Help in understanding my Gage R&R results on a pH meter

G

grs2828

#1
Hello,

I am fairly new to gage r&r studies, and I just finished running one on a ph meter, and got horrible results! (94.45 % Contribution and 97.18 % Study Variation)

10 samples of a Buffer solution were used (all were taken from the same source so they were assumed to be equal), three operators performed the measurements, two trials per operator. I also made sure the ph meter was calibrated and that all the proper procedures were followed when measuring.

I really wasnt expecting to get such bad results and I am not sure whether I did something wrong in the design of the test or if the measurement system is really that bad. I have attached a word document with the data, the minitab output for the crossed gage r&r and graphs. (wouldn't let me upload the minitab file...)

I would appreciate any comments/feedback/insights on what went wrong or any help on how to interpret what are the fallbacks of my measurement system. I noticed that basically all the variability is in repeatability, which doesnt seem that logical.

Appreciate your help,

Thank you!

Gabriela
 

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bobdoering

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#2
Re: Help in understanding my r&r results

You are probably jumping into GR&R right in the deep end!

I think your first problem is you need more variation in you samples - like 3 buffer solutions - not samples of the same buffer solution. You just don't have enough variation in your "samples" to see if you can detect the difference between them (which is kind of the point of the exercise).

What is the specification you are trying meet with the pH meter?
 
G

grs2828

#3
Re: Help in understanding my r&r results

thank you, that is a big help! I actually doubted for a long time on whether i should use the same buffer solution or not for all samples.

But this is where I get confused again. When you say to take the samples from 3 different buffer solutions, do you mean three completely different solutions (say one with ph of 5, another 14 and the third 10), or just three different batches of the same solution (all supposed to have the same ph)?
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#4
Re: Help in understanding my r&r results

you need to have the full range of variation that you would expect - or you know to have of the ph of your buffer solutions. that is why knowing the allowable range is important. Now if your allowable range is extremley tight and you suspect that you have poor measurement repeatability my suggestion woudl be to make buffers that were just outside of the spec limits and use those in the experiment.

If you still get "lousy" results come back and we can help. all is not lost, you will just need to understand that yoru measurement system is more categorical (yes/no or high medium low) than it is exactly correct out to the nth decimal place.
 

Miner

Forum Moderator
Staff member
Admin
#5
For information regarding the selection of samples, see
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 5a: R&R, Part Selection and

Intro to Measurement System Analysis (MSA) of Continuous Data – Part 5b: R&R for more information.

As pointed out, your part variation is not even measurable. If this reflects the actual process variation, your gage is not acceptable. Start looking at the gage resolution. Your pH meter
does not have sufficient resolution to be used for process control or statistical studies. This is based on the test for the Range chart: At least 5 distinct levels of measurement within the UCL, and nor more than 25% equal to zero.

If the part variation does not reflect the actual process variation, you need to obtain samples that do reflect it, or use the variation from a capability study. Minitab allows you to enter this information and will calculate another metric using this data.

Depending on your tolerances, it may be acceptable for inspection to a tolerance.
 
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