We manufacture similar equipment. I would recommend that most of the testing be performed in an automatic shut-off mode.
There are potential issues with both methods, automatic shut-off and fixed time tests. In an automatic shut-off mode, the cleanliness tester will continue the testing procedure until the level of detected contamination stops rising. In a fixed time mode, the tester will shut off after a predetermined time, even if the machine was still detecting contaminants.
The only �danger� to an automatic shut-off mode is if contamination is trapped and requires extra time to �dissolve� into the test solution. In this case, the cleanliness tester may prematurely discontinue the test before any contamination is detected. This is not a common phenomenon.
The danger of a fixed time test is when the time runs out before all of the contamination is detected. This phenomenon occurs more often than the preceding. If one must operate the cleanliness tester on a fixed time mode, then one must choose to review the real-time data produced by the equipment. Most cleanliness testers can be instructed to produce a real-time graph that charts the detected cleanliness. Inspect the graph to ensure that the initial rise in contamination �flatlined� or leveled off for at least 60 seconds prior to the end of the test. In other words, there should be no rise in contamination detected in the last 60 seconds of the test. If the contamination was rising during the final 60 seconds, chances are good that contamination, in addition to the detected amount, was present on the board.