Calibration equipment design

If you need to ask, you shouldn't be building it.

They cost that much in part because they are traceable to NIST. It will cost you an arm and a leg to build traceable equipment - secondary standards are pricey, and keeping those standards traceable is pricey.

Dale H. Cook, Chief Engineer, Centennial Broadcasting, Roanoke/Lynchburg, VA - WZZI / WZZU / WLNI / WLEQ

Dale H. Cook wrote in news: snipped-for-privacy@4ax.com:

Building a frequency standard would be the easiest. an xtal and some divider ICs. You can cal it using WWVB. Electronic Goldmine used to sell a xtal timebase kit for scope cal,with PCB.

You want accuracies at least 4x better than what you are calibrating.

Designing equipment that'll be used for *traceable* calibrations isn't something most engineers can take on by themselves. This stuff is expensive for a reason!

You'll find lots of good deals on older calibrations standards on flea bay. Years ago I bought a very nice Fluke voltage/current standard for $50.

Thanks for the replies,

Reactions: "If you need to ask, you shouldn't be building it. "

What did you do when you build your first electronic whatever? You just designed and built it? I think you asked or looked for info to...

"They cost that much in part because they are traceable to NIST. It will cost you an arm and a leg to build traceable equipment - secondary standards are pricey, and keeping those standards traceable is pricey."

I like to make clear that I do not want to have the equipment to be to the NIST standards as this is a bit to extreme. An above standard precision is ok.

"You'll find lots of good deals on older calibrations standards on flea bay. Years ago I bought a very nice Fluke voltage/current standard for $50."

That's an option and worth considering.

I ask myself" how did the first test equipment mfg's started to calibrate and continue to compare the rest of their production with the first? It all started somewhere and with an average precision (as precision had still to be invented).

Google for "standard cell".

Then when you have this voltage reference you can compare any voltage to this standard with a bridge composed of a null votage detector and a Kelvin-Varley divider (KVD). Now think how about how KVD are built: if you have thermally and age stable resistors a KVD is easy to build to high precision.

It may have started out simple over 100 years ago, but now if it isn't traceable to NIST, it isn't calibrated.

Again you're correct; if only you want to manufacture test equipment and sell them. But here it is for a limited use. We like to calibrate our instruments by our self and not at a price. It may be exaggerated to some but it is not for us. Making electronic equipment or calibration standards do not differ much, except the precision. And for the later we do not have much expertise. We prefer to build it by ourself (and for ourself!) so we know what we are doing. As written before: getting some on eg Ebay may be an option, but how do we know they are correct and precise? Back to square one I would say... Getting the calibrators calibrated by a NIST recognized institute?

Zilog

That is ok for home brew projects, but not for professional work

You have them calibrated as primary, or secondary standards like everyone else. You can't just set your own arbitrary standards.

Its the only way to fly!

If you can't trace it to an internationally recognized standards organization such as NIST, how will you know that it is accurate? How will you know what its precision is? In order to know that your calibrations have a given accuracy those calibrations must be traceable.

The amateur calibrator of electronic equipment is primarily interested in two classes of standards:

You only need one of these, since each can be converted to the other. In practice the amateur calibrator almost always maintains a local frequency standard. I have two. The one that I use is the oven-controlled time base in my frequency counter, which is calibrated to WWVB using an HP VLH comparator. By running the calibration plot for 12 hours I can obtain a short-term accuracy for the local standard of about 1 part in 10^10, and can achieve higher accuracies with longer calibration plots. This method can also determine the ageing rate of the local timebase.

My other standard, which I maintain just for fun, is the oscillator in a very old GR frequency standard.

The appearance of rubidium standards on the used equipment market offers an alternative for those with deep pockets - they can often be found in the $500-$1,000 range.

You need two of these in order to determine the third. In practice the amateur calibrator generally maintains local standards for voltage and resistance. I own two standard cells, a standard resistor, and the potentiometers and bridges needed to scale them, as well as a VTVM calibration set which is calibrated against my local standards. I have the local standards calibrated periodically by a local cal lab. I can calibrate instruments such as VTVMs to an accuracy of 1%, which is what those VTVMs are specified for, and all that I am interested in.

Instruments that I use for work must be traceable to NIST in order to satisfy the FCC. Those other than my frequency counter are periodically calibrated by returning them to the manufacturer or by taking them to a local cal lab.

Dale H. Cook, Chief Engineer, Centennial Broadcasting, Roanoke/Lynchburg, VA - WZZI / WZZU / WLNI / WLEQ

Precisely - either by the manufacturer or by a local cal lab. Your local standards will also have to be recalibrated periodically in order to maintain long-term accuracy.

Dale H. Cook, Chief Engineer, Centennial Broadcasting, Roanoke/Lynchburg, VA - WZZI / WZZU / WLNI / WLEQ

You've answered your own question. There's no reason why you can't design it (though it hasn't been recommended here) and send it out to be calibrated and/or adjusted.

Depending on the calibration lab, some are happy to adjust AND calibrate providing you have instructions on how to do that. If you've designed your own, it's obviously mandatory, but if it's a bought box, the cal lab *might* have procedures on hand to do it - just ask.

Buy and then have adjusted and/or calibrated. This really is the cheapest and easiest way to go about having _known_ accuracy test kit in your lab.

That's not strictly necessary.

What is also doable here in Australia, having something NATA certified (our version of your NIST) is not the only way of having a known accuracy.

Some test labs offer tests and/or adjustments that give the same tests as would the traceable standards, however you don't get a "fancy" certificate at the end, and you don't have traceable paperwork- at a significant discount.

For those where traceability is not important but cost is, it's a very viable option.

At the end of the day, you know where you stand, but just don't have the certificate to prove it. If the traceability doesn't hold much weight for your workshop, then this is very doable.

I don't know if all test labs offer this, even across any type of equipment, but it's worth asking.

Then you have no way of knowing whether their work is accurate.

But is isn't calibration, because there is no traceability to an accepted standard.

You don't know where you stand, because the work is not traceable.

Dale H. Cook, Chief Engineer, Centennial Broadcasting, Roanoke/Lynchburg, VA - WZZI / WZZU / WLNI / WLEQ

Hmm,It seems that the result is as follows:

1. You let your equipment calibrate and do the things you have to do with that calibrated equipment. You just lost an arm and a leg. You can allways show everybody that your test equipment is calibrated but not that your repair or build goods are...
2. You get some calibrated references (or build and let them calibrated)and go on from there. Again here your repairs or build goods are within spec (because of the calibrated calibrators) but can not show that it is due to the missing paperwork. Youjust lost an arm...

I worked for a multinational company that was very strikt on calibrations etc. At the end we were working with test equipment that showed us different results. Fault by the calibration institute? Due to mal use or aging?

It never happened when the calibration was done in house (with calibrated calibraters) But no fancy papers. Iso 9000 etc is no step forward but rather backward at the expence of the industrie.

So I know enough now to get my instruments calibrated.

Thanks all,

Zilog