Why limit the measuring tubes to pipes smaller than DN300?
Measuring tubes are generally recommended for small pipe sizes. Indeed, the upstream and downstream straight lengths to be provided depend on the inner diameter D of the pipe. The greater the diameter D is, the longer the straight lengths are.
Example for an orifice plate: to ensure a good measurement accuracy, the minimum upstream length over which the roughness is critical corresponds to a value of 10D *. For a pipe diameter D of 50 mm, the upstream length to be provided with the measuring tube is therefore at least 50 cm. For a diameter D of 300 mm, the length of the upstream tube to be supplied is at least 3 m. For larger diameters, the measuring tube can reach lengths that are difficult to transport. This is why we generally limit the asssembly of high precision tube to DN300 but, if necessary, we are able to manufacture measuring tubes for larger diameters.
The measuring tube has a great benefit: precision, but also a disadvantage: its size (defined according to the values of the standard). It is therefore essential to find the right compromise. We remain at your disposal to determine the right lengths required for your application and evaluate the possibility of manufacturing a high precision measuring tube.
* This upstream length of 10D corresponds to the minimum length over which the roughness criterion must be respected. However, the upstream and downstream total straight lengths to be respected for a measurement by means of a differential pressure flowmeter are greater than this value. Example, for β = 0.5, according to the ISO5167 standard, upstream straight lengths can reach up to 44D depending on the obstacle upstream of the measurement. Refer to the straight length tables of the standard for more details.