[volt-nuts] Anyone know how to make stable inductors?

[acbern at gmx.de]

Thats a great document, and certainly is a supperior method than doing some DIY coils, provided very stable resistors and capacitors are used. Resistors is straightforward. As capacitors, mica glass is recommended. There are some NOS russian mica high rel capacitors available for little money. I have used those as standards, however I have no history yet, since I only have the inital cal data, second cal verification is still pending, but since they are hermetic, and provided they are used at a constant temperature, should be very good.

It also annoys me that I would have to send C/L meters to calibration regularly. So having own standards is a solution, provided they are stable. Actually, the HP service manual of the older meters says no need to do L verification, as the measurement principle ensures that when doing C verifcation, L automatically is covered, since no good L standards for higher frequencies are available (which seems not really true as we have learnt). Should be true for the 4284A measurement principle too, but I have not checked.

The next question then is, how do you ensure that the drift of these DIY capacitors is good (it is just an assumption they are stable, and 50ppm per year is a challange!!. The IET 1404 primary standards are <20/<40ppm only!!). Actually 50ppm may be pretty hard with anything else than a invar air capacitor. Now these hermetic invar 1404 air capacitors are availabe from time to time on ebay (also ESI SC1000), and thay have that guaranteed low drift over years. Most often they are quite expensive, but from time to time ony pops up at a reasonable price, got mine for about 500 usd. They could be used as a gold standard for at least one value. Together with an inductive decade transformer bridge other capacitances could be calibrated (the decade transformer principle ensures inherent calibration).
And third, one could build up a quadrature bridge and refer a capacitor to a resitor which can easily be measured. Thats how the national labs work to define capacitance values down to the sub-ppm accuracy (besids using calculable capacitors). Building such a quadrature bridge with relatively high accuracy (10E-5 at 10nF) is not too difficult, provided one has a decavider 7 digit ac voltage divider (e.g. for AC voltage calibration purposes) and a dual channel digital 16 bit DDS in the lab anyway.



> Gesendet: Donnerstag, 20. August 2015 um 03:38 Uhr
> Von: "Todd Micallef" <tmicallef at gmail.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] Anyone know how to make stable inductors?
>
> Here is a DIY guide to making some lab standards. It is detailed with some
> component values.
> 
> http://nopr.niscair.res.in/bitstream/123456789/4848/1/JSIR%2065%286%29%20510-513.pdf
> 
> On Wed, Aug 19, 2015 at 8:48 PM, Dave M <dgminala at mediacombb.net> wrote:
> 
> > Here's a paragraph from IETLab's web site on how their inductance
> > standards are made:
> >
> > "Each standard inductor is a uniformly wound toroid on a ceramic core. It
> > has a negligible external magnetic field and hence essentially no pickup
> > from external fields. The inductor is resiliently supported in a mixture of
> > ground cork and silica gel, after which the whole assembly is cast with a
> > poƫting compound into a cubical aluminum case."
> >
> > Sounds like their objective is to isolate the winding from as many
> > external influences as possible.  Of course, the same couild be said of any
> > physical or electrical standard.
> >
> > Cheers,
> > Dave M
> >
> >
> >
> >
> > Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> >
> >> I was looking to make some inductors that I can use as a sanity check
> >> for
> >> my HP 4284A LCR meter. I don't too much care what their values are,
> >> but I want them to be stable with time. Any suggestions about the
> >> best way to
> >> make or buy them? I'd like values in the range of 1 nH to 100 mH.
> >>
> >> The LCR meter has 4 terminal Kelvin connections, with 4 x BNC sockets
> >> on a 22 mm pitch.
> >>
> >> The meter is at Keysight at the moment being calibrated, along with a
> >> free software upgrade they are kindly providing. So I'd like to
> >> measure some inductors when it comes back, and track their values
> >> over time, to see if the meter is drifting.
> >>
> >> The meter covers 20 Hz to 1 MHz, and has a basic uncertainty of
> >> 0.05%, so ideally I'd like to keep inductor changes to less than
> >> 0.005% over a year, so the inductor is an order of magnitude better
> >> than the meter. Maybe that is not practical. As I say, the absolute
> >> value is not important, since I only want a comparison.
> >>
> >> The calibration costs on this meter are not too bad (£207 GBP), but
> >> the calibration interval is 6 months, which is a bit annoying. I'd
> >> rather not
> >> be sending it off every 6 months if I can satisfy to myself it has not
> >> drifted too much. Luckily I don't need to satisfy anyone else.
> >>
> >> Dr. David Kirkby Ph.D CEng MIET
> >> Kirkby Microwave Ltd
> >> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex,
> >> CM3 6DT, UK.
> >> Registered in England and Wales, company number 08914892.
> >> http://www.kirkbymicrowave.co.uk/
> >> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
> >>
> >
> >
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