First, air monitoring should be conducted from outside of the confined space.  A pump may be necessary to draw air from the space.  All levels of the confined space should be checked - the top, mid-section, and bottom.  Gases can be lighter than air, heavier than air, or similar to air.  Testing all levels of a space ensures that all gases are detected.

Continuous air monitoring is a best practice and should be used whenever possible.  It is required where ventilation is the control method for a potentially hazardous atmosphere (i.e. initial air monitoring indicated conditions unacceptable for entry).