Pressure Loss

An additional function estimates the permanent pressure losses. ISO5167 is rather vague for venturis or venturi-nozzles saying the permanent loss is between 5% and 20% of the measurement differential pressure. For a reasonably conservative estimate use 15%.

For orifice plates and other nozzles, the formula in section 5.4.1 of part 2 (or 5.1.8 in part 3) of ISO5167-2003 is used. This estimates the permanent pressure loss caused by the flow element. Since this is often used to estimate the operating cost due to energy loss, the loss should be calculated for the normal flow rate rather than the full scale.