One of the first descriptions of instrument performance characteristics is provided by a transfer curve. A transfer curve shows the output from a sensor as a function of the measurand. The slope of this curve is the static sensitivity, or the change in sensor output based on the change in input. If the response is linear, the transfer curve will be a straight line. In this example, the x-axis shows the measurand and the y-axis indicates the output of the sensor.
A linear relationship between the output and the measurand is given by:
Output = Intercept + (Static Sensitivity x Measurand)
A transfer curve often is determined by calibration and then used inversely to determine an estimate of the measurand based on the output of the sensor. Deviations from a linear transfer equation can be caused by a non-linear response from the sensor, in which case the relationship between input and output might be a transfer curve represented by a higher degree (>1) polynomial, or due to systematic measurement error caused by drift, lag or hysteresis, or failure to calibrate.