Page 1 of 2

Is this the right pump forStar Mode

Posted: Sat Sep 11, 2010 9:51 pm
by Vikram Singh
i have attached a picture of my pump and i just want to know if this appropriate for Star mode

Re: Is this the right pump forStar Mode

Posted: Sun Sep 12, 2010 2:19 am
by Jerry Biehler
A little overkill size wise. Unless you have a pretty large (By fusor standards) chamber. You will still probably want a small diff or turbo in series to do the initial pump down though.

Re: Is this the right pump forStar Mode

Posted: Mon Sep 13, 2010 3:49 am
by Vikram Singh
But would i still need it for star mode and i was thinking of using a bell jar 8x10 would this qualify as a large chamber or not?

Re: Is this the right pump forStar Mode

Posted: Mon Sep 13, 2010 6:52 am
by Nicker
I have one of these backing my turbo. Works excellent.
Alone they can provide an ultimate pressure of 1x10-4 Torr.

Any cons? Heavy (~90lbs with oil) & costly.

But if you want to make a small investment, do it once, do it right.

Re: Is this the right pump forStar Mode

Posted: Mon Sep 13, 2010 7:37 am
by Chris Bradley
The pump is fine. But there is no way of predicting the quality of your vacuum chamber, which will reflect in your ultimate achievable vacuum. Usually, folk's systems, here, include a diffusion or turbo pump to achieve micron levels. If you have to buy such a thing, then this would be fine (if oversized) for backing those pumps.

Personally, I have never found a two stage pump on its own to get my 'dirty chamber' down to sub 20 microns in any reasonable time, whereas others say they have done that with ease. If you have a clean chamber and a big pumping capacity, lower vacuums will be acheived.

Re: Is this the right pump forStar Mode

Posted: Mon Sep 13, 2010 1:57 pm
by Doug Coulter
Nick nailed it. Yes, this is a good pump. No, by itself it's not enough to run a fusor with great success. You would use this as a backing pump for a true high vacuum pump of some kind (diff pump or turbo).

It's not just reaching fusor pressure -- you have to be able to get well below that so that when you let in fuel gas, it's still fairly pure, not 20% or less of the total gas in there -- remember the chamber will keep outgassing during a run.

Nameplate specs on attainable vacuum from mech pumps mean little, and are made on brand new pumps blocked off right at the intake, after pumping overnight. With a small chamber attached, in real life you rarely do better than ten times the rating. Look at a volume vs pressure curve for any of these in the Lesker catalog for an example -- near the nameplate rating the pumped volume is basically zero, and all pumps get to some equilibrium based on how much gas is coming out of the chamber vs how fast the pump is at that speed -- the speed rating is usually at atmospheric pressure, and in real life it's not just one number. As usual, only the most "good for advertising and sales" numbers are the ones published. Look for the caveats and qualifiers and a truer picture emerges.

Re: Is this the right pump forStar Mode

Posted: Mon Sep 13, 2010 11:06 pm
by DaveC
Vikram -

Perhaps, more important than the ability to reach a pressure in the range of a few microns, is the issue that, at these pressures, on pumpdown from atmosphere, the bulk of the remaining gas will be water vapor.

This will take some time - hours - to go away, and it will require require a pressure atleast a factor of 10 below the pump's lower limit to accomplish this.

Add to this, the fact that at very low pressures (for the mechanical or roughing pump) you are approaching the vapor pressure of the pump oil itself...particularly when it is hot.

This all leads to conclusion mentioned by Chris and Doug, that you need a High Vacuum pump on the chamber, and the roughing pump to back it.

Unfortunately, adding the second pump brings some additional valving, the need for an ion gage, along with the thermocouple gage... but.... you will get in return....the capability for getting the water pumped out and keeping the chamber free of high vapor pressure hydrocarbons.

Look for either a small diffusion pump ~ 2 inch inlet or larger, or a small turbomolecular pump (~50 L/sec) ....preferably air cooled either pump type. These will get you to about 10 -6 Torr or below in reasonable time...

Dave Cooper

Re: Is this the right pump forStar Mode

Posted: Mon Sep 13, 2010 11:53 pm
by Vikram Singh
thanks guys for the help i will be sure to get a turbo or diffusion pump asap

Re: Is this the right pump forStar Mode

Posted: Tue Sep 14, 2010 1:20 am
by richnormand
I have one of those. Rebuilt completely from a Dunaway kit. Also several photos during the process. I have the service manual too, should you need a copy.

Bullet proof and definitively an overkill. It will serve you well.
Just add a diff or better yet a turbo pump and you are set.

Re: Is this the right pump forStar Mode

Posted: Tue Mar 08, 2011 6:42 am
by Nikelopez
@richnormand

Would you still happen to have the service manual?