Accreditation of ETL lab

I assume you are referring to setting up an Environmental Testing Laboratories and you asking whether it's necessary to have equipment calibrated by a 17025 calibration laboratory ? It all depends on the performance requirement you need from that equipment and if you need to have a calculated measurement of uncertainty provided by the calibration laboratory. If the equipment has a big impact on the performance of your method, then it is recommended to have external calibration regularly followed by intermediate verification that you would perform in your own laboratory using certified standard.

For more information, have a look at have a look at my response to a similar question at https://community.advisera.com/topic/calibration-of-laboratory-equipment/