Author Topic: How reliable is a calibration?  (Read 10176 times)

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Offline quantumvolt

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Re: How reliable is a calibration?
« Reply #25 on: June 16, 2013, 02:33:42 pm »
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Or you can buy a 4 1/2 digit Voltmeter module on eBay for 10-20 USD (I got one, they claim informally around 0.3% accuracy, but mine is closer to 0.1% within the readings from an Agilent 34401A).

You mean 3 1/2 digit moduls, don't you?


http://www.ebay.co.uk/itm/5-Digit-0-36-red-LED-DC0-33-000V-3-wire-Digital-voltmeter-DC3-5-30V-powered-/350813582680?pt=LH_DefaultDomain_0&hash=item51ae1e7558

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Offline jancumps

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Re: How reliable is a calibration?
« Reply #26 on: June 16, 2013, 02:39:43 pm »
I would definitely question the drift from spec. with them; also because it's a proper company and not a second-hand seller; it's their problem if the item shipped isn't within spec. when it reaches you.

We are judging them by verifying their findings with measurements from uncalibrated equipment.  Even though multiple meters are used, their performance is not verified and 3 multimeters is too small a batch to give reliable probability that the calibration company is wrong.
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #27 on: June 16, 2013, 02:59:39 pm »

We are judging them by verifying their findings with measurements from uncalibrated equipment.  Even though multiple meters are used, their performance is not verified and 3 multimeters is too small a batch to give reliable probability that the calibration company is wrong.

Agreed the DMMs are uncalibrated and I was considering that at first.  But what are the chances that three high quality (well two are high quality, one is RS) agree to within .02 V that the supply is reading high by 0.2V?  I believe that probability is remote, even with only three samples.
« Last Edit: June 16, 2013, 03:43:18 pm by mikepa »
 

Offline quantumvolt

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Re: How reliable is a calibration?
« Reply #28 on: June 16, 2013, 03:09:20 pm »
I am sorry to say so, but most of the posters here are talking about snow during the summer holidays.

This is the instrument in question or something very like it. All the OP complains about is that the Voltmeter MEASURES an UNKNOWN supposed to be 10.0 or 20.0 Volt voltage with a 'large' error.





His 'evidence'  :o  is that he has 3 other meters showing at least 2 other values (within 20 mV of each other) for the same unknown voltage. So in fact the OP claims that based on an unknown value to be measured and at least 3 different measured values on 4 different voltmeters, the instrument under test is out of spec. The OP is probably right - but he does not seem to listen to the technical information given in this thread about linearity of digital meters.

Call Britest, tell them you think the meter is out of spec, and see what they say ... :-DD
« Last Edit: June 16, 2013, 03:31:49 pm by quantumvolt »
 

Offline madires

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Re: How reliable is a calibration?
« Reply #29 on: June 16, 2013, 03:11:44 pm »
If you wanted a device spot on; you'd ask for cal & adj, and if required, a "repair". no?   

Basically if the OP power supply is outside the manufacturers spec. but when adjusted  then drifts faster than the manufacturer stated 8hr stability drift,  it will need a repair.

Doesn't that mean (if there was a log of old calibration data) that you loose calibration history as soon as you adjust?

It's not lost, but you need a second calibration just after the adjustment to be able to continue tracking any drifts.
 

Offline madires

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Re: How reliable is a calibration?
« Reply #30 on: June 16, 2013, 03:16:28 pm »
Calibration != Adjustment

Calibration == adjustment (or recording of correction factors to be used in subsequent measurements).

Measuring without adjustment is called performance verification.

Performance verification provides a level of confidence in measurements already made. Calibration is for future measurements.

Please see  http://www.npl.co.uk/reference/faqs/what-is-the-difference-between-calibration-and-adjustment-(faq-pressure)!
« Last Edit: June 16, 2013, 03:20:00 pm by madires »
 

Offline quantumvolt

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Re: How reliable is a calibration?
« Reply #31 on: June 16, 2013, 03:19:24 pm »
At first I thought it must be my Fluke 179 that was out of calibration (it has never been calibrated), but as I have two other DMMs (Fluke 77 & one from RS) for a total of three that all read within .02 volts of each other, it makes me think the power supply is the one out out of calibration.

When you call them, please write down whether it is 0.02 or 0.002 Volts (as you state above). It might make a difference ... :-DD
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #32 on: June 16, 2013, 03:49:44 pm »
Thanks Quantumvolt for pointing out the error in one of my earlier posts, I meant to say the DMMs were within 0.02V of each other, not .002V.  I corrected that post.

