Our experience technical engineers have been working with pressure sensors for over thirty years – here we give you an insight into the science behind pressure measurement:

What is gauge, absolute and differential pressure? Pressure is a widely measured physical variable within a diverse range of modern applications and industries worldwide. Pressure (P) is defined as the unit force (F) divided by the area (A) over which that force is evenly distributed, e.g. P = F/A. This simple relationship means that pressure increases proportionately with force if the working area remains constant. Atmospheric pressure is often used as a zero reference in pressure measurement, when the resulting measurement is referred to as gauge (g) pressure. Atmospheric pressure is also called barometric pressure and due to gravity the mean value at sea level is approximately 1 bar absolute (1013.25 mbar). Atmospheric air pressure decreases with increase in altitude, creating an increasingly negative pressure (vacuum) until the absolute pressure zero point (total vacuum) is reached in outer space. Conversely, liquid pressure increases below sea level at a rate of approximately 1 bar for every 10 m of static water (hydrostatic) depth and the world’s deepest ocean depth is equivalent to a pressure of over 1000 bar (approximately 1000 times greater than atmospheric pressure). Hydrostatic pressure increases in proportion to depth or level measured from the surface because of the increasing weight of fluid exerting downward force from above. Where pressure is measured in the region below atmospheric pressure, this is known as vacuum or negative pressure. It can be measured as “absolute” pressure (“a”) where the zero point is a full vacuum (e.g. barometric / meteorological measurements, altitude monitoring, sealed container pressure etc) although most applications use a “gauge” reference with zero point at ambient atmospheric pressure. This automatically deducts the effect of ambient pressure, which is usually the most common measurement requirement. For applications monitoring the difference in two input pressures (such as filter condition, orifice plate flow etc) this is known as differential pressure ( “d” or “DP”) and the zero point simply occurs when both input pressure values are zero. How are pressure sensors used for level measurement? Pressure sensors can be used to measure liquid level using the hydrostatic principle, from open water such as reservoir level or sea depth to storage tanks and vessels. This assumes that the media is a static head of liquid with constant specific gravity and the system is compensated proportionally where liquid density is not the same as clear water. For example when measuring oil level in a tank, a lower specific gravity means a proportionally lower pressure for every metre of hydrostatic depth or level. Also if the tank is not vented, or it is sealed under pressure, then it is necessary to measure and compensate for surface pressure above the liquid. In practice hydrostatic level measurement can be achieved using a fully submersible probe with hermetically sealed cable assembly to sense the liquid pressure head above the probe inlet. Alternatively a pressure sensor can be externally mounted at the bottom of tanks or vessels. A differential pressure sensor can be used to provide a single output for sealed tanks representing the difference in two input pressures for tank level and surface gas pressure or they can simply be measured separately using two sensors and outputs. Why use electronic pressure measurement? Electronic pressure measurement is increasingly the preferred method to reliably provide comprehensive and accurate pressure data directly to the user control system. This includes a wide range of cost effective industrial pressure sensors, transducers and transmitters configured for gauge, absolute or differential pressure measurement with analogue, switch or digital outputs and employing high grade wetted materials to suit many different applications. Ranges can be supplied from just a few mbar to thousands of bar. Examples of application specific configurations include miniature, high precision / test, flush diaphragm, high temperature, combined pressure / temperature sensor, hydrostatic / level, CANbus, HART, ATEX / Ex Intrinsically Safe transmitters and high reliability / SIL approved. Often these can be supplied from modular based designs due to the compact and flexible nature of electronic sensor technologies, but for more complex requirements custom designs can often be readily accommodated. What is a pressure sensor? A pressure sensor is the basic module of an electronic pressure measurement system. It converts a physical pressure value into an analogue electrical signal by accurately measuring the deflection of a pressure sensitive diaphragm and providing a high speed proportional output signal that can interface directly with a centralized data monitoring / acquisition system or a local readout or display. Using a variety of sensing principles / materials and enclosure designs, modern electronic pressure sensors (when correctly applied for the intended purpose) will provide a compact, robust and highly accurate cost effective solution for optimizing process measurement performance. Pressure media is typically hydraulic (liquid) or pneumatic (gas) based. In some liquid applications the media can be relatively viscous or contain solid particulates, requiring a flush mount diaphragm. Dependant on specific physical properties of the pressure media and the general environmental / operating conditions, pressure transducers and