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SECTION TITLE


INSTRUMENTATION • ELECTRONICS


Meteorological Institute (FMI). Tanks to the long-term stability and accuracy of these sensors, as well as their ability to tolerate dust and harsh environmental conditions, these technologies are especially suitable for such demanding applications. Te FMI devices onboard the rovers employ standard Humicap humidity sensors and specially customised Barocap pressure sensors. Te modification of the pressure sensor is mainly a slightly thinner membrane to accommodate the lower pressure conditions. In comparison with Earth, Mars is a


dry planet; relative humidity is practically zero (0% RH) during the day and rises during the night as the temperature falls. Tere have been observations of small amounts of water in the atmosphere in the past, though never in liquid form. Martian surface temperatures vary during the night from -135°C to -70°C and the Martian surface layer along its equator achieves positive temperatures during the day time. Considering the temperature and low pressure of the atmosphere on Mars, it was previously estimated that liquid water could not be present on Mars. However, in May 2015, supported by the measurements made by FMI and Vaisala technology, combined with numerical models, the first evidence of ancient liquid water on Mars was confirmed. Curiosity’s mission was to determine whether Mars has ever had the right environmental conditions to support microbial life forms. Te discovery of traces of ancient liquid water was therefore a major success, and led to the mission objective for Perseverance: to search for signs of ancient microbial life.


ENVIRONMENTAL MONITORING ON EARTH Meteorological and aviation measurements rely on the same pressure and humidity technologies that are deployed on Mars. However, terrestrial applications also include building management systems and industrial monitoring where products are dried or moistened, or where process measurements, such as gas levels, require compensation. On Mars, sensor service and calibration is not possible for obvious reasons, but on Earth it is relatively simple to perform. However, terrestrial customers are also looking for sensors with high levels of accuracy and stability, long life expectancy and extended periods between service or calibration. Tis is important because unreliable measurements could have catastrophic consequences and because frequent service and calibration would interrupt production and increase operational costs. Despite the long-term stability of


the Vaisala sensors, it is still necessary to conduct regular calibration to check that the instrument is within specification and capable of producing valid data. Unless required by regulation, it may be possible to extend the period between calibrations as an instrument is proven to be stable.


HUMIDITY MONITORING TECHNOLOGY Te Humicap sensors employ a capacitive thin-film polymer sensor consisting of a substrate (typically glass or ceramic) on which a film of polymer is deposited between two electrodes. Te polymer either


Vaisala’s Barocap pressure sensor


absorbs or releases water vapour as the relative humidity of the ambient air rises or falls, which changes its dielectric properties, and thereby the capacitance of the sensor, which is measured and converted to a humidity reading.


PRESSURE MONITORING TECHNOLOGY Using a single-crystal silicon material, Barocap is a micromechanical pressure sensor that measures dimensional changes in its silicon membrane. As the surrounding pressure increases or decreases, the membrane bends, thereby increasing or decreasing the height of the vacuum gap inside the sensor. Opposite sides of the gap act as electrodes, and as the distance between them alters, the sensor capacitance changes, which is measured and converted to pressure readings.


The Curiosity rover working on Mars


18 www.engineerlive.com


SUMMARY Although participation in projects such as the Mars missions is an exciting and challenging diversion from the daily business, Vaisala’s engineers are keen to point out that invitations to be involved in such work only come when product development aims for ‘best in class’. Torri says: “Our sensors have become popular for three main reasons. Firstly, we focus intensely on the voice of the customer; listening to their needs and driving our R&D to meet and exceed expectations. Secondly, as part of our constant drive for improvement, we seek and encourage innovation in everything we do. Tirdly, our brand stands for quality and reliability, so before any product can be awarded our logo, it has to earn that right by passing a rigorous and demanding performance test programme.”


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