Anytime a measurement is made there are a number of factors that affect how accurate the results are; that is why measurement uncertainty is something we try to minimize at Advanced Inspection Services. Whether you’re using a gage pin, a pressure mic, or an ultra-high accuracy CMM inspection tool, there is always some amount of measurement uncertainty. To simplify, measurement uncertainty is the difference between the measured value and the true (actual) value.
So, if all measurements have uncertainty it is imperative that we have a grasp on how much uncertainty there is, otherwise we will not be able to make any confident decisions based on our measurements.
Here at AIS/PQI we have spent a lot of time and resources in understanding measurement uncertainty and we are active in helping our customers understand and asses they’re internal measurement uncertainty. There are a number of documents out there related to measurement uncertainty, the most helpful documents we have found are from B89.7 series of documents from ASME. In the B89.7.3.2 document there is a chart that shows the various categories of factors that contribute to measurement error.
One of the most commonly used methods of determining measurement uncertainty is to develop “equipment-based” uncertainty budgets. For example, based on a number of different tests (Gage R&R’s, etc.), we determine that the uncertainty of a specific CMM is +/-.0002". The "equipment-based" uncertainty is really just one of the components (although a significant component) of the overall measurement uncertainty. The equipment-based uncertainty does not address specific part or feature characteristics that can induce additional error in the results. So, in addition to the "equipment-based" uncertainty budget, we also need to determine the "part-specific" uncertainty that is added to the "machine-specific" uncertainty. The 3rd component to assessing the true uncertainty of a specific measurement process is the "feature-specific" uncertainty. Various features on a part can have different amounts of error due to the type of feature (for example, a radius, a plane, or cylinder) and how the measurements are taken – CMM inspection (probing force, cosine error, etc) or vision inspection system (pixel size, lighting, etc.).
If measurement uncertainty seems a bit overwhelming, we are here to help. We offer training specifically for understanding measurement uncertainty and also offer services to help you validate you’re internal measurement processes. Check out our training schedule for upcoming classes on uncertainty here.
Please contact us for more information on CMM inspection, measurement uncertainty and vision inspection services. Or Request a Quote today!