pressure transmitters can be supplied in many different configurations and adapted to provide the best cost effective solution for an individual measurement application. This is achieved by a modular standard design that covers most common variations, or by customised design for more complex requirements. “Fit, form and function” of the sensor is paramount for a successful application – this includes pressure media compatibility, accuracy requirements, response time, overload, temperature range, size and weight restrictions, environmental factors such as ingress protection, shock and vibration, EMC compliance and hazardous area / ATEX certification etc. Piezo-resistive silicon sensing technologies There are various methods of measurement at the sensing diaphragm but the most popular is usually made by an integrated electronic circuit using four strain gauges in a Wheatstone bridge configuration. Examples include piezo resistive silicon and thin film silicon diaphragm technologies which can satisfy even critical high reliability applications such as SIL (Safety Integrated Level) or PL (Performance Level) requirements, for example SIL 2 or PL: d rated. Measurement redundancy can be provided within a single pressure sensor, for example by having two measurement points on the diaphragm with completely independent signal processing and output channels. The thin-film and diffused piezo-resistive silicon pressure sensing technology is highly sensitive so produces minimal diaphragm deflection with low material strain, ensuring excellent dynamic performance. Highly adaptable and compact with robust and modular packaging, this provides a fully integrated high reliability sensor readily produced for a wide range of applications from low cost OEM production to high performance R&D and test. The technology is very accurate, compact and lightweight with excellent operational characteristics e.g overload, vibration, shock and acceleration resistance. Product lines offer an extensive choice of cost effective proven solutions for markets ranging from industrial machinery, process and environmental systems to automotive, rail and aerospace. Utilising thin-film Silicon or Steel or diffused silicon diaphragms, very high and lowe pressure ranges can be effectively measured using ranges of just a few mbar up to 4000 bar or more and suitable for media temperatures from -40 up to 200 degC. Gauge, absolute, vacuum and differential configurations are available and flush mount/open face diaphragms. Pressure Transducers and Transmitters Typically the pressure sensing module is supplied with a threaded process connection with an internal pressure inlet/cavity to allow the media to reach the internal diaphragm assembly. An external (flush/open) diaphragm can also be provided without this inlet to suit (for example) hygienic, viscous or slurry applications where it is often located at the thread end face, dependant on the mechanical configuration required. Signal conditioning electronics are also provided behind the diaphragm assembly within a compact rugged cylindrical housing to provide a robust “pressure transducer” or “pressure transmitter” for a wide variety of industrial applications. Signal conditioning provides a rationalized high level output, compensated for environmental errors such as temperature changes and protected against ambient electromagnetic field radiation. This can be provided using purely analogue circuitry, but digital based signal conditioning (for example ASICs) often provide a superior level of linearity and thermal compensation that substantially enhances sensor measurement performance. A “pressure transducer” will usually provide an amplified high level voltage output (e.g. 0 to 10Vdc, 0 to 5Vdc, 1-5Vdc, 0.5 to 4.5Vdc) although some simply offer the basic or “raw” sensor wheatstone bridge output (typically 50 to 100mV FS) whilst a “pressure transmitter” will produce a current output (e.g. 4 – 20mA). The elevated zero point of the 4-20mA or 1-5Vdc signals enables cable breaks and instrument faults to be detected. An external power supply is usually required to provide a fixed excitation voltage typically from 5V up to 32V dependant upon configuration and application. The amplified output pressure transducer is often adequate for short to medium distance transmission within industrial environments, whilst the pressure transmitter is more suitable for long distance signal transmission and also for operating environments with dense / significant electromagnetic fields as the 4-20mA signal produced has lower sensitivity to electrical interference and automatically compensates for cable resistance. 2 wire, 3 wire or 4 wire interfaces The most typical interface configurations for industrial analogue outputs include 2 wire and 3 wire (current or voltage). The 2 wire system shares power supply and signal on the same 2 wires as it generally operates as a current “loop” (e.g. 4-20mA). The 3 wire system typically uses separate cores for supply and signal with a common 0V. This also facilitates low power ratiometric interfaces. The ratiometric configuration enables an output which is proportional to the supply voltage of typically 5V within a 10-90% range, eg 0.5V to 4.5V. This provides a low power solution for remote system applications such as environmental data logging. 