A strain-gauge differential pressure transmitter measures the pressure difference between two points and converts it into an electrical signal for monitoring and control. It works on the principle that the electrical resistance of a strain gauge changes when the gauge is stretched or compressed by mechanical strain.
These transmitters are used in industrial process control, flow measurement, filter monitoring, hydraulic systems, and liquid-level measurement. They typically use a pressure-sensitive diaphragm, strain gauges, a Wheatstone bridge circuit, and signal-conditioning electronics to provide a measurement proportional to the applied differential pressure.
In this article, we will explain the working principle of a strain-gauge differential pressure transmitter, its construction, circuit diagram, advantages, limitations, and industrial applications.
What Is a Strain-Gauge Differential Pressure Transmitter?
A strain-gauge differential pressure transmitter is an instrument designed to measure the difference between two applied pressures. It has two pressure connections: a high-pressure (HP) port and a low-pressure (LP) port.
When different pressures act on the sensing element, the resulting force causes the diaphragm or another elastic element to deform. Strain gauges attached to the sensing structure experience tension or compression, producing changes in their electrical resistance.
The transmitter detects these resistance changes and converts them into a calibrated output signal, such as 4–20 mA or a supported digital communication signal.
The differential pressure is calculated as:
Where:
- = Differential pressure
- = Pressure applied to the high-pressure port
- = Pressure applied to the low-pressure port
For example, if the pressure at the HP port is 300 kPa and the pressure at the LP port is 220 kPa, the differential pressure is:
Therefore, the transmitter measures a differential pressure of 80 kPa.
Working Principle of a Strain-Gauge Differential Pressure Transmitter
The working principle of a strain-gauge differential pressure transmitter is based on the change in electrical resistance caused by mechanical strain.
When a pressure difference acts on the sensing diaphragm, the diaphragm bends or deflects. The strain gauges mounted on the diaphragm or an associated mechanical element experience tensile or compressive strain. This changes their electrical resistance, which is detected by a Wheatstone bridge circuit.
The resulting voltage is amplified, compensated, and converted into a standard output signal.
The working process involves the following steps.
1. Application of Differential Pressure
The two process pressures are connected to the HP and LP ports. The difference between these pressures produces a net force on the sensing element.
2. Deflection of the Diaphragm
The applied differential pressure causes the diaphragm to deform. The amount of deformation depends on the pressure difference, diaphragm geometry, material properties, and mechanical design.
3. Strain Development
Strain gauges attached to the sensing element experience mechanical deformation. Some gauges are stretched, while others are compressed, depending on their location and the direction of diaphragm movement.
4. Change in Electrical Resistance
The deformation changes the resistance of the strain gauges. Tensile strain generally increases resistance, while compressive strain generally decreases resistance for conventional metallic strain gauges.
5. Wheatstone Bridge Operation
The strain gauges are commonly connected in a Wheatstone bridge configuration. The resistance changes unbalance the bridge and produce a small differential voltage.
6. Signal Conditioning
The transmitter electronics amplify the bridge output and apply calibration and temperature compensation. Depending on the design, the electronics may also provide linearization, damping, diagnostics, and digital communication.
7. Output Signal Transmission
The conditioned signal is transmitted to a PLC, DCS, controller, indicator, or monitoring system. The output represents the measured differential pressure within the configured range.
Working sequence:
Differential pressure → Diaphragm deflection → Mechanical strain → Resistance change → Wheatstone bridge → Signal conditioning → Electrical output
Strain-Gauge Differential Pressure Transmitter Diagram
A strain-gauge differential pressure transmitter consists of pressure connections, a sensing diaphragm, strain gauges, a bridge circuit, and signal-conditioning electronics.
The following block diagram illustrates the basic measurement process.
This is a functional block diagram. The physical arrangement of the diaphragm, strain gauges, and pressure isolation components varies between manufacturers.
What Is a Strain Gauge?
A strain gauge is a sensor that measures mechanical deformation by detecting the corresponding change in electrical resistance. It is commonly bonded to a surface or integrated into a sensing structure that deforms under an applied force.
