Calibration of Equipment

You must calibrate, maintain, and use the equipment in accordance with the manufacturer’s recommendations.
If the meter is not properly calibrated, you cannot rely upon its results. Deaths can occur if the instrument underestimates the atmospheric conditions. Most confined-space instrument manufacturers now offer
“docking” stations that can automatically calibrate instruments and print a record of calibration. The stations also recharge and
store the instruments.

How to test the confined space area.

Always test the entrance of the area first. 

Make sure that as much of the space as possible is tested, including the
bottom, mid-level, top, and comers, so that you don’t miss layers or pockets of bad air.  There are some gases that are lighter or heavier
than air. Lighter gases, such as methane, will accumulate near the top, while gases heavier than air will sink to floor level. Gases that are the same weight as air, such as carbon monoxide, will be found
throughout a confined space.
Check for oxygen content, combustible or explosive gases and vapours, and toxic gases and vapours in that order if you use more than one meter.
First, check tor oxygen content. Checking oxygen first is important because you may need adequate oxygen to get a valid result from other tests.
If the oxygen level is adequate, test for explosive atmospheres. Several different calibration gases are available. Methane is used most frequently since it is a common gas found in many places. But you can get
devices calibrated for propane, hexane, heptane, or almost any other
combustible gas. These devices give a result expressed as a percentage of the lower explosive limit (LEL) for the calibration gas used.
The next thing to check for is the presence of toxic gases and vapour using a calibrated Instrument.
If you’re using a multi-gas monitor capable of measuring oxygen, combustibles, and toxic gases simultaneously then the order of testing is not as critical.
All three types of dangerous atmospheres must be evaluated before entry. Users of gas detectors must be competent workers. They must also receive training in the operation, calibration, and maintenance of the devices. Most manufacturers can provide necessary training.
Always test for the three dangerous atmospheres:
 too much or too little oxygen,
 combustible or explosive gases or vapours, and,
 toxic gases or vapours.

You may need to monitor the atmosphere continuously. Continuous monitoring in a confined space is required while hot work is being performed in a potentially flammable or explosive atmosphere or where the flammable or explosive atmosphere has been rendered inert by adding an inert gas. It should also be considered when conditions in the confined space change rapidly.
If continuous monitoring is performed, then test results must be recorded at regular intervals.
Most new confined space instruments have data logging capabilities. Data logging is useful for compliance and record-keeping purposes. If an accident or unusual event happens data-logging may be useful for demonstrating due diligence and diagnosing a problem, to prevent reoccurrence.