1. How do differential pressure sensors work?
Differential pressure sensors (also known as differential pressure transmitters) are instruments used for measuring the difference in the pressure of a liquid or gas between two locations. They are capable of sensing extremely small pressure differences, significantly smaller than would be possible using two ‘regular’ pressure sensors. Reasons for needing to measure pressure differences include for determining flow velocity and for condition monitoring of filters. Differential pressure sensors are enclosed within metal housings, sealed at each end by thin stainless steel or ceramic diaphragms. The external face of each diaphragm is exposed to the process fluid. Two typical configurations are illustrated below, one contains a single capacitive pressure sensing element and the other contains two piezoresistive or strain gauge elements.
Sensors with capacitive elements have a single sensing membrane surrounded by silicone oil. The external fluid pressures applied to the diaphragms cause them to deform. Since the space between the diaphragms is filled with an incompressible oil, both diaphragms deform in the same direction i.e. towards the lower fluid pressure. The deformation of the diaphragm causes the membrane to deform. Fixed capacitor plates are located either side of the membrane. The membrane acts as the opposing capacitor plate and so there are two capacitors; as the plate separation distance of one capacitor increases the other decreases. Both fixed plates are excited by an AC voltage (generated by an LC oscillator circuit) causing their polarity to switch at high frequency. This results in alternating currents, the amplitudes of which are functions of the membrane position. By calibration, the relationship between current and pressure is determined.
Sensors with piezoresistive sensing elements have piezoresistive devices bonded to each of the diaphragms. A piezoresistive device is a semiconductor whose resistance changes as a function of mechanical strain, according to the piezoresistive effect. The fluid pressures applied to the diaphragms cause them to deform. The space between the diaphragms is filled with air so that the diaphragms deform independently of one another. The deformation of the diaphragms results in mechanical strain of the piezoresistive devices and therefore a change in their resistances. The change in resistance is measured using Wheatstone bridge circuits. The piezoresistive devices on each diaphragm sense the external fluid pressure relative to the internal air pressure between the diaphragms. The two voltage outputs (once from each diaphragm) are fed into a difference amplifier which subtracts one voltage from the other. The result is a single voltage proportional to the pressure difference between the two external fluids. Most differential pressure sensors include built in electronics for converting their low voltage outputs into standard signal forms such as 0-10 V or 4-20 mA.
2. General characteristics of differential pressure sensors
Differential pressure sensors detect the relative pressure difference across the two sensor ports. Depending on how they are built, differential pressure sensors are either compatible with liquids and gasses on both ports (known as wet/wet sensors) or have a port that is only compatible with dry, non-condensing gases (known as wet/dry sensors). Differential pressure sensors can measure changes in pressure at frequencies of up to about 1,000 Hz (known as their bandwidth). Most sensors are only compatible with fluid temperatures up to 85 °C or 120 °C. It is possible to measure the pressure of higher temperature fluids by connecting the sensor via small diameter capillary tubes which prevent heat from being conducted to the sensor. However, the capillary tubes dampen the pressure changes, which reduces the effective sensing bandwidth. Most differential pressure sensors include built in electronics for amplifying the output and converting it into to standard output signal form.
3. Input and output signals
Differential pressure sensors need a 10-30 Vdc supply voltage which powers their Wheatstone bridge circuit or capacitor and built in electronics. Piezoresistive sensing elements are excited by a DC voltage. Capacitive sensing elements use an internal LC oscillator circuit to generate a high frequency alternating voltage which is fed to the capacitor plates. Some differential pressure sensors directly output an unamplified mV scale voltage. However, most pressure sensors include built in electronics to amplify the voltage and convert it into a standard output signal form, e.g. 0-10 V or 4-20 mA.
4. Applications of differential pressure sensors
Typical applications of differential pressure sensors include for flow measurement (as part of an orifice plate or Venturi meter), measuring positive pressure differences in clean rooms, negative pressure differences in biology labs and filter condition monitoring.
5. Typical specification
|Cost||Low to Medium cost|
|Measurement range||0-0.01 bar to 0-1,000 bar|
|Fluid temperature||20 to 85 °C (400 °C possible)|
|Bandwidth||100-1,000 Hz typical|
|Long term stability||High|
|Supply voltage||10-30 Vdc|
|Output voltage||0-10 V, 4-20 mA or unamplified|
|Passive / active||Active|
|Contact / non-contact||Contact|
6. Purchasing tips
- Unidirectional or bidirectional: Many sensors are unidirectional meaning that one port is designated as the high pressure side and the other as the low pressure side. unidirectional sensors are cheaper to purchase than bidirectional sensors but are not suitable for all applications.
- Wet/wet or wet/dry: Wet/wet sensors are compatible with liquid (or gas) on both of their ports. Wet/dry sensor are compatible with liquid on one port (the wet port) and only compatible with a dry, non-condensing gas on their other port (the dry port). The electronics are exposed to the dry port so failure to adhere will likely lead to destruction of the sensor.
- Bandwidth: The bandwidth is the frequency range over which the sensor operates i.e. the maximum rate of change of pressure that can be detected. The maximum frequency is limited by both the natural frequency of the sensor and the speed of the built in electronics.
- Overload capacity: The overload capacity is typically 150-300% of the rated pressure. This is the maximum pressure that can be applied to the sensor without damaging it.
- Chemical Resistance: The sensor’s datasheet may provide chemical compatibility data for highly corrosive fluids. Most diaphragms are made of various grades of stainless steel. Minor corrosion of the diaphragm can be enough to reduce accuracy (e.g. hydrogen embrittlement). Sensors with ceramic diaphragms provide enhanced protection from corrosive fluids.
- Built in electronics: Most sensors include built in electronics for amplifying the output signal and converting it into a standard output signal form (e.g. 0-10 V or 4-20 mA). Sensors without built in electronics require an external amplifier to be used.
7. Advantages of differential pressure sensors
Dynamic pressure sensors:
- Achieve significantly higher accuracy than obtainable by measuring the pressure difference with two regular pressure sensors and subtracting one from the other.
- Can be used to measure flow velocity according to the Bernoulli principal.
8. Disadvantages of differential pressure sensors
Dynamic pressure sensors:
- Geometrical constraints mean that they typically cannot be flush mounted.
9. Application tips
- Mounting: In most cases it is not feasible to flush mount the sensor. The intermediate pipes used for mounting the sensor cause damping of pressure changes. If high frequency pressure changes must be measured, aim for pipes of minimum length and maximum diameter so that damping is minimized.
- Fluid sealing: Pressure sensors may include a flat base at the end of their screw thread for sealing with a Dowty seal. Dowty seals often provide improved sealing compared to using Teflon tape or liquid sealant.
- Differential pressure sensors for measuring flow rate: The Bernoulli equation predicts that as flow velocity increases, measured pressure decreases. This principal is often used to measure flow velocity, from which flow volume can be determined. A typical setup, known as a Venturi flow meter, is shown below. A reduction in pipe diameter causes velocity to increase. A differential pressure sensor is used to measure the difference in pressure, from which the velocity can be determined using the equation below. A further common method for measuring flow rate it to calculate the pressure difference across an orifice plate. Orifice plates are cheaper and more compact than Venturi meters. However, they result in larger pressure drops and are more susceptible to clogging and wear.
- Make your own differential pressure sensor: Many of the differential pressure sensors on the market consist of two sensing diaphragms, connected to a difference amplifier. It is possible to assemble a differential pressure sensor of reasonable resolution by connecting the outputs of two regular pressure sensors to a difference amplifier.