[Test-Equipment] Resistor measurement

[Richard Knoppow]

----- 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