By H4RTM4N - 7/18/2013 10:33:03 PM
Hi all,
My name is Andreu and I'm new to this forum. I've just received my order from EC and I'm already trying to make my own carbon fibre parts using resin infusion. For the first trial I wasn't concerned about the final quality of the part (a simple plate) as I only wanted to be familiar with the process. However, I noticed I couldn't reach 'full' vacuum. Instead, the vacuum gauge was stuck at -29Hg (0.98+ bar). I closed all the leaks in the vacuum bag but I kept having that remaining pressure inside. In order to be sure of that, I connected the vacuum pump to the resin catch-pot only. The hose that usually comes from the resin catch-pot was clamped so the vacuum was only made inside the catch-pot. When I pulled vacuum from the pot, I obtained the same result. Taking into account that I live in Barcelona and my house is <100m above sea level, I wonder if my equipment is all right or not. In other words, I want to know if the the vacuum gauge is simply not representing the reality inside the bag because of its precision. I can imagine 2 possible answers to the question: - YES, the equipment is just fine. The precision of the vacuum gauge of the resin catch-pot is somewhat near to 2mbar and that is why I obtain these 0.98bar. - NO, there's something wrong. You should obtain -1bar (-30Hg), which in reality represents the advertised 99.995% vacuum. Could you please help my clarify this issue? Thank you a lot and good luck to everyone! |
By brainfart - 5/14/2014 10:01:12 PM
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.
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