Electrical wiring/stud finder

[JohnW2]

You take me too seriously/literally

Maybe, but how else was I meant to take "The same could be said for any item of test equipment .... " ? - are you suggesting that your comment was not intended to be taken seriously?

Kind Regards, John

 

[Jackrae]

OK I'll bite

The point I was trying to illustrate is as follows :

a) one has a piece of test equipment that one believes actually works (confidence level 99% say)
b) to prove it works one attaches it to a known measurement source (confidence level rises to 99.9% say)
c) one measures unknown source and takes reading
d) at point (c) was the device actually still operating correctly (confidence level falls slightly to say 99.5%)
e) one reproves device works by repeating point (b) (confidence level rises back to 99.9%)

As pragmatic DIYers, technicians or engineers (take your pick) we usually skip steps (a) to (e) for most measurements but there is always the slight chance that our measuring device may fail at any point so one can never be absolutely 100% certain. The best example of this is the old 2-wire test lamp. We always did steps (a) to (e) whether testing dead or live circuits since assuming "no-light" represented a dead bus-bar would mean either the bar actually was dead or the operator might be dead for making an incorrect assumption.

 

[JohnW2]

a) one has a piece of test equipment that one believes actually works (confidence level 99% say)
b) to prove it works one attaches it to a known measurement source (confidence level rises to 99.9% say)
c) one measures unknown source and takes reading
d) at point (c) was the device actually still operating correctly (confidence level falls slightly to say 99.5%)
e) one reproves device works by repeating point (b) (confidence level rises back to 99.9%)

Indeed and, as I said, that's precisely what we (should) do with any bit of test equipment. With the great majority of test equipment, that is possible, and is about as foolproof as one can get - the only further improvements I can think of would be to go round the b/c/d loop multiple times and/or repeating the whole process with a different piece of test equipment.

However, that only works because, in (b), one is able to use a "known measurement source" with closely emulates the real-world 'measurement'/test one will be doing in (c).

However, my point was that (b) is really not possible (certainly not in a general sense) in the case of equipment designed to detect buried cables. What would you use as the "known measurement source"? If one made up a 'proving jig' which consisted of a cable of a certain size, buried at a certain depth in a certain material, with a certain current flowing through it, 'proving' that the equipment was capable of detecting that would give absolutely no information about it's ability to detect a cable of unknown size at an unknown depth in a (possibly) unknown material and carrying an unknown current.

Kind Regards, John

 

[Jackrae]

Agreed
In weld inspection using NDT techniques it was quite common to provide "calibrated" samples of defects to prove that the system was capable of identifying unknown defects at unknown depths in material of "almost unknown" chemistry.

 

[JohnW2]

Agreed

I'm please to hear that. It was my (only) point, but you appeared to be challenging it by reference to 'proving techniques' which would be applicable in many other situations, but not the situation we were discussing!

In weld inspection using NDT techniques it was quite common to provide "calibrated" samples of defects to prove that the system was capable of identifying unknown defects at unknown depths in material of "almost unknown" chemistry.

The implications of what I just wrote was obviously that one theoretically could try doing that for 'cable finders' but it would be impractical (and, obviously, could never be totally exhaustive).

Kind Regards, John