Rockwell Automation 1746-NI8 SLC 500 Analog Input Modules User Manual - Manual (Page 49)

5–8 Channel Configuration, Data, and Status
Publication 1746Ć6.8 - April 1997
Scaling Examples
The following scaling examples show how to convert the channel
data word from the configured data type to “real” units. “Real” units
are the values being measured such as temperature and pressure. To
perform the scaling, you must know the defined voltage or current
range for the channel’s input type. The lowest possible value for an
input type is S
LOW
, and the highest possible value is S
HIGH
. Refer to
the Channel Data Word Value tables on pages 5–5 and 5–6 for the
data channel word ranges.
Engineering Units to “Real” Units
Situation #1: A transducer is being used to measure temperature.
The 4–20 mA signal is proportional to 100–500
°
C (212–932
°
F).
The channel data word is in engineering units format.
The channel data = 5500.
Find: “Real” units equivalent in
°
F.
Equation:
Real Units Equivalent + channel data value ǒscaling factorǓ
ǒ
S
HIGH
* S
LOW
input range
Ǔ
where: S
LOW
= 212
°
F
S
HIGH
= 932
°
F
channel data word = 5500
scaling for engineering units = 1
µ
A per step
Solution:
Real Units Equivalent + 5500 (1ĂmA)
ǒ
932Ă°F * 212Ă°F
20 mA * 4 mA
Ǔ
+ 247.5Ă°F
Situation #2: A transducer is being used to measure temperature.
The 0–10V dc signal is proportional to 0–10
°
C.
The channel data word is in engineering units format.
The channel data = 5000.
Find: “Real” units equivalent in
°
C.
Equation:
Real Units Equivalent + channel data value ǒscaling factorǓ
ǒ
S
HIGH
* S
LOW
input range
Ǔ
where: S
LOW
= 0
°
C
S
HIGH
= 10
°
C
channel data word = 5000
scaling for engineering units = 1 mV per step
Solution:
Real Units Equivalent + 5000 (1ĂmV)
ǒ
10Ă°C * 0Ă°C
10V dc * 0V dc
Ǔ
+ 5.000Ă°C
" TIP
If you are measuring a “real” voltage or current, then selecting engineering units allows the
module to report “real” values without further scaling. For example, a 4.683V dc signal
would produce a channel data word of 4683.