QMT Features: November 2015
Best practice approach to performing a measurement
The fourth in a series of articles designed to help QMT readers in their everyday measurement needs from Keith Bevan, the Delivery Manager for Training at the National Physical Laboratory (NPL)

Our previous article (published in QMT September/October) outlined the advice NPL gives to measurement professionals who are preparing to take a measurement. In this issue we progress to the next stage of the process and examine the questions to ask when performing the measurement.

If you have followed the advice of past articles and planned your measurement well, it is likely to be successful. However, you should still think critically whilst undertaking the measurement. There are fundamental checks and good practices that should be followed. For example, using the details that you have identified in the preparation stage to set the instrument to the required measurement units. Working to the correct units is critical; to overlook this can have huge consequences. A well-known example is NASA’s Mars Climate Orbiter, launched in December 1998 to study the planet’s climate, atmosphere and surface changes. However, programming teams in the US and Europe had used different systems (imperial and metric) to calculate the spacecraft’s trajectory. So when the probe entered the Martian atmosphere at the wrong angle, it quickly disintegrated.

Measurement professionals also need to have confidence in the instruments they are using. The previous article looked at the options available to you when choosing an instrument, once you have made that choice, you need to be sure that the instrument is functioning correctly. The most basic checks can be made visually - ensure the instrument is clean; that there are no visible signs of damage and that the instrument is within its stated calibration period.
Once the visual inspections are complete, check the zeroing of the instrument (if appropriate). If using a micrometer, for example, bring the anvils into contact using the ratchet and observe the reading, it should be zero – if not clean the anvils again and if necessary adjust the setting or take the instrument to the appropriate person who has the authority to make the adjustment. See NPL’s Callipers and micrometers Measurement Good Practice Guide No. 40 for more information.

Ensure that you have aligned the measurement instrument to the work-piece. It is important for measurement professionals to be confident that they are measuring the features the product correctly.
The next step is to ascertain whether there is an identified procedure and ensure you are following it correctly. Repeatability is key here, as the popular proverb says, ‘measure twice and cut once’- highlighting the need to ensure you have made a good measurement before committing to a potentially irreversible decision. Repeating a measurement several times allows you to calculate a mean (average) value and if the repeatability is high, the statistical uncertainty in the mean value will be low.

Repeatability describes the agreement within sets of measurements where the same person uses the same equipment in, the same way, under the same conditions. However, if different measuring equipment is used, a different result may be obtained because of errors and offsets in the instruments. If different measurement professionals each had a go at measuring an item on different days using different tools, a wider range of results would not be surprising. This is known as ‘reproducibility’ and describes the agreement within a set of measurements where different people, equipment, methods, locations or conditions are involved. More information on this can be found in NPL’s Fundamental Good Practice in Metrology Good Practice Guide Number 80.

Now you have confidence in your repeatability you need to determine what level of inspection is needed during your measurement. Inspection is essential to prove that the components meet the design requirements. How much you inspect is determined by the industry involved, do you assess all, or a sample? In some industries it will be impractical and unnecessary to measure everything but in others it will be critical to measure every dimension on every part due to the fact that the component may be safety critical, for example in aircraft components or medical component. NPL’s ­Good Practice Guide number 79, Fundamental good practice guide in the design and interpretation of engineering drawings for measurement processes, has more information on determining the level of inspection needed.

Once you have a set of results, you need to be confident that they enable you to make an informed judgment. You may think that it is a simple matter of the result falling within the tolerance band to prove conformance. But to be sure results conform to a specification they need to meet ISO 14253. To do so the measurement of uncertainty is needed, to determine how and where your results could be wrong.

This can be difficult to evaluate, as you need to identify any possible sources of uncertainty, evaluate the uncertainty from each source and, finally, combine the individual uncertainties to get an overall figure. NPL and IMechE developed a Beginner’s Guide to Measurement in Mechanical Engineering (Good Practice Guide no 131) that includes a useful eight-point plan to evaluating uncertainty:

1. Decide what you need to find from your measurements. Decide what actual measurements and calculations are needed to produce the final result.
2. Carry out the measurements needed.
3. Evaluate the uncertainty of each input quantity that feeds in to the final result (Type A and Type B evaluations). Express all uncertainties in similar terms (standard uncertainties).
4. Decide whether the errors of the input quantities are independent of each other.
5. Calculate the result of your measurement
(including any known corrections for things such as calibrations).
6. Find the combined standard uncertainty from all the individual aspects.
7. Express the uncertainty in terms of a coverage factor together with an expanded uncertainty at a stated level of confidence
8. Record the measurement result and the uncertainty, and state how you got both of these.

Further help
This series of articles are just a microcosm of the measurement expertise and assistance a recognised external body such as NPL can offer. As well as Good Practice Guides, NPL provides a range of training programmes, available through the classroom and via e-learning. This training can enable measurement professionals confidently undertake measurements in the workplace, but also show them the wider impact of measurement in society.
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