+x Lester Populaire - 10/2/2019 4:12:22 PMfirst of all those gauges measure relative pressure and not absolute pressure. so depending on your altitude you are already off. Then those cheap gauges are never accurate. If you connect a couple of them in parallel they all show a slightly different value.For a drop test i usually clamp everything off, let it sit for a while, then when i come back i pull a vacuum in the vacuum gauge with the connection between the gauge and the part closed still. this is now the full vacuum the pump can deliver. then i close the clamp between the gauge and the pump, and then i open the clamp between the gauge and the part. if the needle moves it wasn't airtight.