How to Calculate Differential Pressure Levels

Keeping the pipes in your household protected means making sure they can handle the pressure of water and other liquids flowing through them. Regular maintenance to make sure they operate properly means figuring out whether you might need a differential pressure transmitter. These devices sense pressure levels in water.

Pressure Difference Formula

When water flows through pipes, it exerts a force on the inner walls of the pipe. Expressing this effect as a pressure, force divided by area, helps to demonstrate how strong it is for the flow of liquid. Use units of Pascals (Pa) to atmospheres (atm) to express pressure.

Use the pressure difference formula, the difference between any two other pressures, to compare other pressure values such as the pressures between two pipes. Differential pressure transmitters (DP transmitters) detect differences in pressure between two pipes or chambers and convert the energy from them to electricity. This makes them transducers, devices that convert one form of energy to another, so you may find that word used to refer to them as well.

Differential Pressure Transmitters

Many DP transmitters produce a 4 to 20 mA electric signal that can be sent across long distances and have use in industrial settings. They're engineered to use methods of digital communication to allow researchers and other individuals to maintain pressure even long distances away.

Some DP transmitters are used alongside alarms to warn when pressure levels go beyond a certain limit. DP transmitters are also designed for practical applications in oil and gas flow metering across water and land, monitoring water in treatment plants and for pump systems so they can control flow rate in cooling towers.

Pressure Difference Examples

You can also use the Bernoulli Equation, based on Bernoulli's principle, to describe the flow in DP transmitters. The principle itself is a set of equations that describe different types of flow, but many write the Bernoulli Equation as P/ρ +Vs2/2 + gz =constant for velocity of the fluid in a continuous path Vs and height above a certain section of the pipe z.

The kinetic energy, how much energy the particles of the liquid have due to their own motion, causes these changes in pressure and volume to occur for flowing liquid. As the liquid flows from resting states to states of motion, its potential energy (how much energy it has resting) is converted to kinetic. This observation also lets you set values of energy equal to one another as pressure differences as:

P1/ρ +V12/2 + gz1 = P2/ρ +V22/2 + gz2

for two pressures P1 and P2, two velocities V1 and V2 and two heights z1 _and _z2. Use this equation in conjunction with the differences in pressure between pipes or locations within pipes to determine differential pressure. The liquid must flow in a "steady-state" current, a method of current many fluid systems are designed to use, which means any change in the rate of flow or other factors that may affect the rate of flow are negligible.

You can calculate hydrostatic pressure for a liquid as P = ρ x g x h for density of a liquid "rho" ρ (in kg/m3 but you can find other units of mass/volume, too), gravitational acceleration constant g (9.8 m/s2) and height of the liquid column h (in m or appropriate units of length). Pressure difference examples can show how DP transmitters work with respect to the flow of liquid.


About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.