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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
MinorityCarrier wrote ...
Use of low-tribolelectric effect triaxial cable and triaxial connectors (see picture) are recommended for stable measurement of femtoamp currents (how many electrons comprise one attoamp?).
My (old) Keithly 619 uses triaxial connectors, it is only capable of measuring gigohms.
My Takeda-Riken TR8601 has a PL259 socket for input, and one of those BNC-like HV connectors for high voltage out, and a guard terminal which may be optionally strapped to the earth lug.
It certainly is used for measuring the resistance of polymers, glasses etc, but I can't see any easy repeatable way of doing this. For example, to measure the resistance of a glass rod 5cm long and 1 cm diameter, I can only imagine a procedure something like this:
both ends of the rod would have to machine polished dead flat, and silver plated over aquadag. Terminals could then be soldered on to this.
The assembly is cleaned with Triton X-100
a braided guard would have be slipped over the centre portion of the rod rod, and held in place with adhesive heat shrink.
That the rod assembly should be placed inside a die-cast box with an anhydrous atmosphere, and the shielding box connected to the meter earth by heavy copper strapping.
Does this sound sensible? It sounds like a lot of work! But it would seem necessary to ensure tolerable repeatability.
The whole thing is an absolute breeze for medium high resistances - 5G or 10G resistors - where simple ohmmeter mode gives very satisfactory results against 1% standards.
There are some data output connections on the back of the machine, and I suppose it is possible that measurements might be subject to post-measurement processing for all I know, to compensate for surface leakage, temperature, humidity and what have you. But who can say?
But why do I want to bother with all this? It seems to me that with a miniature EHT source used to power a 10kVp 20uA miniature end window X-ray tube, that half of the high voltage power (what Tesco would call 'meaty goodness') is creeping away somewhere in all sorts of little leakages, so that whilst in theory the miniature PSU should be sufficient, the combined total of losses means that it is not. So a bigger PSU and a bigger battery end up being required, and then it won't fit on the recycled Vickers microscope stand opposed to the miniature fluoroscope image intensifier unit which Plazmatron made and very kindly gave me. The idea is to get the completed X-ray microscope neatly back into the Vickers transit case, so I can take it down to the village pond, or to the autumn woods, and look at in vivo specimens, recording the images on a laptop.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
MinorityCarrier wrote ...
This article may prove useful.
Thank you for that - a very interesting read. I still use thermionic electrometer pentodes as front end devices, partly because I am at home with the technology, partly because they don't fizzle when rogue spikes show up, and partly because they are by their nature 'hardened' against ionizing radiation damage. They are good to femtoamps, but as I have no way of making very accurate calibrations of such tiny currents, I tend to use them in a partly relativistic way. You have to leave them to stabilize for a week before measurement, so it's best not to be in any sort of hurry.
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T-0hm measurement techniques
