Why do we need a separate set of temperature sensors for measurement when there are sensors built within the chillers?

Most chillers in the market are factory-tested to rigorous standards in accordance to ANSI/AHRI Standard 550/590 which specifies an allowable tolerance of 5% for capacity, efficiency and heat balance. Separate meters are used during the factory test rather than the built-in sensors in order to achieve the required uncertainty level. Other measurement standards such as ASHRAE guideline 14 and ASHRAE Guideline 22 have also stated the use of independent sensors rather than the chiller’s built-in sensors.