Argon gas accumulated in the bottom of a large tank killed two workers assigned to clean the tank. The atmosphere in the tank had been tested before they entered it, but the testing was done incorrectly and did not detect the argon. One of the workers died after he entered the tank to save the other. Atmospheric monitoring of confined spaces is an exacting procedure that must be done just right and must be repeated if necessary to account for changing conditions in the confined space. Monitoring equipment itself must be tested and properly maintained.