Pressure is the force applied on a specific area. Pressure can be exerted by all forms of matter; solid, liquid and even gasses. Measurement of fluid pressure is particularly useful in terms of process monitoring and control in industrial automation systems. This article aims to discuss differential pressure transmitters. They are one of the most widely used pressure sensors to get pressure information such as flow rate of fluid based systems.
What is a Differential Pressure Transmitter?
When it comes to monitoring pressure in fluids, there are three techniques of measurement:
- Gauge pressure
Gauge pressure is the pressure difference between the measured fluid and the atmospheric pressure.
- Absolute Pressure
The combined atmospheric and fluid pressure is known as absolute pressure.
- Differential pressure
Differential pressure is the difference of pressure at two distinct points in the same fluid/gas.
While all these techniques may sound different from one another, all three are related to each other.
A differential pressure transmitter (also known as DP transmitter) is a special type of pressure sensor. It can measure two different pressures and subtract one from the other; hence the name differential. Differential pressure sensors are used commonly in fluid transmission mediums to determine the flow rate.
The term pressure transmitter and pressure transducer often go along together. However, functionality wise, they are different from one another. Pressure transducers are simpler than pressure transmitters, and contain very minimal electronic circuitry. Both sensor types have piezo-resistive or thin-film mounted on a process connection.
Pressure transducers directly output analog signals from the piezo element. The signal is usually in the millivolts range. They need external processing circuitry to condition this raw signal to later feed to a controller such as a PLC.
Pressure transducers, however, have additional processing circuitry built-in. This includes linearization, amplification and further conditioning of the signal to be transmitted to a remote receiver. They consist of integrated transducer and transmitter circuitry to form a single unit capable of producing a signal that can be directly fed into a PLC.
Differential Pressure Transmitter Working Principle
As mentioned above, differential pressure transmitters have multiple sections that acquire, condition and transmit the pressure signals. In the mechanical construction, there are three main parts:
- Primary Element
- Secondary Element
The primary element is a special mechanical construction that introduces a pressure difference into the fluid flow. This can be a venturi tube, orifice plate, pitot tube, flow nozzle, or a laminar flow element.
The primary element creates a pressure differential in the fluid flow by adding an artificial barrier using one of the mechanical barriers mentioned above. The secondary element picks up the pressures at two points, before and after the barrier.
The primary element is often known as ‘high-side’ and the secondary element as ‘low-side’. This should not be misunderstood as the high-side to be of higher pressure and low-side to be of lower pressure.
The primary section has a connection to the secondary element through some form of an isolating fluid coupling or a mechanical coupling. The fluid is often based on silicone as silicone is a good thermal insulator and it is chemically stable. There are multiple types of sensors available in differential pressure transmitters as their secondary element:
- Differential capacitance pressure transducers
- Vibrating wire pressure transducers
- Strain gauge pressure transducers
The secondary element converts the sensed pressure differential into a small voltage signal. The electronics pick up this voltage signal and condition it further. This includes filtering, normalization, amplification, sampling and conversion to current/voltage signal.
The electronics section then outputs a linear output related to the pressure range measured. For example, if the sensor is capable of measuring 0-100psi differential pressure and it is a current output type, it will output 4mA at 0psi and will output 20mA at 100psi. Any pressure in between will have the corresponding current value to it. (i.e. 8mA for 50psi)
Differential Pressure Transmitter Elements
As mentioned above, differential pressure transducers have 3 main elements; primary, secondary and electronics assembly elements.
Let’s look at the common primary elements and their construction in brief.
- Orifice Plates
Orifice plates are one of the simplest forms of primary elements. They introduce a pressure drop in the flow by introducing a restriction. The orifice always has a smaller diameter than the pipe being connected.
The orifice plate pressure transducer has two pressure outlet connections to connect the device to an external pressure monitoring device. In differential pressure transmitters, this connects to the secondary element of the device.
- Venturi Tube
On some pressure transmitters, the primary element is a venturi tube. It is a special arrangement which also restricts the flow and creates a pressure differential. Unlike the orifice place, the venturi tube has a more gradual shape and the pressures are measured at two different places.
The venturi tube has an inlet, converging part, a throat (smaller than typical pipe diameter) and a diverging part which increases the diameter back to the original pipe diameter. The pressure measurements are taken from the inlet section and the throat section, which has two diameters.
The principle used to measure the pressure is related to the continuity equation and Bernoulli’s equation found in the theory of fluid mechanics. Venturing type primary elements are found mostly in flowmeters to determine the flowrate.
- Pitot Tube
Pitot tubes are used primarily to measure flow rates. They consist of a bent pipe that has two openings
One end of the tube is open to a static point in the fluid where there is low pressure, and the other is directly in line with the flow as shown below:
As the flow rate increases, the liquid stuck in the bend section of the tube is displaced to create a liquid manometer, which indicates the flow rate.
However, the installation of such two-port pitot tubes can be cumbersome. And has following limitations:
- If the velocity is too low, the pressure difference can be hard to measure.
- If the velocity is too high (i.e. supersonic/faster than the speed of sound), this also nullifies Bernoulli’s equation’s primary requirements, making it invalid.
- If the tube becomes clogged, the resulting pressure deviates from actual pressure values.
