Difference bw precision instruments and non precision instruments?

precision instrument: caliper, micrometer non-precision measuring instrument: your hand, a piece of string

Precision means accuracy. A precision instrument is a top quality one that is super-accurate. You can get an instrument that will just do the job, or you can spend extra money and get a precision...

By calibration in comparison to a standard one.

Make sure you are only measuring what you are after. Not noise and local disterbances. Proper system grounding is needed. Make sure the device you are using has been calibrated periodically properly...

Sturdiness, standardization, and legibility.