11 Appendix RMS 621
66 Endress+Hauser
11.2.1 Differential pressure flow calculation
The device offers 2 possible differential pressure measurements:
• Traditional differential pressure methods
• Improved differential pressure methods
Traditional differential pressure method:
All flow calculation coefficients are calculated using the actual system design and are then brought
together as a single constant.
Improved differential pressure method:
Compared to the traditional cacluation method the coefficient of the flow equation (flow coefficient,
velocity factor, expansion number, density) is continuously being recalculated according to the ISO
5167. This has the advantage that the flow can be exactly calculated even under highly fluctuating
process conditions, which could be far outside the rated condition (temperature and pressure within
their rating) thus guaranteeing a high accuracy on the flow measurement.
For this the device needs the following data:
• Internal pipe diameter
• Diameter ratio ß (the K factor on Pitot tubes)
ƒ = Correction factor (measurement correction, e.g. taking the pipe roughness into consideration)
Pitot tubes
When using Pitot tubes a correction factor needs to be entered instead of the pipe diameter ratio.
This factor (resistance value) is allocated by the sensor manufacturer, On the "E+H Deltatop" this is
in the form of the K Factor.
The input of the correction factor is absolutely required (see following example)!
The flow rate is calculated as follows:
The flow rate is calculated from:
ƒ = Correction factor (K-Factor or from the correction tables)
d = Tube internal diameter
∆P = Differential pressure
ρ = Density under operational conditions
Traditional differential pressure methods Improved differential pressure methods
Only accurate as a complete system (pressure, temperature,
flow)
Accurate to every operating point due to fully compensated
flow measurement calculation
DP transmitter signal is squared, that means scaled to ope-
rating volume or mass
DP transmitter signal curve is linear, that means scaled to
differential pressure
ρ
π
ε
β
⋅∆⋅⋅⋅⋅
−
⋅= pd
c
Qm 2
4
²
1
1
4
⋅
=Qm
∆⋅ p2
k
⋅
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