Traceability and Uncertainty

Traceability & Uncertainty

 

Quote by Lord Kelvin on the importance of measurement 

Introduction

Measurement today is more valuable than ever. We depend on measurement for almost everything - from time keeping to weather forecasts, from DIY work at home to heavy-duty manufacturing, industrial research and medical science.
Since measurement plays such a fundamental part in our lives, it is important that the accuracy of the measurement is fit for purpose, i.e. it fully meets the requirements of the application. Every measurement is inexact and therefore requires a statement of uncertainty to quantify that inexactness. The uncertainty of a measurement is the doubt that exists about the result of any measurement.
One way of ensuring that your measurements are accurate is by tracing them back to national standards. This method of guaranteeing a measurement's accuracy through an unbroken chain of reference is called traceability.

Scientist with scales 

Accurate Measurement

Accurate measurement enables us to:

• Maintain quality control during production processes
• Comply with and enforce laws and regulations
• Undertake research and development
• Calibrate instruments and achieve traceability to a national measurement standard
• Develop, maintain and compare national and international measurement standards

Successful measurement depends on the following:

• Accurate instruments
• Traceability to national standards
• An understanding of uncertainty
• Application of good measurement practice

There are many factors that can cause inaccuracy:

• Environmental effects
• Inferior measuring equipment
• Poor measuring techniques

 

Ruler melting in the sun

Inferior measuring equipment

Poor measurement techniques

In the United Kingdom, the National Measurement System (NMS) is in place to enable measurements to be traced back to their national standards. As the UK's national standards laboratory, NPL is at the pinnacle of this system guaranteeing the accuracy of physical measurements for the nation and abroad.

What is Uncertainty?

No measurement is ever guaranteed to be perfect. Uncertainty of measurement is the doubt that exists about the result of any measurement. By quantifying the possible spread of measurements, we can say how confident we are about the result.

Explanation of the error vs uncertainty 

Expressing Uncertainty

A measurement result is only complete when accompanied by a statement of its uncertainty. A statement of uncertainty is required in order to decide if the result is adequate for its intended purpose and consistent with other similar results.
It does not matter how accurate a measuring instrument is considered to be, the measurements made will always be subject to a certain amount of uncertainty.
In order to express the uncertainty of a measurement, we need to evaluate as accurately as possible the errors associated with that particular measurement.
For example - we might say that a particular stick is 200 centimetres long, plus or minus 1 centimetre, at a 95% confidence level. This is written:

200 cm ±1 cm at a level of confidence of 95%

This means we are 95% sure that the length of the stick is between 199 centimetres and 201 centimetres.

Why does Uncertainty Matter?

Calculating and expressing uncertainty is important to anybody wishing to make good quality measurements.
It is also crucial where uncertainty can influence a pass or failure in a particular test, and must therefore be reported on a calibration certificate.
There are established rules for the evaluation of uncertainty. [More information can be found in NPL's Good Practice Guide (011) 'A Beginner's Guide to Uncertainty of Measurement'.] Of course, we must all make every effort to 'control' the uncertainty in our measurements. This is done by regular inspection and calibration of our instruments, careful calculation, good record-keeping.

                        Various accuracies 

What is Traceability?

Traceability is a method of ensuring that a measurement (even with its uncertainties) is an accurate representation of what it is trying to measure.

What is Traceability to National Standards?

The simple and basic concept behind calibration is that measuring equipment should be tested against a standard of higher accuracy.
It should be possible to demonstrate an unbroken chain of comparisons that ends at a national standards body such as NPL. This demonstrable linkage to national standards with known accuracy is called 'traceability'.
National standards laboratories such as NPL also routinely undertake international comparisons in order to establish worldwide consensus on the accepted value of fundamental measurement units.
Representatives of seventeen nations signed the Convention of the Metre (Convention du Mètre) on 20th May 1875 in Paris. This diplomatic treaty provided the foundations for the establishment of the Système International d'Unités (International System of Units, international abbreviation SI) in 1960. Since then, national standards laboratories have cooperated in the development of measurement standards that are traceable to the SI.
Any organisation can achieve traceability to national standards through the correct use of an appropriate traceable standard from NPL.

Who is Who in the Measurement World?

International Committee for Weights and MeasuresCIPM - Comité International des Poids et Mesures) the world's highest authority in the field of measurement science.
International Bureau of Weights and Measures(BIPM - Bureau International des Poids et Mesures) co-ordinating body for international metrology, based in Sèvres, France.
National Physical Laboratory(NPL) is the UK's national standards laboratory, a world-leading centre in the development and application of highly accurate measurement technology and material science.

What is the Difference Between ACCURACY and PRECISION?

The difference between accuracy and precision is illustrated below by 4 different archers… each with varying degree of ability. The bull's-eye in the target represents the true value of a measurement.
 

