EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: gus789 on July 24, 2013, 09:18:03 pm
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Hello, I have a problem that maybe a few other folks here can relate to. I work in a university lab and we commonly have to use test equipment that due to either lack of money or time (or both) has not been sent in for calibration in a few - or quite a few - years. In my case, this includes equipment like vintage keithley 175A bench multimeters, miscellaneous handheld multimeters, as well as higher end (but older) gear like a few Keithley 236 source measure units and a HP4145 semiconductor parameter analyzer. It would be very nice to have a few "standards" around to quickly check if the equipment in question meets certain specs before usage.
I was wondering if anyone here can provide some suggestions/insights as to how such standards could be setup.
For most applications, I'd be looking to generating DC currents in the range of tens of nA to tens of mA and voltages in the tens of uV to a volt or so. I have seen the video where Dave discussed some precision reference resistors he uses to check ohmeters. For checking current and voltage meters I was considering using a few such resistors with one or more voltage reference ICs to produce stable currents or voltages. The tricky part would be to produce very small voltages as that would require very low value yet high precision resistors.
For checking current and voltage sources I'd imagine that the best way would be to setup some kind of balanced bridge with a voltage reference on one side and the equipment to be checked on the other, but suggestions here would be particularly welcome. Thanks.
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Although not really good enough to adjust or calibrate your equipment, you might find the products offered by Doug Malone good enough for your purposes.
http://www.voltagestandard.com/Home_Page.html (http://www.voltagestandard.com/Home_Page.html)
If you want you equipment adjusted to their full accuracy then you need references to be be an order of magnitude better than the desired calibration/adjustment accuracy. Give him a call or email and see what he suggests.
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Conrad Hoffman's Mini Metrology Lab articles are a good start. He covers a precision reference, null detector, and kelvin-varley divider. Combined these can be used for calibrating most DC measurement devices.
http://conradhoffman.com/mini_metro_lab.html (http://conradhoffman.com/mini_metro_lab.html)
If nothing else the articles should give you some ideas on the difficulties and techniques you can use to keep your equipment requirements down.
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Yes, I've seen Conrad's articles. Building a K-V divider as he did is an option, albeit a time consuming one. I even filled up a cart at mouser with the parts required for that build to get a cost estimate for such a project. What used to be $75 in the early 90's is more like a $160 now. Still not too bad, just some inflation. I have also considered ebay for second hand decade resistor bridges, etc, but for those the specs are just not quite there, plus I'd have to send the bridge in for calibration, which defeats the purpose somewhat. New ones (like those from IET) are out of the budget for me at the moment.
The idea of course is not to calibrate the instrument, but simply to check if specs are met with some degree of confidence within a particular range. It does not help that we do a lot of work with lower level signals, otherwise Doug Malone's gadget would serve the purpose well. Still may be good for checking handheld multimeters though.
In any case, I just sent one of the keithley 236's to New Jersey for calibration today. $375 aint's cheap but maybe that will become the reference to check others against in the near future. Gus
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One option for US users is to rent or borrow a calibrated 6.5 digit DMM, like the 34401a, and compare it against your device under test using some samples in your lab. The samples need only be stable for a few minutes enough to compare the DUT, and the calibrated DMM. All you'd need is to stock components that have low tempcos and low noise to use as test samples. You can make these checks yearly in lieu of sending it in for calibration.
For example, take an ordinary 1.25V AAA primary alkaline or LSD NiMH rechargeable; they have very low self discharge rates. Measure it with the calibrated DMM to 6.5 digits and say you read 1.250 000 DCV with resolution to 1uV. Now compare that reading against your DMM; if off you can either adjust it or sent it to a cal lab for formal cal, particularly if you need the traceability certificates current as mandatory for grant work.
Note, when using DMMs of similar accuracy you get into problems with TUR if yo intend to adjust the meters. You can compensate for that formally by renting the 3458a when comparing a 6.5 digit DMM.
