Author Topic: How reliable is a calibration?  (Read 10174 times)

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Offline mikepa

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How reliable is a calibration?
« on: June 16, 2013, 12:20:30 am »
I just picked up a Agilent E3615A 20V 3A power supply from eBay and it came with a calibration certificate saying it was just calibrated a few days ago.  The company I got the supply from was Brltest and they did the calibration, so I was expecting the calibration certificate to mean something.

But on testing the output voltage, I see the power supply voltage indicator is reading 0.07V low at 10V and 0,22V low at 20V.  (This is with or without a 10 ohm load on the supply).  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.

Thoughts?  Is this expected or should I ask Brltest to re-calibrate - or should I just fix it myself?
 

Offline EEVblog

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Re: How reliable is a calibration?
« Reply #1 on: June 16, 2013, 12:24:45 am »
The calibration itself is practically 100%
But remember that calibration is only valid for the instant that calibration was done.
The rest of the time has to do with the confidence you have in that instrument. This is built up with calibration history that gives you a confidence level in a particular instrument.
Technically, calibration labs also offer confidence level based on certain gear.
But it's all a statistical thing of course, you could have a complete dud.
This is why some critical things mandate the use of two meters, to ensure that one has not drifted or is not playing up.
Welcome to the interesting world of metrology.
 

Offline EEVblog

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Re: How reliable is a calibration?
« Reply #2 on: June 16, 2013, 12:25:33 am »
For your actual problem, what is the PSU spec? Most PSU do not have tight specs.
 

alm

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Re: How reliable is a calibration?
« Reply #3 on: June 16, 2013, 12:30:55 am »
You should not expect the company to make any adjustments if it's well within specs. Many will make adjustments if it's off by more than for example 70% of its specified tolerance.
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #4 on: June 16, 2013, 12:39:19 am »
E3615A meter accuracy is stated as ±(0.5% of output + 2 counts).  So reading 9.99V while output is actually 10.07V would be just out of spec.  Reading 20V while output is actually 20.28V is way out of spec.
 

alm

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Re: How reliable is a calibration?
« Reply #5 on: June 16, 2013, 12:51:35 am »
Either the supply drifted a lot the past few days (defect?), you're really unlucky with three DMMs off by the same amount, or someone botched the calibration. I would call them and see what they have to say.
 

Offline Galaxyrise

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Re: How reliable is a calibration?
« Reply #6 on: June 16, 2013, 01:02:25 am »
Just another sanity check: How long did you leave the PSU on before testing the voltages?
I am but an egg
 

Offline mikes

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Re: How reliable is a calibration?
« Reply #7 on: June 16, 2013, 02:32:14 am »
It's eBay. Let me help:

For parts = someone put this one through a crusher.
Won't power up = this experienced a direct lightening strike
Powers up/can't test = something changes when you turn it on, but it's broken with valuable pieces removed.
Working = it's broken in some way, but the return shipping costs will keep you from returning it.
Calibrated = we checked a few things, and our China meter says they're close.
 

Offline vk6zgo

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Re: How reliable is a calibration?
« Reply #8 on: June 16, 2013, 03:41:30 am »
0.7% out at 10volts is hardly the end of the world! ;D

Neither is 1.1% at 20volts.

After all,the thing was on eBay,not brand new,so I assume it was cheap.

The easy answer is,just use your Fluke to set the voltages when they are critical.

There are a lot of other more important specs for power supplies than the readings on the front panel meters.
Have you checked for long term drift in settings,hum level,short term stability,etc?

By the way,I notice you changed the figures in your later posting--which is right?
« Last Edit: June 16, 2013, 03:44:44 am by vk6zgo »
 

Offline quantumvolt

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Re: How reliable is a calibration?
« Reply #9 on: June 16, 2013, 04:34:46 am »
I second the last post. Those 2 1/2 digits meters are probably of the same quality that you get for a few dollar today. The quality of the supplied voltage/current is what matters.

