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172-082010_400 I Series Software Page 7 of 35
different set of coefficients. Any gas dependent values will have a different value stored in
each gas record.
This linearized shunt value is multiplied by the following factors: a shunt factor (ShF, “s46”,
nominal nitrogen equivalent range of the laminar flow element), a time correction factor for
the flow unit (TcF, “g5”), a volume correction factor for the flow unit (VcF, “g8”), a gas
dependent calibration span adjust factor (SpF, “g14”) and if the flow unit is volumetric (“g15”)
then a reference temperature correction factor (Tc, calculated from “g19”). The end result is
the flow rate
in
engineering
units that is
reported when the “f” command is received.
Six different multiplication steps could be performed on each data reading or alternatively an
internal factor could be maintained that is the product of the six factors. This internal factor
could be updated whenever one of the other factors is changed.
The Tc value is calculated as follows:
1.3. Analog Output
The resultant flow value (“f”) is divided by the full scale flow value (FSF,“g2”). This dividend
is multiplied by the DAC full scale code in counts (DFS, “s 52”). The resultant counts value
then has the DAC zero code (DA0, “s 51”) added to it. This final result (DAC) can be sent to the
analog output Digital to Analog converter.
1.4. Analog Set point Input
The flow reading starts with acquiring a value (AD4) from the set point A/D. This value will be
in counts (16 bits signed, -32768 – 32767 corresponds to -2.048 volts – 2.047 volts). The input
voltage that corresponds to a zero flow command (4Z, available via s69 command) will be
subtracted from this. The result is multiplied by the full-scale factor (FS4) (see “s24”
command) to generate a full scale input range equal to the full scale analog output value.
If the original analog input signal was a 4-20 mA or 1-5 VDC signal the zero flow command value
is not numerically equal to 0.00 in these cases the value reported back for the s26 command is
the Sp value plus 1.00 or 4.00 dependent on the value stored in the Product Configuration word
s64.
()
SpFSZAD
=
−
444
()()
(
)
(
)
(
)
(
)
[]
fTcSpFVcFTcFShFSHL
=
()
[
]
(
)
[
]
(
)
SHLSLFSLESLD =++
42
Tc
T
=
+
15.273
15.273
DACDADFS
FSF
f
=+ 0