1. How do pressure sensors work?
Absolute and gauge pressure sensors are devices used for measuring the pressure of liquids and gases. What we refer to as pressure is the sum of two components: static system pressure and hydrostatic head. There is an additional component of total pressure called dynamic pressure which is the velocity component of pressure and not directly measured by the sensor. However, dynamic pressure is responsible for the reduction in static pressure which occurs as fluid velocity increases (as predicted by the Bernoulli equation). Dynamic pressure is equal to the kinetic energy of the fluid. The Bernoulli equation dictates that any increase in dynamic pressure causes an equal decrease in static pressure. We can therefore deduce that the reduction in static pressure (and therefore the pressure detected by the sensor) is more significant in high density fluids such as liquids and high pressure gases. The graph below shows the reduction in static pressure of water with increasing velocity, from an initial pressure of 101 kPa.
Static pressure of water with increasing flow velocity
Pressure sensors detect pressure by measuring the mechanical deformation of a thin stainless steel or ceramic diaphragm. One side of the diaphragm is exposed to the process fluid and the other side of the diaphragm is exposed to either atmospheric pressure (gauge sensors) or to a vacuum (absolute sensors). The difference in pressure causes temporary deformation of the diaphragm. The magnitude of deformation is measured using either a strain gauge, capacitive, or piezoresistive (often referred to as a silicon strain gauge) sensing element. The sensing element outputs a mV magnitude voltage as a function of applied pressure. Most pressure sensors include built in electronics to amplify the mV output and convert it into a standard output signal such as 0-10 V or 4-20 mA.
Strain gauge sensing element
Capacitive sensing element
Pressure sensors are described as measuring either gauge or absolute pressure. The distinction between the two is that gauge pressure sensors measure pressure relative to atmospheric pressure and absolute pressure sensors measure pressure relative to a vacuum. This is achieved by maintaining the opposing end of the diaphragm at either atmospheric pressure (for gauge sensors) or vacuum pressure (for absolute sensors). It is simple to convert between atmospheric and gauge pressure because Pabsolute = Pgauge + Patmosphere. As stated in the previous paragraph, the deformation of the diaphragm is measured using either strain gauge, capacitive, or piezoresistive sensing elements:
Strain gauge: A strain gauge is a thin laminated foil pattern used to measure mechanical strain. It is bonded to the diaphragm, so that deformation of the diaphragm causes the strain gauge to stretch. This results in elongation of the strain gauge and a decrease in its cross sectional area, thereby increasing its electrical resistance. By measuring the change in electrical resistance, it is possible to determine the amount by which the strain gauge has stretched. The sensor includes multiple strain gauges connected in a Wheatstone bridge arrangement, to provide improved sensitivity and temperature compensation.
Piezoresistive: A piezoresistive sensing element is a silicone semiconductor used to measure mechanical strain and is the most commonly used sensing element in pressure sensors. It is bonded to the diaphragm, so that deformation of the diaphragm causes the piezoresistive element to stretch. The strain results in an increase in electrical resistance due to the piezoresistive effect. The resistance is measured using a Wheatstone bridge, which provides improved sensitivity and temperature compensation. Piezoresistive elements are higher sensitivity and have a longer lifetime than strain gauge elements.
Capacitive: A capacitive sensing element detects the deformation of the diaphragm according to the change in capacitance between the diaphragm and a fixed metal plate positioned above the diaphragm so that a small air gap is formed. An AC voltage is applied to the fixed capacitor plate, causing its polarity to switch at the frequency of the AC waveform. The AC voltage results in an alternating current flow. Deformation of the diaphragm reduces the size of the air gap, thereby increasing the capacitance and therefore the flow of current detected by the sensor. Capacitive sensing elements consume very little power compared to strain gauge and piezoresistive sensing elements.
The sensor configuration described above is known as a dry or oil free pressure sensor because the sensing element directly measures the diaphragm deformation without the use of oil to transfer the force. An alternative configuration known as a fluid or oil filled pressure sensor is also available. In fluid filled sensors the sensing element is located on a separate membrane located above the diaphragm. The space between the diaphragm and the membrane is filled with oil, which transfers the force from the diaphragm to the membrane. The oil provides mechanical damping of pressure spikes. However, it also increases the sensors response time and the risk of the system being contaminated with oil in the case of a diaphragm rupture.
