What Really Are the Zero and Span Calibration Controls?

Fair Enough Question.  

The answer can be expressed in a couple of ways depending on your background and interest.

Moisture, oil/fat and coat weight etc., are all components measured with established Near Infrared (NIR) Technology. Moisture is measured at 1.94µ or 1.42µ. Organic coatings are measured at 2.34µ and oils/fats at 1.72µ. These are well established applications in theory, practice and documented literature.

The accuracy of an NIR sensor is dependent upon calibration samples across a realistic dynamic range. Normally 3 to 5 samples throughout the dynamic range and including the set point target are the best approach for calibration. Near Infrared absorbance and constituent content (lab value) enjoy a linear relationship. The sensor must be most accurate at the set point.

The zero and span calibration controls are used to make the Near Infrared sensor display values match the lab values within the limitations of laboratory accuracy and sample handling. Collecting lab vs. NIR sensor values allows for a linear regression to be performed that will yield a correlation co-efficient, standard error, slope and y-intercept.

Our “Zero” is an offset adjustment, meaning that if the measured value is 2% up or down, that it just needs an offset or zero adjustment around the set point. The “Span” is a sensitivity adjustment, meaning that for a given change in absorbance, there is a given response in the digital measurement. The fully automatic calibration sequence adjusts the zero and span for the operator based on these calculations to provide the correct Zero and Span for the analyzer to accurately measure and display the NIR measurement.

Calibration Graph

The objective of gauge calibration is to obtain a straight-line graphical relationship between the sensor’s reading and the true moisture value of a series of samples as shown in Figure 1 below.

Adjusting the zero and span is easy. Simply enter the calibration parameters of lab vs. MCT sensor and the correlation co-efficient, standard error and new zero and span are calculated.