Vacuum system connections question

[joniverson]

I am making a device like this for the purpose of freeze drying insects. I am basically following along the same guidelines as the author. However, I am wondering about something. For his aspirator valve, he elected to save costs and use what I believe is a brass or copper 1/4" valve like is used for plumbing purposes. Since I assume the valve has threaded female inserts, he is then taking most likely a copper or brass threaded nipple pipe of appropriate length, cutting it in half, and then threading each pipe into each female side of the valve. All well and good except he doesn't discuss what adhesive/ cement, etc to use with the valve-pipe coupling. Since this part of the system will be in the freezer, along with the jar, for over a month while the freeze drying occurs, and needing to maintain 15mm Hg, I'm wondering what substance should be used on the nipple threads when screwed into the valve? I can't think something as simple as teflon tape wrapped around nipple threads before screwing in would be sufficient.

Thank you!

 

[Asymptotic]

Does it have to come apart again?
If easy disassembly isn't an issue, then Loctite 271 threadlocker (red) can be used.

 

[joniverson]

The value, and both pipes coming from it, does not have to come apart again. I keep some of that Loctite red here, but never thought of it for this purpose. Would it maintain the 15mm Hg required within the system for a month?

 

[Asymptotic]

The value, and both pipes coming from it, does not have to come apart again. I keep some of that Loctite red here, but never thought of it for this purpose. Would it maintain the 15mm Hg required within the system for a month?

I believe so.

I've never tried holding a 15mm Hg vacuum for a month, so cannot say for sure, but took a look around, and it appears at least one https://www.omega.com/pressure/pdf/PSW-681.pdf manufacturer has used it to seal their "wetted" components within.

 

[joniverson]

Ok, thanks. The gentleman who wrote the article didn't use any sort of gauge to monitor vacuum, but I will be using one, a cheap one from Harbor Freight. I'll do a weekly or biweekly test and if vacuum has failed, I'll know.

 

[joniverson]

Just an update here from the OP. The parts arrived this week,and I did a first test and assembly yesterday. It took about 45 sec or so to pull the vacuum the author described. I had a cheap, oil filled gauge in-line and it drew just beyond 29" Hg. All seemed to go ok, except I ran into the following difficulties:

1) I haven't yet been able to find a suitable adapter for my faucet so that I could couple the washing machine hose to it! My faucet with aerator removed has 3/4-27 threads and apparently there is no direct way to get that to garden hose size threads, so I have been using water from the outlet just outside the door which works for tests, but no good once I place the actual caterpillars & dehydrator.

2) Not sure why, but a couple of min after I draw vacuum and hook up the gauge, it's reading like 20" Hg. I think there's air loss happening just from gauge hookup. I'm using 1/4" vinyl tubing and barbs, maybe I should have made the connection a lot more direct like I see with the vacuum chambers, where the gauge sits directly on top.

3) The biggest disappointment is that vacuum is not held overnight. Not sure why: I did draw full vacuum outside of the freezer first and then placed the jar into the freezer overnight, maybe the difference in temp caused a leak? I know the author didn't remove his already frozen bug jars for more than a min to draw vacuum and then back into the freezer. Either the temp difference, or I have a leak somewhere but when I immerse the jar in water, no water is drawn when the valve is closed.

Thanks for any help!

 

[dlgoff]

Since this part of the system will be in the freezer, along with the jar, for over a month while the freeze drying occurs, and needing to maintain 15mm Hg, I'm wondering what substance should be used on the nipple threads when screwed into the valve? I can't think something as simple as teflon tape wrapped around nipple threads before screwing in would be sufficient.

15mm Hg isn't much of a vacuum, so teflon tape would be okay IMO. But you could get a better seal by soldering; flux and let solder flow down the threads. However you'd need to somehow protect the valve from the heat of a, say, propane torch. I've used both in my old system.

3) The biggest disappointment is that vacuum is not held overnight. Not sure why: I did draw full vacuum outside of the freezer first and then placed the jar into the freezer overnight, maybe the difference in temp caused a leak? ...

One reason, other than leaks, is outgassing. Even glass outgassing the water it's absorbed is a problem when trying for a high vacuum.

 

[joniverson]

I've been able to maintain vacuum simply by using small hose clamps around all connections. This not only makes for more vacuum friendly connections, but better vacuum performance as it seemed to add an additional half inch Hg or so.