Posted By Gavin Francis When working for BA we purchased a large numebr of detectors and we used the manufacturer to deliver training to key personnel who would then be responsible for changing the filters on the portable gas detectors. The reasons for this approach is that the OEM is best placed to know the correct procedure and parts reference details so as to minimise problems. Also, the issue of recalibration should be addressed and, dependant upon use, it could be carried out by the company who calibrate your detectors.