Instruments should measure directly what we need to know

6 INSTRUMENTS SHOULD MEASURE DIRECTLY WHAT WE NEED TO KNOW 

Actually the air was not vented. The 1-in. vent line on the air supply was choked by a wasp nest. Whenever possible we should measure directly what we need to know and not some other parameter from which it can be inferred [1]. 

Other incidents in which operators relied on automatic valves and did not back them up with hand valves are described in Sections 17.3 (b) and 17.5 (c). 

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