For horizontally-orientated venturi tubes and orifice plates, inter-conversion between pressure energy and kinetic energy is not ideal; energy is lost as heat (Q) and as internal energy (Uf > Ui). The amount of energy lost is correlated to a discharge coefficient, C(Re), that is itself a function of Reynolds number. C(Re) is a measure of the deviation from ideal energy conversion and varies from 0 to 1.
As the absolute pressure changes are relatively small, the gas can be approximated as an incompressible fluid. (You might want to validate this approximation.) Consider ideal energy conversion (Qt?2 = 0; U2 = Ut) between the throat of the venturi or orifice plate (meter) to the free stream at point 2