[Translate to en_uk:] Einführung Durchflussmessverfahren bei Gasen
Differential pressure method
By narrowing the diameter of a pipe the velocity of flow is increased at identical volume flow, whereby a change in pressure energy or temperature is caused. The pressure drop between the normal pipe diameter and the narrowed diameter is called the differential pressure DP. This method is used with:
- orifices, nozzles and Venturi-pipes
Supercritically operated Laval nozzles
If nozzles are operated with a sufficiently big pressure difference between inlet and outlet, a stable condition is reached at certain pressure conditions leading to sonic velocity at the narrowest nozzle passage.
Critical nozzles have been successfully used for years as transfer standards for measurement tests, calibration and gauging.
Flow measurement on the basis of pressure drop in straight pipes
Flow measurement on the basis of pressure drop in straight pipes
Friction resistance in pipes increases as flow increases. To overcome this resistance, a pressure gradient must be provided along the pipe. On the basis of the pressure differential between inlet and outlet and the laws of resistance the flow can be calculated.
Flow measurement with Laminar Flow Elements
Pressure loss of a laminar flowed pipe segment is directly proportional to the velocity of flow (Hagen-Poiseuille equation). On this basis flow can be calculated. An LFE consists of a multitude of capillaries, thus splitting the volume flow and creating a laminar flow. Setup of an LFE measuring line:
