Silicone tubing with hose barbs OK to pull low 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.
Shireesh Apte
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Silicone tubing with hose barbs OK to pull low vacuum?

Post by Shireesh Apte » Wed Oct 11, 2017 9:43 pm

We are in our first preliminary stages of building a plasma (not necessarily a fusion reactor) at this point.
We are still trying to get a low enough vacuum. We obtained a Fieldspace micron vacuum pressure gauge but we have been unable to pump down to below 500 microns (its upper set reading). I am going to change out the plastic tubing we are using for SS tubing sometime this week. We are using a 2 stage, 4 CFM rotary vane vacuum pump for now. The pump indicates that it can go down to 3 X 10^-1 Pa, or 2.25 X 10^-3 mmHg (2.25 microns). I realize that this magnitude may not be reached, but it seems to me that it should be able to pump down to <100 microns, where one could get a plasma. I have attached the gauge to the vacuum pump inlet.

My objective is to get plasma before I plunge into the fusion part of this project. Is it at all possible to get a vacuum of < 500 microns using silicone tubing? I am currently using garage door grease on the barb fittings before inserting the tubing over them (I received a reply as to why this was the incorrect approach so I will abandon it) to hopefully get an airtight seal but I am beginning to have serious doubts as to whether this is possible - hence the planned change to SS tubing with proper NPT adapters.

As regards the fusion part, I have a 4 way SS cross (2.75 inches) with CF flanges. I have ordered three CF to female NPT adapters with copper gaskets and one CF quartz view port. We have a deuterium supplier that we found and contacted thanks to these posts. Our Chinese Electroprecipitator unit is still functioning and the (extremely) brief user-guide on ebay says it can do a maximum of 60 KV, but we have never measured the output voltage. We will order a diffusion pump when we have plasma.

Best,
Shireesh Apte

ian_krase
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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by ian_krase » Wed Oct 11, 2017 10:55 pm

Regarding the electroprecipitator: This may work, or it may not -- to do fusion you must be able to run negative polarity with high current (5-50 mA) and high voltage (25-100 kV) at the same time. Please keep in mind that safety precautions for these very high voltages are much more stringent than those for lower voltages to the tune of 5 to 15 kV. Voltage measurement above 40 kV is likely to be expensive or homebrew. Keep in mind that at these voltages the electricity can leak through the air, cause corona, blow through a good many mm of insulating material, and attract lightweight objects with electrostatic attraction. It can also leak along the surface of an insulator.

A much safer option for initial tests is a neon sign transformer or oil burner ignition transformer (often cheaper and easier to find in the old-fashioned iron core type which is needed).

Can you post a picture of your setup and your parts? While NPT connections can be used in vacuum, they are generally considered undesirable because they are sometimes tricky to get leak-free (the best option is to glue them together with Loctite 209).

Silicone hose would probably work for what you are planning to do, as long as it seals to the hose barbs and does not collapse, but it's generally avoided in high vacuum because air can seep through it. Other rubbers (such as Buna-N and Viton) are better.

I am curious what kind of stainless tubing you are planning to use. If it's just "a tube made of stainless", that's great. If it's flex hose, you'll want to be careful that it is actually stainless on the inside as well as the outside and capable of being reliably connected.

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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by Jerry Biehler » Sun Oct 15, 2017 9:08 am

Silicone tubing should be fine as long as you can get it in a thick enough wall that it wont collapse or shove a spring down the length to support it. Even generic pvc tubing is fine at roughing pressures as long as you can keep it from collapsing. Tygon vacuum tubing is just a flexy PVC. Gum rubber too, that is the traditional vacuum line from way back, it's a reddish rubber.

Shireesh Apte
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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by Shireesh Apte » Sun Oct 15, 2017 3:34 pm

Thank you Ian, Jerry,

The pictures - second iteration (without the vacuum set up - as that is at school) are below:

https://drive.google.com/drive/folders/ ... sp=sharing
https://drive.google.com/drive/folders/ ... sp=sharing

My first iteration was mason jars with playdough as vacuum holder. I have moved on to the second iteration. I went to the container store and bought SS containers with acrylic lids. I drilled holes for the high-voltage feed-through and for the vacuum line into the acrylic lid and JB welded the vacuum hose barb. For the high-voltage feed-through, I have a flared 1/8th inch compression fitting that I JB welded to the acrylic. I acquired 1 foot, 0.094 inch ID ceramic tubing from McMaster and cut it into 2, 6 inch sections using a dremmel with a 1 inch diamond tipped cutting blade. I then inserted an 11 gauge (diameter 0.0907 inches) copper wire through the ceramic tube and joined one end to the inner grid (that i made from 18 gauge galvanized steel wire. The other end of the copper wire was JB welded to the ceramic tube (away from the plasma locale) so that some copper wire projected out of the ceramic tube. This apparatus was then fed through the compression fitting and the nut tightened at the appropriate spot on the ceramic tubing such that the inner grid was approximately in the center of the SS jar. ( I might add that I also applied JB weld to the compression fitting to ensure a vacuum seal).
The acrylic lid with the vacuum fittings and the high-voltage-feed-through was then JB welded to the SS container (see pictures for details). I plan to use this next week with the silicone tubing and the pump with my chinese electro-precipitator to see if I can get plasma. The center grid will be negative and the SS container itself will be positive. I will post pictures of the whole set up next week.
I realize that this content is better suited for the "fusor construction" forum so I will post it there as well. I thought I would post it here so that there is continuity when I post the vacuum set up (and the complete set up) pictures.
As you can see from the pictures, this apparatus will be used to generate a plasma. The 4 way 2.75 inch cross with the CF flanges and 1/4 inch, 1.5 inches hex bolts, nuts, washers and copper gaskets are for the fusion part of the project. For the fusion, I plan to use a 1 mm thick tungsten wire that I acquired from ebay-china. I also need to get a diffusion pump. I have a deuterium gas supplier that I found on these forums. Is there a store that I can get SS rigid tubing (for starters -I am looking at 1/8th inch male fitting to connect to my 1/8th inch female built in the CF flange).
Best,
Shireesh

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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by John Futter » Sun Oct 15, 2017 6:30 pm

Shireesh
we prefer that pictures are embedded on this site not a link o another picture sharing host.
Some time in the future the external host will dissapear leaving dead links making this whole post somewhat worthless
To get to the required size use irfanview a free picture manipulation app that you can resize resample to 1024 x 768 which is the best size for this forum

Shireesh Apte
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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by Shireesh Apte » Mon Oct 16, 2017 12:29 am

Image,
Obviously, this method of inserting the link between two Image HTML tags does not work.
John, This is going to take me some time until I can figure out how to do this. Best, Shireesh

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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by John Futter » Mon Oct 16, 2017 8:56 am

Shireesh
you are not getting it
no links
put the picture on this site natively
check out the attachments button directly below (next to options) all of this that we type
go and select file
then put it inline

no links to external sources!!!

all should be internal to this site

Shireesh Apte
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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by Shireesh Apte » Mon Oct 16, 2017 10:54 am

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Testing to see if this works for inserting pictures. Shireesh
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Dennis P Brown
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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by Dennis P Brown » Mon Oct 16, 2017 8:19 pm

Acrylic lids will not work very well - even for a simple plasma. It will allow you to get a plasma but it will be a mess after a while. Do make a glass lid system sooner rather than later.

Also, as the electrode heats up, epoxy will really out gas and the acrylic lid may melt. All that may even fail rather quickly so be aware of those issues.

Shireesh Apte
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Re: Silicone tubing with hose barbs OK to pull low vacuum?

Post by Shireesh Apte » Mon Oct 16, 2017 11:25 pm

Thanks Dennis. Is there a way to cut holes in glass similar to my acrylic set up please?
I ran the reactor today in my lab. I do not think I obtained a plasma but did get some very good ionization glow. As you can see from the pictures, the plasma seems to be confined to the region in between the central grid and the bottom of the SS container. I suspect this may be because, I made the central grid too 'dense' with more metal than air (partial vacuum) so that there is hindered acceleration in the grid ( I may be dead wrong about this and I welcome corrections and suggestions). I operated the Variac at 120 V. The white (step up transformer) device you see plugged into the variac increases the voltage to 230 V that the Chinese electroprecipitator (CE) unit operates on. I operated the CE at full voltage. Again, I am unable ( thus far) to measure the actual voltage fed into the fusor. I taped the positive to the SS vessel and the negative was alligator clipped to the copper wire that jutted out of the high-voltage feedthrough from the ceramic tube.
Since this is a vacuum post, I come finally to the pictures of my vacuum pump. I tried to pull a vacuum in the morning with the Fieldspace micron gauge attached to the inlet but it did not get past an overload (OL) reading. I hence swapped out the micron gauge for a coarse vacuum gauge. This is what is depicted in the pictures. It read approximately -29.5 inches Hg. I don't know why one would have a gauge that is supposed to be read with a negative sign. However, I figured, that -29.5 inches Hg was equivalent to -(29.5 x 2.54 x 10) mm Hg or -749.3 mm Hg. I took this to mean that the actual pressure in the vessel was [768.1 (barometric pressure today in Euless, TX) -749.3] = 18.8 mm Hg or 18800 microns Hg. This is my real conundrum as to why the pressure does not decrease to at least in the 100 micron region range (especially when the pump says it can pull to 5 microns).
I then shut off the vacuum pump. The pressure remained steady at -29.5 inches Hg for a good 10 minutes indicating that I did not have any major leaks in the system. The plasma is when we ran the apparatus in the afternoon. By the way, I used glycerine to attach the silastic tubing to the hose barbs on the vacuum pump and on the reactor.
At this point, I think I will make another version of the same reactor with the following design changes: make the inner grid so that it has a lesser metal/air ratio, position the inner grid so that the distance between the inner grid and the bottom wall of the SS container is increased. I still need a way to get to at least 1000 microns ( don't know how yet but will ruminate on this). Suggestion welcome.
Best,
Shireesh Apte


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