Electronics > Metrology

New Precision Resistors

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MisterDiodes:
I have one more thought on resistors / humidity protection - and this is a cheap, practical and classic way to do it - just something to add to your toolbox when working with possible humidity-sensitive stuff:

Toss in a desiccant packet into say your precision resistor enclosure and put the covers back on.

For instance, this was done on a lot of Precision resistance decade dividers.  We have a few 7-decade dividers built this way - circa late 60's.  All PWW resistors, none in hermetic packages, and these boxes are checked for NIST cal occasionally - and they are always within their expected +- 0.3ppm range or better - and have been perfectly fine for decades. 

BUT there is a purpose-built screen holder inside the cabinet to hold a 4oz desiccant packet.  We use the type that turns color when it's time to change, and we change it out once or twice a year.  You can bake the packet out in the oven and re-use it.  There really isn't a lot of airflow in and out of the cabinet, and the humidity inside is usually right around <30% or so.

While you change out the packet you might hit a switch contact with DeOxit or grease a shaft  if required but very rare and very sparingly.  That's it.

So that's another simple, practical approach that will work fine for some items that don't run warm, and we copy the idea for other boxes around the lab.

branadic:
I wonder if some Vishay VAR resistors with feedthrough capacitors (4 for 4W connection) in a copper case and the case filled with Galden could give some stable resistor without humidity problems. I think I need to check this some time...

Andreas:
Hello branadic,

according to my measurements they have a hysteresis that is not better (even worse) than epoxy-packaged resistors.

https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879467/#msg879467

It seems there is some paint/epoxy over the resistor which should be removed before packaging.

with best regards

Andreas

branadic:
Thanks Andreas, have you ever checked what happens if you put them into some hydrophobic fluid?

If it is an epoxy only mechanical removing is possible, which should be avoided. Paint could probably be removed with some solvent, but could also dissolve the glue between ceramic substrate and foil?

JS:
  Just as a thought exercise, what's the case of resistors living under oil, like the fluke 720 first decade. I guess the humidity shouldn't get to the resistors and now instead of hermetic packaging only something able to contain the oil, which is much easier to stop than humidity in the air. Am I missing something? I'd still guess fluke did it just, or mainly, for thermal tracking there.

JS

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