Low cost hermetic seals - Page 1
Forum section: electronics / projects

author: splin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 04:22:02 pm
content: Does anyone know where in the UK I could buy low cost hermetic seals in small quantities (around a dozen)?I’d like to encapsulate some resistors in a small metal box; Kovar eyelet seals were suggested here in an earlier thread, whereby a Kovar tube is hermetically sealed axially in a metal ring with a glass seal between. This can be soldered into a hole in a tin or copper box and a copper wire fed through the Kovar tube and soldered to it to complete the seal. This avoids the thermal EMF problems of Kovar leads.Are feed through capacitors an alternative? The ones sold by Digikey etc. don’t seem to specify they are hermetically sealed or the lead material and can be quite expensive at $5+.So what are the options and who sells them? What about re-purposing second hand components with appropriate seals? I'd guess most suitable components would be expensive as new but perhaps there may be some which go cheaply on Ebay?

author: rvalente
author_rank: Frequent Contributor
timestamp: September 12, 2015, 04:59:35 pm
content: Panel mounting BNC connectors would fit your needs?They're are shielded, can be soldered in the metal box or add some RF shielding.... and are CHEAP!

author: TimFox
author_rank: Super Contributor
timestamp: September 12, 2015, 05:05:49 pm
content: Another style of BNC connector achieves reasonable hermeticity through an O-ring seal on the shell, and a non-permeable inner connection.Seehttp://www.amphenolrf.com/031-4238.htmlI remember using these at reasonable vacuum levels in grad school, down to maybe 10^-6 mm Hg.For real vacua, < 10^-8 mm Hg, we bought ceramic feedthroughs from the usual suppliers (Ceramaseal and Varian).

author: rvalente
author_rank: Frequent Contributor
timestamp: September 12, 2015, 05:14:27 pm
content: May these would fit your needs...http://www.keyelco.com/category.cfm/Terminals-Terminal-Boards/Terminals-PTFE-Insulated/p/509/id/515

author: AF6LJ
author_rank: Unknown
timestamp: September 12, 2015, 05:20:01 pm
content: Are you trying to keep moisture out, or a gas such as dry nitrogen in?

author: splin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 05:51:53 pm
content: Quote from: rvalente on September 12, 2015, 04:59:35 pmInteresting but I'd be surprised if these provide a hermetic (gas-tight) seal as it wouldn't typically be required for most applications. Could you solder the body to a metal box without subjecting the plastic (PTFE?) to excessive heat thus damaging the seal?

author: rvalente
author_rank: Frequent Contributor
timestamp: September 12, 2015, 05:54:44 pm
content: Nope, probably will not keep the gas inside under pressure.Was not mentioned gas hermetic, thought would be moisture, dirt or RF tight.Fluid hermetic and low cost are words that simply wont match, better reach top vendors like Amphenol and Harting connectors...

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 12, 2015, 06:06:16 pm
content: Make your own.Not sure how much pressure you need to seal against, but HVAC stuff typically operates at up to a few hundred PSI.

author: AF6LJ
author_rank: Unknown
timestamp: September 12, 2015, 06:21:04 pm
content: Quote from: splin on September 12, 2015, 05:51:53 pmYou can get gas tight BNC connectors but you might not like the price, we use to use them and TNC connectors that were rated to provide a hermetic seal. We tested every one that was installed on one of our transmitters.

author: splin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 06:30:10 pm
content: Quote from: AF6LJ on September 12, 2015, 05:20:01 pmGood question. It looks like I misunderstood the term hermetic which I assumed meant gas tight but actually only seems to mean water-tight (or better). I want to keep both water vapour and oxygen out to prevent corrosion of the resistors to minimize drift for many years. They wouldn’t be pressurized beyond those caused by normal room temperature variations so no high vacuums but I’d prefer a seal that I could have confidence in for many years which suggests a glass-metal seal. I guess I could pressurize the enclosure with something like dry nitrogen and periodically top it up if it lost pressure but it would be a pain to have to do and would still risk the ingress of fresh oxygen if not done carefully.Quote from: rvalente on September 12, 2015, 05:14:27 pmI’m not sure how you could provide a gas tight seal between the PTFE and the hole in the metal box - I guess it would have to be a very tight interference fit but as PTFE doesn't have much elasticity I would not be confident of its integrity.Quote from: TimFox on September 12, 2015, 05:05:49 pmI'm not sure the BNC connectors would do the job which although specified as hermetic don't specify they are gas-tight.Can small quantities of the Ceramaseal/Varian seals be obtained easily at reasonable prices?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2015, 06:45:42 pm
content: Depending one the degree of gas tight, it might work to use emaneled copper wire and expoxy glue. The wires should be long enough, not to heat the glue to much on soldering. Here it helps if the material the wires pass through is rather thick, not just 1 mm or less. In the old days sometimes a kind of tar is used for seals.The ceramig feed throughs specially for vacuum are often quite expensive.

