Monitoring & metering
In addition to good stability, high-quality instruments are also easy to maintain, calibrate, adjust, and fine-tune. Having convenient tools for verifying and maintaining sensors and transmitters enhances reliability and provides peace of mind throughout the life of the instrumentation.
Sophisticated transmitters are also capable of providing calculated parameters (such as humidity) straight from the transmitter, which also improves both reliability and the ease with which the system is commissioned. Other factors to consider include modularity, exchangeable probes, and convenience – installation and commissioning should be flexible, while calibration and maintenance should be as easy as possible. For example, a multi-use transmitter like the Vaisala HMT120 meets all of these requirements, allowing users to easily switch probes for calibration, for example. The measurement probe is either attached to the transmitter itself, or attached by a cable, so the transmitter can be somewhere easy to read while the measurement is close to the process it needs to measure.
SMALL INACCURACIES LEAD TO LARGE COSTS Vaisala participated in a trial to simulate how much more energy would be used if there was just one-degree of temperature measurement error, causing too much cooling in a datacentre. The results showed that this one-degree error increased energy consumption by more than 8.5 per cent. With the size of datacentres and assuming the same deviation can occur over a decade, this seemingly tiny error translates into many millions of euros of extra costs, particularly if server uptime is affected.
much humidity can cause damage from condensation. Although the tolerances of electrical equipment have increased, the consequences of inaccurate measurements and unreliable building systems can be dramatic.
CHOOSING A SENSOR SOLUTION There are several important criteria to consider when choosing a sensor solution, including reliability, stability, accuracy, and the varying requirements of different applications. Stability is the most important criterion; sensors must be accurate and reliable even in demanding environments, both now and in the long term. When buying sensors, the datasheets of different suppliers may state similar accuracies, but this does not provide insights or verification of sensor accuracy after one year, or a few years.
Instrumentation Monthly August 2024 A low-quality sensor might break down
faster, requiring maintenance or even replacement. However, a more serious risk is that the sensor might have drifted and be operating inaccurately without the user being aware. ‘Drift’ means that the measurement is no longer a valid input for the control system, and this can go undetected unless there is a visible consequence, like equipment being damaged by sub-optimal conditions. All sensors drift, but some drift a lot or quickly, while others have good stability with minimal drifting; maintaining measurement accuracy for years. Therefore, the stability of sensors should be the most important factor to consider because it influences performance over a long time and is reflected in calibration and maintenance requirements.
INVEST IN LONG-TERM THINKING Cheaper sensors will require more maintenance and need replacing more often, but the invisible threat is that they may drift without users even realising, leading to sub-optimal indoor conditions and poor energy efficiency. It is wiser to think long-term over the sensor lifetime and to understand and compare the total cost of ownership – when it comes to stability of measurement, the comparative unit cost is only a small part of the story. Real lifetime value is obtained when a decision is made to invest in reliable, high-quality sensors that maintain their accuracy in the long-term. The more critical the building, the more important the sensors. Minimising the total cost of ownership is different to minimising the cost of investment. High-quality sensors will last 15–20 years, so it is worth purchasing those that maintain their accuracy over time and are designed for effortless calibration and maintenance. This will help to keep the facility running optimally, saving energy, saving money and improving the sustainability of the business.
Vaisala
www.vaisala.com 67
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