Subject: Use of Cut-off Key F6 Introduction With aluminum, and especially fiber wrapped cylinders, the cylinders are slow to expand to the fully stretched condition. If the pump has stopped (because test pressure has been reached), any further increase in size causes a reduction in pressure. If the pressure drops below the set test pressure, the machine of course aborts the test (per DOT requirements as test pressure "must be maintained for the duration of the test"). Also the machine checks for pressure drop at the start of the test. Basically this is a leak check, as falling pressure at the start of the test, if excessive, is likely to result in a failure due to pressure drop, so the machine also aborts the test at this point rather than waste further time.
By using the F6 key, you can enter a value that will cause the pump to stop at a pressure higher (or lower if a negative value if used!) that the test pressure. See the chart below.
1. In the chart, chosen test pressure is 6,000 psi. Normally the pump would stop at this point. This is point 1. 2. If cutoff is set to 50 psi, the pump will now actually stop at 6,050 psi, although test pressure is still 6,000 psi. This is equivalent to manually pumping a little above test pressure. This is the section of the curve at 2.
3. If the pressure rise during 2 is controlled with the pump slow valve to either be VERY slow, or manually so that it does not rise above the cutoff pressure (6,050) then when the pressure finally reaches the cut-off pressure (6,050) the cylinder will have virtually stopped expanding.
4. If it does expand very slightly, or there is, perhaps a small system leak, even if the pressure drops a little, the test will not abort unless it drops below 6,000 psi.
Note: that if the rate of drop after the cut-off pressure is too high, the system will also abort the test, hence the need to control this pressure carefully.
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