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A pump is connected to a glass tube to evacuate it. The reading shows 0.4 mbar when the pump is on because of a vacuum. As soon as the pump is turned off, the pressure in the tube becomes equal to atmospheric pressure.

Fred
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Rahul
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    I'm not 100% sure, but I suspect that there is a safety valve in the pump which releases back pressure when the pump is not running. Vacuum pumps aren't designed to hold a static pressure gradient when they aren't running, and I suspect trying to do so for long periods of time can damage the pump. – Chris Mueller Sep 15 '15 at 12:13
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    If Chris is correct, all you need to do to fix the problem is to include a valve between the tube and the pump, and shut off the valve before turning off the pump. I know this is standard practice when evacuating refrigerant lines for AC systems, which must be under vacuum before the system can be charged with refrigerant. – Matthew Sep 18 '15 at 22:47
  • I'm closing this question as additional detail is required from the OP in order to provide a meaningful answer. –  Nov 25 '15 at 19:20

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