circuit. This output offers the best dynamic response since it is only limited by the natural frequency of the diaphragm assembly. Ratiometric - has an output which varies proportionally to the supply voltage. Most sensors with millivolt outputs have ratiometric outputs and some amplified outputs such as 0.5 to 4.5 Vdc are dependent on supply voltage. RS232 - being slowly replaced by more modern alternatives USB and RS485, this interface is still found on older computers inside and outside industry. Typically easier to interface than RS485 for most users and can be converted to USB without too much trouble. In a similar way to USB, many manufacturers provide RS232 converter and software packages for quick and easy installation. RS485 - has mostly replaced RS232 in industrial applications providing a robust digital signal which can be transmitted over distances as far as 1200 metres. Interfacing with a RS485 either requires the writing of a software program using the manufacturers provided protocol or supplied software and hardware. Span Sensitivity - is the amount of output signal produced per unit of pressure, e.g. a 50 psi sensor with a 0-5Volt would have a span sensitivity of 0.1 V/psi. For a millivolt output sensor this would also incorporate supply voltage, e.g. a 50 psi sensor with a full scale output of 30mV with 10V supply would have span sensitivity of 0.06 mV/psi/V. Universal Serial Bus (USB) - now the most convenient and popular way to connect to a computer, sensors with USB interfaces are normally provided with all the converters ad software to enable the user to be up and running very quickly. Electrical Connectors and Cables DIN plug - the most popular and lowest cost electrical connector is used in all general applications indoors and outdoors. If correctly installed a DIN plug will achieve IP65 but care should be taking to not restrict the vent path on a gauge reference pressure sensor. FEP Cable (Fluorinated Ethylene Propylene) - higher level of chemical resistance than PTFE. FEP is often used with substances such as gasoline and landfill site leach agents. Integral Cable Gland - cable is retained by sliding a grommet over the cable and then compressing it to seal via a hex nut. These types of cable seal mostly offer an IP65 protection but can be lower at IP63 or higher to IP68 but can only be submerged to a few metres. For higher rated cable glands the sealing arrangement is more complicated and normally involves 2 or 3 stages of sealing for greater water tightness. IP Ratings - indicate the level of protection against dust and moisture ingress, the first number is for dust and the second number is for moisture. The higher the number is the greater the protection. M12 Connector - small and compact used predominantly in automotive applications. PTFE Cable (Polytetrafluoroethylene) - high temperature rating and chemically resistant to aggressive substances such as hydraulic fluids, oils and lubricants and is commonly used in automotive and aerospace applications. PUR Cable (Polyurethane) - more durable and chemically resistant than PVC it is recommended for applications where the cable is longer and is exposed outdoors. PVC Cable (Polyvinyl Chloride) - this is the most common outer jacket for a sensor cable and offers all round chemical resistance. Vented Cable - has 1 or 2 vent tubes, typically made from nylon, running through its length to provide a breathing path from outside atmospheric pressure to the reference side of gauge reference pressure sensor. Particularly for lower ranges it is advisable to use a vented cable on gauge reference pressure sensors to ensure that changes in atmospheric pressure are easily transmitted to the reference side of sensing element. Short lengths of un-vented cable do vent but a bend or longer lengths may restrict the vent path and therefore the use of vented cable is always recommended. Some manufacturers provide a vent via the sensor housing and in these cases vented cable is not needed. Pressure Connections BSP (British Standard Pipe) - a universally adopted thread of fixed pitch for a given thread size which is either parallel (G) or a tapered (R) thread. The G1/4 (1/4BSP) and G1/2 (1/2BSP) parallel threads are the most widely used standard in European countries. In addition to thread size there is also a fitting style that should be specified which is typically DIN3852, DIN16288 or EN837. DIN3852 uses a bonded or Dowty seal around the outer diameter of the thread and DIN16288 or EN837 a small o ring mounted on a protrusion from the tip of the male thread. Dowty or Bonded Seal - is a metal washer with an elastomer seal bonded to its inside diameter. The seal is made by pushing together 2 flat faces with the seal sandwiched between the 2 surfaces. EPDM Seals (Ethylene Propylene) - offer excellent resilience against solvents, antifreeze and aviation hydraulic