The rental cost of a 34401a in the USA is about $50-100 a month, and a 3458a about $500 a month. Having the instrument in your lab will allow you to check a lot of devices yourself and be truly confident of the results. The drawback is having to do the labor yourself, knowing some metrology, and being skilled to do low level measurements properly to avoid noise and stray voltages. I do this now in lieu of getting a Geller or a Malone as you can confirm entire ranges and the rental instruments are fully documented.
http://www.abtechtechnologies.com/node/427 (http://www.abtechtechnologies.com/node/427)
http://www.abtechtechnologies.com/node/425 (http://www.abtechtechnologies.com/node/425)
The above is only an example, rental companies have different policies and rates depending say on the duration and number of units.
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DC test samples, collected via eBay over 3+ year span:
DCV
For precision DCV to 100nV to 10V, Krohn Hite MV106, $100
For precision 1V-90VDC, serially connected Power Designs precision DC supplies 50V+20+20, $150 for all
For 30V-3kV: surplus electrophoretic supply, stable to 1V, $10
ohms:
ESI Tegam Decabox : 1megaohm and 10k ohms, stable to 10 milliohms [ ?? 1 milliohm, I forget] , $40 for both
1-100 megaohms: 10 megaohm resistors x 10 $20, Vishay RNX series1
100-1000 megaohms, 100 megaohm resistors x 10, $20, Vishay RNX series1
1http://www.alliedelec.com/search/productdetail.aspx?sku=70201686 (http://www.alliedelec.com/search/productdetail.aspx?sku=70201686)
bought new
Current:
?? uA to 2A, I take it directly out of the Power Designs PSU
I lack a high DC current source, 10A, but my most pressing needs is low power < 2A, so its not been as anal in acquiring one. Same as with AC.
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Thanks for the suggestions. Yes, for now at least my rationale is along the same lines as saturation proposed, i.e. having one unit officially calibrated once a year and hopefully use the calibrated one to check the other 236 units as well as other DMM around the lab. I hadn't considered the option of renting a unit for that purpose, though, which seems likely to work too. If all this works out then ~$400 yearly in calibration is reasonable. A little over $30 a month buys some peace of mind knowing that what you are measuring is not complete garbage.
Still would be a nice challenge to hack a good low level reference from off the shelf parts. Gus
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BTW, saturation, you have enough to start your own cal lab :)
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Thanks, in a way that was the idea. We are blessed in the USA that we can buy the cal equipment of 3-4 decades ago fairly cheap, and that's what I've been doing. These are not cal lab quality, rather what I have are field calibrators; made to adjust instruments in their installed locations. The difference is 1 year accuracy and they are 'portable' by 1960's definition :-DD. The cal lab references from the 1960s/70s remain fairly costly, about 10-100x what I paid, and boat anchor in size and weight, but field devices have very good short term, i.e., 24 hour, stability and easily make for very reliable 'transfer references' comparing a known calibrated DMM against the DUT right at your bench.
If you total the cost, ~ $340 is spent for a DC transfer setup; plus the annual cost of renting if not 'zero' cost borrowing someone's calibrated DMM, the $340 is just a capital cost, not recurring cost, and is really a bargain. In fact, the devices I purchased can be sold for near or greater than what I paid, it all depends on eBay supply-demand; so there is no depreciation, its fully depreciated as is and a stable value. Spread over 3 years it cost to acquire it, that's ~ $113/yr, both the annual or total cost is far underneath most capital equipment costs in organizations and can be taken from 'petty' cash.
BTW, saturation, you have enough to start your own cal lab :)
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(http://www.pa4tim.nl/wp-content/uploads/2010/10/calibratorek.jpg)
Over here it is a bit more difficult but it can be done. My calibration rack. On top a guildline with four standardcells and full history. Then a bunch of gear from Fluke, Philips and ESI. Calibrators, stanards and KV dividers.Besides that I have stanard resistors, inductors and capacitors and some home made standards.
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Very nice PA4TIM :clap: