The maintenance and use of Portable Gas Monitors should be part of your plant operation and safety procedures. OSHA Safety and Health Information Bulletin SHIB 09-30-2013, Verification of Calibration for Direct-Reading Portable Gas Monitors provides guidance on the need for regular testing and calibration.
The International Safety Equipment Association ISEA Statement On Validation Of Operation For Direct Reading Portable Gas Monitors, revised March 5, 2010, defines bump test, calibration check, and full calibration.
Both of these documents recommend testing with gas before each day’s use and a full calibration in accordance with the manufacturer’s instructions. Beware that the new ISEA statement no longer recommends calibration frequency up to 30 days under certain circumstances as mentioned in an earlier ISEA guideline. See ISHN article from 1/5/2011.
I recommend that you have a policy for the maintenance and use of Portable Gas Monitors. Procedures should require bump tests or calibration checks before use and regular full calibration with certified calibration gas following manufacturer’s instructions. Calibration gas should be traceable to NIST and should not be used after the expiration date.
Remember that batteries, whether rechargeable or not, have a limit to their life. Also, Oxygen sensors typically last 1 to 2 years, depending on the manufacturer, and can fail suddenly at the end of their life.