Instrument are calibrated to make them accurate within the manufacturer's specification. Accuracy is an essential factor in instrument performance. In instrument calibration "accuracy" has a special meaning.

• The closeness of the agreement between the result of a measurement and the the true value of the measurement. 

A precise instrument will produce the same representation or output every time it receives an identical input.

Repeatability of measurement

The closeness of the agreement between the result of successive measurements of the same measurement carried out subjected to the following:

• Same method of measurement
• Same observer 
• Same measuring instrument
• Same location 
• Same condition of use 
• Repetition over a short period of time 

Reproducibility of measurement

The closeness of agreement between the result of measurements of the same measured, where the measurements are carried out under changed conditions such as:

• Principle or method of measurement
• Observer 
• Measuring instrument 
• Location 
• Condition of use 
• Time. 

Error of measurement

The result of measurement minus the of the measurand. 

Correction

The value which, added algebraically to the uncorrected result of a measurement, compensated for an assumed systematic error. 

Uncertainty of measurement

At estimate characterising the range of values within the true value of measurand lies.

Traceability

The property of a result of a measurement whereby it can be related to appropriate standards, generally international or national standards. 

All measuring equipment should be traceable to the National standards which are traceable to international standards.