Inaccurate vacuum?

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Conrad Farnsworth
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Inaccurate vacuum?

Post by Conrad Farnsworth »

So, my science teacher is ever so graciously letting me borrow his vacuum pump (Fischer LAV-3 http://www.fischertech.com/VACUUMPUMP.htm) and i looked up the specs and it goes down to 25 microns. Well he let me hook up my dv-6m vacuum gauge and tube (analogue) to the pump's hose (partially snug, but we figured it would work). According to the gauge, it pumped all the way down to 20 microns. Is this wrong or does it have something to do with the volume of the tube? IF the TC is inaccurate, will it matter as long as its consistant? Thanks!
EDIT: I could hear the pump pulling so im sure it was pumping at least most of the air out. Also, how would i go about calibrating an analogue vacuum gauge?
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Steven Sesselmann
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Re: Inaccurate vacuum?

Post by Steven Sesselmann »

Conrad,

What you have there is a two stage rotary vacuum pump. When they are new and in good condition they are capable of pulling 20 micron, but more commonly they hover around 40-45 micron. You always get a better result when you put the gauge directly on the pump, because you will have less leaks. The efficiency also changes as the pump and oil gets hot.

Are you hoping to do fusion experiments, or just plasma experiments.

If you just want to experiment with air plasma, then it should be capable of entertaining you for a while, however if you intend to do fusion, you need to get down into the 1-10 micron range.

To do this you will need a secondary pump, such as an oil diffusion pump or a turbo pump. The second pump is connected between the first pump and your fusor chamber, preferably with a valve on either side.

Diff pumps come up on ebay fairly cheap, from time to time.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
Conrad Farnsworth
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Re: Inaccurate vacuum?

Post by Conrad Farnsworth »

I bought a diffusion pump from Chad Ramey already so that base is covered (except for the flanges which i will have to find). Will a Fischer work for a primer pump? And i do hope to achieve fusion some time but as of now, I am only doing a demo fusor for my project! Why valves? And could I use barb fittings for the project? I used no fittings what so ever (just the tube fitted over the TC) when i tested it. Thanks!
John Futter
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Re: Inaccurate vacuum?

Post by John Futter »

Conrad
First 20 microns is at the low end of a thermocouple gauge capability
second calibration of the head electronics usually states that a vacuum much better than a rotary pump can do is used as a zero vacuum. It maybe that who ever calibrated the gauge head did not have access to a better vacuum than a rotary pump can supply
third I wouldn't worry about a result that is within an order of magnitude, within that you are in the margin of error for almost all gauges.
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Rich Feldman
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Re: Inaccurate vacuum?

Post by Rich Feldman »

Let's let a diff or turbo pump user explain the valve thing, if it's not in the FAQs.

Gauge accuracy:
When you buy a gauge to measure the pressure of air in your tires, or compressed gas in a cylinder, a 10% accuracy rating is pretty cheap.
But medium and high vacuum gauges span orders of magnitude, and our degree of caring tends to be sort of logarithmic (like measuring sound levels, or photographic light levels).
[edit] An indicated:actual ratio of 3:2 or 2:3 is an error of less than 1/5 decade -- good enough for most applications. The scale markings on analog gauge help to put that in perspective. And many gauge types change sensitivity by more than a factor of two (0.3 decades) according to the gas species.

Rotary vane pump specs:
The pumping speed depends predictably on displacement and motor speed. But AFAIK, the ultimate vacuum rating is more about leakage, manufacturing tolerances, and oil properties. So a reading slightly lower than the pump datasheet is good news, and no reason to question the gauge. It's no cause for alarm if a high-end industrial vane pump with sub-micron rating levels off at a few microns on a TC type gauge, but that story is told elsewhere.

Calibration:
Granville-Philips gives these instructions for calibrating their "Convectron 275" Pirani gauges, whose analog scales are marked from 1 micron to ATM:
After pumping to 0.1 micron or lower, adjust the Vac trimpot so needle points to zero (or so analog output voltage is at standard value for zero pressure). Then at atmospheric pressure, adjust the Atm trimpot so needle points to 1 Atm (w/ optional offset for your altitude). IMHO it would be reasonable to adjust your gauge to read correctly at the lowest pressure available in your lab, if you have a more trustworthy gauge as a reference. Remember that in micron-level work, the pressure can be different at opposite ends of a hose!

The pump I reported here
viewtopic.php?f=10&t=3977#p25527
had its cartridge repaired at the factory 6 months ago. I put it back together last week, and got an indicated pressure of 15 microns with gauge plumbed directly to inlet port.
Your teacher's pump has the same physical configuration, so please only run it right-side up.
All models are wrong; some models are useful. -- George Box
Chris Roberts
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Re: Inaccurate vacuum?

Post by Chris Roberts »

The simplest way to do a rough calibration on these types of gauges is to hook it up to something like a diffusion pump or anything similar which will pump below the measuring capability of the gauge. Once you know the pressure is below what your gauge can distinguish, just adjust the reading until it reads zero. Sure the pressure is still not perfectly zero but as far as a thermocouple gauge can tell, it is at zero.

Once you do that your gauge will then give you about as accurate a reading as these thermocouple gauges can get. A $3000 capacitance manometer might show that you are off by a few microns or so, but that is of little concern. Some of the fancier types of gauges have you calibrate the gauge at both atmosphere and "zero", but thermocouple gauges only have the one adjustment, and for the purpose they serve that one adjustment suits them well.

Even if you do not get it zeroed off exactly, your gauge will still be very useful, for example, now that you know the roughing pump will do "20" by itself, when you hook it up to your chamber and read "50", that might be expected since your chamber is bigger, outgasses more, maybe it has a very small leak, etc. However, if you get "500", you then know that something is up, and it is time to start suspecting leaks.

-Chris
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Richard Hull
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Re: Inaccurate vacuum?

Post by Richard Hull »

Get up close to that pump's plate and squint tightly.... you might note it says 0.25 micron and not 25 microns as its base pressure. The decimal point might be obliterated or hard to see. Pumps always claim far LOWER than they ever really pump. I have never heard of a company claiming worse ultimate pressures than it can actually pump. They are always off in dreamland and claim something you will never get.

A TC gauge that has the TC tube's current adjusted to spec is considered fairly accurate on air down to 10 microns. In the 100 to 10 micron range expect 10% accuracy. Below 10 microns expect +/- 30 %

If your pump gets down to 20 microns on a good TC gauge at the head, it is probably a wee bit worn but is considered good enough to back a diff pump.

A new pump with perfect oil might realize 10 microns or just below on pump down at the head.

Again, all this is discussed in many past postings and FAQs in this forum. Use the search function next time you have a question. Search on FAQ in the forum you have a question in.

Richard Hull
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The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Conrad Farnsworth
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Re: Inaccurate vacuum?

Post by Conrad Farnsworth »

Thanks for the help! Apologies for not using the search function!
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