On 01/01/2015 8:41 AM, trader_4 wrote:
....

So, if that's right, then the question remains, what failed in Danny's
detector? There have been so many posts, I'm unsure of is what exactly
he measured, what the readings were, etc.
I think he measured that there was infinite resistance between rod and
furnace metal, ie no shorting of the insulator. I think he verified
conductivity from end of wire to tip of rod, but not sure what exactly he
measured there. So, I don't understand what could be wrong with the old
one......

I'd guess the measurements he took aren't precise enough (or weren't
actually measuring what thought he was is also a possibility of course).

Being as it's a minute current, _any_ breakdown in the insulation and/or
hermetic seal could lead to having enough leakage current to essentially
short out the actual current but still measure a high (but not infinite)
impedance.

Hypothesis; we don't have enough info to be able to diagnose remotely
exactly. The key is it doesn't work; a new one does. That's good
enough for me...

In a "former life" was responsible for incore self-powered neutron
detectors for a particular commercial power reactor vendor. These are
essentially a current source also in the 100-1000 nano-amp range and are
a swaged leadwire on a Rh emitter surrounded by a ceramic in a metallic
sheath. (Sound familiar?)

Their OD is only about 1/16", the lead wire and ceramic are thus tiny.
Turned out that even though one could measure a very high bulk
resistance between the emitter and sheath that miniscule locations along
the length of the detector could have very localized thin spots and
become a shunt path for leakage currents. Also turned out that moisture
could infiltrate the end seal and gradually diffuse down the length of
the detector over time. This also reduced the overall resistance and
showed up as a systematically lower output signal but of great
variability as again the existence of the surface roughness and local
thickness made exponential changes in local conductivity.

The detectors had been tested for years at the corporate research
reactor without ever discovering the issues--turned out it required the
higher power density of the real thing for the issue to become
significant. While they're not identical, given the similarity in
construction and function I can imagine many such similar scenarios with
these devices. Mostly I imagine it is a moisture infiltration problem
with these as a first guess.

In the end, the reactor detector problems turned out to be a secondary
blessing; I got a MS thesis out of it while it kept me gainfully
employed to pay for the course work while working around the issues for
operating reactors w/o requiring warranty replacements and solving the
manufacturing problems for new product.

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