Sutron 8310 - Compute Sensor Slope and Offset

Sutron Corporation 8310 & 7310 Users Manual 8800-1125Rev. 2.7 (BETA) 4/16/2014 pg. 176
Compute the Slope and Offset for a Sensor
Most sensors produce an analog output or frequency. 8310 measurements report the values in
the units of volts or Hz unless some action is taken to scale the value into its final units. The
slope and offset are used to perform this scaling. As long as your sensor is a linear sensor, slope
and offset can be used to scale it. (If you have a non-linear sensor and want the 8310 to do the
computation, you will need to use the BASIC capability of the 8310). A linear sensor is one that
has output that can be graphed as a straight line. Examples of linear sensors are:
tipping bucket
shaft encoder measuring water level
some temperature sensors
some pressure sensors
wind speed/wind direction.
Examples of non-linear sensors are:
most thermistors,
shaft encoder measuring gate opening
stage/discharge computations.
Whenever the 8310 takes a sample or measurement and has the processing set to Slope-Offset,
it applies the equation:
value = raw_value * slope + offset.
With slope = 1 and offset = 0 the final value is equal to the raw_value.
To determine the slope and offset for your sensor, you will need to know the sensor output for
two different points. This information is typically available on the sensor's data or calibration
sheet. These two points are then used to compute the slope and offset. The two points are
represented as (x1, y1) and (x2, y2) where x is the voltage or raw reading and y is the
corresponding value.
The slope and offset are then computed from these two points as follows:
slope = (y2-y1)/(x2-x1)
offset = y1 - x1*slope,
as long as the "x" values are in the same units as the raw_value.
Consider a wind direction sensor. The sensor is a potentiometer excited by 5 volts. The output at
0 degrees is 0 volts and the output at 360 degrees is 5 volts. (x1, y1) = (0, 0) and (x2, y2) = (5,
360). The slope would be (360-0)(/5-0) = 72 and the offset would be 0 - 0 * 72 = 0.
Some further examples will help demonstrate the use of these equations. Remember that the x
values are the raw values (volts, hz, counts etc.) and that the y values are the final values.