LINEARIZER (Cont’d)
The computer program in Appendix “C” outputs the display and process (input)
scaling points (the location of each linear segment) as a percentage of the full scale
input (eg. 4-20 mA) and full scale display (eg. 0-1000 gpm). To obtain the actual
input and display scaling points, multiply the respective percent of full scale values
by the respective full scale range for the input and the display. “Pro 2” is then used
to enter these values into the IM unit. Certain linear sections of a given curve may
have a slope which exceeds the measuring resolution of the instrument. The effect
will be an erratic display in that part of the curve, if not corrected (generally, if
slope 2counts/uA). Correcting for this condition consists of three steps: increase
digital filtering to level 1 or level 2, decrease display resolution to 2 or 5 or add
dummy right hand zeros by programming 10 or 100 for “round”.
The following display and process percentage tables were derived from the
computer program in Appendix “C” (Reference Appendix C section for more
details). The display and process percentage tables listed below can be used for
ANY square law (square root extraction) device.
The results of the computer program are as follows:
Number of linear segments = 9
Curve fitting error (%) = .35
Display Values Process Values
(% of Range) (% of Range)
0.00 0.00
6.32 0.20
10.43 1.00
18.04 3.10
27.58 7.40
39.09 15.00
52.61 27.30
68.18 46.00
85.74 72.90
100.00 100.00
As an example, to linearize a 4-20 mA flow transmitter, with the following
flow equation:
Flow(gpm) = 44.7
D P
where: P = 0-500 PSI
Flow = 0-1000 gpm
Now, using the above percentages, the actual process (input) signal and
display scaling points must be calculated. To do this for the display values, it is
simply the display percentage times the full scale display range. i.e. 18.04% x
1000 gpm = 180.04; 68.18% x 1000 = 681.8 etc.
To calculate the process (input) values, multiply the process percentage times
the full scale process differential, then add in the process offset value. i.e. For the
process range of 4-20 mA, [3.10% x (20 mA-4mA)]+4mA=4.496 mA;
[27.30% x (20 mA-4mA)]+4mA=8.368 mA. If the process range were 10-50
mA, then the results would be as follows: [3.10% x (50 mA - 10 mA)] + 10 mA =
11.24 mA; [27.30% x (50 mA - 10 mA)] + 10 mA = 20.92 mA.
The tables below show the scaling points calculated for the 4-20 mA process
(input) along with the corresponding display values.
DISPLAY PROCESS (INPUT)
(%FS,
0-1000 gpm*) (%FS 4-20 mA*)
0.00 0 0.00 4
6.32 63.2 0.20 4.032
10.43 104.30 1.00 4.16
18.04 180.40 3.10 4.496
27.58 275.80 7.40 5.184
39.09 390.90 15.00 6.400
52.61 526.10 27.30 8.368
68.18 681.80 46.00 11.360
85.74 857.40 72.90 15.664
100.00 1000.00 100.00 20.000
* The values shown in this column are user calculated.
Note: These full scale percentage values for process and display can be applied
to ANY
square law device.
The example below shows a 10-50 mA square law transducer using the same
percent of full scale numbers with nine segments as the 4-20 mA device.
Flow(gpm) = 42.4
D P
where: P = 0-1250 PSI
Flow = 0-1500 gpm
-26-
To Next Page
Loading...
Need help?
General
