Old and New Primary Standards show a difference between them

L

lucasso

#1
Hello all,

I've been searching the forum for two hours and was not able to find a similar topic. So, this is the problem:

Our laboratory had a primary (certified) reference material for a few years. It was bought at and traceable to NPL (UK). As the old one was coming to it's end, recently we bought a new one, also traceble to NPL.
When we compared the two standards in our laboratory, they showed a few percent difference between them.
The difference is bigger than the uncertainty stated on the certification of the CRM.
We also participated in inter-laboratory comparison measurements with the old CRM, so we don't think it is off.

If we start using the new standard as it is, we will face a significant few percent step in our measurements.

Any thought and suggestions, please.
 
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BradM

Staff member
Admin
#3
Hello there!
I see that you compared against the uncertainty stated on the standard certificates. However, I see no mention of any uncertainty of the systems in your lab. I would think that whatever methodology you are using has error in it also, would it not?

It may not be much, but might be enough to create differences between the two samples. :)
 
L

lucasso

#4
Thank you.

Yes, both suggestions seem reasonable. There is a possibility that both standards are not accurate enough and have different values.
Also, it is possible that the difference is because of our methods.


I also would like to ask a similar question:
Let's say that the difference is tolerable, it is within uncertainty limits, eg:
Both were bought as 50units ±10%.
When measured in our lab:
Old CRM: 51units
New CRM: 49units.

The difference is much smaller than our measurement uncertainty, which is about 20%, but the move from the old to the new one will still create a small drop in measurements.
Are there accepted and established ways to deal with this?
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
Reference materials are a bit more challenging than for a simple thing like a primary 1.1VDC standard. For those who may not know me, I was joking a bit there.

Anyway, the original post suggested that the second material may not have been that far off, unless I misunderstood the statement.

If that statement is correct then the first obvious question is what changed at the lab.

However, it is a reference material, so as an accreditation assessor, my first question would be if the two were from the same lot. If they are not, then a difference is expected.

Next, since this is likely not traceable to SI, then also look at the various other requirements of ISO/IEC 17025:2005 Clause 5.9.1, and assure at least a few of those requirements are in place.
 

BradM

Staff member
Admin
#6
The difference is much smaller than our measurement uncertainty, which is about 20%, but the move from the old to the new one will still create a small drop in measurements.
Are there accepted and established ways to deal with this?
If you are using the actual values of the standards in your baselining activity, why would there be a concern as to the direction of the difference in value? Or, given the particular sample, does that indicate some level of degredation in the standards?
 
L

lucasso

#7
If you are using the actual values of the standards in your baselining activity, why would there be a concern as to the direction of the difference in value?
The concern is not about the direction. The concern is about the fact that there will be a step in the measurements. Let's say we measure a parameter of one item on daily basis and the results are:
45.6 45.7 45.6 45.5 45.6 etc. (±20%)
After we start using the new CRM, it will change to something like this:
43.4 43.5 43.4 43.6 etc (±20%)


Or, given the particular sample, does that indicate some level of degredation in the standards?
It is possible. Although we did participate in interlaboratory measurements with the old one. The other is brand new.
 
S

step30044

#8
The concern is not about the direction. The concern is about the fact that there will be a step in the measurements. Let's say we measure a parameter of one item on daily basis and the results are:
45.6 45.7 45.6 45.5 45.6 etc. (±20%)
After we start using the new CRM, it will change to something like this:
43.4 43.5 43.4 43.6 etc (±20%)



It is possible. Although we did participate in interlaboratory measurements with the old one. The other is brand new.
It doesnt make a difference you are still within the accepatable limits of the new CRM's actual value. I would just list the new standard value in your records.
 
L

lab_analyst

#9
I have similar problem. Used an old standard as a calibration standard, then used the same standard to verify the calibration. Then we came to know that we need to have a second standard (to verify calibration), so we bought new standard. Both traceable to NIST. Anyway after calibration with new standard, when we tested the old standard (as a verification) the result is quite off. I am sending to another lab for cross check. Any thoughts on this?
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#10
Yes, make sure you keep the results, then compare them including the En score if possible.

Download APLAC PT 001, and in that document are the formulas for calculation of En scores. This will help you determine where an issue may be. You can also document it as an ILC but whether that helps with the next assessment I can't say, your AB will have to determine that.
 
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