Be safe, contact the manufacturer of your instruments and set up your program according to their guidelines. For your protection, and the protection of your employer, always document calibrations and preserve these records in a log. The calibration log should be maintained, current, and include all the gas monitors used at your facility. These records should be on file for no less than 1 year although retention for 5 years is advised. Monitoring the gases in one area of a confined space is a dangerous oversight.
Some gases are heavier than air, some are lighter.
The molecular weight of a gas determines where it will accumulate within a confined space. Air has a molecular weight of approximately 28.8. Common gases like hydrogen sulfide and methane will stratify naturally at different levels. Hydrogen sulfide, for example, has a molecular weight of 34 therefore we can expect it to accumulate closer to the floor. Methane will be present closer to the top of the space due to its molecular weight of 16.