CALIBRATION FEATURE HOW TO BUILD A FLOW CALIBRATION LABORATORY
Brendan Robson, consultant engineer at TUV SUD NEL, outlines the factors that need to be considered when designing and building a calibration laboratory
C
reating a flow calibration laboratory. “How do I build a flow
calibration lab?” said a potential client during an unexpected phone call. “Wow. Okay, let’s have a think about it...,” I said. The time spent on your requirements
specification will pay for itself very quickly, both in terms of eliminating unintended over-specification, and preventing under specification that requires costly rectification later on. The first item is to have a target performance for your facility. What fluid/s do you need? What is the flowrate range? Temperature and pressure conditions? Variation in flow meter type, size, signal transmission, etc. Now add to your list the infrastructure which supports all of this: tanks, pumps, conditioning plant, valves, control systems; your list will continue to grow. This should be an iterative design
process, particularly when you start to cost all this equipment, the installation, commissioning, running, and maintenance. It also requires a careful assessment of one-off, up-front costs (capital expenditure) versus running costs (operating expenditure). Sometimes more capital expenditure up-front can buy better performance that leads to lower operational costs, saving money over time. Calibration has a very definite definition
as “a comparison between the reading of a device and that of a standard (or reference system).” As for ‘testing’, you may be conducting a prescribed protocol, like OIML R49 or R117. Alternatively, the customer may only be paying for the reference results and keeping their meter’s metrics to themselves.
Take fluid flow, unlike many
measurements, flow is a derived quantity. A flowmeter’s calibration depends on mass, time, electronics, temperature, pressure, fluid properties (density, viscosity, etc), amongst others. So, your instrumentation extends beyond the common, control system tolerance and into the rigorous world of metrology.
ARE YOU CERTAIN? All measurements have a level of uncertainty: the ‘+/-’ bit (along with a statistical ‘confidence’ level). What is it worth to your calibration business if your measurements are imprecise or ‘wrong’ (maybe only by a fraction)? To answer such questions, we need to take a closer look at measurement uncertainty and its related analysis. The proper assessment of your new facility’s instrumentation will lead to a fact-based Uncertainty Budget (‘UB’). It is this document and its foundation of impartial, scientific records that will be the ultimate selling point of your facility which will satisfy your original (or revised) requirements ‘spec’. The collation, design, upkeep and
refinement of this critical UB must be supported by your traceability system. Traceability, in calibration parlance, is
the process of proving the performance of equipment for each stage of calibration (Figure 1). It is very rare, perhaps impossible, for a single organisation to calibrate all of the measurements that are mandatory to derive flow. Take temperature for example, the traceability chain ends in baths of molten metal or the standard critical point of water.
National Standard
Calibration House
Company ‘Master’
Produced Product
Figure 1: The pyramid of traceability
Returning to the aspect of “know what
you actually want”, this could be at the company level: (bottom level of the Traceability Pyramid): checking one in ten of your products before the batch leaves your factory, say. If you are aiming to build your country’s
Brendan Robson, consultant engineer at TUV SUD NEL
National Measurement Institute, the pinnacle for any measure as it would be recognised by the international scientific community (e.g. BIPM), and fiscal charges, then that requires a massive difference in uncertainty, build costs, operating expenditure and annual audits to maintain your ISO 17025 ‘license’ (the world standard in the required competence for testing and calibration laboratories). Consider having phases to your
MULTI-DEVICE CALIBRATOR COMPLIES WITH INTERNATIONAL STANDARDS
Fluke Calibration has introduced the 5322A Electrical Tester Calibrator, a multi-device calibrator built to comply with international standards, including the UK’s BS7671 17th Edition, IEC/EN Standards. The 5322A combines many functions into a single instrument, replacing discrete resistors, decade boxes, and other custom solutions commonly used to calibrate electrical testers. This single-box solution speeds and simplifies calibration because users only need to learn, operate, and maintain one calibrator rather than multiple instruments. The 5322A can be automated with Fluke Calibration MET/CAL Calibration Management Software in 5320A emulation mode – further increasing speed and throughput. The 5322A can handle a spectrum of electrical safety tester workloads. From older hand-cranked insulation
testers to modern insulation resistance testers, hipot testers, RCD testers, earth group resistances testers, loop/line impedance testers, ground bond testers, portable appliance testers (PATs), and multi-function installation electrical safety analysers, the 5322A has it covered! The calibrator is available in multiple model configurations offering the flexibility to select the features
best suited to the user’s workload. The base 5322A model offers 1.5 kV high resistance sourcing. The 5322A/5 offers 5 kV high voltage resistor sourcing. Add to either model active loop compensation and a 600 V precision ac/dc output source for calibrating devices under test with metering capabilities, or a characterised 40 kV probe accessory for precision measurement of very high voltages to 0.5% accuracy. Fluke Calibration
www.flukecal.com/5322A
implementation by allowing steps in your scope for future development in technology and costs. Perhaps, initially, aim for Calibration and Measurement Capability (‘CMC’) and uncertainty that starts high. Then build the experience, adapt the design and collate the evidence to make incremental improvements. Taking a too big bite of the calibration apple and you’ll soon choke. And, the most valuable ingredient in
your calibration lab? Experienced, motivated, professional, ethical staff. It may seem a cliché, but it is a vital truism: if you do not build a competent team of calibration engineers, then your ‘rig’ may just be a lovely lump of pipes. Not every technician can be a calibration
engineer since they must have continuing competence, interest, patience, and a natural eye for detail. And do not underestimate the skill of consistency, handling unplanned events or results, and calmness under pressure when applying impressive diagnostic precision.
TUV SUD NEL
www.tuvnel.com
PROCESS & CONTROL | APRIL 2019 7
Decreasing Uncertainty Increasing Cost
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