When the material expands or contracts, the strain gauge experiences a change in length and cross-sectional area. Its electrical resistance changes accordingly.
The resistance of a conductor is expressed as:
Where:
- = Electrical resistance
- = Electrical resistivity
- = Length of the conductor
- = Cross-sectional area
A strain gauge detects small dimensional changes in the sensing element and converts them into a measurable resistance variation.
Strain gauges used in pressure transmitters may be metallic foil gauges, wire gauges, or semiconductor gauges, depending on the sensor design. Metallic gauges are commonly used in bonded strain-gauge transducers, while semiconductor gauges can provide higher sensitivity in suitable applications.
Strain Gauge Formula and Gauge Factor
The sensitivity of a strain gauge is described by its gauge factor (GF). It is defined as the ratio of the fractional change in electrical resistance to the applied mechanical strain.
Rearranging the equation:
Where:
- = Gauge factor
- = Change in resistance
- = Initial resistance
- = Mechanical strain
Mechanical strain is defined as the change in length divided by the original length:
For example, consider a strain gauge with an initial resistance of 120 Ω, a gauge factor of 2, and an applied strain of 500 microstrain.
First, convert the strain into a dimensionless value:
The fractional resistance change is:
Therefore, the change in resistance is:
The final resistance under tensile strain is:
This small resistance change can be measured using a properly configured Wheatstone bridge and signal-conditioning circuit.
Wheatstone Bridge Circuit in a Strain-Gauge Pressure Transmitter
A Wheatstone bridge is an electrical circuit used to detect small changes in resistance. In a strain-gauge differential pressure transmitter, the bridge converts the resistance changes of the gauges into a differential voltage.
A typical full-bridge arrangement uses four resistive elements. Depending on the design, these may include active strain gauges positioned so that some experience tension while others experience compression.
When no differential pressure is applied, the bridge is balanced or electronically zeroed. The output voltage is ideally zero, although practical transmitters may have a small offset that is corrected during calibration.
When differential pressure is applied:
- The sensing diaphragm deforms.
- The strain gauges undergo tension or compression.
- Their resistance values change.
- The Wheatstone bridge becomes unbalanced.
- A differential voltage develops across the bridge output terminals.
For small resistance changes, a suitably arranged full bridge can provide an output approximately proportional to the applied strain.
The bridge output is generally small, so an instrumentation amplifier or dedicated signal-conditioning circuit is used before the signal is converted into the transmitter’s standard output.
Construction and Main Components
A typical strain-gauge differential pressure transmitter contains the following components.
1. Pressure Ports
The transmitter has HP and LP connections for receiving pressure from two process locations. The pressure connections may be threaded, flanged, or designed for a specific manifold arrangement.
Correct identification and connection of the ports are essential for accurate differential pressure measurement.
2. Sensing Diaphragm
The diaphragm is the mechanical element that responds to the pressure difference. It deflects when the pressure applied to one side differs from the pressure on the other side.
The diaphragm’s material, thickness, diameter, and geometry determine its pressure range, sensitivity, and mechanical strength.
3. Strain Gauges
Strain gauges are mounted on or integrated with the sensing element. They detect the mechanical strain produced by diaphragm movement.
Their placement is selected to maximize sensitivity and reduce unwanted effects such as common-mode strain and temperature variation.
4. Wheatstone Bridge
The strain gauges are connected in a bridge circuit to convert resistance changes into a voltage signal. A full-bridge arrangement can provide greater sensitivity and temperature compensation than a single active gauge when properly designed.
5. Pressure Isolation System
Industrial transmitters may use isolation diaphragms and a suitable fill fluid to prevent the process medium from contacting the internal sensing element. The exact arrangement depends on the transmitter’s construction and intended application.
6. Signal-Conditioning Electronics
The electronic circuit amplifies the bridge output and converts it into a usable pressure measurement. It may include temperature compensation, linearization, filtering, calibration settings, and fault diagnostics.
7. Housing and Electrical Connections
The enclosure protects the sensing assembly and electronics against environmental conditions. Industrial models may provide protection against dust, moisture, vibration, and electromagnetic interference, depending on their ratings and design.
