Basics of A Five Point Calibration

Owing to the physical limitations of measuring devices and the system under study, every practical measurement will always have some errors.Several types of errors occur in a measurement system. These include;

Static Errors:

They are caused by limitations of the measuring device or the physical laws governing its behaviour.

Dynamic Errors:

They are caused by the instrument not responding fast enough to follow the changes in measured variable. A practical example can be seen in a situation where the room thermometer does not show the correct temperature until several minutes after the temperature has reached a steady value.

Random Errors:

These may be due to causes which can not be readily established; could also be caused by random variations in the system under study.

Basic Steps in Instrument Calibration

Calibration is a process where by we ascertain the output of an instrument after being used over a definite period, by measuring and comparing against a standard reference and to carry out the necessary adjustments required to confirm whether its present accuracy conforms to that specified by its manufacturer.
There are three basic steps involved in the calibration of an instrument. These include:

(a) To collect measured values (Output values) of standard values(Input values) provided by a standard input reference.
(b) To complete verification/calibration tables for upscale and down scale values( 5 or 3 points)
(c) To calculate the error on the output signal and to compare the result with the expected accuracy.

If ERROR = EXPECTED ACCURACY, no adjustment is required. In other words we are verifying the accuracy of the instrument.

If ERROR is greater than the EXPECTED ACCURACY, we carry out necessary adjustments to reduce this error within the expected accuracy (calibration process)

Five Point Calibration Basics

In a five point calibration for an instrument, the output is measured at 0%, 25%, 50%, 75% & 100% of the calibration range of the instrument. In the five point calibration process, output readings are taken for upscale and down scale values of the calibration range to determine the repeatability and hysteresis of the instrument.
In a five point calibration, LRV = 0% Input; URV = 100%

Steps Involved in a Five Point Calibration

In a five point calibration exercise, three basic steps are involved. They include:
(a) Zero Adjustments (at LRV)
(b) Span Adjustments(at URV)
(c) Linearity Adjustments – at 25%, 50% & 75%

These basic steps above are illustrated in the flowchart for a five point calibration below:

You should familiarize yourself with other basic concepts of instrument calibration from Basic Principles of Instrument Calibration