Water / Wastewater 29 • Specifi cations, as these tell you what to expect from a product.
• Accuracy, as this is the true value of a measurement that can never be known.
Common sources of uncertainty The functional relationship will defi ne all the input variables. The next step is to list all the factors that can infl uence the measurement of each input. The provision of a defi nitive list is impractical, and the individual engineer must use their own judgment to decide the factors to consider in any analysis.
The essential point is to recognise that the sources of uncertainty extend far beyond the simple aspects of reading the meter output. For example, a fl owmeter will expand and contract with temperature and, if calibrated at room temperature and then used to meter hot crude oil direct from a wellhead, the fl uid temperature may affect the results obtained from the meter.
While it is not possible to list all the likely sources of uncertainty, the following list gives an indication of the types of factors that should be considered. Every measurement engineer should be aware of the possibility of additional sources that may be particular to the process being analysed.
• The environment – many instruments are sensitive to changes in pressure, temperature, humidity and vibration, as well as voltage fl uctuations and electromagnetic and radio-frequency interference.
• Measured quantity – in many industrial processes the parameter being measured is dynamic. For example, a fl ow rate may vary as a pump speed varies due to changes in electrical input.
• Calibration uncertainties – when an instrument is calibrated, its uncertainty can never be better than that of the calibration.
• Operator bias – when industrial measurements are taken manually, the results will be subject to additional uncertainty.
• Instrument used – many aspects, such as resolution, bias, hysteresis, ageing and non-linearity affect the result of the measurement.
• Measurement procedure – measurement procedures should be fully documented and adhered to. However, there will be occasions on which different instruments must be used or readings taken over a different time interval. The impact of this on the uncertainty should be assessed when applicable.
• Usage effects – it is common practice to calibrate an instrument in ideal conditions rather than in the conditions of its normal usage.
Calculating uncertainty
The ISO/IEC “Guide to the expression of uncertainty in measurement (GUM)” specifi es two distinct methods of uncertainty analysis. These are classifi ed as Type A and Type B analyses.
Type A is based upon the statistical analysis of multiple readings of the same measurement. It includes:
• Arithmetic mean • Spread or standard deviation • Degrees of Freedom
• Normal or Gaussian distribution
Type B is essentially a non-statistical approach. For example, these could be: • The uncertainty quoted on a calibration certifi cate
• Engineering judgement based on experience of a measurement system • Manufacturer’s specifi cations
For most uncertainty analyses we usually have to apply a mixture of both Types A and B. Before you are able to combine uncertainties from the various sources to get an overall uncertainty for a given quantity, a series of criteria must be
About TÜV SÜD National Engineering Laboratory
The company is a global centre of excellence for fl ow measurement and fl uid fl ow systems and is the UK’s Designated Institute for Flow and Density Measurement, with responsibility for providing the UK’s physical fl ow and density measurement standards.
TÜV SÜD National Engineering Laboratory is a trading name of TUV SUD Ltd, a company of the TÜV SÜD Group, an international service organisation. More than 24,000 employees work at over 1,000 locations in about 50 countries to continually improve technology, systems, and expertise. They contribute signifi cantly to making technical innovations, such as Industry 4.0, autonomous driving, and renewable energy, safe and reliable.
Carl Wordsworth, Head of Water Sector at TÜV SÜD National Engineering Laboratory, a world-class provider of technical consultancy, research, testing and program management services. Part of the TÜV SÜD Group, it is also a global centre of excellence for fl ow measurement and fl uid fl ow systems and is the UK’s Designated Institute for Flow Measurement.
Author Contact Details Dr Carl Wordsworth • TÜV SÜD National Engineering Laboratory • Address: East Kilbride, Glasgow G75 0Qf, UK • Tel: 07590 724069 • Email:
carl.wordsworth@tuvsud.com • Web:
www.tyv-sud.co.uk/nel
met, and calculations performed. Before combination, each uncertainty should be reduced to at least a common confi dence level and at best to a standard uncertainty. To add uncertainties together they must be in the same units. The most appropriate units are those of the output quantity whose uncertainty we wish to calculate.
The cost of uncertainty
In all measurement systems, it is generally the case that the lower the required measurement uncertainty, the higher the fi nancial cost required to achieve it. Thorough evaluation of the system uncertainty helps ensure that the metering system is properly designed, cost-effective and fi t for purpose, meeting any uncertainty constraints specifi ed legally, through partner agreements or through internal requirements.
Uncertainty analysis is an essential component of the design and use of any measurement system. Without a thorough uncertainty analysis time and money will be wasted on inappropriate instrumentation. As discussed, the techniques used for performing the analysis are not complicated but must be based on the solid foundation of a detailed review of the entire measurement process.
Convenient sonic water level meter
The Solinst Sonic Water Level Meter is a handheld, acoustic ranging instrument designed to simply and quickly measure depth to static water level in a well, piezometer, sounding tube, or any closed pipe from surface. It works in straight wells, but is also ideal for wells that are diffi cult to access due to crooked pipes, narrow tubes, or other instrumentation in the well. Because there is no equipment to lower down the well, the Sonic Water Level Meter is also ideal for use in contaminated or corrosive environments. Depth measurements to 600 m (2000 ft) are possible. It is supplied with a plastic disc for covering larger well openings and a convenient Solinst Field Bag.
More information online:
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For More Info, email: email:
For More Info, email: email:
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