Appendix F.Use Of The Regression Zero When Using The Linear Gauge Factor

It is normal for load cells, having an annular design and for solid load cells that do not have ‘button heads’ or spherical seated bearing plates, to be susceptible to irregular load distributions at low loads. This is because there is a 'bedding in' process that takes place while the surfaces at both ends of the load cell conform to the surfaces they bear against causing the load cell to deform in an unpredictable way giving rise to strange strain patterns and faulty readings at low loads.

This irregularity of load disappears once the load cell surfaces have bedded in and from that point on the load cell behaves in the a more linear fashion such that there is a constant relationship between the applied load and the observed change in readout as quantified by the linear gauge factor shown on the calibration sheet.

Because of this the linear gauge factor shown on the calibration sheet has been calculated after excluding the often anomalous zero reading from the data points. And this gauge factor best describes the performance of the load cell at moderate to higher loads

This linear gauge factor describes the slope of the best fit line drawn through the calibration data points and the reading where the line intersects the zero load point on the load axis is called the 'Regression Zero' shown on the calibration sheet.

It is important when using the linear gauge factor to calculate loads that the value of R₀ in the linear equation be equal to the regression zero.

For greater accuracy, a second order polynomial can be used to map the data points. In this case, the regression zero is replaced by the factor C shown on the calibration sheet.

It may be, for a variety of reasons (for example if the load cell is used repeatedly on a number of jobs), that the no load zero reading might change significantly. Again, for greater accuracy, the value of the Regression Zero can be adjusted by an amount equal to the observed change in the no load zero from that shown on the calibration sheet. Similarly, the C factor of the polynomial can be adjusted by the amount of the zero load change multiplied by the linear gauge factor to convert it into the corresponding load change.