Dry pressure sensor
Fluid filled pressure sensor
2. General characteristics of pressure sensors
Absolute and gauge pressure sensors detect fluid pressure relative to either a vacuum or to atmospheric pressure respectively. They do so by measuring the mechanical deformation of a diaphragm exposed to the fluid pressure on one side and either a vacuum or atmospheric pressure on the other side. Deformation is measured with either a strain gauge, piezoresistive or capacitive sensing element. Pressure sensors achieve high measurement accuracy, resolution and long term stability (far superior to that of manual pressure gauges). Most sensors are only compatible with fluid temperatures of up to 85 or 120 °C. However, metal capillary tubes can be used to cool higher temperature fluids to below 85 or 120 °C. Sensors are available with either stainless steel or ceramic diaphragms and may be oil filled (for mechanical damping) or dry. Most sensors include built in electronics for amplifying the outputs and converting them into to standard output signal forms (e.g. 0-10 V or 4-20 mA), though unamplified mV scale outputs are also available.
3. Input and output signals
Pressure sensors require a fixed 5-24 Vdc supply voltage to power the Wheatstone bridge or capacitor and post processing electronics. Strain gauge and piezoresistive sensing elements are excited with a DC voltage. In contrast, capacitive sensing elements utilize an internal LC oscillator circuit to generate a high frequency alternating waveform from the DC supply voltage. Some strain gauge and piezoresistive pressure sensors directly output an mV scale voltage without amplification. However, most pressure sensors include built in electronics for amplifying the voltage and converting it into a standard output signal form such as 0-10 V or 4-20 mA).
4. Applications of pressure sensors
Absolute and gauge pressure sensors are used in fluid monitoring and control applications requiring high accuracy, high resolution measurement and data transfer to a process controller or data logger (the alternative is to use a mechanical pressure gauge). Pressure is the second most measured process variable after temperature. A few example applications of pressure sensors include for control of hydraulic and pneumatic force actuators, tank fill level detection, monitoring of steam boilers and monitoring of engine exhaust gases.
5. Typical specification
|Cost||Low to Medium|
|Measurement range||0-0.1 bar to 0-5,000 bar|
|Fluid temperature||-50 to 120 °C (up to 300 °C available)|
|Long term stability||High|
|Supply voltage||5-24 Vdc|
|Output voltage||0-10 V, 4-20 mA|
|Passive / active||Active|
|Contact / non-contact||Contact|
Comparison of sensor elements
6. Purchasing tips
- Absolute vs gauge: Pressure sensors measure pressure relative to either a vacuum (absolute sensors) or to atmospheric pressure (gauge sensors). Gauge sensors are preferred in systems which are open to or interact with the atmosphere, e.g. pneumatic cylinders and open top process tanks. Absolute sensors are preferred in closed systems where atmospheric pressure is irrelevant, e.g. in an organic Rankine cycle.
- Piezoresistive vs strain gauge vs capacitive: The basic operating principals of piezoresistive, strain gauge and capacitive pressure sensing elements are described in section 1 and there advantages & disadvantages are discussed below. In most cases the sensor can be selected based on its technical specification, without concern for the type of sensing element. Keep in mind that most pressure sensors have piezoresistive sensing elements, often referred to as silicon strain gauges.
- Dry vs fluid filled: In dry pressure sensors, the sensing element is bonded directly to the diaphragm that separates the process fluid from the sensor. Fluid filled pressure sensors include a thin membrane above the diaphragm, to which the sensing element is bonded. The space between the membrane and diaphragm is filled with oil which transfers the force from the diaphragm to the membrane. Fluid filled pressure sensors provide damping of pressure spikes. However, they increase response time and carry the risk of contaminating the system with oil in the case of a diaphragm rupture.
- Ceramic vs Stainless steel diaphragm: Diaphragms are made of either stainless steel or ceramic. Stainless steel is most common but ceramic diaphragms provide superior wear and corrosion resistance. Most sensors with stainless steel diaphragms are fluid filled whereas those with ceramic diaphragms are usually dry.
