Q about Resistor Aging

[Watson A.Name - \"Watt Su]

I have a precision resistor that I use (among several others) to
calibrate my DMMs etc. It's 987.3 ohm, .01% with radial leads in a
black cylinder that's about 3/8" (9mm) by 1/2" (13mm). The date code
looks like it's from 1968, so my question is, how much would aging
change the resistance of a resistor like this, assuming it's over 30
years old. I don't know if it's wirewound, but almost all of the
precision resistors that I have from that long ago are obviously
wirewound because they're on a bobbin. The top of this cylinder looks
like the element was inserted and then epoxy was poured over it, level
with the top. So I'll assume that it's wirewound. Any ideas? Thanks.


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"Watson A.Name - \"Watt Sun, the Dark Remover\""
<NOSPAM@dslextreme.com> wrote:

I have a precision resistor that I use (among several others) to
calibrate my DMMs etc. It's 987.3 ohm, .01%

Probably depends on the construction of it. Do you know the
temperature compensation to use?

 

[Tim Shoppa]

"Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com> wrote in message news:<10pe3v97op2l8b0@corp.supernews.com>...

I have a precision resistor that I use (among several others) to
calibrate my DMMs etc. It's 987.3 ohm, .01% with radial leads in a
black cylinder that's about 3/8" (9mm) by 1/2" (13mm). The date code
looks like it's from 1968, so my question is, how much would aging
change the resistance of a resistor like this, assuming it's over 30
years old. I don't know if it's wirewound, but almost all of the
precision resistors that I have from that long ago are obviously
wirewound because they're on a bobbin. The top of this cylinder looks
like the element was inserted and then epoxy was poured over it, level
with the top. So I'll assume that it's wirewound. Any ideas? Thanks.

The most likely thing that could do you in is leakage. To change
a 987.3ohm resistor by 0.01% downwards, you only need a 10Mohm resistance
in parallel to it. Gunk building up on the outside of the resistor
or a decay in the internal epoxy coating could do that. Typically such
things are in the Gigaohm range, but 40 years of grunge might add up.

If two turns of wire shorted inside the resistor you
would see a drop in resistance that was a big step, could be well over
one percent (depending on the geometry of the short).

Tim.

 

[Watson A.Name - \"Watt Su]

"Tim Shoppa" <shoppa@trailing-edge.com> wrote in message
news:bec993c8.0411140511.3dd506a0@posting.google.com...

"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com> wrote in message

news:<10pe3v97op2l8b0@corp.supernews.com>...

I have a precision resistor that I use (among several others) to
calibrate my DMMs etc. It's 987.3 ohm, .01% with radial leads in a
black cylinder that's about 3/8" (9mm) by 1/2" (13mm). The date
code
looks like it's from 1968, so my question is, how much would aging
change the resistance of a resistor like this, assuming it's over 30
years old. I don't know if it's wirewound, but almost all of the
precision resistors that I have from that long ago are obviously
wirewound because they're on a bobbin. The top of this cylinder
looks
like the element was inserted and then epoxy was poured over it,
level
with the top. So I'll assume that it's wirewound. Any ideas?
Thanks.

The most likely thing that could do you in is leakage. To change
a 987.3ohm resistor by 0.01% downwards, you only need a 10Mohm
resistance
in parallel to it. Gunk building up on the outside of the resistor
or a decay in the internal epoxy coating could do that. Typically
such
things are in the Gigaohm range, but 40 years of grunge might add up.

If two turns of wire shorted inside the resistor you
would see a drop in resistance that was a big step, could be well over
one percent (depending on the geometry of the short).

Tim.

So what're you saying? Are you saying that I shouldn't be concerned
with aging, that I should look at other avenues for errors? Maybe I
should just send the resistor into a calibration lab for verification?
Or buy a standard precision resistor that's traceable to NIST? Or..

Maybe I should just trust it. After all, when I measure it with the
DMMs, it's usually right on, allowing for several hundredths of an ohm
for the test leads, etc.