There are acceptable modifications to most guidelines with the exception of SAFETY. This applies to the standards that are used for accurate measurement of anything. Scales for example must be verified and calibrated on a repeatable basis for the food industry. Gas pumps are verified by local government agencies to protect the consumer. Gas monitors must be verified and calibrated relative to the environmental conditions to which they are exposed. All gas detection manufacturers specify a calibration interval as well as situations such as high gas concentration exposure, where the instrument should be verified and calibrated if necessary.