- Recessed Vs flush Diaphragm: Most sensor diaphragms are recessed behind the screw thread to provide protection. A flush diaphragm is ‘flush’ with the end of the sensor. They are useful for measuring the pressure of viscous fluids. Furthermore, they are used in the food processing industry where a recessed diaphragm could provide an opportunity for bacteria to flourish.
- Natural frequency: This is the frequency at which a system oscillates in the absence of driving or damping forces and is therefore also the frequency at which the oscillation amplitude is greatest. Avoid purchasing a pressure sensor with a natural frequency less than three times the vibration frequency of the system (e.g. vibrations from a gear pump). The natural frequency of the pressure sensor is often stated in the manufacturer’s datasheet.
- Built in display: Pressures sensors can be purchased with small built in screens displaying measured pressure in real time without the need to connect the sensor to a HMI panel.
- Overload capacity: The overload capacity is typically 150-300% of rated pressure. This is the maximum pressure that can be applied without damaging the pressure sensor.
- Chemical Resistance: The manufacturer’s datasheet may provide information on chemical compatibility with highly corrosive fluids. Most diaphragms are made of various grades of stainless steel. Even if it appears that stainless steel is compatible with the fluid, minor corrosion of the diaphragm can still reduce accuracy (e.g. hydrogen embrittlement). Sensors with ceramic diaphragms are often preferred for use with highly corrosive fluids.
7. Advantages of pressure sensors
Each sensing element has the following advantages:
- Have a fast response time.
- Are suitable for measuring large strain and high pressures.
- Are high accuracy sensors.
- Are high sensitivity sensors.
- Have a fast response time.
- Have a long lifetime.
- Are high accuracy sensors.
- Are high sensitivity sensors.
- Have a long lifetime.
- Consume less power than piezoresistive and strain gauge elements.
8. Disadvantages of pressure sensors
Each sensing element has the following disadvantages:
- Are of lower accuracy than piezoresistive and capacitive elements.
- Are of lower sensitivity than piezoresistive and capacitive elements.
- Are of higher cost.
- Are prone to damage due to overload because they cannot endure as much mechanical strain as strain gauges.
- Have a slow response time due to the time constant of the capacitor.
9. Application tips
- Pressure probes for static and total pressure: Pressure sensors normally measure pressure at the boundary of the pipe or vessel. A pressure probe is a cylindrical tube that connects to and extends the pressure sensor into the bulk of the fluid. Pressure probes are available with either a straight end for measuring the sum of static and hydrostatic pressure or a 90° angled end for measure total pressure. The 90° angled end is aimed up stream (i.e. into the flow) causing a stagnation point (a point where the flow velocity is zero). When flow velocity is zero, dynamic pressure is also zero and so total pressure is equal to the sum of static and hydrostatic pressure. Dynamic pressure is equal to kinetic energy. By measuring pressure with both the straight and angled probes we can determine the dynamic pressure and therefore calculate the flow velocity.
- Operating with high temperature fluids: Pressure sensors can be used in high temperature applications by attaching a coiled capillary tube between the sensor and the pipe or vessel. The capillary tube is cooled by the atmospheric air, protecting the sensor from exposure to the high temperature fluid. Capillary tubes enable pressure sensors to be used with fluids at temperatures in excess of 300 °C. However, they cause mechanical damping which increases the sensors response time.
- Measuring fluid level: Pressure sensors can be attached near the bottom of fluid vessels and tanks to measure hydrostatic pressure. If the density of the fluid is known, the fluid level can then be determined.
- Water hammer: Water hammer is a transient pressure spike that occurs as a result of fluid inertia when a control valve is suddenly closed. The phenomenon can cause permanent damage to the pressure sensor. To prevent damage, purchase sensors with sufficient overload capacity and take action to minimize water hammer. The location of the sensor within the system can also affect its susceptibility to water hammer.
- Fluid sealing: Most pressure sensors have a flat base at the end of their screw thread. This enables a Dowty seal washer to be used for providing a leak tight seal. Dowty seals are easier to use and often provide a more reliable seal than Teflon tape and liquid sealants.