Endress+Hauser Deltatop DP71B, Руководство по эксплуатации

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Deltatop DP71B, DP72B, DP73B Operation and commissioning
Hauser 37
5.2.2 Calculation formula for temperature and pressure compensation
The starting point for compensation first has to be defined. The starting point is the calculation sheet
which accompanies every primary element. Layout data for specific operating conditions (pressure
and temperature) can be found on the calculation sheet.
The relationship between flow and differential pressure is described by a square root function:
for the mass flow (or volume flow at normal or standard conditions)
and
for volume flow
where
ρ
= the density of the medium.
If the current output of the Deltabar transmitter is set to flow values, the square root function is
already implemented. Otherwise the square root function must be computed externally, e.g. in a
PLC. Please make sure that the square root function is not applied twice.
If the real operating conditions differ from the conditions used in the calculation sheet, the density
of the gas will change and thus also the calculated flow rate will change according to the above-
mentioned formula.
where
P = absolute pressure
T = absolute temperature (K)
Z = compressibility factor
1 = operating condition according to the calculation sheet
2 = operating condition currently measured
The compensation can now be computed as follows:
for mass flow (or volume flow at standard conditions)
for volume flow
The compressibility factor Z can be neglected if tis value is close to 1. If the compressibility factor is
to be included in the compensation, the value must be determined according to the pressure and
temperature currently measured. Compressibility factors are available in the corresponding
literature in tables or graphs or can be calculated using the Soave-Redlich-Kwong procedure for
example.
2 p
D r
m
Q =
2 p
D
r
Q =
v
r r
=
2 1
P
P T
T Z
Z
2 1 1
1 2 2
Q = Q
2 1
P
P T
T Z
Z
2 1 1
1 2 2
Q = Q
2 1
P
P T
T Z
Z
1 2 2
2 1 1