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and enter keyword(s): flowmeter or pressure gauge.
TECHNICAL PAGES
Differential
pressure
gauges
A differential pressure gauge is a common, low cost flowme- ter that does not restrict the flow or add any pressure loss in a flow system. It is well known that these instruments can display the difference between two pressures and are available in a wide selection of ranges, but there are two interesting facts that are not widely known.
Two things most dealers may not know.
Square root dials First, the differential pressure gauge can easily be calibrated with a “square root dial” to show the parabolic relationship between liquid flowrate and pressure drop in a piping system.
In the piping system shown in picture one, the pressure in the 3-inch pipe section and the pressure in the 2-inch pipe section are recorded at the system normal flowrate, which hopefully can be measured by a timed collection. Separate Bourdon Tube pressure gauges can be used here, as the ranges are usually not yet known. The higher pressure will be realized in the larger diameter pipe, because the flow velocity is lower. As we know from Bernoulli’s energy equation, the pressure difference is proportional to the square of the velocity, and the velocity is proportional to the volumetric flowrate. Once we have the two pressures and the associated flowrate, a differential pressure gauge can easily be custom made (see picture two) showing flowrate in any units such as gallons per minute (gpm) at the various pressure differentials. The flow rate can be measured by collection or by calculation, based on known pipe sizes. The flow data will have a parabolic distribution (non-linear)
Picture 1
Higher pressure will be realized
in the larger diameter pipe because
flow velocity is lower.
