4-20ma Input Calibration
Note: These features can only be accessed by Client Admin users. Your Wildeye devices may be managed by a third party as a service.
Accessing
The Input Calibration page can be accessed from the Input Settings page by clicking on the calibration link in any Input row.
Data recorded from a 4-20mA analogue input can be calibrated using a linear or piecewise calibration by entering the relevant calibration data for the input.
Stabilisation Time
Whenever the Wildeye device logs a value from a 4-20mA Input, the 12 Volt switched power output is activated to power the sensor. The stabilisation time parameter is a time period to keep the power switched on before reading the sensor. Different sensors require different time periods to settle their power draw before providing an accurate reading. Take note of whether the stabilisation time is measured in seconds or milliseconds.
Linear calculation
This calculation method follows the simple linear relation between 4-20mA input value and calibrated output measurement value. See Wikipedia article Linear Equation.
Data required:
A 0mA or 4mA low point and its corresponding output measurement value
A 20mA high point and its corresponding output measurement value
Stabilisation time - time required by sensor attached to input to power up before it can provide an accurate measurement
Unit of Measure - All Inputs require a Unit of Measure.
The linear calculation is stored according to the generic linear function:
f(x) = mx + c
where m and c are constants.
The input is therefore calculated where:
f(x) = calibrated output measurement value
x = input measurement value
m = reciprocal of difference between raw values
c = difference of calibrated values
Piecewise calculation
This calculation method is used when there is a non-linear relationship between raw analogue value and output calibrated value. The piecewise function is written as x,y pairs. The y values are the desired output values for given raw values x. See Wikipedia Piecewise Function.
It is most often used for temperature sensors, where the relationship between sensor resistance and temperature is non-linear, and given by the manufacturer in a table of resistance to temperature values.
If the x value is outside the range of the given piece-wise values then the piece-wise function is extrapolated by using the 1st two or last two x values.
Piecewise calculations are entered as per below syntax:
<raw value 1>,<calibrated value 1>;<raw value 2>,<calibrated value 2>,...
eg:
760,100;781,99;795,98;816,97;837,96;858,95;878,94;899,93;920,92;941,91;969,90;989,89;1017,88;1038,87;1065,86;1093,85;1120,84;1148,83;1182,82
Piecewise direction
IMPORTANT: The input values (1st value in each pair) need to be ordered from low to high throughout the piecewise function