ISSUE FOCUS FEED & ADDITIVE MAGAZINE May 2023 49 A linear relationship between the moisture and the sensor value allows a much simpler calibration. The sample results follow a straight line allowing the use of the standard gain/offset calculation usually expressed as y=Bx+C (where B is the gain and C is the offset), shown as ‘Hydronix Digital Technology’ in Figure 2. DIGITAL MEASUREMENT Sensors with a digital measurement technique have a naturally linear output, which gives a sensor reading directly and proportionally related to the moisture content. This method allows systems to achieve optimal calibration with a few points. With the linear measurement, it is possible to achieve a calibration with only two points. As already demonstrated, there is still likely to be errors in the sampling and laboratory test for each of the points. It is vital to gather samples and corresponding sensor values with as wide a spread as possible between the low and high moisture samples for an accurate result. In Figure 3, all the samples have a 0.3% error from the actual moisture. By using low and high moisture samples that have a wider range (shown in blue), the influence of the errors is reduced. This provides a much more accurate calibration line than using samples that are close together (shown in red). In real-world applications, it is best practice to add more samples, reducing the errors from only using a small set of samples. As Figure 4 shows, using a linear sensor and a simple mathematical formula, it is possible to generate a line of best fit, which reduces the influence of the errors from individual points. Hydronix sensors use a digital multi-frequency measurement technique that gives a linear measurement, where the sensor value and water content are directly proportionally related. This method allows the sensor to be more accurate throughout the moisture range and calibrated more precisely, removing the need for recalibrations after commissioning. About Alessandro Mario Alessandro Mario started to take things apart to see how they worked as soon as he could hold a screwdriver… later, during his teenage years, he learned how to put things back together. Now a Chartered Electrical and Automation Engineer, with 10 years of experience in customer service, continuous improvement and engineering, he now works with Hydronix to help companies improve the efficiency of their processes, and increase their revenue, by providing expert advice on moisture measurement solutions. He shares the Hydronix vision that by helping customers to succeed, he helps to build a more sustainable future. Figure 3. The importance of the spread of the calibration points Figure 4. Using a larger number of samples to increase accuracy
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