[Test-Equipment] Resistor measurement

[Brooke Clarke]

Hi Richard:

Why did they use a DC  measurement method?  Using AC gets rid of the 
problem of DC offsets caused by different metals.  See:
http://www.prc68.com/I/Kelvin.shtml
Where the DC problems put a limit on how low a resistance you can measure.

Have Fun,

Brooke Clarke
http://www.PRC68.com


Richard Knoppow wrote:
> ----- Original Message -----
> From: "Fuqua, Bill L"<wlfuqu00 at uky.edu>
> To: "Discussion of Electronic Test Equipment"
> <test-equipment at mailman.qth.net>
> Sent: Tuesday, July 05, 2011 10:33 PM
> Subject: Re: [Test-Equipment] Resistor measurement
>
>
>    
>> If you had 4 of them you could make a bridge and use a
>> sensitive meter
>> to check the balance. If any one of them was off there
>> would be an error or imbalance and
>> it is unlikely all  of them would have the same error.
>> 73
>> Bill wa4lav
>>
>>      
>       If you have two of something you can compare them to
> the degree of precision of the measuring device. There is a
> difference between accuracy and precision. An instrument may
> have a high degree of precision, that is, be able to detect
> and read very small differences repeatably and reliably, but
> not be accurate. However even an uncalibrated instrument can
> make a comparison reading within the limits of its
> precision. An instrument can not be calibrated to an
> _accuracy_ greater than its precision, however it can make
> differential readings up to that limit.
>       In fact, the substitution method is a good one for
> calibration of devices such as resistors since it eliminates
> the tolerance of all components other than the method of
> reading out the differences.
>       The usual standard for resistance calibration is a
> Thomas Ohm cell. These are extremely precise one ohm
> resistors of known error. They can be certified by NIST and
> used to calibrate working standards at a DC standards lab.
> The less accurate and stable standard is called a Rosa
> resistor or Rosa bottle because they resemple bottles. These
> can also be certified by NIST but are usualy used as working
> standards after being calibrated against a certified Thomas
> Ohm or group of them.  Its been a long time since I
> maintained a DC standards lab for a resistor manufacturer. I
> think our master bridge was an Otto Wolf bridge which was in
> an oil bath to maintain its temperture constant. We had a
> group of about a dozen Thomas Ohms, which were cycled
> through (at that time) NBS. Precise resisance measurements
> can be difficult despite being DC. Especially difficult is
> the measuement of very small resistances since the
> difference in potential of the metals at the terminals can
> affect the readings. We had a very good Kelvin Double Bridge
> to measure fractions of an ohm. Oh well, another life...
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
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