Replacement Roughing Pump?

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Jackson Oswalt
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Replacement Roughing Pump?

Post by Jackson Oswalt » Tue Sep 19, 2017 12:22 am

Hello all!

Before I begin, I'm not a new user, but this is a basic question I'm having quite a bit of trouble with.

Alright, so the very first piece of hardware I purchased for my fusor was a Robinair 15500 pump. According to the manual, it's rated to reach 75 microns. However, now that I've purchased a working a TC gauge (at least I think it's working. Maybe it's just way off), I know that I'm not even getting down to a full Torr, which is just horrible. So I've come here to ask your opinion on what my next move should be. Should I get a new pump (if so what model should I get?) or should I be looking for huge leaks in my chamber? If you'd like to see pictures of my chamber and pump, see my other posts.

Thanks!

Rex Allers
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Re: Replacement Roughing Pump?

Post by Rex Allers » Tue Sep 19, 2017 12:40 am

Since you mention leaks in your chamber as a possibility, I assume you are measuring the pressure on your whole system. First step should be to put your gauge as directly as possible onto the vacuum pump to remove the possibility of many places that could be leaking. The connection between the pump and the gauge should be short and guaranteed as leak free as possible. Teflon tape on any screw fittings is Not recommended.

Don't leave the gauge on the pump after you turn it off or you could suck oil back into the gauge.

There should be lots in the facs on testing and plumbing suggestions. I'd guess you have big leak(s) somewhere.
Rex Allers

Jackson Oswalt
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Re: Replacement Roughing Pump?

Post by Jackson Oswalt » Tue Sep 19, 2017 1:56 am

Thanks for the quick response!

The only Teflon I have is on the 1/4 not to kf25 adapter, and there's now 2ay around that. Everything is is Kf or Cf. I tried attaching the gauge directly to the pump, but I got the same result. A little more than 1000 microns. I'm starting to think my TC gauge is faulty, which would make it the 3rd gauge I've gotten that hasn't worked.

Niels Geerits
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Re: Replacement Roughing Pump?

Post by Niels Geerits » Tue Sep 19, 2017 12:41 pm

Jackson which TC gauge is it? Are you using the proper controller? If not did you make your own? Keep in mind some TCs use AC current for the heater while others use DC (for example the tube from the roll your own gauge FAQ). You would think it doesn't matter for the heater wether the current is AC or DC, but it does matter for the thermocouple unit. http://www.repairfaq.org/sam/lasercva.htm#cvagtc Check out the part about internal wiring. If you want to know how to make a controller for an AC heater I can share the circuit I have built. That is all I can think off at the moment.

Jackson Oswalt
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Re: Replacement Roughing Pump?

Post by Jackson Oswalt » Wed Sep 20, 2017 1:18 am

I apologize for not mentioning the following: My TC gauge is a Consolidated battery-operated gauge. The TC tube is also Consolidated. When a vacuum is pulled and the gauge is turned on, it moves to quite a bit more than a torr, but it does move. If I turn the adjustment knob all the way to the right, I can get it to read out a small amount more than a full Torr, but it still starts at Atmosphere before the vacuum is pulled. I let the pump run for 30 seconds to 5 minutes. No matter what, wether the gauge is on the chamber or directly on the out or how long I run the pump, the vacuum stays the same.

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Richard Hull
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Re: Replacement Roughing Pump?

Post by Richard Hull » Wed Sep 20, 2017 5:28 pm

There is so much still missing here that diagnostics for you by us is still floating about in the dark. "Vacuum heads" know consolidated as CVC. CVC's stuff is old, but typically sturdy.
Battery operated TC guages do not typically involve any active electronics at all. A 1.5 volt "D" cell battery a meter and a potentiometer for zeroing or calibrating the instrument and perhaps a well hidden screw driver adjustment pot for TC tube current and perhaps a resistor or two, and that is it.

Does the CVCtube have a number on it?

Try and attach photos of the tube label and the front of the instrument showing the meter and controls.

Rchard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Replacement Roughing Pump?

Post by Dennis P Brown » Tue Sep 26, 2017 10:49 am

The elephant in the room is that your pump will not be useful for anything but a demo-fusor; 75 microns maybe is not the pump you want (at best; unlikely to even reach that considering that is the mfg 'best' performance for a new pump.) If you intend to do anything real than you need a proper two stage vacuum pump that can reach below 5 microns (most are rated sub-micron but never can do that - again, mfg claim and reality are often two very different issues.) Whether your TC is lying or not isn't terribly relevant until you decide what type of vacuum you will need - demo or real fusor system.

Frankly, calibration of a TC isn't really easy nor something worth doing (for exact pressure (i.e. +/- a few microns)) unless one has another gauge that can be trusted (aside: if you have a local public university/college, call their physics department; a Prof there will likely help you check your gauge.) If you want to do real fusion in the future, focus on getting a proper pump and then, maybe another TC unit to cross reference. Yes, one can make their own but again, that requires a level of calibration experience that isn't something a newbie should focus upon.

Aside: of course, if one can get down to about 100 microns, a good turbo can handle the high vac using that pump depending on leaks/etc. But the cost of a turbo/controller/cable far exceeds the average good two stage "sub-mircon" pump and a diffusion pump combo that can often be found on ebay. Having a decent two stage pump is really a good idea if you ever want to do real fusion in a fusor.

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Richard Hull
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Re: Replacement Roughing Pump?

Post by Richard Hull » Tue Sep 26, 2017 7:33 pm

It is a very rare brand new Welch or any two stage pump that can go sub micron even though all their labels state it. We have been through this before. A brand new two stage welch might easily hit 5 microns or almost get to 2 or 3 microns in a flawlessly sealed system or at the head. A diff pump will work at 50 microns maybe a little above, but it is not recommended and don't expect anything better than barely sub-micron from the diff pump and this will demand a perfect seal.

A decent, usable, two stage pump should bottom out at the head, prefereably, below 20 microns. This assumes a good diff pump or turbo as a secondary pump for a working fusor. One can generally expect a bottom pressure in the 10e-5 torr range or even lower. At these lower pressures, sealing is the main bad guy you have to fight.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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