author: Gyro
author_rank: Super Contributor
timestamp: September 12, 2015, 06:46:46 pm
content: If your application is DC, then solder-in feedthrough caps might give you a combination of reasonably gas-tight sealing and EMC filtering.Edit: Eg.http://www.ebay.co.uk/itm/Ceramic-Feed-Through-Capacitor-100pF-FOUR-CAPACITORS-/400979029100?hash=item5d5c366c6c(one of many listings)

author: splin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 06:48:02 pm
content: Quote from: NiHaoMike on September 12, 2015, 06:06:16 pmYes that looks tempting; however I guess the leakage of the refrigerant through the epoxy would be pretty low and so it wouldn't be noticed given the relatively large amount of refrigerant available. In my case the metal box would cause positive and negative pressures to be applied to the seal as the room temperature and air pressure varies. Standard epoxies are permeable to oxygen so I'd have to source one that is but I still couldn't be sure that I'd made a perfect seal without getting them helium leak tested or similar. Or I'd have to ensure the box was positively pressured at all times which could be a pain to do.

author: splin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 07:00:07 pm
content: Quote from: Kleinstein on September 12, 2015, 06:45:42 pmMy understanding is that special oxygen-impervious epoxy would be required which might not be easy to source. I guess it might work if the epoxy seal was sufficiently long and narrow by passing the wire through a thin copper or brass tube but it would be tricky to completely fill the gap between the tube and wire with epoxy without getting a lot of trapped air bubbles which would defeat the purpose. I'd have no idea how to calculate how long the seal would need to be to be effective though.QuoteI suspected that would be the case - but the pressure difference shouldn't exceed 2 or 3 psi I'd have thought if the the metal box is thin enough. Any idea how expensive?

author: daqq
author_rank: Super Contributor
timestamp: September 12, 2015, 07:46:23 pm
content: I've heard of spark plugs being used in a similar manner - they are abundant, cheap, and must withstand a lot of abuse in environmental terms.

author: voltsandjolts
author_rank: Unknown
timestamp: September 12, 2015, 08:11:10 pm
content: Search ebay for "glass feedthrough", there are a few options.Or cut the can lid off of a TO3 package and use the two glass-metal feedthroughs

author: splin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 09:12:56 pm
content: Quote from: voltsandjolts on September 12, 2015, 08:11:10 pmAh-ha - thank-you - Perfect!http://www.ebay.co.uk/itm/Russian-military-miniature-glass-Feedthrough-insulators-lot-of-100-/251041737116?hash=item3a7340e99cQuoteExcept that I'd have to drill axially down the centre of the Kovar leads without damaging the glass-Kovar seal to allow me to pass copper wires through - not impossible but very tricky! (I want to avoid the very high thermal EMFs which would arise from the resistor lead to the Kovar lead connection on the inside of the enclosure and the Kovar lead to copper wire connection on the outside).

author: LaserSteve
author_rank: Super Contributor
timestamp: September 12, 2015, 11:57:08 pm
content: Get a Minicircuits mixer in the metal can, such as a SRA-1 or SRA-7. Unsolder can, which takes some heat as they use a solder with a bit of silver in it. Voilà! 8 PIN solderable seal with pins on a multiple of standard 0.1 inch spacing. Plus the kovar or 52 alloy shell is nickel plated so you can solder it as well.Steve

author: langwadt
author_rank: Super Contributor
timestamp: September 13, 2015, 12:15:48 am
content: a pcb with few pads on? solder on both sides, bolt or solder to box

author: splin
author_rank: Frequent Contributor
timestamp: September 13, 2015, 12:50:31 am
content: Quote from: LaserSteve on September 12, 2015, 11:57:08 pmBut wouldn't I have exactly the same problem as using a TO3 package? Presumably the feed through pins are solid Kovar so how would I get my copper wire through the seal? As I said previously I don't want to have two copper - Kovar junctions in the circuit because of thermal EMFs.