While I concede that using un-calibrated DMMs does not provide conclusive evidence, I do believe that their very close agreement provides enough evidence to question the validity of the calibration.

I have contacted Britest and will see what they have to say.  If they won't entertain checking the calibration I will do it myself.
 

Offline Rufus

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Re: How reliable is a calibration?
« Reply #33 on: June 16, 2013, 04:50:51 pm »
Calibration != Adjustment

Calibration == adjustment (or recording of correction factors to be used in subsequent measurements).

Measuring without adjustment is called performance verification.

Performance verification provides a level of confidence in measurements already made. Calibration is for future measurements.

Please see  http://www.npl.co.uk/reference/faqs/what-is-the-difference-between-calibration-and-adjustment-(faq-pressure)!

One of many opinions on the ill defined difference between calibration, verification, and adjustment.

I would argue that calibration requires adjustment. For example when a 10g weight it calibrated and determined to be 9.999g that value can only be used to adjust measurements made using that weight as a reference or to at least adjust the claimed accuracy of measurements made using that weight. If that calibration value is not used to adjust something it has no value. Calibration without adjustment is a nonsense.

Many instruments provide the facility to store calibration results and automatically apply adjustments to future measurements using them. Storing calibration results in the instrument (usually without even seeing them) is still calibration and I consider adding or removing a bit of metal to/from a weight to be storing results and automatically applying them to future measurements.

Calibration and verification are used more or less interchangeably. I would argue that verification implies adjustment to measurements already made (you can't verify something that hasn't yet happened).

So I argue that calibration used to verify measurements already made does not involve adjustment to future measurements and should be called verification. Calibration to adjust future measurements should be called calibration and if the instrument has the facility to store calibration results and automatically make measurement adjustments that is a required part of the calibration.

Verification may show that no calibration is required.

 

Offline Bored@Work

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Re: How reliable is a calibration?
« Reply #34 on: June 16, 2013, 05:43:48 pm »
One of many opinions on the ill defined difference between calibration, verification, and adjustment.

The terms are well defined. E.g.  http://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2012.pdf It is just that the definitions aren't properly taught and that "practitioners" pretty much hate the definitions and ignore them to cover up their sloppy behavior and practice.

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Calibration without adjustment is a nonsense.

Well, if you say so.
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Offline grumpydoc

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Re: How reliable is a calibration?
« Reply #35 on: June 16, 2013, 06:09:45 pm »
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Calibration without adjustment is a nonsense.

No, not at all - in fact adjusting precision instruments can be a nightmare.

If it's a reasonably modern piece of equipment where the adjustment is done electronically it might not be too be, however it's best avoided if the instrument is in spec.

If it's a mechanical adjustment then you can get all sorts of mechanical settling/whiplash type effects which can make it extremely difficult to improve the accuracy, even it it starts out of spec.

I do tend to adjust stuff that I sell on ebay, if it's out of spec, but then I find myself running things for days afterwards to make sure they're not going to drift right back out of spec on me by the time the customer receives the item.

Even then, I can only reliably say that it was in spec when it left my hands as I'm sure a careless courier could bounce something which had trim pots or variable capacitors out of adjustment in all sorts of interesting ways once I have handed it over to their tender mercies.
 

Offline madires

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Re: How reliable is a calibration?
« Reply #36 on: June 16, 2013, 07:08:34 pm »
I would argue that calibration requires adjustment. For example when a 10g weight it calibrated and determined to be 9.999g that value can only be used to adjust measurements made using that weight as a reference or to at least adjust the claimed accuracy of measurements made using that weight. If that calibration value is not used to adjust something it has no value. Calibration without adjustment is a nonsense.

The calibration would tell you how the weight drifts over time and if it's within a defined range/spec. If the weight keeps beeing 9.999g over some years you know that you got a stable weigth. In case it drifts and exceeds the allowed range then you would adjust it or buy a new one. If it drifts a lot I would buy a new one anyway because it's a bad weigth, I can't trust it. A good weigth with no or very little drift is usabe much longer. Would you adjust a bad weight every year (paying for adjustment and a second calibration) or try to get a good one?

Quote
Many instruments provide the facility to store calibration results and automatically apply adjustments to future measurements using them. Storing calibration results in the instrument (usually without even seeing them) is still calibration and I consider adding or removing a bit of metal to/from a weight to be storing results and automatically applying them to future measurements.

