Two-Point Multiplier and Offset Calibration

Two-point multiplier and offset calibration uses a linear fit technique against two different known value conditions of the sensor’s measurement. The sensor is placed into the first condition, and the known value for that condition is provided to the datalogger program. One or more measurements of that first condition are stored, and then the datalogger informs the user that the second known condition should now be applied to the sensor. The second condition is applied and its known value is then provided to the datalogger. The datalogger then measures the second condition. When the measurement of the second point condition is complete, a linear fit of the two points is calculated. The results are a slope value (m value, or multiplier), and a y-intercept (b value or offset). Thus the simple form y=mx+b is a representation of the linear fit, where m is the new multiplier value used and b is the new offset used.

A simple example of a two-point multiplier and offset calibration would be placing two objects of known weight (such as 5 lbs. and 15 lbs.) on a conventional scale at two different times. With the first known weight on the scale (5 lbs), this is the first condition in which the scale should give a known reading for its output. The calibration is triggered, the datalogger is informed of the value of the known weight, and the measurement is read. The datalogger then notifies the user that it is ready to measure the second point. The second known weight is placed on the scale (15 lbs), and this is the second condition in which the scale should give a known reading for its output. The second point of calibration is triggered, the datalogger is informed of the value of the second known weight, and the measurement is read. At this point the new multiplier and offset are calculated by the datalogger and the variables are adjusted accordingly to ensure that the scaling gives a properly matched reading for those two conditions. For future measurements (unknowns), a linear response will be used based on the line defined by those two points.

To perform a two-point multiplier and offset calibration, use an argument of 2 (the number two) for the calibration type in the FieldCal() instruction of your CRBasic program. The Calibration Wizard can be used to calculate and apply the two different known conditions while the program is running in the datalogger.