author: splin
author_rank: Frequent Contributor
timestamp: September 13, 2015, 12:54:09 am
content: Quote from: langwadt on September 13, 2015, 12:15:48 amA simple solution, but I doubt that the glass-fibre/epoxy PCB material is impervious to oxygen.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 13, 2015, 02:11:55 am
content: Prepare for a confusing post, you will have to browse some web sites... Also low cost and hermetic in the same sentence doesn't happen unless you like nickel plated solder cup DB9 connectors with a glass core or BNCs. Those can get down to 28$ each or so, Digikey or Mouser stock them.Surplus Sales of Nebraska has some solderable hollow capacitor FTs in stock cheap:http://www.surplussales.com/Feedthrus/FTsolder3.htmlIf you need pure copper to glass or copper to ceramic seals, those CAN be made, but they are NOT easy to make, nor are they reliable. For the most part they went away when Dumet, Kovar and Nickel to Glass seals were perfected. With the exception of some specialty applications, pure copper hermetic seals went away long ago.For copper hermetic seals thru glass, at first, Houskeeper Glass to Metal Seals were used. Not fun to make as the oxidation state on the copper must be exactly right. Which is why Dumet Glass to Metal Seals were invented for light bulbs and vacuum tubes. On a Dumet seal, drawn copper is the outer layer but it has a nickel-iron alloy inner. So basically these were for soft leaded glass, and not very useful for military and aerospace connectors. The copper melting point is very close to the hard glass (Pyrex, Nonex, Aluminosilicate) working point, thus good seals are difficult to make. So pure copper feedthrus are reserved for TC and High Power, High Vacuum work.Omega has some low cost ones, but not that low cost:http://www.omega.com/pptst/PFT2.html?gclid=CPGJw7iB88cCFQmpaQodXhkB6wAs for the copper EMF portion, Sorry, I Missed that part. I have some expensive ones here in a drawer here at work. Actually a common part in university research. They are made, and they show up from time to time on Ebay. However they are not cheap. You can use the copper halves of TC vacuum flanges or solder a wire into a hollow feedthru or flange. High power Vacuum Flanges have copper leads, but are not inexpensive.http://mpfpi.com/stock-products/thermocouple.aspxhttps://www.lesker.com/newweb/menu_feedthroughs.cfm?section=tctypek&init=skiphttp://www.mdcvacuum.com/DisplayContentPage.aspx?cc=d68eacb2-f915-42dc-bc1e-9496cfe70ffaIf your looking for solderable hollow tubes thru glass or ceramic, you can try contacting any of the classic feedthru makers:http://www.ceramicseals.co.uk/netscape/index.htmlhttp://www.ametekinterconnect.com/http://www.teknaseal.com/http://specialtysealgroup.com/ceramic.phpUsually I ask if they have over-runs laying around. Sometimes they do, Sometimes they don't...If it were me, I'd order the Ebay bag of 100 Ex Soviet seals...Would you be happy with a ceramic loaded epoxy? Hysol 1C White is what we use for vacuum work.. It is basically the core material used for making Torrseal Epoxy which is used on high vacuum chambers . As well as for sealed glow discharge tubes that only need to last a year or two. Its basically 70-80% MgO dust.Steve

author: babysitter
author_rank: Frequent Contributor
timestamp: September 13, 2015, 01:03:52 pm
content: Go for hollow feedtrough caps, I just think they are fine - Stettner used to make them, you can get surplus from Oppermann Elektronik, germany, for example.edit:http://www.oppermann-electronic.de/html/durchfuhrung.htmlDFK types

author: TerraHertz
author_rank: Super Contributor
timestamp: September 13, 2015, 01:54:04 pm
content: Quote from: splin on September 12, 2015, 09:12:56 pmAh, that's nice, thanks. Ordered two boxes.Does anyone know what the best solder and bulkhead materials for these would be? Aim is to make high vacuum feedthrough ports.Something I have not done before.I'd assume a brass bulkhead, silver solder, and copper wires soldered only in the extended bit of feedthrough tube. But I don't know how critical expansion matching and soldering temp would need to be to avoid fracturing the glass seal.

