Sensor specification is an important consideration when choosing process monitoring and control equipment. However, in the following article, Juhani Lehto from Vaisala explains why the quality of a sensor can be difficult to determine from a spec sheet alone.
The cost of high-end sensors can be negligible in comparison with the processes and assets that they help to manage and protect. Process managers should therefore invest in measurement technologies that have been proven to deliver stable, accurate reliable measurements in the long-term.
Sensor selection.
The suitability of a sensor for a specific application can be determined by a quick examination of its datasheet. For example, the sensor will need to operate within a known range of temperature and pressure, it will provide an acceptable output signal and it will deliver measurements within a claimed level of accuracy and precision. However, there are several important factors to consider when choosing sensors. Beyond accuracy, these include stability, reliability, and any variance in performance due to process or environmental conditions.
Stability is perhaps the most important criterion because sensors must be accurate and reliable even in demanding environments, following installation and in the long-term. The specifications of different sensor suppliers may state similar accuracies, but this does not provide insights or verification of sensor accuracy after one year, or several years. All sensors drift, but some drift more than others, and some drift more 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 and costs over a long time and is reflected in calibration and maintenance requirements.
Given the potential costs of long-term unreliability, it is important to consider the lifetime cost of sensors, and not just their purchase cost.
In addition to good stability, high-quality instruments are also easy to maintain, calibrate, adjust, and fine-tune, which lowers lifetime costs and is good news for operation and maintenance staff. Having convenient tools for verifying and maintaining the sensors and transmitters enhances reliability and provides peace of mind throughout the life of the instrumentation.
The cost of inaccuracy can be large.
Cooling represents a high proportion of the costs and carbon footprint of running a datacentre. Vaisala participated in a datacentre trial to simulate how much more energy would be used if there was just one-degree of temperature measurement error, causing excessive cooling. The results showed that this one-degree of error increased energy consumption by more than 8.5%. Given the size of modern datacentres and assuming the same deviation can take place over a decade, this seemingly tiny error translates into many millions of euros of extra costs, particularly if server uptime is affected.
Vaisala Echo measurement infrastructure brings enterprise muscle to SMEs Traditionally only large corporations have had the resources to build measurement infrastructures with advanced features such as remote monitoring, firmware updates and easy scalability. Vaisala Echo connects Vaisala measurement devices and monitoring software to create an intelligent measurement infrastructure, without the requirement, and cost, of developing a bespoke solution.
In practice, customers will not need to buy Echo separately, because it is a built-in feature for Echo-compatible Vaisala measurement products. Simple to set up and run, Echo provides reliable, always-on access to data and alerts from all compatible devices, with remote firmware updates for all connected devices.
A major benefit of Echo is easy access to comprehensive reporting – both current and historic. Echo reports current conditions in real-time and enables notifications/alerts, and all data is encrypted in transit and rest, and secured with digital certificates.
Case study: sausage casings.
A team of British process engineers has developed a technique for manufacturing collagen sausage casings which protects product quality and trebles output. The process relies on precise monitoring of temperature and humidity with Vaisala sensors, combined with multi-stage feedback control.
The manufacture of collagen sausage skins from animal hide is a good example of sustainable production because it recycles a waste product into a valuable commodity that helps to improve the quality and consistency of sausages. However, all sausage casings are delicate and require precise conditions and careful handling during manufacture, storage and distribution, and throughout the subsequent sausage manufacturing process.
Natural casings from the small intestine of meat animals tend to be variable in length, diameter and thickness, so they are not ideal for high-speed sausage manufacturing processes. In contrast, collagen casings offer reliable, consistent features, and are therefore ideal for fast, efficient processes. Also, collagen casings do not need to be stored in chilled conditions and have a longer shelf-life than traditional casings. However, sausage casings are delicate, so precise management of handling processes and environmental conditions is essential.
Collagen casings are ready to use straight out of the package, horn loading is quick and easy, and there is no requirement for soaking and untangling of bundles. In comparison with sausages in natural casings, there are no seasonal variations, and sausages manufactured in collagen casings offer improved cooking coverage which delivers a superior cooked appearance
Prior to the development of a new product handling system, the sausage skin manufacturer utilised a conveyorised flat sheet collagen dryer, but this created friction that could lead to ripping and product wastage. The process was slow and wasteful, so the engineers were challenged to develop a new, less damaging technique for drying and processing the extruded cylindrical collagen cases. A key feature of the new process would be to minimise friction and optimise both temperature and humidity, irrespective of the plant’s geographical location.
The new processing solution allows the casings to be inflated with hot, dehumidified process air provided by a silica-based desiccant rotor. New equipment was also developed for transporting the inflated gel using an innovative handling system.
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| HMT120 |
The new system employs three precision sensors. A Vaisala HMT120 humidity transmitter monitors environmental humidity, so that dry air can be recirculated when outside conditions are too humid. In addition, two Vaisala DMP5 dewpoint transmitters monitor process air to ensure that the collagen products are maintained in ideal conditions by a multi-stage feedback control system.
The HMT120 transmitter contains Vaisala’s HUMICAP® technology; a thin-film polymer-based capacitive humidity sensor, which delivers accuracy, reliability, long-term stability and is insensitive to condensation or contamination.
The DMP5 dewpoint transmitters incorporate the Vaisala DRYCAP® sensor, which is also designed for accuracy, reliability and stability. The sensor is condensation-resistant and is immune to contamination, however, the high level of performance delivered by this technology is based on a capacitive thin-film polymer sensor and an autocalibration function. The sensor also has a rapid response time and fast recovery after wetting.
Case study: baking ovens.
Climate change, environmental concerns and rising energy prices are driving demand for improved efficiency across all industries; not least in food production where processes involving baking ovens can run at temperatures of up to 325°C. Bühler’s latest indirect-fired convection ovens, therefore aim to increase efficiency in line with the company’s strategic commitment to help customers make substantial savings in energy, water, and product wastage.
By monitoring humidity levels accurately inside an oven, it is possible control the process to maintain a consistent baking profile, even with fluctuations in ambient conditions or ingredient quality. This enables a more consistent product in terms of spread, thickness, colouring, and other important indicators of quality. This improved consistency also helps to minimise wastage.
Ovens with built-in measurement technology enable process managers to control the humidity and temperature of their ovens more effectively, which can be particularly valuable for those producing multiple products with different baking profiles.
Summary.
The applications highlighted above demonstrate the critically important role of accurate stable measurements, which highlights the need to invest in proven, high-quality sensors, particularly when the costs of not doing so can be so great.
In datacentres accurate temperature measurement can save enormous energy costs, in baking ovens accurate sensors can improve product quality, save energy and reduce wastage, and in sausage skin manufacture accurate sensors save time, reduce waste and increase productivity three-fold.
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