Precise Flow · Calibration · Test Equipment

Orifice and nozzle evaluation – how is it done?

Pressure drop devices are working according to Bernoulli's principle and measure the average flow rate and the mass flow. They are operated subcritically.


According to the Bernoulli equation the mass flow follows the differential pressure and the density in a square root relationship at subcritical flow by an orifice, nozzle or venturi tube. A range of flow of 1:4 consequently requires a differential pressure range of 1:16. Due to the square root relationship between flow and DP it has to be made sure that in case of a low flow of, e.g., 10% of full scale, a differential pressure of approx. 1% of full scale is not exceeded as maximum measurement.

Operating principle of the throttle measurement
Operating principle of the throttle measurement

Test section design and flow evaluation with throttle device:


For flow measurements with throttle devices a differential pressure sensor, absolute pressure sensor and a temperature sensor have to be provided. The measurement of humidity is optional.

The current viscosity and density of the medium is calculated from the sensor data. The flow depends on the geometrical data of the throttle together with the medium data and the differential pressure.

The recording of sensor data and the calculation of the flow values can be carried out easily by our high-capacity flow computer.


Due to the turbulent flow partly occur strong differential pressure variations of 1% – 5% of reading. To reach high accuracies of the evaluations it is recommended to always use a flow computer with average filter and automatic evaluation. In addition, the calibration of the measuring section clearly improves the measuring accuracy under operating conditions. A throttle device can be used then with an accuracy of better than 1% of reading.

Orifices and subcritical nozzles

Flow calculation according to Bernoulli

More information and technical details can be
found in the data sheet
and on the product page

Additional information
on this subject:


Flow calibration

Flow element software

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