Output Signals of a Strain-Gauge Differential Pressure Transmitter
The output signal depends on the transmitter model and its intended application.
Common output options include:
- 4–20 mA: A widely used industrial analog signal for PLC and DCS systems.
- Voltage output: Some models provide outputs such as 0–5 V or 1–5 V.
- HART communication: Available on compatible transmitters for configuration and diagnostics.
- Digital communication: Certain models support interfaces such as RS-485 and Modbus.
Not every transmitter supports all these options. Always check the manufacturer’s datasheet for the available output, supply voltage, electrical connections, and communication protocol.
Example of 4–20 mA Output Scaling
Suppose a differential pressure transmitter is calibrated for a range of 0–100 kPa and configured for a 4–20 mA output.
If the measured differential pressure is 60 kPa, the output current is calculated as:
Where:
- = Output current in mA
- = Measured differential pressure
- = Calibrated pressure span
Substituting the values:
Therefore, a differential pressure of 60 kPa corresponds to an output of 13.6 mA, assuming a linear 0–100 kPa calibration and no additional output scaling.
Advantages of Strain-Gauge Differential Pressure Transmitters
Strain-gauge differential pressure transmitters offer several advantages for industrial measurement.
1. High Measurement Sensitivity
Strain gauges can detect small mechanical deformations. With a suitable diaphragm and bridge circuit, the transmitter can measure relatively small pressure differences.
2. Good Accuracy and Repeatability
Properly designed and calibrated instruments provide repeatable measurements over their specified operating range. Actual performance depends on the sensing element, electronics, temperature, and installation.
3. Suitable for Static and Changing Pressure
Strain-gauge pressure sensors can measure steady pressure as well as changing pressure. Their usable dynamic response depends on the mechanical design, pressure connections, damping, and electronics.
4. Simple Electrical Measurement
The Wheatstone bridge converts resistance variations into a voltage signal that can be amplified and processed using conventional electronic circuits.
5. Industrial Automation Compatibility
Transmitters with standard analog outputs can be integrated into PLCs, DCSs, control panels, and process monitoring systems.
6. Flexible Sensor Construction
Strain gauges can be incorporated into different diaphragm and mechanical arrangements to suit specific pressure ranges and installation requirements.
Limitations of Strain-Gauge Differential Pressure Transmitters
Despite their benefits, strain-gauge transmitters have limitations that must be considered during selection and installation.
- Temperature effects: Temperature can change the gauge resistance and mechanical properties of the sensing structure. Compensation is often required.
- Long-term drift: Strain gauges, bonding materials, and mechanical components may exhibit drift over time.
- Overpressure damage: Excessive pressure can permanently deform or damage the sensing diaphragm.
- Installation sensitivity: Incorrect pressure connections, vibration, and unsuitable impulse-line arrangements can affect measurement performance.
- Calibration requirements: Periodic calibration may be necessary to maintain accuracy.
- Material compatibility: Wetted components and seals must be suitable for the process fluid.
Correct transmitter selection, installation, and maintenance help reduce these effects.
Applications of Strain-Gauge Differential Pressure Transmitters
Strain-gauge differential pressure transmitters are used in applications where the pressure difference between two locations provides information about flow, equipment condition, or process operation.
1. Flow Measurement
A differential pressure transmitter can measure the pressure drop across an orifice plate, venturi tube, or another primary flow element. The measured differential pressure is then used to calculate the flow rate using the appropriate flow equation and calibration data.
For many conventional differential-pressure flow installations, flow rate is approximately proportional to the square root of differential pressure under the applicable operating conditions.
2. Filter Condition Monitoring
The transmitter measures the pressure difference across a filter. As the filter accumulates dirt or particles, the pressure drop generally increases at a comparable flow rate.
This information helps operators identify filter blockage and schedule cleaning or replacement.
3. Liquid-Level Measurement
Differential pressure can be used to determine the level of liquid in a tank by measuring the hydrostatic pressure produced by the liquid column.
