Differential pressure is measured by comparing pressure at two distinct points and calculating the difference between them. The simplest version uses a liquid-filled tube that shows the gap visually, while modern electronic transmitters convert that difference into an electrical signal. The method you choose depends on what you’re measuring: airflow velocity, fluid flow rate, filter condition, or liquid level in a tank.
What Differential Pressure Actually Tells You
Every differential pressure measurement boils down to one formula: take the pressure at one point, subtract the pressure at another point, and the result is your differential pressure (ΔP). Unlike gauge pressure, which compares a single point against the atmosphere, differential pressure compares two points within the same system. That difference reveals something useful: how fast a fluid is moving, how much liquid is in a tank, or whether a filter is clogging.
The physics behind it comes from energy conservation in flowing fluids. In any steady, incompressible flow, the static pressure plus the energy from the fluid’s motion stays constant. So when a fluid speeds up (say, through a narrow section of pipe), its static pressure drops. Measuring the pressure on either side of that restriction gives you the flow rate. This principle drives most industrial differential pressure applications.
Measuring With a Manometer
A U-tube manometer is the most straightforward tool for measuring differential pressure. It’s a transparent U-shaped tube partially filled with a known liquid, often water, mercury, or oil. Each end connects to one of the two pressure points you want to compare. The liquid shifts toward the lower-pressure side, and the height difference between the two columns gives you the pressure difference directly.
The math is simple. For a differential manometer where both ends connect to the same system (like across a valve or filter), the equation is:
P1 − P2 = (ρ_fluid − ρ_process) × g × h
Here, ρ_fluid is the density of the manometer liquid, ρ_process is the density of the fluid in your system, g is gravitational acceleration, and h is the height difference between the two liquid columns. If you’re measuring air pressure differences, the process fluid density is so small you can ignore it, and the equation simplifies to just ρ × g × h.
To read a manometer accurately, make sure it’s mounted vertically and your eyes are level with the liquid surface. Read the bottom of the curved meniscus for water-based fluids, or the top for mercury. The height difference is typically measured in millimeters or inches of the manometer fluid, which is why you’ll see pressure expressed in units like “inches of water column” (inH₂O) in HVAC and ventilation work.
Electronic Differential Pressure Transmitters
In most industrial settings, a differential pressure (DP) transmitter replaces the manometer. These devices connect to two pressure taps and output an electrical signal proportional to the difference. They’re more accurate, easier to read remotely, and can feed data directly into control systems.
The most common type uses a capacitive sensing element. A flexible diaphragm sits between two chambers, each connected to one of the pressure taps. When pressure differs between the two sides, the diaphragm flexes toward the lower-pressure chamber. That movement changes the distance between capacitor plates, which changes the electrical capacitance. The transmitter’s electronics convert that capacitance change into a pressure reading.
Other sensing technologies include piezoresistive elements, which change their electrical resistance when mechanically stressed by a pressure difference, and vibrating-element sensors, which detect shifts in the resonant frequency of a vibrating component as pressure changes. Capacitive sensors dominate general industrial use, while piezoresistive types are common in smaller, lower-cost applications.
Installing a DP transmitter requires connecting high-pressure (HP) and low-pressure (LP) impulse lines to the correct ports. Getting these reversed will give you a negative reading or peg the instrument in the wrong direction. The impulse lines (the tubing connecting the transmitter to the process) should be as short as practical, sloped to drain condensate in gas service or vent bubbles in liquid service, and protected from freezing.
Measuring Flow Rate With Differential Pressure
One of the most common reasons to measure differential pressure is to determine how fast a fluid is flowing. The technique works by placing a restriction in the pipe, such as an orifice plate, venturi tube, or flow nozzle, and measuring the pressure drop across it. As fluid squeezes through the restriction, it speeds up and its pressure drops. The bigger the flow, the bigger the pressure drop.
The key relationship is that flow rate is proportional to the square root of the differential pressure. Double the flow and you’ll see roughly four times the pressure drop. This square-root relationship means that at low flow rates, small changes in flow produce very small changes in ΔP, which is why these devices lose accuracy at the bottom of their range.