fluids such a Skydrol with an operating temperature range from -54 to +150 deg C (-65.2 to +302 deg F). NBR or BUNA N Seals - general purpose seal material for use with fuels, oils and lubricants. The operating temperature range is -55 to +120 deg C (-65 to +248 deg C). NPT (Tapered National Pipe Thread) - originating in the USA this thread is widely used throughout the world. An NPT thread is sealed by wrapping a seal tape or sealant around the male thread before tightening to the appropriate torque setting. Viton or FKM Seals (Fluoroelastomers) - are the most commonly used form of pressure seal and have excellent resistance to hydrocarbons and solvents. They also. Can be used up to a high temperature of -40 to +200 deg C (-40 to 392 deg F). Media Compatibility Ceramic Alumina (Al2O3) - used as a diaphragm material due to its hardness and highly elastic properties this material has excellent chemical resistance and can be used with substances that would quickly corrode thin stainless steel diaphragms such as seawater and acidic solutions. Dry/Dry - differential pressure sensor that can only accept a dry non-condensing and non-condensing gaseous media applied to both the positive and negative pressure ports. Hygienic or Sanitary means the sensor has a completely flush diaphragm for easy cleaning and is often made from highly polished stainless steel to discourage build up harmful bacteria and other contaminants. Any fluid used to transmit pressure to an inner sensing element behind a flush isolation diaphragm must be edible, e.g. olive oil, in order to prevent contamination if a diaphragm fails and leaks into the process media. These types of sensor are primarily used in the Food, Drink, Pharmaceutical and Biotechnology industries where there are extreme levels of cleanliness and protection against product contamination. Oxygen Service or Oxygen Cleaned means that the sensor is suitable for measuring the pressure of pure Oxygen or other media including high concentrations of Oxygen. The sensor must be designed so that no fluids or materials are used in the construction which might cause a fire or explosion when brought into contact with pure Oxygen. The part of the sensor which will be in contact with pure Oxygen is also cleaned to a stringent procedure to remove any contaminants that may have been introduced during the manufacture of the sensor. PVC (Polyvinyl Chloride) and PVDF (Polyvinylidene Fluoride) Sensor Housings - offer corrosion resistance from solvents, acids and bases which would otherwise corrode stainless steel housings. Stainless Steel Grades - there are a few types used in the construction of pressure sensors which include 303, 304, 17-4ph, 316Ti, 316L and Duplex. All offer high levels of corrosion resistance against most common media, particularly 316L which is often specified which offers the best protection. Duplex is typically only used in offshore applications which require the protection from seawater and salt spray corrosion. Wet/Dry - differential pressure sensor that can accept a liquid media applied to the positive pressure port and only dry non-condensing and non-corrosive gaseous media on the negative side. Wetted Parts - These are the sensor materials that are exposed to the liquid media being measured. Wet/Wet - differential pressure sensor that can accept a liquid media applied to both the positive and negative pressure ports. Approvals ATEX - is the European Directive for governing health and safety in explosive environments. For instrumentation to be used in an area where explosive gases or dust are or maybe present they must first be approved by a recognised body such as Beseefa and then installed with other ATEX approved equipment appropriate to the level of protection required for the installation. Intrinsically Safe or Intrinsic Safety - is a method for limiting the amount of energy, heat or ignition sources entering a hazardous zone containing or potentially containing an explosive atmosphere. Typically a pressure sensor installed in the hazardous area is connected to instrumentation in the safe area via a zener diode or galvanic (transformer) isolated barrier system located also in the safe area. The pressure sensor must be matched with the barrier system so that the barrier only allows voltage and current levels through which are within those defined in the pressure sensor approval. Any electrical energy exceeding the barrier limits will be redirected to ground/earth in the case of the zener barrier or blocked in the case of the galvanic (transformer) isolated barrier. Marine Approval - means that a devise has been tested and certified by a recognised body such as Lloyd's Register, Det Norske Veritas or Germanischer Lloyd for use on a ship. 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Site Last Updated: 13/05/08
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