Best way for Decade Resistor

[splin]

Some more information on epoxy seals with the interesting statement (wrt helium leak testing):

Most high-performance glass or ceramic seals have leak rates of less than 1x10-5 std cc/sec at 1 atmosphere differential. Pave epoxy seals have leak rates typically between the 10-7 to 10-10 std cc/sec. This means that the leak rates of epoxy seals as measured by the helium test are as low or lower than those of glass or ceramic seals.

Of course 'glass or ceramic seals have leak rates of less than 1x10-5 std cc/sec' doesn't mean much - it could mean that most glass or ceramic seals have leak rates well below 10E-10. I'm not sure I believe that 1E-5 figure, given that leaks > 5 x 10-6 std ccs/sec are classed as 'gross leaks'.

http://www.pavetechnologyco.com/pdf/hermetic.pdf

From this:

http://nepp.nasa.gov/workshops/etw2013/talks/Tue_June11_2013/1630_McManus_Pressnell_Hermeticity%20Leak%20Testing.pdf

If I'm interpreting this correctly, a 1cc enclosure with an 'equivalent standard leak rate' of 1E-7 atm cc/s air will exchange 50% of its atmosphere within 80 days which isn't very long. I would have expected a hermetic seal to be much better than that. In any case it seems that a 1E-8 seal and larger enclosures, say > 10cc, are required for lifetimes of more than a few years.

[2N3055]

Aren't those specification with vacuum on one side...?

If box is pressurised on the inside, seals would have to hold only differential pressures, making leakage miniscule...

I apologise it if my question is stupid, I find this topic interesting but haven't got  much knowledge about it.. 

[e61_phil]

pressure isn't pressure

If there is hydrogen on one side and no hydrogen on the other side for example. There will be diffusion no matter of the partial pressure of other gases.

[Kleinstein]

The leak rates (especially when small) also depends on the gas. This is why they often test with helium. The leakage rates for water vapor are usually much smaller than for helium, but difficult to measure.

The leakage rates are usually for vacuum and with less pressure difference, leakage can also go down. Though there is still the partial pressure problem, so still some tendency for humidity to come into a dry container, even if pressurized inside. A sealed container will also see pressure changes, from atmospheric pressure and temperature changes.

For a resistor circuit it is not so much keeping any humidity out, but to hold it low at a value below about 40% RH - the low value because often surface leakage gets significant better below a limit of about 50% (depends on the material).

For the volume, a small amount of getter material can absorb quite a lot of humidity, so increasing the effective volume to really large values. This also makes is reasonable to start dry.

Form my experience with vacuum, if would expect epoxy sealing to be good enough.

[2N3055]

I understand a physics behind it, but have no material knowledge...

So you're saying NASA tested container that had same pressure in and out, just helium inside and then tested how much of it diffused...
That would be a good measure of material porosity, yes...
But here we would be interested in water vapour diffusion rate, which should be slower ..

Also air has different  water vapour capacity at different temperatures and air pressure...

Would desiccant in the box be able to help (together with epoxy seals) or it will deteriorate long term (it will saturate eventually?)?

[Gyro]

I wonder if it would be better to introduce a non-oxidising atmosphere, such as dry Nitrogen or even better, an inert gas like Argon. I suppose it's possible that oxidation could play a part in long term drift of non-hermetic parts, in addition to the shorter term drifts caused by humidity changes.

Argon is readily available for TIG welding as well as some food (wine) preservative products. It is also significantly denser than air, so even less likely to diffuse through seals. It doesn't sound too difficult to introduce two tiny holes in the container so that it could be flushed through and quickly sealed with a couple of solder blobs.

[Gyro]

as it is a noble gas [doesn't chemically react with most things]

In fact, nothing so far established at temperatures warmer than 40k, so you've got a couple of hundred degrees to spare. 

