Electronics > Metrology

NML 1 ohm standard resistor

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lowimpedance:
 After reading Zylmex's excellent thread on a number of resistor standards I had a rummage around in some cupboards for some old parts relating to the 1 ohm standards that were produced a very long time ago. See pics.
Managed to scrounge a few of the 'heat treated' coils, an outer can and inner mount, (will have to dig through some other boxes to see if I can find enough bits to assemble a complete one  :). Unable to really call this "bitsa" a standard though, however most likely good for the hobby lab I reckon!. (even without an oil bath!).

A spec. sheet found is also posted as is a pic of the earlier style of construction with the outer can removed.
Also some 'light' reading here and a mention of the NML 1 ohm, if interested.
www.nist.gov/calibrations/upload/std_resistor_2009_metrologia.pdf
Unfortunately I dont have access to better measuring equipment or the oil baths etc (might raise a few eye brows  :D)

TiN:

--- Quote ---might raise a few eye brows
--- End quote ---
Indeed.  :-/O

Transportability = hand-carried +/-0.02 ppm/journey gives a nice smiley.  8)

zlymex:
This is great. Is it the new Thomas selling by MI? Would the coils and the supporting frame be sealed? Here are something related.

Calibration Procedures For Direct-Current Resistance Apparatus
digicoll.manoa.hawaii.edu/techreports/PDF/NBS39.pdf

Automated Resistance Measurement Systems w/ Sub-PPM Accuracies
www.mintl.com/media/pdfs/tech/subppmresistancemeasurement.pdf

Temperature and pressure coefficient of Resistance for Thomas 1R Resistors
www.nist.gov/calibrations/upload/coefficients-of-resistance.pdf

http://www.mintl.com/DC/Products/Resistance/Resistors_and_Shunts

Vgkid:
Nice job lowimpedance.
Thanks for the reading materials zlymex.

manganin:

--- Quote from: lowimpedance on March 14, 2016, 03:47:29 am ---I had a rummage around in some cupboards for some old parts relating to the 1 ohm standards that were produced a very long time ago.

--- End quote ---

There are a few major differencies compared with the Thomas type.

The NML wire was Evanohm, not Manganin and the heat treatment process of the Evanohm wire allowed much lower temperature coefficient. The TC of the Thomas type was quite high even for Manganin, the process was not TC optimised.

The Thomas was tightly wound on a brass cylinder, but the NML had a self supporting structure, secured with silicon rubber strips*. That solved the main problem of the Thomas type, the very high sensitivity to temperature variations. (A thermostat fault in the early 1970's resulted in abandoning five of the ten Thomas resistors in the NBS legal ohm group. The temporary rise in the oil bath tempererature was around 10 degrees, but that was enough to make the resistors unstable.)

The NML was not hermetically sealed, even though a flange type sealed container was designed.

* Silicon oil bath will ruin the resistor!

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