Expansion of the calibration range of a sensor

Interface has added a new technical white paper to our library entitled Extending Transducer Calibration Range by Extrapolation. This detailed technical report addresses the concept of extrapolating partial capacity calibration to full capacity, potentially increasing confidence in the extended range. Below is a brief introduction to the white paper and an explanation of how extrapolation can increase confidence in your data.

Preface

Force and torque transducers must be calibrated in a laboratory to be useful for their intended application. Transducer applications range from relatively simple process measurements to relatively critical calibration of other transducers or devices. Laboratory calibration consists of loading the transducer with known masses and lever arms or using a comparison method in which a load is generated by hydraulic or pneumatic means and the transducer under test is compared to a reference transducer. With both methods, the cost of calibration equipment increases rapidly as capacity increases.

Many calibration laboratories have the means to calibrate forces up to about 10.000 lbf and from torques up to approximately 20.000 lb-in. But the opportunities for higher areas are rare. In fact, there are only a very limited number of laboratories in the United States capable of calibrating forces in excess of 200.000 lbf and torques in excess of 100.000 lb-in.

In the past, some transducer manufacturers have used the practice of calibrating a high capacity transducer at a fraction of the capacity, leaving the owner to hope and guess at the sensitivity at the upper end of the capacity. This gives rise to the concept of extrapolating partial capacity calibration to full capacity, potentially increasing confidence in the extended range.

When full capacity calibration is not an option

Strain gauge transducers are fundamentally linear. That is, the output follows the input in an almost constant ratio. Nonlinearity is routinely measured and is typically in the range of ±0,10%FS or less. This allows values ​​to be interpolated between calibration points with near zero error. However, the same cannot be said for extrapolation, which is actually about estimating values ​​that lie outside the observable range. Conventional wisdom says, and this is logical, that extrapolation is not a valid calibration method.

Extrapolation is similar to prediction, and this idea helps one realize the responsibility that comes with it. But the different methods of extrapolation are not all the same. The aim of this article is to investigate a method that has reasonable validity when economic considerations do not allow full capacity calibration.

Extrapolation methods

There are several methods of extrapolation. The white paper presents three methods: Linear (0 and last point), Linear (last 2 points) and Poly (calibration points). We also discuss the best extrapolation methods by comparing these three methods and showing how the different methods are performed. The aim of the white paper is to explain how to achieve the best results using extrapolation.

The white paper goes into extrapolation in detail and provides our customers and partners with a roadmap to expand the calibration range of transducers. You can request the white paper here.

If you have technical questions about interface transducers and calibration, please contact our application engineers.