For an orifice plate, you install pressure taps upstream and downstream of the plate and connect them to a DP transmitter. The transmitter reads the pressure difference, and a flow computer or control system applies the square-root extraction along with correction factors for fluid density, temperature, and the size ratio of the orifice to the pipe.
Measuring Airspeed With a Pitot Tube
A pitot tube is a specialized differential pressure instrument used to measure air velocity, commonly seen on aircraft and in duct airflow testing. It works by comparing two types of pressure simultaneously.
The tube has a center opening pointed directly into the airflow, which captures both the random molecular pressure and the pressure from the air’s forward motion. This is the total pressure. Surrounding holes on the side of the tube sit perpendicular to the flow and sense only the random molecular motion, giving you the static pressure. A pressure transducer inside the instrument measures the difference between total and static pressure.
That difference is the velocity pressure, and you can solve for air velocity using: V = √(2 × ΔP / ρ), where ρ is air density. In practice, you connect the pitot tube to a differential pressure gauge or transmitter, read the ΔP, and look up or calculate the corresponding velocity. HVAC technicians use this method routinely to check airflow in ductwork.
Measuring Liquid Level in Closed Tanks
Differential pressure transmitters can measure liquid level in sealed, pressurized vessels where a simple gauge pressure sensor won’t work. The concept relies on the fact that the pressure at the bottom of a column of liquid is proportional to the height of that column.
The HP port connects to a tap at the bottom of the tank, where it senses both the gas pressure above the liquid and the hydrostatic pressure of the liquid column. The LP port connects to a tap above the maximum liquid level, where it senses only the gas pressure. Because the transmitter subtracts LP from HP automatically, the gas pressure cancels out, and the reading reflects only the liquid height.
There are two common configurations for the LP impulse line. A “dry leg” setup leaves the LP line filled with air or gas, which works well when the process vapor won’t condense in the tubing. A “wet leg” setup fills the LP line with a reference liquid (water, glycol, or glycerin) to prevent condensation from creating unpredictable liquid columns. Each configuration requires different calibration because the reference liquid in a wet leg adds a constant offset to the LP side that must be accounted for.
The preferred installation places the transmitter at the same elevation as the HP (bottom) tapping point. Mounting it below that point introduces additional liquid head in the impulse lines, which can be compensated through calibration. Mounting above the HP tap is generally avoided because air bubbles can form in the impulse line and cause erratic readings.
Common Units and Conversions
Differential pressure is expressed in the same units as any other pressure measurement, but certain units dominate in specific industries. HVAC and cleanroom work typically uses inches of water column (inH₂O) because the values are small and readable. Industrial process control often uses pounds per square inch (psi) or bar. Scientific and international applications use Pascals (Pa).
- 1 psi = 6,895 Pa = 0.0689 bar
- 1 bar = 14.504 psi = 100,000 Pa
- 1 Pa = 0.000145 psi = 0.00001 bar
When working with manometers, you’ll often see readings in millimeters of water column or inches of water column. One psi equals about 27.7 inches of water column, which gives you a sense of why inH₂O is useful for low-pressure HVAC measurements: a typical filter pressure drop of 0.5 inH₂O would be an awkwardly small 0.018 psi.
Choosing the Right Method
For a quick, one-time check of a pressure difference in a low-pressure system, a simple inclined manometer or handheld digital differential pressure gauge works well. These are inexpensive, portable, and need minimal setup. HVAC filter checks, room pressurization verification, and duct velocity measurements all fall into this category.
For continuous monitoring in an industrial process, a DP transmitter with a 4-20 mA or digital output is the standard choice. These instruments can be calibrated for the specific range you need, connected to a control system, and configured to trigger alarms when the differential pressure moves outside acceptable limits. Typical applications include monitoring flow through an orifice plate, tracking filter or strainer condition, and measuring tank levels.
Accuracy requirements also matter. A water-filled manometer is accurate to perhaps ±1 mm of water column, which is fine for many field checks. A quality DP transmitter can achieve ±0.04% of its calibrated span, making it the better choice when precision affects product quality or safety. Whichever method you use, zeroing the instrument before measurement (confirming it reads zero when both ports are exposed to the same pressure) eliminates offset errors that would otherwise skew every reading.