What Is Differential Pressure and How Is It Measured?

Differential Pressure

Differential Pressure is the difference between the pressure measured at two different points in a system. Instead of measuring pressure relative to atmospheric pressure or a vacuum, differential pressure compares two process pressures directly.

It is commonly represented as the pressure at the high-pressure side minus the pressure at the low-pressure side. Differential pressure is particularly useful when the pressure difference itself provides information about flow, level, filter condition or equipment performance.

What Is Differential Pressure?

Consider a pipe carrying fluid through a restriction. The pressure upstream of the restriction is higher than the pressure downstream. The difference between these two pressures is the differential pressure across the restriction.

The two pressure connections are commonly referred to as the high-pressure (HP) and low-pressure (LP) sides. The instrument determines the pressure difference between these two sides.

How Is Differential Pressure Calculated?

Differential pressure is calculated by subtracting the lower pressure from the higher pressure. The same pressure units must be used for both measurements.

How Is Differential Pressure Measured?

Differential pressure can be measured by connecting an instrument to two pressure points. One connection senses the high-pressure side and the other senses the low-pressure side. The instrument then measures the difference between these two pressures.

Depending on the application, the output may be a local mechanical indication, a 4–20 mA electrical signal or a digital communication signal. The selected instrument should match the required pressure range, accuracy and process conditions.

What Is a Differential Pressure Transmitter?

A differential pressure transmitter is an instrument designed to measure the pressure difference between two process connections and convert that measurement into a usable output signal.

The transmitter contains a sensing element that responds to the pressure difference. Internal electronics process the sensor signal and produce an output that can be transmitted to a PLC, DCS, SCADA system or other monitoring equipment.

A common industrial output is 4–20 mA. Modern transmitters may also support digital communication and provide additional diagnostic information depending on their design.

How Does a Differential Pressure Transmitter Work?

The transmitter continuously compares the pressure applied to its high- and low-pressure sides. The resulting differential pressure is converted into an electrical signal corresponding to the configured measurement range.

For example, a transmitter configured for 0 to 10 bar differential pressure may represent 0 bar differential pressure with approximately 4 mA and 10 bar with approximately 20 mA, subject to the transmitter configuration.

Differential Pressure vs Gauge Pressure

Differential pressure and gauge pressure are not the same measurement. Gauge pressure compares process pressure with atmospheric pressure, while differential pressure compares two process pressures.

Applications of Differential Pressure Measurement

Differential pressure measurement is used in many industrial applications because a pressure difference can provide useful information about the process without directly measuring the desired variable.

Differential Pressure for Flow Measurement

One of the most important applications of differential pressure is flow measurement. A restriction such as an orifice plate creates a pressure difference between the upstream and downstream sides.

The measured differential pressure can then be related to flow rate using the appropriate flow equation and application-specific parameters.

This principle is widely used with differential-pressure flow measurement systems. For additional information, see this guide to orifice plate flow measurement .

Differential Pressure for Filter Monitoring

Filters create resistance to fluid flow. As a filter becomes loaded with particles or contaminants, the pressure difference between the upstream and downstream sides can increase.

Monitoring differential pressure therefore provides a convenient way to determine when a filter may require inspection, cleaning or replacement, depending on the process and manufacturer's specified limits.

Differential Pressure for Level Measurement

Differential pressure can be used to measure liquid level in tanks and vessels because the pressure produced by a liquid column is related to the liquid's height and density.

In a typical closed vessel, a differential pressure transmitter can sense the pressure at the bottom of the vessel and compare it with the pressure at the top. The difference can then be used to infer the liquid level when the application and transmitter configuration are suitable.

The relationship between pressure, liquid density and height means that changes in fluid density can affect the inferred level. Process conditions should therefore be considered during instrument selection and configuration.

Differential Pressure Across Pumps and Equipment

Differential pressure can also be used to evaluate pressure gain or pressure loss across equipment. Measuring pressure at the inlet and outlet of a pump, heat exchanger, valve or other component can provide useful information about system operation.