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2015, 04:02:21 pm
content: Not silver solder, ordinary 63/37 lead based is best. It is soft and will not whisker, plus will not crack the glass metal seal. Those feedthroughs are vesigned for it in any case, and are pre coated with it as well. Case material best is brass plate, drill it and then dip in molten solder to coat it, then wipe down while the solder is still molten to leave a thin film and clean the holes, then solder in the feedthroughs, then do the leads and finally seal with solder. You probably will want to fill with either vacuum degassed water free oil or with wax.

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2015, 06:48:44 pm
content: I haven't dealt with Ceramaseal since graduate school (during the Nixon administration), but they were reasonable to deal with back then. The indivicual feedthrus require welding.seehttps://www.ceramtec.com/ceramaseal/feedthroughs/

author: SteveP
author_rank: Contributor
timestamp: September 14, 2015, 07:57:05 pm
content: Hi,Regarding the statement that "epoxy is permeable to oxygen", people may not want to discard it as a solution quite so quickly. Based on some research I did a while back, the issue isn't *whether* epoxy is permeable, it is *how* permeable it is. For instance, if only one molecule of oxygen gets through per decade of use, then it would probably be deemed adequate as a barrier.So just how permeable is epoxy to oxygen? I don't have the reference handy at the moment but the best answer I found was in a report issued by some researchers investigating the permeability of a large number of materials to various substances from the point of view of radioactive containment. The number they gave was 0.128 barrers. OK, what the heck is a "barrer"? And how do we use that information?It is named after Professor R.M. Barrer who was, apparently, an early pioneer in the field of permeation and the barrer is the unit applied to the coefficient of permeability of a substance. This coefficient is given by the formula P = q*t/A*deltap where:P is the permeability (in barrers),q is the mass flux of the permeant (10E-10 cm**3/sec),t is the thickness of the membrane (cm),A is the area of the membrane (cm**2), anddeltap is the difference in pressure across the membrane (cmHg).In other words, if our material has a permeability coefficient of 1 barrer we will get10E-10 cm**3 of gas across the membrane, per second, if our membrane has an area of 1cm**2, is 1 cm thick, and we have a pressure difference of 1 cmHg (about 0.193 psi = 0.013 atm = 0.013 bar).Since we want the contents of our container to last much longer than a second, let's do the calculation for a somewhat longer time period, say 100 years. That turns out to be about 3E9 seconds which, if we take the permeability of epoxy to oxygen as 0.128 barrers, gets us 0.128 * 10E-10 * 3E9 = .384 cm**3 of oxygen. At STP, the density of gaseous oxygen is about 0.00143g/cm**3, so we would get 0.00055 grams of oxygen in 100 years!Since our membrane area (diameter of the hole in the container) isn't likely to be 1 cm**2, the thickness of the membrane (the thickness of the wall of the container) isn't likely to be 1 cm, and our deltap isn't likely to be 1 cmHg, let's do the calculation using more realistic numbers.Take deltap: here's where things get a bit tricky. If we fill our chamber with dry nitrogen, it will presumably be at atmospheric pressure, as will the atmosphere on the outside of the container. So the pressure difference is zero, and there won't be any movement of oxygen across the membrane, right? Nope. Here, we need the concept of "partial pressure" which is simply the pressure not of the atmosphere as a whole, but of the *oxygen* in the atmosphere compared to the pressure of the oxygen on the other side of the membrane. Well, the pressure of the oxygen on the inside of the container is zero, since there is zero oxygen present (because we filled the container with nitrogen). On the outside of the container, the pressure of the oxygen is simply the percentage of oxygen present in the atmosphere (21%) times the pressure of the atmosphere which is 1 atm, giving us an "oxygen pressure" of 0.21 atm and therefore a pressure difference (deltap) of 0.21 atm which is about 16 times the 0.013 cmHg of the barrer definition.As for the membrane, let's make it easy on ourselves and consider a hole 1 mm in diameter