Why do we calibrate measuring instruments?

Dear Taurai

for any electronic device -including electronic measuring instruments- a deviation occurs on the characteristics curve of the device output due to aging of electronics. So, we do that calibration to ensure that there is no deviation in our curve -or the deviation is acceptable-, and incase a deviation noticed we may re-adjust the curves again -if it's applicable for the device- by assignning a new zero and a new span points (for a linear characteristic curves)

Also the same occurs for non-electronic measuring devices. And the cause is usually the same: aging of components. But instead of an aging diode or an aging capacitor, it will be an aging bourdan tube or an aging spring.

I hope my answer is helpful for you

Taurai , we calibrate the measuring instruments in order to make sure we get the most accurate readings or actuate accurately as possible which in turn would lead to the increase in the efficiency of the process . why do we need to do recalibration is due to many reasons like the process errors or the field errors , like the existance of water in the instrument air , the fluctuation of electric current , leaks and the quality of the manufacture of the instrument it self and its aging , i found some honeywell DPT that has been working for10 years in my power plant without any one calibrating them and they were perfectly fine !hope this helps :)