If the hazard assessment determines that there is an atmospheric hazard in the confined space, you must perform atmospheric testing.
a) The employer must appoint a competent worker to perform adequate
tests safely before and during the time a worker is in a confined space to
ensure that acceptable atmospheric levels are maintained. The
competent worker who will perform the test must receive training in the
operation, calibration, and maintenance of the instruments. Most
manufacturers can provide necessary training.
b) If the confined space has been left unoccupied and unattended, you must perform the testing again.
c) The competent worker performing the tests must use properly calibrated and maintained instruments appropriate for the hazards in the confined space.
d) Results of every sample of a test must be recorded on the entry permit. If continuous monitoring is performed, test results must be recorded at
adequate intervals.

Gas detection Instruments
Gas detection instruments can take many forms “personal” or “area” single-gas or multiple-gas detectors, detectors with dedicated sensors or those with interchangeable sensors.

If a monitor is worn by the worker, it is referred to as “personal monitoring”. Personal monitoring gives information about the concentration of hazardous substances surrounding the worker. It is particularly useful when the worker is moving from place to place within the confined space.

Area sampling is done before entry or re-entry. As much of the confined space area as possible should be tested, including the bottom, mid-level, top, and corners.
Single-gas detectors measure only one gas while multigas monitors are
available with several toxic sensor options and have the flexibility of measuring many gases simultaneously. Most multi-gas monitors include an oxygen sensor, a flammable/combustible gas sensor, and one or two sensors for detecting specific toxic gases. Newer single and multi-gas instruments offer the flexibility of interchangeable sensors. You can change the sensors to suit the application at hand. For example, a single-gas detector used to check hydrogen sulphide levels can be used to monitor carbon monoxide concentrations only after you change the sensor.