through which our wire will pass and neglect the diameter of the wire itself. This will give us an "A" (area) of 0.785 mm**2 or 0.00785 cm**2. Let's further assume that our container's wall is 1 mm thick, giving us t = 1mm = 0.1 cm.So now we have a deltap that is about 16 times greater than 1 cmHg, and an A/t of about 0.0785 resulting in a factor of about 16 * 0.0785 = 1.256; this means that for a hole of that size we would get about 0.00055 * 1.256 or about 0.0007g oxygen per 100 years.You can take the math from here: ten such holes would result in 10x that amount of oxygen, if you only cared about a ten-year period it would be 1/10 of that amount, if you doubled the thickness of wall material it would be 1/2 that amount, and so on.As it happens, manufacturers of LEDs care about sealing out oxygen and I ran across a paper that reported the results of sealing LEDs with epoxy. They used tritium as a tracer and found that the oxygen permeation was about 3x the above calculation for the type of epoxy they used. Epoxies differ, of course, so the exact number for something like JB Weld would probably be slightly different, but I think that the calculation gets us in the right ballpark.Given that some slight amount of oxygen will cross the membrane, we might simply "scavenge" it with an oxygen absorber. These are often used in packaged food products (and we've all seen moisture absorbers in electronics packages)--they're simply small sachets of a material designed to react with oxygen (or moisture) thereby removing it from the air. For oxygen, iron oxide is one such material (but it requires moisture to do its job, and we don't want moisture in our container any more than we want oxygen, so that's out) and ascorbic acid (vitamin C).Based on what I've read, it takes one unit (mass) of ascorbic acid to absorb 2 units of oxygen, so even a single gram of ascorbic acid would be far more than we'd need. What I haven't been able to ascertain is whether any byproducts in the reaction (which is fairly complex) would have any effect on electrical components (solder, wire, etc.). I also don't quite understand whether moisture is required for the absorbing reaction to take place, so those two things would have to be investigated if one were to decide that 0.0007g of oxygen per century was intolerable. As noted above, in 1 cc of atmosphere (at STP) there is about .00143g of oxygen, so over the course of a century, with no oxygen absorber, the oxygen content inside the container would rise to about half the level of what it is in the normal atmosphere.I'd be interested in the results if someone cared to take the idea of an oxygen absorber further and discover what, if anything, could be safely and effectively used around electronics.Oh, before someone asks, the permeability of epoxy to moisture was listed as 0.013 barrers or about 1/10 that of oxygen.Hope all this helps,--Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: September 15, 2015, 02:17:19 am
content: In light of the above post, I give you Torr Seal:http://www.agilent.com/cs/library/datasheets/public/Data%20sheet%20TorrSeal3ok.pdfand my beloved 1C Epoxy...https://www.duniway.com/part/ep-8034Both are "low outgassing" materials as well.Steve

author: richard.cs
author_rank: Super Contributor
timestamp: September 15, 2015, 11:43:36 am
content: Quote from: SteveP on September 14, 2015, 07:57:05 pmThoughts on using a reactive metal as the absorber? I'm thinking magnesium ribbon or similar which will react with oxygen without water present and produce a solid oxide which should be reasonably easy to contain.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 15, 2015, 04:24:52 pm
content: OK, there are US Military Specifications for solderable feeds. See this manufacturer's site:http://www.shp-seals.com/aboutfeed.htmlSteve

author: SteveP
author_rank: Contributor
timestamp: September 17, 2015, 12:05:33 am
content: Re the use of magnesium (or aluminum) as an oxygen absorber, the idea doesn't work because the oxidation creates an impermeable layer on the outside of the metal which halts further oxidation after the layer is only about 10 nanometers thick--far too soon to use up the quantity of oxygen we're talking about. There are, of course, more energetic metals one might consider--sodium and potassium, for instance--but they have the unfortunate side effect of bursting into flame upon exposure to the atmosphere.--Steve

