[volt-nuts] Resistance standard

Alan Scrimgeour scrimgap at blueyonder.co.uk
Wed Dec 16 06:49:35 UTC 2009

Hi Rob,

I'm new to this area but thinking a lot about it lately, so here these a 
just ideas rather than any sort of 'inside knowledge'.

I'd be suspicious of any perfumed oil and more inclined to trust 
pharmaceutical grade products. The stuff for horses is presumably cheaper 
than the human variety, but would it be as pure? It may well be identical. I 
don't know if it's capable of absorbing any moisture but you could heat it 
to say 140C for a while to drive off volatiles. You could have a small 
capsule of silica gel in the sealed resistance box to keep the oil dry.
I think you may have problems casting resistors into blocks of resin. If you 
make castings too big the centre of a volume can get very hot and go into 
thermal runaway during the curing process. Even with a small casting the 
resin will develop mechanical stresses as it sets and shrinks which will be 
transfer to your resistors. I would stick to surrounding your resistors with 
a fluid, or perhaps a vacuum or gel. I wonder if convection currents within 
mineral oil are a source of error? Fumed silica could be blended with it to 
increase its viscosity. What about using any oxygen free gas instead of oil? 
CO2, Argon, Nitrogen? And are your lower grade resistors really not already 
hermetically sealed? Or perhaps they just need a spray with conformal 
coating to perfect the seal.

How about avoiding the need for leads and their thermal EMF issues and 
instead fitting your (small and light) resistance box with banana plugs so 
it can plug straight into a meter for calibration?

The electrical connections may act as thermal leaks into your sealed box. 
This can be improved by having connecting leads in thermal contact with the 
container over an extended distance both outside and inside the container 
and using a coiled connection leading to the resistor within the container. 
Not sure if that would impact on the resistance value significantly.

Soldering can have a big effect on a resistor as I found out recently. 
Having made such a large investment I would consider the possible advantages 
of crimping rather than soldering. If soldering is considered more reliable 
I would use home made and very effective heat sinks between the solder point 
and the resistor body and try not to let any heat get to them beyond your 
85C bake out temperature.

Hope there might be something useful in that lot for you!


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