Appendix h. calculating multipliers and offsets, Appendix h – Campbell Scientific AL200 ALERT2 Encoder, Modulator, and Sensor Interface User Manual
Page 41
Appendix H. Calculating Multipliers and
Offsets
Unlike ALERT which could only represent measurement values between 0 and
2047, ALERT2 has the ability to represent a much larger range of floating
point numbers. It is no longer necessary to scale a reading to a unit-less
number less than 2047. Instead, with ALERT2, you can transmit values in
engineering units with a high degree of resolution. The simplified example
below demonstrates how to calculate the multiplier and offset that are needed
to convert a pressure transducer’s output, 0-5 volts, to feet of water.
For this example we will use a pressure transducer that has a 0 to 30 psi range
and a 0 to 5 volts output. From the manufacturer supplied calibration report it
is known that the sensor outputs 0.0123 volts at 0 psi and 4.987 volts at 30 psi
(12.3 and 4987 millivolts respectively).
The relationship between the sensor’s output in millivolts and pressure is
defined by the equation of a line (assuming linearity), Y = mX + b, where Y is
pressure, m is the calculated multiplier, X is the sensor’s output in millivolts,
and b is the calculated offset.
Y = mX + b
OR
Pressure = Multiplier • Sensor Output + Offset
The multiplier is determined by dividing the sensor’s measurement ranges by
the sensor’s range of output. For this example the multiplier will be:
1
X
2
X
1
Y
2
Y
Multiplier
−
−
=
OR
mV
per
psi
0060
.
0
)
7
.
4974
(
30
)
3
.
12
4987
(
0
30
Multiplier
=
=
−
−
=
The offset is determined by entering the multiplier into the pressure equation
and solving for the offset using a known pressure and sensor output.
Offset = Pressure – (Multiplier • Sensor Output)
OR
(
)
mV
per
psi
074
.
0
3
.
12
•
0060
.
0
0
Offset
−
=
−
=
H-1