Edit: Argon is 37% denser than air but has much lower thermal conductivity... I have no idea whether that would be better or worse for convection currents (thinking heated Vref type applications here).

[Kleinstein]

A desiccant inside can help a lot, as it can adsorb really large quantities of water (e.g. gramms for a small bag). A small bag of desiccant could thus be equivalent to a volume of about 0.1 m3, when it comes to absorbing water. They are long time stable at normal temperatures.  There could be a slight problem if the unit gets warm, as this will shift the humidity to higher values, or would need a really dry bag and thus less possible adsorbtion.

There would also be the option to use dehydrated salts / boron oxide instead - can be even more efficient at low humidity levels. It could be just difficult to keep dust away.

There is a more or less smooth transition from small leaks to material porosity. Some of the leakage is just diffusion through bulk material, especially with something like epoxy or as a really bad material silicone. Transport through microscopic cracks is similar and may include short distance through the material. The standard for testing is helium, because it is usually the fastest and there is a low background. Water is rather difficult to measure because there is so much background and it sticks to surfaces - so if you have a small vial, more water could be at the surface than in the volume gas. The adsorbed surface layer of water also influences surface leakage.

For the DIY glass sealed resistors, I would initially keep the other end open. This allows testing for leakage and than filling with the right gas and sealing the glass end as a last step. This is relatively easy by pulling it thin and twisting. This also avoids gas leakage, no matter which orientation as you have the tube end to fill.

[Nuno_pt]

Well today I got a quote from Rohpoint for the Caddock resistors.

USF240-1.00M-0.01% - £33.11
USF240-10.0M-0.01% - £33.11
USF371-25.0M-0.01% - £42.85 x 4 = £171.40

All of them have the delivery schedule for 12 weeks from now.

[Alex Nikitin]

Well today I got a quote from Rohpoint for the Caddock resistors.

USF240-1.00M-0.01% - £33.11
USF240-10.0M-0.01% - £33.11
USF371-25.0M-0.01% - £42.85 x 4 = £171.40

All of them have the delivery schedule for 12 weeks from now.

Digikey has USF340 in stock. If you can measure the actual value, 0.1% version is about half of the price for 0.01% .

Cheers

Alex

[Nuno_pt]

I ask the price of this because of the TC ~2ppm/ºC, to see if was much difference in price.

Since the price is the double, and all the resistors will be equal, 4x25M to make 100M, and the 1 & 10M, the .1% 5ppm/ºC vs .01% 2ppm/ºC shouldn't present a problem, correct ?

[e61_phil]

If you want a 100Meg resistor with best TC possible you can ask Caddock to match some USF370 for you.

I bought some matched sets of 5 USF370 with 25Meg, 0,01%, <0,003ppm/V and guaranteed below 1ppm/K. One of these sets cost about 250€+VAT. Not much more than 5 USF370 alone. Maybe you should ask Caddock for a quote.

[Nuno_pt]

Thanks e61_phil, that price is not more the what they are asking for the 4x25Meg, since they are asking £171.40+VAT.
I've send an email to sales@caddock but the email bounce, and was not deliver.
Still waiting for a replay from VPG with the quote.

[Gyro]

Just in case anyone's interested, I've just spotted some nice cheap and compact glass hermetic feedthroughs from a seller in Poland. They're much less bulky than the previous ex-soviet ones that have been discussed. Outer diameter is a mere 5mm, with solderable center hole of 0.8mm. Being hollow there's no worry about thermocouple effects. They come in lots of 10 but no additional postage for multiples. Looking at other items, he also has some bigger ones. There are a couple of other sellers in Poland too, someone must have unearthed a crate.

http://www.ebay.co.uk/itm/10pcs-Glass-Hermetic-1-Pin-Feedthrough-Feedthru-Military-Product-/322318095449?hash=item4b0ba7d459:g:X-YAAOxy4t1SoC-4

Now, do I ask Santa for some Argon? 

P.S. No connection with the seller.