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Uncertainty and Significantly Different Means - Buffer Solutions

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Post Number #1
27th September 2017, 11:03 AM
 PETERROW Total Posts: 7
Uncertainty and Significantly Different Means - Buffer Solutions

Hi there

I have discovered an "interesting" (meaning I do not know how to proceed) problem.

I am estimating an uncertainty budget for a measurement of buffer solutions. We measure the buffer solutions with two pH electrodes in the same solution at the same time. Temperature is controlled (25°C +/- 0.2). The electrodes are calibrated using primary standard buffer solutions with pH values either side of the solution being measured. Around 300 measurements are collected over 15 minutes after which the mean of the measurements of each electrode are combined to deliver the total mean, which is attributed to the pH of the buffer.

Now my problem. The method that I have just stated, I believe, is valid IF there is no significant difference between the measurements from the individual electrodes. Unfortunately, there is a significant difference.

In practical terms, the differences are small (i.e. less than the uncertainty of the primary standard buffer) but never the less statistically significant.

My question is can I use distribution of the difference between the buffers to calculate the measurement uncertainty of the electrodes or because of the statistical significance, must I go back to the drawing board and re-examine my measurement method.

Any ideas are much appreciated.

Post Number #2
11th October 2017, 09:39 PM
 Hershal Total Posts: 2,498
Re: Uncertainty and Significantly Different Means - Buffer Solutions

Have you tried Guardbanding to address this?
Post Number #3
12th October 2017, 12:37 AM
 PETERROW Total Posts: 7
Re: Uncertainty and Significantly Different Means - Buffer Solutions

Guardbanding? You will have to help me more here, I have never heard of guardbanding. Could you post a link please?
Post Number #4
12th October 2017, 03:11 PM
 Hershal Total Posts: 2,498
Re: Uncertainty and Significantly Different Means - Buffer Solutions

The most current version of the GUM has it I believe. The current edition of The Metrology Handbook should also have some information. The following link may also help. I make no claim regarding the website, but it appeared to have a good explanation.

Guard-banding | Applications of statistics in Measurement & Testing
 Thanks to Hershal for your informative Post and/or Attachment!
Post Number #5
12th October 2017, 05:02 PM
 Marc Total Posts: 26,289
Re: Uncertainty and Significantly Different Means - Buffer Solutions

Thank you for your help, Hershal!

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