Differential Pressure Gauges, Transmitters and Sensors Explained

Quick answer: a differential pressure (DP) instrument measures the difference between two pressures, using two ports instead of one. It comes as a gauge (a dial for a person), a transmitter or sensor (an electrical signal for a controller), a switch (an on/off contact) or a digital manometer (a handheld meter). Typical uses are checking when a filter is clogged, measuring flow across an orifice plate, and measuring level in a sealed tank. Choose the range from the pressure difference you expect, not the line pressure, and make sure the instrument can survive the full line pressure on both ports. This guide explains each type with worked examples, and tells you honestly what to check before ordering.

What is differential pressure?

Differential pressure is high-side pressure minus low-side pressure. If the pressure before a filter is 400 kPa and after it is 360 kPa, the differential pressure is 40 kPa. That 40 kPa is what the filter is costing you in pump energy, and it rises as the filter clogs.

A differential instrument has a high port and a low port, normally labelled HP and LP or + and -. If one port is left open to the atmosphere, it behaves like a gauge pressure instrument. If both ports are connected to the process, it reads only the difference, whatever the line pressure.

Types of differential pressure instrument

For the difference between a sensor, transducer and transmitter, read our pressure transducer vs transmitter vs sensor guide.

Use 1: filter and strainer monitoring

A clean filter has a small pressure drop. As it blocks, the drop rises. A differential gauge across the filter tells you when to change the element.

Worked example. A hydraulic return filter shows 20 kPa when clean. The maker says to replace the element at 150 kPa. A 0 to 250 kPa differential gauge fits, with a red mark at 150 kPa. The operator can see the trend at a glance, and a differential switch set at 150 kPa can add an alarm.

Use 2: flow measurement

When fluid flows through a restriction, such as an orifice plate or a venturi, the pressure drops. The bigger the flow, the bigger the drop. The relationship is a square root:

Flow is proportional to the square root of the differential pressure.

Worked example. An orifice plate gives 16 kPa of differential pressure at 100 litres per minute. At half the flow, 50 litres per minute, the differential pressure falls to a quarter, which is 4 kPa. That means at low flows the signal is small, so the transmitter range and accuracy matter. A transmitter set for 0 to 16 kPa gives 20 mA at 100 L/min. At 50 L/min the differential pressure is 4 kPa, so the output is 4 + (4 ÷ 16) × 16 = 8 mA, which is only 25% of the signal span for 50% of the flow. The PLC or a flow computer takes the square root to show flow correctly.

Use 3: level in a sealed tank

In an open tank, a gauge pressure sensor at the bottom measures liquid level. In a closed tank the gas pressure above the liquid also acts on the bottom sensor, so you measure the difference between the bottom and top pressures. That removes the effect of the headspace.

Worked example. A closed tank holds water to a depth of 2 m. The differential pressure between the bottom and top ports is 1,000 × 9.81 × 2 = 19,620 Pa, which is 19.6 kPa. A 0 to 30 kPa differential transmitter would show it at about 65% of range. See hydrostatic level measurement for more.

Use 4: air, ventilation and cleanrooms

Very low differential pressures, measured in pascals, control airflow and room pressure in cleanrooms, labs and hospitals. A digital manometer or a low-range sensor with a range such as 0 to 250 Pa is typical. These are far smaller than the pressures that a standard Bourdon gauge can read. For low pressure gas, see gas and LPG pressure gauges.

How to choose a differential pressure instrument

1. Define the pressure difference. Choose the range around the expected difference, with room for a blocked condition.
2. Know the static (line) pressure. Both ports must be rated for the maximum line pressure, and over-range on one side must not damage the instrument.
3. Choose the output. Dial for people, 4 to 20 mA for control systems. See 4 to 20 mA wiring and calibration.
4. Check the media. Wetted parts must suit the fluid on both sides.
5. Check the connection. Port threads and mounting. See connection types and thread sizes.
6. Plan the installation. Valves, a manifold and vents are almost always needed.

Installation and zeroing

• Fit a three- or five-valve manifold so you can isolate the instrument and equalise the ports for a zero check.
• Keep impulse lines short, sloped and free of air or trapped liquid.
• Mount the instrument so that the lines stay full of the same fluid, to avoid head errors.
• Zero the instrument with both ports equalised, then close the equalising valve.
• Never open one side without equalising first, or a large static pressure can slam against one side of the sensing element.

Common mistakes

• Choosing the range from line pressure instead of from the pressure difference.
• Ignoring the static pressure rating and overloading one side.
• Forgetting to take the square root for flow.
• Letting air bubbles sit in liquid impulse lines.
• Using a single-port gauge to read a difference by subtracting two gauge readings. With large line pressures, the errors add up and swamp the small difference.

What we can supply

Our current range includes single-port pressure gauges, digital pressure gauges and the WIKA Model A-10 pressure sensor. Differential pressure instruments are a different product family, so please tell us your port connection, differential range, static pressure and output via the contact page, and we will confirm what we can supply for your job. A related instrument you may already have is the manometer, which measures small pressure differences. See gauge vs transmitter vs manometer.

Worked example: choosing the range for a filter gauge

A water filter runs at 400 kPa line pressure. The maker says the clean pressure drop is 10 kPa and the element must be changed at 60 kPa.

1. Differential range: choose a range above the change-out point with some headroom, such as 0 to 100 kPa. Choosing a 0 to 1,000 kPa range because of the line pressure would make the 10 kPa clean reading almost invisible.
2. Static pressure rating: confirm the gauge can withstand at least the line pressure on both ports, with a margin for surges.
3. Marking: mark 60 kPa in red on the dial so the operator knows when to change the filter.
4. Alarm: add a differential switch set at 60 kPa if you want an automatic warning.

Digital manometers and air balancing

For ventilation, cleanroom and filter-bank testing, technicians use digital manometers with ranges in pascals. These show small differences clearly and often allow zeroing at the push of a button. They are a testing tool rather than a permanent installation, and they need regular calibration like any other instrument. Hold the instrument level, allow it to warm up and zero it with both ports open to the same pressure.

Maintenance

• Check the zero regularly by equalising the ports.
• Purge impulse lines to remove trapped air or debris.
• Replace elements that have been overpressured on one side.
• Keep calibration records. See pressure gauge calibration.

Which type for which job?

A note on static pressure and safety

Always treat both ports as live. Isolate and vent in the correct order, and never remove a differential instrument from a pressurised line without following the isolation procedure for the manifold. This protects both the sensing element and the technician.

Frequently asked questions

What is the difference between a differential pressure gauge and a normal gauge?

A normal gauge compares one pressure with the atmosphere. A differential gauge compares two process pressures.

Can I use two gauges and subtract the readings?

Only for large differences. With high line pressure and small differences, the combined gauge errors can exceed the difference you want.

What is static pressure?

The common pressure present on both ports. The instrument must withstand it even though it measures only the difference.

Do I need a manifold?

For transmitters, almost always. It allows safe isolation, zeroing and venting.

Can a differential transmitter measure flow?

Yes, with a primary element such as an orifice plate or venturi, and a square root conversion.

Next steps

Read what is a pressure gauge for the basics, gauge vs absolute vs vacuum pressure for the pressure types, and browse the pressure sensor category.