Campbell-scientific AL200 ALERT2 Encoder, Modulator, and Sensor Interf Manuel d'utilisateur télécharger pdf (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

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:

1X2

Multiplier

−

mVper psi 0060.0

)7.4974(

)3.

124987(

030

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)

( )

mVper psi 074.03.12•0060.00Offset −=−=

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Campbell-scientific AL200 ALERT2 Encoder, Modulator, and Sensor Interf Manuel d'utilisateur Page 41