Omega FC-21 - Manual (Page 42)

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FC-21 Flow Computer
7. Principle Of Operation
7.1 General:
The FC-21 Flow Computer uses several internal calculations to compute the
compensated flow based on specific data input. Several computations are performed
to arrive at the uncompensated flow, temperature, density and viscosity. This information
is then used to compute the Corrected Volume Flow or Mass Flow.
7.2 Orifice Flowmeter Considerations:
Head class flowmeters are supplied by the manufacturers with a 4-20 mA output span
which is already in flow units. The FC-21 permits the user to enter this flowmeter
information directly. However, closely associated with this information is the density
that was assumed during flowmeter calibration. This information must also be input if
the user is to obtain maximum accuracy.
It is assumed that the user has the printout from a standardized orifice sizing program
for the particular device he will be using. Such standardized printouts list all the
necessary information which the user will then be prompted for.
Several specialized flow equations are listed that are not intended for the standard
unit but to be offered to appropriate OEMs or as special order items. These are
designated by a “†”.
Note concerning Fluid Information
The user will be prompted for Fluid Information during the setup of the instrument.
The Factory will be preparing application information for several common fluid types.
7.3 Flow Equations:
Input Flow Computation:
Linear or External SQRT
Input Flow = [% input span * (flow FS - flow low scale)]+ flow low scale
Orifice
Input Flow = [(√% input span) * ( flow FS - flow low scale)] + flow low scale
Input Flow Computation:
General Case
Tf = [% input span * (temp FS - Temp low scale)] + temp low scale
RTD Case
Tf = f(measured input resistance)
Input Density Computation:
Temperature Transmitter
density = reference density * (1 - Therm.Exp.Coef. * (Tf-Tref)) 2
Density Transmitter
density = [% input span * (density FS - density low scale)] + density low scale
General
Operation
Orifice Flowmeter
Considerations
Flow Equations