I don't know if applying calibration results is already some kind of adjustment or not. If not, the instrument would require a special mode in which it doesn't apply the calibration results to be able to be re-calibrated, I guess.
 

Offline Rufus

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Re: How reliable is a calibration?
« Reply #37 on: June 16, 2013, 07:12:11 pm »
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Calibration without adjustment is a nonsense.

No, not at all - in fact adjusting precision instruments can be a nightmare.

Adjusting measurements made with an instrument (for example dividing by 1.01 because calibration showed it read 1% high) is still adjustment. Adjusting the claimed accuracy or tolerance of a measurement made with the instrument because calibration showed it was 'out' is still an adjustment. If you don't use the results of a calibration to adjust something it doesn't matter what the results are and there was no point calibrating in the first place.
 

Offline Christe4nM

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Re: How reliable is a calibration?
« Reply #38 on: June 16, 2013, 07:38:34 pm »
One very important question: where exactly did you measure with your DMM's? If right at the binding posts, that's OK. But any test lead /wire connected will introduce a voltage drop. Your DMM leads' drop might be very very low while measuring voltages though. So if you measure at your load you will find a lower value than the PSU displays.

[off topic: it works the other way around too. So don't trouble yourself to set your PSU to 3.30 V precisely if you need 3.3 V for example. Measure at the load and adjust accordingly if needed, while keeping in mind that for general use the supply voltages for IC's can be +-5% or even +-10%.]

Also, I'd first do a worst case calculation to the values measured by my DMM's. Same for the PSU display. Then see what's left of the error.

I'm not saying there is nothing wrong with your instrument. Just make sure that you don't introduce errors yourself that might lead to false conclusions.
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #39 on: June 16, 2013, 08:48:53 pm »
Thanks Christe4nM, the leads to the DMMs were connected right at the binding posts using Pomona 4mm plug patch leads.

I have certainly been thinking, "why do I really care".  No circuit that I know of requires high precision voltages.  5.1V is as good as 5.0V.
 

Offline grumpydoc

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Re: How reliable is a calibration?
« Reply #40 on: June 16, 2013, 09:20:25 pm »
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Adjusting measurements made with an instrument (for example dividing by 1.01 because calibration showed it read 1% high) is still adjustment. Adjusting the claimed accuracy or tolerance of a measurement made with the instrument because calibration showed it was 'out' is still an adjustment. If you don't use the results of a calibration to adjust something it doesn't matter what the results are and there was no point calibrating in the first place.

Surely that's compensation rather than adjustment..... :)
 

Offline SLJ

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Re: How reliable is a calibration?
« Reply #41 on: June 17, 2013, 01:54:30 am »
I worked for a defense contractor for several years.  If the contract called for it (which it often did), test equipment would be calibrated to the spec called for in the contract or by the design engineers, used in production test, and then a performance evaluation would be run on the test equipment.  If the test equipment failed the performance evaluation after use, every product it tested would be pulled and the process would be started all over again.

Offline mikepa

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Re: How reliable is a calibration?
« Reply #42 on: June 24, 2013, 11:09:38 pm »
Good news.  BSLTesst agreed to calibrate the power supply and have returned it to me (they paid shipping both ways).  I'm pleased to report the unit is now "bang-on". 18.00V indicated shows 18.00 on my Fluke.  The current reading is equally accurate.  Thanks BSLTest.
« Last Edit: June 25, 2013, 12:21:41 am by mikepa »
 

Offline Stonent

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Re: How reliable is a calibration?
« Reply #43 on: June 25, 2013, 02:12:08 am »
Good news.  BSLTesst agreed to calibrate the power supply and have returned it to me (they paid shipping both ways).  I'm pleased to report the unit is now "bang-on". 18.00V indicated shows 18.00 on my Fluke.  The current reading is equally accurate.  Thanks BSLTest.

 :-+

That's pretty good service, do they list a website? Because I could not find one.  I'm curious as to what the cost for this calibration was.
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jucole

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Re: How reliable is a calibration?
« Reply #44 on: June 25, 2013, 03:18:16 pm »
Great result! ;-)
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #45 on: June 25, 2013, 07:43:59 pm »

That's pretty good service, do they list a website? Because I could not find one.  I'm curious as to what the cost for this calibration was.

http://esitest.com/

This was a calibration (adjustment) on a power supply I purchased from them on eBay, so there was no charge.
 


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