[volt-nuts] Fluke 731A Transfer Standard and Fluke Wire WoundResistors
J. L. Trantham
jltran at att.net
Sun Sep 16 21:54:46 UTC 2012
Thanks for the reply.
I was just looking at the properties of the wire. I guess I don't
understand the 'resistivity' of 43-48 uOhmcm for the wire that is about the
same size as what the resistor is made of. Also, it is quite expensive,
about $300 per 100 feet.
It lists a tempco of .00001 per K.
As best I can tell, the 5903 ohm resistor is about 15.5 mm long, on a 1 inch
flat form about 1 mm thick (about 2 inches per turn) and about 14 turns per
mm as best I can tell. I measured the wire at .05 mm thickness. That would
put it at about 217 turns, 434 inches, and about 13.6 ohms per inch. My
original calculation appears to be wrong.
So, how to proceed?
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
Behalf Of Pete Lancashire
Sent: Sunday, September 16, 2012 4:03 PM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Fluke 731A Transfer Standard and Fluke Wire
There is a good chance the wire is Manganin or an allow that is similar.
If what you use to connect to the resistance wire is not the same you run
into things like EMF (think thermocouple) being created at the repair.
On Sun, Sep 16, 2012 at 1:36 PM, J. L. Trantham <jltran at att.net> wrote:
> I have been following the discussion about the LM399 heater, noting
> of the Fluke Wire Wound resistors used often in Fluke precision equipment.
> I have a 731A that I have been resuscitating and have found that the R8A
> wire wound resistor is failing. Initially, the source was very unstable
> then went to about 8 VDC. This led me to an open R8B. I was able to
> reconnect the broken ends, loosing about half a turn of wire. Then,
> again, unstable and went to about 13 VDC. This led me to an open R8A.
> Again, able to reconnect, and, again, loosing about ½ turn.
> It appears that the wire is corroded with corrosion leading to failure.
> This brings up several questions.
> 1. What are the benefits of these wire wound resistors? Tempco?
> Ability to construct precise resistance?
> 2. Is there a source of the resistance wire used in these resistors
> that the resistor can be reconstructed? I think the resistance is in the
> to 40 ohms per foot range. Something like 36 to 38 gauge Nichrome 60
> work but it has to be insulated.
> 3. Would it be better to look for a collection of commercial
> to replace R8A, R8B? If so, what? The manual states that IC2, the
> voltage reference, R6, R7, R9, and R10, are factory selected and all
> appear to be OK.
> 4. Would it be better to completely abandon the voltage reference
> section of the 731A in favor of a new (perhaps LM399, LTZ1000A, etc.,
> based) reference to generate 10 VDC and still use the output divider of
> 731A to generate the remaining voltages?
> The 731A manual and schematic are here:
> R8A is the high side of a voltage divider with R8B, R9, R10 and R11 (a
> ohm, 20 turn pot) forming the low side, dividing 10 VDC to feed the
> voltage reference, IC2, a DH80417B. The op amp, IC1, used to generate the
> 10 VDC is an LM301AH.
> R8A measures about 5903 ohms, R8B measures about 11558 ohms, R9 measures
> about 78.02 ohms, and R10 measures about 399.8 ohms.
> Thanks in advance for any and all information and suggestions.
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