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Silver Soldering Platinum - Job's Done


Hi everyone, The silver soldering of the platinum contacts is done. I
thought you might like to know how it went.

The device itself is a heating and temperature measuring chamber,
the size of a fat coffee cup, but not as high. In this, sunk into two
wells, side by side, sit two minute platinum cups, barely big enough
to hold a pea. Each cup has a heating circuit and a measuring
circuit. A known substance is put in one, the substance to be
investigated into the other; both are heated in an argon atmosphere,
and by comparing and contrasting the flow of electrical current in
the two cups the behaviour of the unknown can be deduced. The
temperature range of the device is from liquid nitrogen to 1,000
degrees Kelvin; at the moment my physics prof friend was working on
the behaviour of borax glasses.

The heating/measuring cups sat atop a stalk which emerged from a
quarter-size round base plate; four tiny ribbons of platinum ran
from the base of each cup to L - shaped contacts in the base plate;
two of these had broken off from the contacts in the base plate and
needed to be re-attached.

| |
| | Heating/measuring cup

/ || \ Platinum foil leads
/ || _
|______________| Base plate

Fortunately the contacts in the base plate were robust enought that
I was able to clamp them in a hemostat to hold the device steady and
also to serve as a heat sink. A dot of solder remained on the posts
where the leads had detached, so the whole job essentially became a
matter of holding the cup device steady and then positioning the
platinum tinsel on the solder dot. For soldering, the smallest tip on
the air-acetylene torch served quite nicely; the solder ran visibly
up the glowing platinum foil. All told it took about two hours.

Having checked the resistances of the circuits my physics prof
friend pronounced everything to be in order again. What, I asked,
would have been the consequences if we hadn’t been able to re-attach
the leads? Well, he said, it would have been a matter of several
thousand dollars, which would have entailed putting the present
project on hold until the next research grant had come in.

So that was a gratifying result for an enjoyable and unusual

Hans Durstling


Hans, Pardon my ignorance, but if the device is to work up to 1000K
(that’s about 730C isn’t it), won’t the solder remelt and the leads
detach again?


 Hans, Pardon my ignorance, but if the device is to work up to
1000K (that's about 730C isn't it), won't the solder remelt and the
leads detach again? Tom 

Hi Tom, No, it’s only the tiny cup itself which gets so hot; the
platinum foil leads that run down from it are very very thin - 30
microns - and the metal posts they are soldered to are vastly thicker
(about 1/16") by comparison, plus there’s just shy of 1/2" distance
between the bottom of the heating cup and the top of the contact
posts. So the contact posts act as a heat sink, plus such a tiny
flimsy foil lead isn’t going to conduct a whole lot of heat down to
them in the first place.