author: Ian.M
author_rank: Super Contributor
timestamp: September 17, 2015, 12:47:32 am
content: The addition of a small quantity of Mercury allows Aluminium to be rapidly oxidised by air at room temperature. The Mercury dissolves some of the the Aluminium which then preferentially oxidises at the surface of the resulting amalgam liberating the liquid Mercury to dissolve more Aluminium. As the Aluminium oxide formation causes the surface under it to form a liquid Mercury film, the normal stable, firmly attached Aluminium oxide film cant form so the reaction continues. The Aluminium surface *MUST* be initially free of oxide as the Mercury cant break through the oxide film, but even a small fresh scratch is enough to start it off.I would expect a similar reaction with Magnesium.However the use of Mercury is problematic, and even apart from the toxicity and environmental issues, the presence of Mercury vapour in the enclosure is likely to seriously affect long-term reliability as any condensed drops will alloy themselves with most metals if a metal surface is exposed.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 17, 2015, 04:44:24 am
content: Now that you know how to figure out how much oxygen and water get across a given barrier you need to figure out how much will cause you a problem. Even harder question. Depends on material and construction of your resistors.As an aside, why not eliminate the thermal EMFs by the classic technique of paired junctions? In your application it should be relatively easy to keep the junction pairs at the same temperature. Then you will open up your material selection problem.

author: richard.cs
author_rank: Super Contributor
timestamp: September 17, 2015, 07:36:59 am
content: I've certainly had bits of magnesium ribbon (pure, not an alloy with aluminium) turn completely into white dust after ~10 years in a corked test tube. Clearly there are some circstances in which it doesn't pasivate but it's possible that I had moisture ingress as well as oxygen.Another option might be a finely powdered reactive metal, freshly milled alumiumium dust that's been kept away from air. Normally you'd introduce oxygen slowly after milling it to oxidise the surface without it catching fire but if you don't then there's alot of surface area there to absorb oxygen on before it passivates. Might be more work and danger than sodium, which is normally fairly safe and can be handled in air, small pieces usually don't catch fire in water either.I just prefer a solid oxide to an unknown mix of organics which may break down into water later.

author: SteveP
author_rank: Contributor
timestamp: September 17, 2015, 12:10:51 pm
content: The OP's requirements aren't super-clear to me even at this point, but apparently two of the objectives are (1) for a small number of wires to pass through to his resistors and (2) he wants to do this for less than $5 per wire. He stated that he wanted to guard against moisture and oxygen, but did not explain why.In my own case, I was investigating putting an entire (small) board in an oven and didn't want the bulk and cost of a bunch of pass-thruough devices and simply wondered if wires/epoxy would do the trick. I had further noted that while some pass-through capacitors are made from glass/kovar, many are actually made with epoxy! (Remember, a lot of these are primarily aimed at keeping out RF rather than moisture.)Regarding *oxygen*, as far as resistors go, modern precision resistors are, as I understand it, made from Evanohm which, being a nickel-based alloy, is basically impervious to oxygen. Manganin, on the other hand, is susceptable to surface oxidation, so the Thomas-type resistors made with it during the 1930s by the NBS (and others) were sealed (with solder) except for the opening through which the wires passed--that opening being sealed with silk impregnated with varnish. The air inside the Thomas resistors was not treated specially--it was not evacuated or replaced with dry nitrogen; they just made sure there was as little of it as possible and used large-diameter wire to reduce the surface area/volume ratio so that surface effects would be held to a minimum. Five of these resistors are still (or were as of a report issued in 2003) being used as working standards for the Ohm (the QHE device being used as the primary standard).So I don't know why the OP was concerned with oxygen unless he knows his resistors are made with Manganin. If they are, then I suspect, based on the experience of the NBS, that replacing the air with dry nitrogen and simply sealing around small diameter wires with epoxy would be more than adequate. If they're made with Evanohm, then the concern with oxygen might be misplaced.Dealing with *moisture* is far easier--simply replace the air with dry nitrogen, include a packet of silica gel which is commonly used with electronics today (and as far as anyone knows, does not turn to mush over time), use epoxy to seal the openings around the wires, and call it a day. For that matter, if the volume is small enough, you might even skip the dry nitrogen step.Back on page 1, the OP expressed a concern that it wouldn't be possible to know whether the openings were actually sealed if epoxy were used. I can assure him that I have done this dozens of times for non-electronics related projects and the seals have proven gas-tight at pressures up to around 1,000 psi for tens of seconds. A thick epoxy like JB Weld (don't use laminating epoxy--it's not viscous enough) still flows well enough that if you coat a small section of the wire with an excess of epoxy and spin it slowly as you feed it through the opening, the epoxy will flow well enough to completely seal the opening. If the container has very thin walls, then simply surround the wire with a blob of epoxy on each side of the wall.(I have mentioned JB Weld a couple of times; I do so merely because I've used it on many occasions over the years and find it to be reliable, relatively inexpensive, and widely available. Other brands may be better (or worse).)--Steve

