Resistance (ohm) Electrical Measurements with a DMM - Verify Shunt?

A

AlbertPaglinawan

#11
This is what we look for in a calibration certificate (normally). Now I will add the <i>scope of accreditation</i> . Although honestly, I can't grasp the idea of the scope of accreditation. what exactly is it? Is it a single number/uncertainty which you will include in each of the testpoints? Is it estimated uncertainty (as stated below)?


- Name and address of laboratory
- Report identification
- Name and address of customer
- Description and identification of item calibrated
- Characterization and condition of item
- Date of calibration
- Calibration procedure identification
- Reference to sampling procedure , as necessary
- Any deviation, additional, exclusions from calibration procedure, and other information relevant to specific calibration
- Any failure identified/out-of-tolerance conditions, adjustment made and recommendations.
- Measurements, examinations & derived results, supported by tables, graphs, sketches and photographs, as necessary.
- Estimated uncertainties
- Signature and title of persons accepting responsibility for the content of calibration report.
- Special limitations of use, if any
- Traceability statement (standards used)
- Logo of calibration laboratory

Is this complete or do you think we should add more?

thanks again, <i><b>a lot!</i></b>

cheers!
 
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D

D.Scott

#12
Take a look at 17025 - it gives a good list of what needs to be included in a report from a calibration lab.

To give you an idea of a lab scope, I have attached our scope for our in house lab.

Hope this helps.

Dave
 

Attachments

R

Ryan Wilde

#13
Originally posted by Lord Ituralde
This is what we look for in a calibration certificate (normally). Now I will add the <i>scope of accreditation</i> . Although honestly, I can't grasp the idea of the scope of accreditation. what exactly is it? Is it a single number/uncertainty which you will include in each of the testpoints? Is it estimated uncertainty (as stated below)?
Lord (I rarely address posts as such, but it is appropriate this time),

I am assuming that you are using accredited sources. Every true accrediting body, along with the certificate of accreditation, includes a "Scope of Accreditation". The scope of accreditation defines the range of tests/calibrations that have been assessed and found to be within the requirements of ISO 17025. It is an entirely separate document from the calibration certificate, and should be looked at prior to sending work to a calibration supplier.

In other words...

The location where I am employed has a scope of accreditation 16 pages long, and encompasses a lot of stuff. It does not include optical calibrations (such as optical attenuators), although we actually do quite a few of them. We have the capability, we have the knowledge, but we made a business decision to not add optical calibration. Therefore, if you sent your optical equipment to me, I could calibrate it, but I could NOT provide an accredited calibration.

The reason that this is important is:

We have a competitor that is accredited. Their scope of accreditation encompasses anything that can be done using a Fluke 5520A. They perform a lot of calibrations that do not involve the Fluke 5520A, but still advertise themselves as accredited, which they are. Without knowledge of exactly what they can calibrate under the umbrella of accreditation, the one who suffers is the customer. You send items to a calibration lab that is accredited, only to find out that they cannot, in fact, provide an accredited calibration of your item, or even worse, they tell you that it is accredited when it is not (which, if reported to the accrediting body by you, the customer, will have the effect of your supplier possibly losing their accreditation, and even legal action). It would be horrible to send your 1281 to this company, as they may calibrate it using their 5520A, even though the 1281 is almost accurate enough to calibrate the 5520A with a 4:1 TUR.

So, what you need to do is to ask each of your calibration suppliers for their scope of accreditation, they will provide it. Make sure that the scope is on the letterhead of the accrediting body, not something generated by the lab itself (a self generated scope of calibration is great for an internal lab, and I commend Dave for actually having one, as they are rare). This will define to you exactly what you can and cannot send to a particular lab, what their capabilities are, and what level of uncertainty they can achieve.

I know this is a lot, so if you have any more questions, let's here them, because you are now touching on the things that many people don't understand, and it is good to increase the collective knowledge level. I know that I've learned quite a bit from this forum.


Ryan
 
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Jerry Eldred

Forum Moderator
Super Moderator
#14
I had a really interesting experience regarding an un-named accredited 3rd party calibration vendor. I won't mention any details that give them away. I'll describe the circumstance in generic details.

A vendor solicited our business recently, and sent us a rather lengthy bid list. For my own interest, I looked up their 17025 accreditation.

I was pretty disgusted, and determined I ought to communicate generically what I found.

This is a large vendor with a very wide range of capabilities. Many companies look for accredited vendors to do their calibrations for them. And I would speculate that it is common to go only to the extent of looking to see that they are accredited, and possibly what parameters are included in their scope.

I went to the next step. I reviewed best uncertainties and what standards are used for that uncertainty. As this might give away the identity of the vendor, I'll cloak it with vague descriptions.

For a parameter that requires normally let's say 1 ppm accuracy, they list standards with 0.1%. They were deceptively INaccurate standards.

This multi-purpose vendor who no doubt does highly accurate calibration work in many disciplines is accredited only in a few very low accuracy areas. In at least one case, the accredited uncertainty is much worse than any standard anyone might normally expect to ship to them.

This really bothers me. It appears to be in a grey area of smoke and mirrors. These labs are picking easy and useless parameters for accreditation to hang on to business coming from customers who care more about looking good in an audit.

I don't at all wish to discredit the process of accreditation. But this seems so blatant to me that I would specifically not want to use the services of any company accredited in that way. I will presume the accreditation process was totally honest and credible. But careful selection of useless parameters, low-accuracy parameters for the sake of maximizing dollars from customers with similar attitudes really bothers me.
 
R

Ryan Wilde

#15
Jerry,

There are some companies (my employer included) that include everything that they can possibly get onto their scope, within reason. There are others (I know of a few) that become accredited for only the bare minimum, and fly the accreditation for all to see, implying that you will receive an accredited calibration of your items. The only way around this that I know of is:

Your Request for Quote must be specific. If you need the item calibrated with accreditation to manufacturer's specifications, you should state it on the request. Leave nothing to chance. If the return your request with a quote, and when you look at their scope it isn't possible, make a complaint to their accrediting body. The only way to stop them is to weed out those with questionable ethics by getting their accreditation revoked. It shouldn't take much for them to understand your point that you are the customer and you don't have to deal with them, and neither should anyone else with less knowledge to know to dig into their information. That, and they make our profession appear shady, which really makes me angry.

I've personally had to report two labs to their accreditation bodies, and one had its accreditation revoked. The other fixed the problem VERY quickly.

Ryan

P.S. I don't think I have anything with 1 PPM accuracy other than my reference cells, so don't ask me to do it either. :biglaugh:
 
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