Regular calibration is crucial to ensure that your measurement equipment is measuring accurately. Learn the important standards, on-line resources, and facilities that help ensure success.
This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.
There is an increasing need for accurate current measurement. In the automotive EV industry, high performance, reliability, and accuracy are key criteria for electronic components and systems. Test benches used to analyse the entire drive system can require several conversion stages—electrical and mechanical—and for a deep understanding of where losses occur, therefore, high accuracy is required. Another significant market for current measurement is renewable energy.
With traditional hydrocarbon-fuelled power plants, it is relatively easy to monitor current levels because the energy flow is highly predictable and comes from a centralised source. With solar and wind, the energy load is much more dynamic, with many more smaller power generation sources and unpredictable fluctuations as the weather conditions change.
A third application is large physics institutions, such as CERN, where accuracy is an inherent requirement. These users are the most demanding, pushing for parts per million accuracy levels, whereas other applications might only require, essentially, a functional test.
Figure 1. For a test setup to be reliable and repeatable, the instruments used must be calibrated regularly in accordance with international standards. Image used courtesy of Bodo’s Power Systems [PDF]
Two Aspects to Calibration
A measurement can only be as accurate as the instrument itself, and calibration provides a level of confidence in the veracity of that measurement. Essentially there are two aspects to calibration: a functional ‘pass/fail’; and the accreditation traceability issue. Calibrations are performed to an international standard, ensuring that results from anywhere in the world will be consistent. ISO/IEC 17025 is the international standard for testing and calibration laboratories, focusing on their competence, impartiality, and consistent operation. ILAC G8 assists laboratories in the use of decision rules when issuing statements of conformity to a specification or standard as required in the 2017 edition of ISO/IEC 17025.
All calibration relies on a comparison of the device under test (DUT) with a known good, or ‘golden’, reference. One of the issues with current sense transducers lies in ensuring the quality of the reference transducer. Reference instruments of sufficient quality are rare, so much so that Danisense has created its own, which has been certified by the Federal Institute of Metrology METAS in Switzerland, which serves as that country’s National Metrology Institute (NMI).
Fundamentally, the calibration process is thus: a known reference current is passed through both the device under test and the reference, and a measurement is performed on each simultaneously. A burden (or reference) resistor is used to convert the current into a voltage, so it is also very critical to use the highest quality resistor, since resistors can suffer from instability.
How Long Between Calibrations?
We have discussed why it is important to have current sense transducers regularly calibrated. But what should the period between calibrations be? Danisense produces both voltage and current output sensing devices. Voltage output products employ a resistor module, and as such, may need calibrating more often than current output units, which do not require the resistor module. Certified test houses are not allowed to give guidance on the frequency of calibration; however, periods of one and even two years between calibrations are common. More often, companies have their own quality procedures, which state the frequency.
One of the greatest inconveniences caused by calibration is the length of time it can take. Test houses routinely quote a four-week turnaround. This may be because they are usually generalist companies that offer a wide range of test services. As such, they may not attract much business from the rather specialised current sense transducer sector, so when they do, they will have to set up from scratch each time.
Another challenge that faces current sense transducer users when they request a unit to be calibrated is that they may be asked a multitude of questions concerning the nature of the data that the user requires from the test. The person in charge of organising the calibration for the user is likely to be a quality manager who may not have specific knowledge of the measurements required, causing further delays and headaches.
ISO/IEC 17025-Accredited Calibration Lab
To address these issues,the current sense transducer manufacturer, Danisense, has created an ISO/IEC 17025-accredited calibration laboratory at its Taastrup headquarters in Denmark. Because the only calibration that the facility will undertake is current measurement equipment, the setup is permanent. This means that the process has been streamlined and automated, so the company is promising a 10-working-day turnaround service.
When considering accuracy, Danisense created its own reference, which benefits from a very high accuracy, so customers can have a very high degree of confidence in the accuracy of the results. The Danisense lab is also able to help customers with the process of defining conformity limits which best suit their application.
Figure 2. Danisense has its own ISO/IEC 17025-accredited calibration laboratory and offers brand-agnostic calibration services for current transducers. Image used courtesy of Bodo’s Power Systems [PDF]
Intriguingly, the Danisense current measurement test facility is brand-agnostic and will accept equipment made by Danisense’s competitors for testing, as well as the Danish company’s own products.
Customers can use a new personal online portal[1] to book their ISO/IEC 17025-accredited calibrations. The portal provides customers with 24/7 availability of the calibration reports, regular online and email updates during the calibration process, and detailed order tracking, as well as access to an online payment function.
Calibration is Crucial
Calibrating equipment is crucial to ensure the accuracy, reliability, and safety of measurements and operations. It maintains equipment within specified tolerances, preventing errors and ensuring consistent results, which is vital for quality control, regulatory compliance, and ultimately, saving money by reducing downtime and waste.
When dealing with demanding applications, such as electric vehicles, power grid analysis, and international research organizations such as CERN, it is of paramount importance that users can have the fullest confidence in the accuracy and reliability of their measurements.
Danisense’s current sense transducers have been proven to be amongst the most accurate instruments of their type, and the new independent calibration service will reduce time lost while instruments are being calibrated, as well as demystifying the process.
[1]: https://danisense.com/webshop/
This article originally appeared in Bodo’s Power Systems [PDF] magazine.