4 wire systems are also used in some test and research applications, for example isolated input/output voltage or raw millivolt output transducers and other configurations. Selecting a suitable pressure sensor The full range output of a pressure transducer or transmitter is usually calibrated for the full pressure range of the device (usually a standardized / DIN range eg 0 to10 bar, with the zero point being atmospheric pressure for gauge (g) units or a sealed vacuum reference for absolute (a) units. Gauge (g) atmospheric reference applications are the most common type of measurement, where the ambient barometric pressure is compensated by using a breather “vent” tube within the cable assembly or by a fine porous element. The ambient air pressure is directed through a vent hole or a vent tube to the back of the sensing element. A vented gauge pressure transducer / transmitter enables the local air pressure to be exposed to the negative side of the pressure sensing diaphragm so that it always measures with reference to ambient barometric pressure. Therefore a vented gauge (g or vg) pressure sensor always reads zero pressure when the process pressure connection is held open to atmospheric air, whereas an absolute pressure sensor will actually measure the atmospheric pressure. A sealed gauge reference (sg) or sealed reference (sr) is similar except that atmospheric pressure is sealed on the negative side of the diaphragm at the time of manufacture. This is generally for high pressure applications (60 bar+ hydraulic) where atmospheric pressure changes will have a negligible effect on sensor accuracy. The zero set point is simply the ambient pressure value sealed into the reference chamber when the sensor was originally made. This means that the sealed gauge / sealed reference unit does not require a local atmospheric reference or vented cable when installed for operation. For standard type sensors, to provide the most cost effective and readily available solution, popular fixed (DIN) ranges are manufactured and typically specified by the user allowing for sufficient overpressure whilst considering the working range and accuracy required (accuracy is usually expressed as a maximum error related to the full scale calibrated pressure range). They can be supplied calibrated to the user preferred pressure units (for example bar, PSI, KPa, mH2O, InHg etc). For higher quantity / volume requirements the pressure ranges can also be calibrated to intermediate / bespoke values as this will not compromise manufacturing costs. Overall full scale operating ranges can be supplied from just a few mbar to several thousand bars. Typically the maximum overpressure specified for a sensor is the highest value of pressure that can be applied without affecting sensor performance once the pressure returns to its usual operating range. Beyond the overpressure limit, permanent offset or damage may occur that affects the performance of the device but the pressure media will be contained up to the burst pressure rating. “Burst Pressure” is the value where pressure can no longer be safely contained within the device and the diaphragm may rupture. These electronic sensors can be supplied for use in a relatively wide media temperature range, typically from -40 to +125 degC or even 180 / 200 degC. The higher temperature ratings can be accommodated directly at the sensor inlet by means of an integral cooling fin design or remote electronics package, avoiding the need for precooling of the pressure media using a separate component such as a temperature reducing coil or pigtail syphon. Consideration must also be made regarding the ambient temperature environment where the sensor will be installed / operating, as the rating is often lower, for example typically in the region 85 -105 degC maximum. Solutions can also be supplied that offer a span “turndown” ratio, enabling for example a 10 bar (g) device to be reset by the user to a lower value such as 4 bar (g). The zero signal point can also be elevated or suppressed to a value above or below atmospheric e.g 1 to 4 bar(g) or -1 to 4 bar(g). Alternatively, for “differential” pressure (dP) transducers or transmitters, full range output represents the maximum working difference in two input pressures. The zero point occurs when both input pressures are equal value (zero differential). Maximum system pressure is important, otherwise known as line pressure. This is the maximum pressure that can be applied to both of the differential pressure ports simultaneously without degradation in performance. Consideration also needs to be given if the pressure differential occurs in the positive, negative or both directions as a custom calibrated / bilateral output may be required. E.g to measure P1 - P2, P2 - P1 or both. In addition to analogue versions, digital outputs can be provided such as the CANbus (CANopen 2.0A or 2.0B) or SAE J1939 protocols often used in automotive / mobile hydraulic applications, also the superimposed digital HART signal widely used for 2 wire communications in the process industries. Commercially available pressure transducers and transmitters are often priced according to accuracy, from low cost OEM to premium performance units for test / R&D applications. For the user it is therefore very important to determine exactly what performance is required and the prevailing operating conditions in order to secure the most cost effective solution for each particular application. For example measurement accuracy at room temperature conditions can vary from 1% to 0.1% with uncertainty factors such as non-linearity, hysteresis, repeatability and zero/span offset. If the temperature varies considerably then these values can be much greater. However manufacturers do not always use consistent and inclusive terms for measurement uncertainty so it is important to carefully compare the product performances offered by each prospective supplier.