Mass flow measurements
To measure the mass flow of a fluid in a duct through measures of pressure, it is necessary that the station in which the measure has to be made be free of vortices and that the axis of the duct be straight (no centrifugal forces and thus no pressure gradient in the direction normal to the axis). The international regulations provide, for any kind of flowmeter, the length of pipe (in diameters) with constant section and straight axis that must separate the measuring station, both upstream and downstream, from valves, elbows, intersections with other tubes and changes in section. For difficult cases, a flow conditioner, consisting of a circular or hexagonal (honeycomb) block of tubes or by one or more perforated plates, should be included in the duct upstream of the measuring station.
Only under these conditions is the static pressure constant in the section and measurable on the wall; for greater safety, usually four holes, placed at 90° in the same section, are placed in communication with each other in a groove around the tube, measuring the resulting pressure gives, somehow, an average of the possibly different pressures prevailing in the four holes.
As with all probes, any reason to disturb the magnitude of the variable to be measured must be avoided; in this case, one must avoid generating a drag in the duct that modifies the flow rate.