Vac system Cycles?

[Chief High Horse]

I have a Vac pump set up for testing my seals and cycle times, before anything is laid up for a wing. My current plan is to use the vbag to sheet some cores with 1/32" ply. My cores were cut from deckmate plus blue foam, I believe 25lbs.

At 15 PSI on the gauge I started with 5 minutes between the pump turning on to hold pressure. I packed some plasticine clay around each fitting and have the cycle up to 18 to 25 minutes between pump downs.

Questions:

1. I have read many different lines of advice on Vac bagging and the recommended pressures. Some say to pull 20 or more if possible, some say 8 to 12, which is it, and why?

2. This system seems to cycle at +/- 1psi. so will a total of 2psi fluctuation over 18 minutes. Will this cause any problems?

Thanks.

Dave

[wyowindworks]

I don't like vacuum systems that cycle. I feel this can induce air between the skin and the core. But, many people are successful at using a cycling system. I'm sure you will be fine with a system that only fluctuates 1 or 2 atmospheres.

On 25 psi foam, I pull to 20 when using 1/64" ply.

Adam

[jfrickmann]

I pull 18-20 PSI and cycling time depends on leaks. If it cycles too often it is not good for the pump, therefore I have added a continuous running switch. But so far, I have made two wings both with the system cycling, and it worked very well.

[A6INTRUDER]

My system will hold vacuum for 30 minutes or more with no bag attached and the hose pinched off. It has 1.5 inches dead band.

The big problem comes with the bags and fittings and seals.
In theory if there are no leaks the system would never cycle but of course that is not really possible.

Yours system sounds like it is working well just the way it is.

I also agree with others that with 25 lb foam I use 18-20 hg and with white foam I use 9-10.

I have also experimented with using 15 hg on 25 and 60 lb foam and it seems to work just fine and actually glass wings come out with less humps and bumps in them too.

TIM

[Troy]

18-20 minutes sounds pretty good. I found that in many cases the bag material can be a major culprit if you use some cheap plastic drop cloth from the box stores. I found some that never pulled over 15hg. Others worked fantastic.

[Tisr]

Dave

Do you have a reservoir and one-way valve in your vac system? Since I added both of these components, I've found mine will take a couple of hours to cycle on a similar test (no bag, hose pinched off).

Kind regards

Dan

[V1VrV2]

I think the reservoir is a good idea.

Most can be made from a length of PVC plumbing pipe capped at the ends and a fitting put on at one end for the hose/switch assembly.

Put a one way valve between the pump and the reservoir to keep the air from entering through the pump side once the vacuum is pulled.

Simple and it works :) ...

Tony