For example, comparing pump suction and discharge pressure can help operators understand whether the pump is producing the expected pressure increase under the current operating conditions.

Types of Differential Pressure Instruments

What Is High Side and Low Side in Differential Pressure?

A differential pressure instrument has two process connections. The connection exposed to the higher pressure is normally called the high-pressure or HP side, while the connection exposed to the lower pressure is called the low-pressure or LP side.

The transmitter measures the difference between these two inputs. Correct identification and connection of the HP and LP sides are therefore essential for obtaining the intended measurement.

Differential Pressure Measurement in Closed Vessels

Closed vessels can create additional pressure conditions because pressure may exist above the liquid surface. A differential pressure transmitter can compare the bottom pressure with the vessel's upper pressure to compensate for the pressure acting on the liquid surface.

This makes differential pressure measurement useful for level applications where the vessel is pressurized. The transmitter configuration must account for the vessel arrangement and installation conditions.

Factors Affecting Differential Pressure Measurement

Reliable differential pressure measurement depends on more than the transmitter itself. Process conditions, installation and impulse connections can all affect the measurement.

• Pressure range and differential pressure range.
• Static pressure in the process.
• Process temperature.
• Fluid density and viscosity.
• Impulse line installation.
• Blocked or restricted impulse lines.
• Leaks in pressure connections.
• Incorrect high- and low-pressure connections.
• Elevation differences between pressure connections.
• Instrument calibration and configuration.

Common Differential Pressure Measurement Problems

Incorrect HP and LP Connections

Reversing the high- and low-pressure connections can cause the transmitter to indicate an incorrect differential pressure or a reversed measurement.

Blocked Impulse Lines

Deposits, contamination or process material can restrict impulse lines and prevent the transmitter from accurately sensing process pressure.

Leaking Connections

Leakage in impulse piping or fittings can affect the pressure reaching the transmitter and produce measurement errors.

Unequal Installation Conditions

Differences in elevation, temperature or fluid conditions in impulse lines can influence the measurement, particularly in liquid and high-temperature applications.

How to Improve Differential Pressure Measurement Accuracy

Correct instrument selection and installation are essential for dependable differential pressure measurement. The transmitter range should be appropriate for the expected differential pressure, while the instrument must also be suitable for the maximum static pressure and process conditions.

• Select the correct differential pressure range.
• Verify maximum static pressure requirements.
• Correctly identify the HP and LP connections.
• Install impulse lines according to application requirements.
• Minimize leakage and unnecessary restrictions.
• Consider process temperature and fluid properties.
• Protect the measurement system from excessive vibration where necessary.
• Calibrate and verify the instrument according to the required maintenance program.

Our guide on types of pressure measurements provides additional information about the different pressure measurement methods used in industrial instrumentation.

Differential Pressure vs Pressure Drop

The terms differential pressure and pressure drop are often used together, but their context can differ. Differential pressure simply describes the difference between two pressure measurements. Pressure drop generally refers to a decrease in pressure as fluid passes through a component or section of a system.

For example, the pressure difference across a filter may be called differential pressure, while the same difference is commonly described as the pressure drop across the filter.

Example of Differential Pressure Measurement

Differential Pressure Measurement Selection Checklist

Conclusion

Differential Pressure is the difference between the pressure at two points in a process system. It is one of the most useful measurement principles in industrial instrumentation because the pressure difference can provide information about flow, liquid level, filter condition and equipment performance.

Differential pressure can be measured using mechanical differential pressure gauges or electronic differential pressure transmitters. In automated systems, transmitters can provide a 4–20 mA or digital signal to PLC, DCS and SCADA systems for remote monitoring, alarming and control.

Reliable measurement depends on selecting the correct differential pressure range, considering static pressure and process conditions, correctly connecting the high- and low-pressure sides, and installing the pressure connections and impulse lines properly.