I've just finished bagging an airbox cover...

I usually get perfect bagging, but this time, even if I could clearly get the tone change  and I could reach the full vacuum on the gauge, when I stopped the pump for the vacuum check, after 10 minutes I could already see some pressure drop (from -1bar to -0,8 bar)... so the bag is not perfectly sealed.. maybe some puncturing (hard to find, maybe it's faster to do the bag again, since it's not a big piece)

So IMHO... no matter tone change and no matter super gauge.... only drop test will tell you if your bag is perfectly sealed..

Of course if you don't hear tone change and don't see full vacuum on the gauge... you have some issue to solve before the drop test....


Alessandro

Hi Mike
I think mine are airtech (off the top of my head). I think the liquid keeps the needle from sudden movements caused by vibrations.
Fred 

An accurate guage will only tell you accurately what the leak has dropped to - which is irrelevant.

Its the fact you have a leak full stop thats important, and the cheapo gauge will tell you theres a leak just as effectively.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Although I have found there is a difference in total vacuum pulled between two different pumps. So having a good gauge will also tell you if the pump is not as efficient and needs an oil change or replacing.. A gauge which is not so accurate will have you allowing for a cheap gauge and thinking the difference is down to many factors but ultimately leave you thinking the gauge is cheap and it is at fault not the pump ect... 

Thanks for the interesting feedback everyone.

Warren made some valid points but I agree that there are situations where a very accurate vacuum gauge is called for. For day-to-day applications a simple vacuum gauge is all that's required, particularly if you're not trying to compare two pumps etc. and of course changes in atmospheric pressure will affect the pressure shown on the gauge anyway which is why we often suggest that people don't rely too heavily on the vacuum gauge as an absolute truth and instead learn to read other signs (like the pump's sound) as indicators of leaks etc. No vacuum gauge I've ever used will indicate the minute difference in pressure between a tiny leak and no leak in the way that opening and closing the valve to the vacuum pump will and so I understand why Warren was emphasizing that point.

I'll certainly look into sourcing some high-end vacuum gauges as an upgrade option for our product range. 'morepower' is quite right that a good gauge doesn't need to cost the earth.

Best regards, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Sorry for adding my two cents in this old thread, but I had to reply. Maybe it will help others reading this

If you guys want a precision instrument capable of reliably and repeatedly measuring such a good vacuum you have to pay quite a lot more money. These round mechanical gauges simply cannot measure such small pressures. Whether the gauge shows -0.98, -0.99 or 0.999 or whatever is pure luck, these numbers shouldn't be taken too literally, it just doesn't have the required resolution. That's like trying to interpolate milliseconds by the movement of the hands on your wristwatch. Does not work, period.
If you want that kind of accurate resolution you have to invest quite some money into laboratory grade equipment. There used to be mercury filled devices which were able to measure low pressures reliably but they are not available anymore for obvious reasons, these days it's all electronic.
Not that it really matters, if your bag seems to hold its vacuum for 15 minutes or longer all is fine as our hosts Warren and Matt have said before. It's still better to have a cheap, somewhat inaccurate gauge than no gauge at all!

Oil pumps are indeed capable of sub-millibar pressures. Under ideal conditions, which means new uncontaminated oil and nothing attached to them. Only then will they reach their final vacuum. If you attach all kinds of cheap plastic tubing and a few square meters of thin plastic bag (which ARE permeable to gasses) and a few hundred grams of liquid resin (which has low volatility, but it does have a vapour pressure) then the pump will NOT reach its final vacuum. That's a physical impossibility. That doesn't mean the pump is faulty, it means it runs under less-than-perfect real world conditions.