Measurements & Instruments

Before any sensor can be connected to a d ata acquisition system one has to find a suitable set of cables, connectors and cable ties. In our case, these need not only to be weather- and UV-resistant but also specified for low temperature usage. Currently, modern sensors, such as ultra-sonic anemo m eters and data acquisition networks can be connected via fibre optical cables. However, fibre cables for cold climate operation need to be adopted for such use by, for example, using non-freezing gel that is pumped into conduits surrounding the interior c a bles to prevent water ingress and subsequent ice formation. One such example is shown in ref [1]. The gel will also protect a cable against breaking if exposed to a) unforeseen external loads by a maintenance crew (or reindeers) and b) movements when cab l e attachments are deteriorating. Cable attachments occasionally will break and weather resistant cable ties are not sufficient in cold climates. Weather Resistant Nylon 6.6 [2] has greater resistance to UV light, which damages natural nylon, but one shou ld use Weather Resistant Nylon 12 in cold climate and/or high moisture conditions. A similar reasoning can be applied to connectors.

Instruments for cold climate measurements, including humidity, temperature, wind speed, wind direction, precipitation and radiation, have to be properly heated under icing conditions to maintain their accuracy. Instruments, more or less suitable for cold climate measurements, are continuously being developed and evaluated by manufacturers and users [3]. Depending on the required accuracy and in standard conditions, the exact location of an instrument might be required to adhere to IEA recommended practices or standards, which ensure proper mounting including sufficient distances to surrounding objects. IEA recommended practices are not available for icing conditions, one is typically recommended to stay away from such events, like ice storms, which is one reason for the creation of this IEA Annex. Icing conditions most probably require, depending on the required accuracy, neighbouring objects such as attachment and connecting booms/tubes to be heated as well. In practice, the latter is often overlooked, thus diminishing the value of such cold climate instrument testing.

In the field of wind energy a properly acquired wind speed is of outmost importance. We have two basic categories of needed accuracy when measuring wind speed in flat terrain:

• resource estimation, required accuracy: ± 3-4%
• verification of wind turbine performance, required accuracy: approximately ± 2%

In addition, accurate wind speed measurements in complex terrain is difficult on its own and does not need the uncertainty introduced by improperly heated or mounted wind speed sensors.

Detection of ice is similarly complex. Traditional ice-detectors used to be extremely unreliable. Ice detection technology has improved considerably, as has our knowledge about the occurrence of ice. Organisations are also developing and sharing extensive research on ice accretion versus temperature, humidity, radiation, wind direction, wind speed and precipitation.

1. http://www.polywater.com/icefree.html
2. http://onlinecatalog.panduit.com search for "cable ties".
3. Tammelin et al, Meteorological measurements under icing conditions Eumetnet SWS II project, FMI reports 2001:6.

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Mail: Tomas Wallenius