Let’s assume we are measuring flow by differential-creating primary device for example an orifice plate and a DP cell as illustrated in Figure 1 below:
The differential head across the primary device is corrected for temperature variation in the first calculation block, but there is no need for pressure correction because a liquid is virtually incompressible. The square root is then extracted and the result is the ‘true’ volumetric flow. Multiplying the volumetric flow by the liquid density will give the mass flow needed.
For the mass flow measurement of gas, let’s consider Figure 1 below:
The flow is measurement by a differential-pressure primary device for instance an orifice plate and a DP cell. The procedure is the same as described for the liquid however, for gas mass flow measurement, we have pressure variation correction in addition to temperature variation correction. Note, the square root is taken after the signal from the DP cell has been corrected for both pressure and temperature variation because the fluid is compressible. A densitometer is also needed for the fluid density measurement which is used in the calculation of the mass flow.
Note that, when using flow measuring primary devices that produce an output directly proportional to flow, we can omit the square root extraction in the arrangements shown in Figure 1 and 2 above. The result can be used to give corrected volumetric flow measurements of either a liquid or a gas, depending on the configuration used, for example we can measure the volumetric flow directly with an electromagnetic flowmeter or a vortex flowmeter with a densitometer, then calculate the mass flow. This could replace the more expensive Coriolis mass flowmeter.
Related: How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter
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