Also we do not know the real voltage you are measuring (your other meters have unknown absolute inaccuracy and differ in readings), so the PSU meters might err less than you think.

Since both readings are low and the 20 V error is bigger, your meter could be quite linear but with a too low resistance to generate the voltage reading from the tapped current. You can look for a trimpot to lift the 10 V reading to 10 (or a tad over) and also at the same time lifting the 20 V reading closer to 20. Or you can put in an op amp with variable offset and some 1.005x trimmable amplification. What is clear though is that you will never get both readings accurate (because of unlinearity in the meter).

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).
 

Offline eevblogfan

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Re: How reliable is a calibration?
« Reply #10 on: June 16, 2013, 10:13:50 am »
hey

simply open the case and adjust the pot(s) right there , that's easy to do and takes no efforts ( unless you are looking for +- 1 LSD 0_0 )

your flukes are more then enough so feel free , 

if you will state your country , I am  sure there will be one eevbloger who might have the will to either calibrate or simply borrow you some meter say fluke 87V or any other goodies :) 

Cheers :)
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #11 on: June 16, 2013, 11:52:40 am »
Thanks for the replies guys. 

I left the unit on all night set at 10.00v and it's still 10.00 volts this morning, so that's good, zero drift.  But the actual output voltage still is .07V high at 10V and 0.2V high at 20V.

Alm seems top have the best advice (call brltest), while Mikes seems a little cynical of eBay goods.  If an auction emphasizes a calibration is included, one expects the thing to be calibrated.  (Cosmetically the condition is like new, I don't know how old the unit is.)

I am tempted to just do it myself as Eevblogfan suggests, but when one has essentially paid for a professional calibration …   

I see very little noise on the output < 4mV.
 

Offline G0HZU

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Re: How reliable is a calibration?
« Reply #12 on: June 16, 2013, 12:09:34 pm »
I don't know about your Agilent supply but I have various Thurlby PL320 dual and triple PSUs here and some of them are 20 years old.

I just checked all the PSUs when set to display 20.00V and they all measured 20V within 0.015V.
Note I set each rail so the display 'just' clicked over to 20.00V from 19.99V to try and get some consistency in the way I set up each one.

I also have a Thandar dual supply that is a lot newer and it only reads to two decimal places up to 18.00V but both rails are within 0.015V at 18V.


I might have 'calibrated' the older supplies about 15 years ago but they still seem to be pretty accurate today. I didn't really expect them all to be this accurate but that's the results I got :)

So I would be a bit annoyed if I was sent an Agilent PSU that was supposed to have been calibrated and it was out by 0.20V at 20V.

However, I think the best option is to tweak it yourself :)




« Last Edit: June 16, 2013, 12:16:14 pm by G0HZU »
 

Offline branadic

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Re: How reliable is a calibration?
« Reply #13 on: June 16, 2013, 12:15:45 pm »
Quote
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?
Metrology Meeting 2020 is canceled. Looking forward for MM2021
 

alm

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Re: How reliable is a calibration?
« Reply #14 on: June 16, 2013, 12:48:44 pm »
Agilent specs are usually realistic and quite conservative, even though the indicators are not exactly the most critical part. In general I would not buy calibrated equipment on eBay with the expectation that they performed a factory calibration, but that doesn't mean I wouldn't complain. The value of a cal certificate depends on the credibility of the company, and my impression of Britest is that they seem fairly credible. If they give you a piece of paper that it met specs (careful they didn't do a partial cal), then they should behind it or admit that the equipment drifts way more than specified.
 

jucole

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Re: How reliable is a calibration?
« Reply #15 on: June 16, 2013, 01:08:12 pm »
A calibration should be reliable, but if the item was shipped a long distance, or it's old gear - i think you'd be unrealistic to assume it would be have to be anything other than spot on;  but the best thing is you can tweak it yourself!  :-/O
 

Offline madires

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Re: How reliable is a calibration?
« Reply #16 on: June 16, 2013, 01:12:27 pm »
Alm seems top have the best advice (call brltest), while Mikes seems a little cynical of eBay goods.  If an auction emphasizes a calibration is included, one expects the thing to be calibrated.  (Cosmetically the condition is like new, I don't know how old the unit is.)

