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Contact protecting coating?

I've just experienced a problem with my temperature controller, a Cal

9400, and I have helped a friend with a few Cal 9500P with the same problem from time to time, basically sporadic behaviour and erroneous temperature readings or thermocouple failure indication. I know the problem is related to the classic problem of solder or tin plated contacts oxidising as I was aware that the guts of these controllers can be pulled from the panel mount by design to allow hot swapping such as when one goes down in a large bank as in an extruder line, it saves having to access the back of the board and removing and replacing wires when the electronics can be easily changed from the front. I just pulled mine and put it back and it's going correctly again as that action wipes the contact surface. Now I'm wondering if there is some suitable contact sealant or protective coating that I can apply to the contacts to prevent this happening again, the environment isn't hot or damp in my case but this problem comes up from time to time, my friend now pulls her's as part of the yearly workshop service. I have seen dielectric grease which seems like it might do the job but it needs to be something which won't effect the low currents in the thermocouple. Any suggestions would be appreciated. BTW I know of another kiln that is suffering the same sort of issue with one thermocouple reading low and tightening the screws helped for a few weeks but it's back but I haven't been able to check as it was being used and the 2nd thermocouple seemed OK, in that kiln the thermocouple connection is next to the case so will get hot in use so if anything is available then maybe 100C capability or higher would be good for the 2nd application.
Reply to
David Billington
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Thermocouple failure is tested with a resistance check, and eventually thermocouples DO age and die (if you can uncouple the thermocouple from the meter, and measure its resistance with an ohmmeter, you can match the resistance you measure with the controller's specified maximum ohms value).

If the resistance is from corrosion/age, you need to replace the thermocouple.

If it's the resistance at a screw-down spot, this can be improved by welding the thermocouple wire to a washer (or lug), and screwing down the lug with a star-type lockwasher under it. The little points that dig in make a gas-tight metal/metal connection that doesn't get 'loose'.

If the problem is in protected connectors (that don't get dusty or wet) inside a protective housing, you might try contact enhancers like

after a little compressed-air dusting and maybe a wipe with an alcohol-soaked swab.

Reply to
whit3rd

You prompted me to test the insulation strength of this:

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which is a grease mixed with conductive particles of zinc and graphite. I've been experimenting with it to repair contacts in old instruments and vehicles that I can't disassemble to polish with an ink eraser.

The fixture is two sheet aluminum cutoffs from under the shear. I covered one with two parallel strips of Scotch tape about 1mm apart, smeared Ox-Gard along the gap and taped the other over it. The tape is

0.0045" thick in two layers (to avoid adhesive on the mike anvil).

My analog, hand-cranked 1KV Megger correctly shows the ratio between applied voltage and leakage current (= resistance) regardless of the cranking speed. At low speed it read above 800 Megohms. The meter showed ~200VDC on the Guard terminal when the grease started to break down. I don't know how closely Guard matches the Sense voltage. The resistance reading dropped below the 10 Megs of a DVM at around

300-400VDC.

In other words that brief test shows it's promising, the metal particles don't short together at low voltages. I think the dissimilar-metal voltages in a thermocouple connection would cancel as long as both terminals are at the same temperature.

-jsw

Reply to
Jim Wilkins

You can buy spade lugs made of thermocouple alloy:

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I use red and yellow nail polish to mark K thermocouple ends and terminals. It doesn't withstand much heat. Wood stove gasket cement keeps the braid from unraveling to at least 2000F.

-jsw

Reply to
Jim Wilkins

Try looking up fretting corrosion. There is a lot of history on this type of problem.

One early remedy was microcrystalline Wax in mineral oil. Another remedy is Cramolin.

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It works well, but the techs did not like it as it smelled. They much preferred Gold Mist.

Dan

Reply to
dcaster

There is a fortified RED gasket RTV that is up to 6or 7000F as I recall. The standard would burn out in my wood stove every other year. Once I put on the RED with a new fiberglass rope it lasted longer than 5. I sold the house at 5 so I don't know if they trashed the wonderful stove or use it another 17 years.

Martin

Reply to
Martin Eastburn

My own personal favorite was something called Cramolin, which got replaced by DeOxit (same company, but less ozone-depleting chemicals in the propellant). Looks like they have been going through more name changes -- but everything from this company is *very* good for contact cleaning and preservation. This is not a company website, but someone's review and experience.

Good Luck, DoN.

Reply to
DoN. Nichols

Yes -- the Cramolin smelled toxic -- the DeOxit a bit less so, and apparently they have a mix of new chemicals from the same company. I just posted a link to a review of sorts which discusses the name change game. :-) But I did use Cramolin in spite of the smell -- it was

*that* good.

Enjoy, DoN.

Reply to
DoN. Nichols

Thanks for the advice. In the end I found a UK seller on ebay that sold a 4 pack of 2ml Deoxit tubes

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so I've ordered that and will see what happens. The 4 pack contains all that would seem to be required for cleaning and protection so if it works I may report back in a decade or so, if not much sooner.

Reply to
David Billington

[ ... ]

Hopefully you won't have anything to report for decades. :-)

Good Luck, DoN.

Reply to
DoN. Nichols

"DoN. Nichols" fired this volley in news: snipped-for-privacy@Katana.d-and-d.com:

For decades in our computer shop, we used an AMP product. It was just a 'numbered' stock item, in a quart metal can. One can would last our whole shop (six to ten techs) a year. It was a slightly oily liquid that smelled of nitrobenzene. It cleaned, and left the merest of residues which both shielded against oxidation and lubricated board-edge finger connectors. That helped prevent the major difficulty with those, which was not oxidization, but fretting corrosion due to vibration. (Big boards, 200 fingers, each.)

Lloyd

Reply to
Lloyd E. Sponenburgh

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