Hi guys, just wonder the statement "the uncertainties are for a confidence probability of not less than 95%" means in my calibration cert. How do we calculate for the measurement uncertainty? Do we need to add the measurement uncertainty value into our measurement data?
Interesting questions.
First, we need to realize that we can never take a perfect measurement. We can get very close to determining an actual value, but there will always be some error involved. That's measurement uncertainty. It is a reality, and we need to deal with it.
How do you calculate for measurement uncertainty? There are books and classes dedicated solely to this subject. I would recommend a publication from the UK's National Physical Lab (NPL). The link follows, and the NPL site has a good deal of information regarding measurement uncertainty:
GPG11 A beginner
Basically you need to determine potential sources of error in your measurement process and equipment and statistically combine the terms.
The "uncertainties are for a confidence probability of not less than 95%" indicates that your calibration provider has performed the uncertainty analysis for a measurement, and is reporting that 95% of the time the true value will be within the uncertainty quoted of the value they give you. This is a way of showing how good a measurement they made. Lower uncertainties are a way to compare the quality of measurement results. They help when you are shopping for calibration services - is the lab providing good enough results for your requirements, or are you paying too much for accuracy that your process doesn't need.
Do you need to add in the measurement uncertainty? That is a requirement when you are stating that your measurements are "traceable" to a national lab like NIST or NPL. You need to account for how far off the true value your actual measurement could be. Consider weighing something, you end up with a result of 100.02 g. But the uncertainty calculated for your scale is ±0.017 g. That means that 95% of the time the true value will be between 100.003 and 100.037 g. Your actual process will determine if you need to include the measurement uncertainty.
