Applying Taylor's series approximation for a small change in the value of

where the series has been truncated after the first term. Setting and , we get

Here, the quantity is the gradient of the calibration curve evaluated at the operating point . The error due to truncating the Taylor series is usually small if . The truncation error in calculating can be reduced by working with an extended Taylor's series expansion

wherein the higher order term has been included. This approach is useful provided the higher order derivative can be determined accurately. In practice, the calibration curve is experimentally constructed and these higher order derivatives can be computed only with substantial error.