+x

 

Warren (Staff) - 10/2/2019 9:40:42 AM

Assuming there has definitely been no leaks, then it could be a variation in atmospheric pressure.  There is a little rubber grommet on the gauge, gently pierce it to equalise the pressure between the gauge and atmosphere and it should be a bit more accurate. Be careful not to pierce deeply as you can hit the internals which would definitely knock it out of sync. 

+x

 

Hanaldo - 10/3/2019 12:25:10 AM

Yeh as above, these analogue gauges don't really provide useful measurements of vacuum, they only provide an indication of vacuum. So the way to use them is to do a drop test over half an hour to an hour (the longer you drop test, the better your indication will be), then unclamp the part while watching the gauge to see if it moves.

The number it shows is really pretty incidental, it's just about making sure that needle stays rock solid when you unclamp.

+x

 

Lester Populaire - 10/2/2019 4:12:22 PM

first 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.