Calibration != Adjustment

A lot of people mix up both or think both are the same :-) Calibration is the process of comparing what your device displays and what a calibration device (traceable to a standard) displays. It's about finding out how stable your device is over time. If you like to have a device which is spot on you'll adjust it to display a correct value. In case you want both (cal & adj) it becomes more complex :-) First you calibrate your device, then adjust it and calibrate again. The first calibration is to track the changes since the last one and the second calibration is the starting point for the new values. Otherwise the next calibration would show a drift of 0.07V in your case.
 

alm

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Re: How reliable is a calibration?
« Reply #17 on: June 16, 2013, 01:26:50 pm »
The cheapest calibration is calibration without data. This is a certificate that states it was within specs at a certain time. If it was outside of the stated tolerance, then it should have been noted on the certificate. These calibration levels will often involve adjustment (eg. Fluke CalNet), since you can't track the stability without data anyhow.
 

jucole

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Re: How reliable is a calibration?
« Reply #18 on: June 16, 2013, 01:47:48 pm »
If you like to have a device which is spot on you'll adjust it to display a correct value. In case you want both (cal & adj) it becomes more complex :-) First you calibrate your device, then adjust it and calibrate again.

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.
 

Offline jancumps

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Re: How reliable is a calibration?
« Reply #19 on: June 16, 2013, 01:59:14 pm »
If you like to have a device which is spot on you'll adjust it to display a correct value. In case you want both (cal & adj) it becomes more complex :-) First you calibrate your device, then adjust it and calibrate again.

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?
 

Offline mikepa

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Re: How reliable is a calibration?
« Reply #20 on: June 16, 2013, 02:18:29 pm »
The certificate states:

As Found: In Tolerance
As left: In Tolerance
Adjustments: N/A

With adjustments stating N/A, not sure if it's not applicable because it wasn't needed or it wasn't part of the deal.

Regardless, a 0.2V deviation at 20V I believe I am correct as stating the device meter is not within tolerance, so I will contact Britest (is it Britest or Brltest?) and see what they say.  It was shipped only 100 miles and was very, very well packed so I doubt rough shipping caused it to drift.  I suspect the calibration was botched.

P.S. The current reading is equally out of tolerance.
 

jucole

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Re: How reliable is a calibration?
« Reply #21 on: June 16, 2013, 02:23:00 pm »
If you like to have a device which is spot on you'll adjust it to display a correct value. In case you want both (cal & adj) it becomes more complex :-) First you calibrate your device, then adjust it and calibrate again.

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?

I'm not sure i'd be that interested in the historical calibration data for an Agilent E3615A with an accuracy of +- 0.5% + 2 counts.
 

Offline jancumps

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Re: How reliable is a calibration?
« Reply #22 on: June 16, 2013, 02:29:02 pm »
I'm not sure i'd be that interested in the historical calibration data for an Agilent E3615A with an accuracy of +- 0.5% + 2 counts.

You're probably right.
 

Offline Rufus

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Re: How reliable is a calibration?
« Reply #23 on: June 16, 2013, 02:29:23 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.
 

jucole

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Re: How reliable is a calibration?
« Reply #24 on: June 16, 2013, 02:29:58 pm »
The certificate states:

As Found: In Tolerance
As left: In Tolerance
Adjustments: N/A

With adjustments stating N/A, not sure if it's not applicable because it wasn't needed or it wasn't part of the deal.

Regardless, a 0.2V deviation at 20V I believe I am correct as stating the device meter is not within tolerance, so I will contact Britest (is it Britest or Brltest?) and see what they say.  It was shipped only 100 miles and was very, very well packed so I doubt rough shipping caused it to drift.  I suspect the calibration was botched.

P.S. The current reading is equally out of tolerance.

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.
 


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