	Low accuracy and low precision (poor repeatability) Stone age man missed the bull's-eye and the 3 attempts were not near each other.
	Low accuracy but high precision Robin Hood's Merry Man missed the bull's-eye but the 3 attempts were near each other.
	Higher accuracy but low precision Native American's 3 attempts were near the bull's-eye, but were not near each other.
	High accuracy and high precisionOlympic archer hit the bull's-eye 3 times!

Accuracy is a qualitative term relating the mean of the measurements to the true value, while precision is representative of the spread of these measurements. Even when we are precise and accurate, there will still be some uncertainty in our measurements; the scientists challenges are to evaluate the uncertainty and make this as small as possible. When the uncertainty of a measurement is evaluated and stated, then the fitness of purpose for a particular application can be properly understood.

For more details and to download a poster with this information visit:-
http://www.npl.co.uk/educate-explore/factsheets/traceability-and-uncertainty/

Measuring Conductivity

The conductivity (or specific conductance) of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter (S/m).

Conductivity measurements are used routinely in many industrial and environmental applications as a fast, inexpensive and reliable way of measuring the ionic content in a solution. For example, the measurement of product conductivity is a typical way to monitor and continuously trend the performance of water purification systems.

In many cases, conductivity is linked directly to the total dissolved solids (T.D.S.). High quality deionised water has a conductivity of about 5.5 μS/m, typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m (i.e., sea water's conductivity is one million times higher than that of deionised water)

Conductivity Meters - Two electrodes with an applied AC voltage are placed in the solution. This creates a current dependent upon the conductive nature of the solution. The meter reads this current and displays in either conductivity (EC) or ppm (TDS).

Our two part guide will help you to measure conductivity accurately.  The guides answer the 7 most asked questions regarding conductivity and the second part is a comprehensive guide on theory and measurement.

Just click on the links below to download your copies.

Part 1 - http://www.prlabs.co.uk/news/article.php?Id=232
Part 2 - http://www.prlabs.co.uk/news/article.php?Id=233

Using digital SLRs to measure the height of Northern Lights

Scientific research doesn’t often start from outreach projects. Yet, Ryuho Kataoka from the National Institute of Polar Research in Tokyo, Japan, came up with an idea for a new method to measure the height of aurora borealis after working on a 3D movie for a planetarium. Kataoka and collaborators used two digital single-lens reflex (SLR) cameras set 8 km apart to capture 3D images of Northern Lights and determine the altitude where electrons in the atmosphere emit the light that produces aurora. The results are published today in Annales Geophysicae, a journal of the European Geosciences Union (EGU).

“We had initial success when we projected the digital SLR images at a planetarium and showed that the aurora could be seen in 3D. It was very beautiful, and I became confident that it should be possible to calculate the emission altitude using these images,” recalls Kataoka, who also works at the Graduate University for Advanced Studies (Sokendai) in Hayama, Japan. He teamed up with other Japanese researchers and an American scientist to do just that.

The separation distance between the human eyes is what allows us to see in 3D. When we look at an object, the images captured by the left and right eyes are slightly different from each other and when combined they give the brain the perception of depth. But because the distance between our eyes – about 5 cm – is small, this only works for objects that are not very far away.
Since aurora extend between about 90 and 400 km in altitude, a much larger separation distance is needed to see them in 3D. The researchers used two cameras, mimicking the left and right eyes, separated by 8 km across the Chatanika area in Alaska. Their two digital SLRs, equipped with fisheye lenses and GPS units, captured two simultaneous all-sky images that the researchers combined to create a 3D photograph of the aurora and measure the emission altitude.

“Using the parallax of the left-eye and the right-eye images, we can calculate the distance to the aurora using a [triangulation] method that is similar to the way the human brain comprehends the distance to an object,” explains Kataoka. Parallax is the difference in the apparent position of an object when observed at different angles.

Scientists have obtained altitude maps of aurora before. They are useful because they provide information about the energy of the electrons that produce the lights. But this is the first time the emission height of Northern Lights has been measured using images captured with digital SLR cameras. As the authors explain in the new Annales Geophysicae paper, the altitude maps obtained in this way are consistent with previous observations.
The technique is low cost and allows researchers to measure the altitude of small-scale features in the aurora. Further, it opens up the door for citizen scientists to get involved with auroral research.

“Commercially available GPS units for digital SLR cameras have become popular and relatively inexpensive, and it is easy and very useful for photographers to record the accurate time and position in photographic files. I am thinking of developing a website with a submission system to collect many interesting photographs from night-sky photographers over the world via the internet,” says Kataoka.

The researchers believe this may lead to new scientific findings, while working to engage the public in auroral research. After all, it was the beauty of 3D imaging of auroras that inspired Kataoka to develop a new tool for scientific research in the first place.

For more information, the scientific article is available online, free of charge, at http://www.ann-geophys.net/31/1543/2013/angeo-31-1543-2013.html.