Tolerance, accuracy, precision, error and uncertainty – a measurement terminology primer
In engineering measurement terms such as error, precision, accuracy, tolerance and uncertainty are used frequently and occasionally interchangeably. Yet in metrology, the science of measurement, each of these terms means something different and must be used correctly. Engineering projects depend on getting the details right – in the oil and gas industry, which OMS specialises in, understanding metrology terms is vital for safe and efficient business operations.
Read on to find out what each term means and how it affects measurement.
Error
In measurement, “error” means the difference between the value, or measurement, recorded and the true, theoretically correct, value. Error, whether intentional or unintentional, is unavoidable so engineers design projects to mitigate or avoid its effects. In the design stages projects must be carefully specified to take account of the inevitable error. Measurement tools rarely provide an accurate answer so the chosen measurement system must be properly defined to meet project specifications and increase the probability of success. This includes specifying error “tolerance”.
Tolerance
“Tolerance” in engineering and manufacturing is the extent by which a value is permitted to vary. Specifying tolerance defines the acceptable limits of variance required for a particular project. For example, if a project requires pipes of 12.2m in length, the specified tolerance, or allowable variation from that value, may be ±100mm.
Why is tolerance important?
Using an example from our industry, when an oil or gas pipeline is being built pipes must be within a certain tolerance to ensure the pipeline is welded and laid correctly and safely. Achieving the acceptable tolerance helps ensure welding will meet the required fatigue design, particularly in stress-sensitive areas such as steel catenary risers. It means pipes will meet their straightness requirement, fit properly within the J-lay tower on a pipelay vessel and should end up in the drop box. Getting the tolerance right means pipelines are welded and laid safely and costs are managed – when pipelay vessels cost upwards of $500,000 per day this can make the difference between project success and failure.
Accuracy and precision
The terms “accuracy” and “precision” are often used when discussing random and systematic errors – but they should be applied with care. “Accuracy” indicates the closeness of a measurement to the true value but it cannot be quantified because it is not possible to know the true value. “Precision” is a description of repeatability and is sometimes used, incorrectly, to mean accuracy. The figure below of target shooting practice illustrates the difference between accuracy and precision.
Target (a) shows inaccurate and low precision shooting.
Target (b) shows better precision but a similar low level of accuracy. Both target (a) and target (b) show a consistent offset, or bias.
Target (c) is more accurate.
Target (d) is the most accurate and most precise.
Accuracy and precision
In our experience, “precision” is not a useful measurement term and we usually avoid using it. As for “accuracy” – that can more correctly be described as uncertainty.
Uncertainty
Uncertainty is the expression of the level of doubt we have about any measurement. Unlike tolerance, which is specified by engineers and expressed as a value, uncertainty is described in statistical terms. Most oil and gas projects have a reasonable expectation of a U95 uncertainty measurement.
Uncertainty in measurement tools
All measurements are subject to uncertainty, since none can be exact, and understanding the level of uncertainty tells us how reliable a measurement is. Measuring tools likewise can never be 100% correct. During a project’s design stages engineers must define the uncertainty of their chosen measurement system so that it is compatible with requirements. The agreed tolerance value must take measurement tool uncertainty into account.
Understanding both sides
Understanding both tolerance and uncertainty means engineering dimensions are more likely to be correct. In the coming weeks we will be looking at the concept of uncertainty in more depth.
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Updated 28.03.2023
[5 minute read]