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: September 17, 2015, 05:06:32 pm
content: Or you could do it the easiest way, just buy resistors that aren't sensitive to humidity, such as in the Ultra Precision Reference LTZ1000 thread, look for Ultrohm Plus resistors. They are way cheaper than Vishay hermetics.

author: splin
author_rank: Frequent Contributor
timestamp: September 18, 2015, 05:13:02 pm
content: Quote from: SteveP link=topic=55033.msg757 002#msg757002 date=1442491851Apologies - I should have made it clearer. I have a fair number of Vishay Beyschlag UXB 0207 precision thin film resistors that I acquired at very low cost. They don’t specify the materials in the datasheet but the stability spec is very good by thin film standards:1000h < .02%8000h < .04%225000h < .12%Ok, thats way worse than wire-wound resistors but mine are old stock so should be drifting much less by now. Much more significantly, I assume that the primary cause of long term drift is corrosion so I’d like to try putting them in a gas-tight hermetic enclosure to exclude oxygen and water vapour. They were also cheap enough to allow me to use multiple resistors in serial/parallel combinations to give some statistical improvement in the temperature and time drift characteristics.I also have some Vishay foil S102K resistors which I also would like to try enclosing. They aren’t especially stable due to moisture absorption by the plastic encapsulant; I’d hope they behave more like the hermetically sealed versions which are very stable (spec’d to be <2ppm in 6 years), but very expensive.This may well turn out to be a complete waste of time but it won’t have cost me much given that I have ordered 50 of the ex Russian military seals for < £6 including delivery.QuoteEpoxy probably would be good enough but those Ebay glass seals are cheap enough that it’s not worth messing about with home made seals. Of course they might turn out to be faulty/rejects so I might try a simple pressure test. You’re right that if I had high quality wirewound resistors then I wouldn’t bother putting them in sealed environment given Edwin Pettis’s statements elsewhere that they are minimally affected by humidity or corrosion.I have to say thank you to everybody who have contributed to this thread with way more high quality responses than I expected - especially LaserSteve and SteveP for his excellent analysis on calculating the permeability of epoxy – it’s one thing knowing the basic theory but getting real numbers and formulae for these relatively obscure topics is what makes these fora so wonderful.Quote from: Edwin G. Pettis on September 17, 2015, 05:06:32 pmTrue, but the drift spec for the Vishay hermetic, < 2ppm/6 years, is quite a lot better than for yours at 2 (or3?) ppm/year. I’ve love to see some real-world measurements though as I wouldn’t be surprised if the stability of your resistors improve considerably over time (in a benign environment).Actually I do intend to order some of your resistors but I’m having trouble deciding exactly how many and which values to order.

author: calexanian
author_rank: Super Contributor
timestamp: November 15, 2016, 04:51:48 am
content: So this a year late, but......I was told by an old timer that when they needed a quick hermetic seal for wide voltage and current ranges spark plugs were commonly used. They can be hard soldered or brazed. External connections can be made with non resistor plug wires, and you can grind off the ground tang and tack weld directly to the electrode. Best of all they are cheap as!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 15, 2016, 11:34:51 am
content: Spark plugs are also better than pretty good in ultra high vacuum setups, according to "Building scientific apparatus" by Moore, Davis and Coplan. If you have some large stainless manifold with a nice thick blank to thread and mount them in, but they are pretty damn bulky for enclosure mounting!