Calibration in measurement is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Calibration is the task associated with establishing an accurate correspondence between any instrument’s input and it’s output signal.
We need to calibrate different types of instruments efficiently, there are Linear instruments, Nonlinear instruments, Discrete instruments.
Linear instrument calibration is so-called as the Zero-span method. Step to do the procedure:
Apply the low range value (LRV) to the instrument and note the reading.
Rotate the Zero adjustment if there is any irregularity in the reading, adjust until the instrument register accurately at this point.
Now after setting the LRV, apply the upper range value (URV) to the instrument and note the reading.
Rotate the span adjustment until the instrument shows the correct URV reading if there is any malfunction in the readings.
Thus we can say our instrument is calibrated and if there is a need for more accuracy in calibration, add for more points between LRV and URV. Which is called free-point calibration. Where an instrument is checked at more points such as in 25%, 50%, 75%, between 0% and 100%.
There is Up-down, where calibration is done at five points in increasing order and decreasing order.
Zero - Span adjustment can’t be used in non -linear instruments because the characteristics of the instrument will be in square root chara or any other complex chara, so we cannot predict the accuracy by calibrating by only calibrate five-points in the range.
Each specific instruments have it’s own calibration procedure, calibration technique will be explained in the manufacturer’s manual. As the calibration of nonlinear instruments is not simple as for linear calibration reset option will be allowed for instruments. You can simply press the reset button to go back to the original position if you are confused or stuck.
The discrete instrument is referred to as have to state operation True or False otherwise On/off function. A discrete instrument makes the condition true or false if the process reached the setpoint.
The most discrete instrument has one calibration adjustment switch, the set-point and there are some other instruments having two adjustment switch one the set-point and deadband adjustment. The porpuse of deadband adjustment is to provide an adjustable buffer that must be transfered before the switch changes the state. A set-point switch switch is used to increase or decrease the set point