The equation given in the DIN 1952 (Edition 1982) for the calculation of the pressure loss in dependence on the pressure drop measured is wrong. Large opening conditions of the orifices result in negative pressure losses. This fact had already been pointed out in (1). But the error is not only due to the supposed confusion of the flow- and contraction number. Incorrect is also the conversion between these values, which in general is not allowed at all, which, however, was used in (1) again. In contrast to this, clear conditions are received by narrowing down the problem with the help of the exact calculation of an upper and a lower limit for the pressure loss concerning the pressure drop. On this occasion, there is in addition demonstrated again the simple presentation of the fundamental principles merely on the basis of the principle of linear momentum without any empirical watering down, whose consequent application would make the DIN 1952 considerably cleare