For a simple open tank:
Where:
- = Hydrostatic pressure
- = Liquid density
- = Acceleration due to gravity
- = Liquid height
For a closed tank, the transmitter arrangement must account for the pressure above the liquid as well as the pressure produced by the liquid column. Installation elevation and liquid density must also be considered.
4. HVAC Systems
In HVAC applications, differential pressure transmitters monitor air filters, ventilation ducts, air handling systems, and building pressure differences.
The measurements help maintain ventilation performance and identify excessive pressure losses.
5. Hydraulic Systems
Hydraulic equipment uses differential pressure measurements to monitor pressure drops across valves, filters, pipelines, and other components. These measurements can help detect restrictions and assess system performance.
6. Chemical and Process Industries
Process plants use differential pressure transmitters for flow monitoring, filter condition monitoring, vessel applications, and other pressure-drop measurements. The selected materials must be compatible with the process medium and operating environment.
7. Water and Wastewater Treatment
Applications include monitoring filter pressure drop, measuring differential pressure in selected flow systems, and assessing pumping and treatment equipment.
8. Laboratory and Test Equipment
Strain-gauge pressure transducers are also used in calibration equipment, test benches, and measurement systems that require electrical pressure signals.
Strain-Gauge vs. Piezoresistive Differential Pressure Transmitter
Strain-gauge and piezoresistive differential pressure transmitters both convert pressure-induced mechanical stress into an electrical signal. However, they differ in how the sensing material produces the electrical response.
| Feature | Strain-gauge transmitter | Piezoresistive transmitter |
| Measurement principle | Resistance changes associated with mechanical strain | Stress-induced change in electrical resistivity |
| Common sensing materials | Metallic foil, wire, or semiconductor gauges | Commonly silicon or other piezoresistive materials |
| Sensor construction | Gauges bonded to or integrated with a sensing element | Piezoresistive elements fabricated in or integrated with the sensing structure |
| Signal detection | Usually a Wheatstone bridge | Commonly a Wheatstone bridge |
| Typical applications | Industrial pressure measurement and force transducers | Compact pressure sensors and differential pressure instruments |
| Temperature effects | Depend on gauge, bonding, and mechanical design | Depend on semiconductor properties and sensor design |
The two categories can overlap: semiconductor strain gauges operate through the piezoresistive effect. Therefore, the terms are not always mutually exclusive. The manufacturer’s description and actual sensor construction determine which terminology is most appropriate.
How to Select a Strain-Gauge Differential Pressure Transmitter
The following parameters should be evaluated before selecting a transmitter for an industrial application.
- Differential pressure range: Choose a calibrated range that covers normal operation and provides adequate resolution.
- Maximum static pressure: Check the maximum pressure that can be applied to both ports, even when the differential pressure is small.
- Overpressure rating: Verify the permissible pressure on each port and the allowable differential overpressure.
- Accuracy and stability: Review reference accuracy, temperature effects, long-term drift, and repeatability.
- Process fluid compatibility: Select suitable diaphragm, seal, and wetted materials.
- Output signal: Confirm compatibility with the PLC, DCS, indicator, or monitoring system.
- Environmental conditions: Consider ambient temperature, humidity, vibration, dust, moisture, and hazardous-area requirements.
- Installation arrangement: Check the pressure connections, mounting position, impulse lines, manifold requirements, and calibration access.
Correct selection improves measurement reliability and reduces unnecessary maintenance.
Conclusion
A strain-gauge differential pressure transmitter measures the difference between two pressures by detecting strain-induced changes in the resistance of sensing elements. Its main components include a pressure-sensitive diaphragm, strain gauges, a Wheatstone bridge, and signal-conditioning electronics.
The transmitter converts the measured pressure difference into an electrical output suitable for process monitoring and automation. Its applications include flow measurement, filter monitoring, liquid-level measurement, HVAC systems, hydraulic equipment, and industrial process control.
Understanding the working principle, gauge factor, bridge circuit, output scaling, and installation requirements helps engineers select and operate a strain-gauge differential pressure transmitter effectively.
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