Campbell Scientific AL200 ALERT2 Encoder, Modulator, and Sensor Interface - 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