Therefore, there are special, single point pitot tubes named ‘averaging type pitot tubes’ which have a number of upstream sensing tubes, as shown below.
This setup overcomes the problems mentioned in the pitot-static tube.
- Flow Nozzle
Flow nozzles are another type of primary element found in differential pressure transducers. They are similar to orifice tubes, but have several advantages over them.
There are few types of wolf nozzles: flanged flow nozzles, flanged flow nozzles with integrated low pressure taps and weld-in flow nozzles.
The special wel-din type flow nozzles are particularly useful in high pressure applications and small diameter piping where flanges cannot be used. The figure below shows a weld-in flow nozzle, which is placed between the pipes and welded permanently in place.
Here are some of the advantages of flow nozzles compared to orifice plate elements:
- Flow nozzles do not contain any sharp edges as orifice plates. This makes flow nozzles less prone to wear and tear over time.
- Flow nozzles are preferred in measuring high-velocity liquids.
Widely used in high pressure and high temperature applications such as high velocity steam flow in turbines.
- Laminar Flow Element
Another interesting primary element is the laminar flow element. This consists of multiple tubes that are much longer than the diameter of the main pipe to slow down the flow and make it laminar.
These laminar tubes introduce a permanent pressure drop that cannot be recovered downstream due to the friction caused by the tubes. The pressure drop is quantifiable by using the Hagen-Poiseuille equation.
Laminar flow elements are used to obtain a linear relationship between the flow rate and the pressure drop, which eliminates the need for a square root characterization to linearize the reading.
However, laminar element based devices need temperature compensation as the temperature affects the viscosity of the liquid, thus, the final reading.
The secondary element of a differential pressure transmitter consists of the devices that convert the physical attribute (pressure) to an electrical signal. This is done by using a ‘transducer’ that can take any of the following forms:
- Strain gauge pressure transducer
Strain gauge type transducers are used for narrow-span pressure and differential pressure measurements. They have a strain gauge, a resistor that changes its resistance according to the strain applied to it. The strain gauge is attached to a diaphragm to allow the pressure to translate into strain.
They can be used in all types of pressure transmitters as the transducer element to measure gauge, absolute and differential pressures.
- Capacitance pressure transducer
The capacitance type transducers use a moving diaphragm according to applied pressure to determine the pressure differential. The diaphragm is connected to the primary element using a fill fluid such as liquid silicone.
This involves a complex circuit that oscillates. The oscillation frequency is affected by the change in capacitance, which ultimately translates into a DC signal as the output from the sensor.
- Resonant wire pressure transducer
A resonant wire pressure transducer consists of a resonating wire according to an integrated oscillator circuit. Any change in pressure causes the wire tension to change. The change in tension changes the oscillation frequency. Since this can be very accurately measured, resonant wire transducers are commonly found in low pressure differential applications.
Resonant wire pressure transducers are very stable under stable temperature conditions, but are non-linear. Therefore, these require the assistance from a microprocessor to compensate for the non-linearity.
The electronics of a pressure transmitter further amplifies, conditions and converts the raw signal to be transmitted to a PLC or another controller. The output can be,
- Voltage output
A 0-10V or 0-5V output depending on the configuration.
- Current output
A 4-20mA current output to feed into a PLC input card.
- Digital output
A thresholded output or a digital communication data stream such as an RS232 or RS485 compatible output which provides a highly accurate, digitally sampled pressure reading.
Differential Pressure Transmitter Construction
Differential pressure transmitters consist of three main sections, the primary element, secondary elements and the electronics housing.
The primary element is directly mounted into the tubing and acts as the pressure observing element. This introduces a pressure difference into the fluid flow, and provides two outputs from two points with known, different fluid pressure.
The secondary element is usually mounted right on top of the primary element, outside the piping. This converts the physical pressure into a small electrical signal.
The final element is the electronics, which has the signal conditioning and transmitter circuitry built in. This stage reads the measurement from the secondary element, amplifies, filters and further conditions to transmit the reading to a PLC or another suitable receiver.
Where is Differential Pressure (DP) Measurement used?
The most common application for differential pressure is the differential flow rate measurement. This type of application can be found in both domestic and industrial environments such as the measurement of liquid flow rate in oil/water dispensing systems.
Other applications for DP include filter monitoring, liquid level measurement and in some cases, drill head torque monitoring. In filter monitoring, DP is used to constantly monitor the filters for clogging. If a filter is clogged, the differential pressure is increased, and the reading is then used to identify the problem.
In some applications, gas pressure, liquid pump pressure monitoring and water pipe leak detection is also done by measuring the differential pressure.
What is a Differential Pressure Transmitter Used for?
A differential pressure transmitter is capable of outputting a differential pressure measurement, according to a fine calibration. The output can be a voltage, current or a digital output compatible with industrial standard equipment.
External pressure transmitters are used to obtain the raw readings from differential pressure transducers, and convert them into electrical signals that are linear and quantifiable according to the pressure values measured.
Differential pressure transmitters are integrated devices that can be used to measure the pressure difference in a fluid system. The measurements from a DP transmitter can be used to measure the flow rate, pressure (gauge, differential and absolute pressure) and even the presence of fluid/gas in some cases. This article airmed to give an introduction to industrial differential pressure transducers, their construction and a brief idea on the different types of DP transmitters available.