Fyrsteilene

Measurement principals

The collected data is presented on a daily basis with a new plot starting each day. One week of data from the previous days is available on the first page.

Measurements are made every 10 minutes and the data is presented as soon as the measurement is completed. Measurements that requires averaging over a period of time are, hence, not real time in the true sense of the word since there is a delay resulting from the averaging period that is, the measurements are not instantaneously measured and presented.

Meteorological Data

The meteorological data are measured with a Vaisala WX510 multiparameter sensor. The sensor measures wind, atmospheric pressure, temperature, precipitation and humidity. The sensor is configured to report data every 10 Min.

The sensor is mounted such that it measures the undisturbed wind 10 meters above the water surface and 7 meters above land.

Wind

The sensor works on the time of flight principle from an array of 3 ultrasonic sensors. Both wind speed and direction are measured with this technique. During the 10 minute measurement period the sensor reports the mean speed and the mean direction as well as the maximum speed (gust). Wind scaling

The average values of wind speed and direction are calculated as a scalar average of all samples over the selected averaging period (10 minutes). The number of samples depends on the sampling rate, which with the Vaisala WX510 is 4 Hz. The maximum values of wind speed represent the corresponding extreme speeds during averaging periods. Measurement intervals are non-overlapping. This means that reported measurements are purely independent of previous measurements.

As one might expect, the wind velocity profile will exponentially increase from zero at the surface to a maximum at a certain height. The Shape of this profile is presented in the plot to the left. Here we have adjusted the plot so that the scaling coefficient is 1.0 at the sensor height of 10 meters above sea level. In order to adjust to the height of interest one should scale the measurement according to the "scaling coefficient". As an example, a measurement from Steilene met sensor of 10 m/s would be about 7 m/s at a height of 2.0 meters above the sea surface.

Precipitation

The precipitation sensor consists of a steel cover and a piezoelectric sensor mounted in the surface of the cover.

The precipitation sensor detects the impact of individual raindrops. The signals from the impact are proportional to the volume of the drops. Hence the signal of each drop can be converted directly to accumulated rainfall. Advanced noise filtering techniques is used to filter out signals originating from other sources than raindrops.

Detection of each individual drop enables computing of rainfall and intensity with high accuracy and resolution.

Pressure – Temperature – Humidity (PTU)

The PTU module contains separate sensors for pressure, temperature and humidity measurements. The measurement principle of the pressure, temperature, and humidity sensors is based on an advanced RC oscillator and two reference capacitors against which the capacitance of the sensors is continuously measured. The microprocessor of the transmitter performs compensation for the temperature dependency of the pressure and the humidity sensors. The PTU module includes a capacitive silicon sensor for pressure measurements, a capacitive ceramic sensor for air temperature measurements, and a capacitive thin film polymer sensor for humidity measurements.

Oceanographic Data

Oceanographic data is measured with an AWAC (Acoustic Wave And Currents) instrument. The instrument is deployed at a mean depth of 20.5 meters , and lies 150 meters east of the northern point of Steilene island.

The AWAC measures a wide range of wave parameters as well as full current profiles at a large number of discrete levels. However, for clarity we have limited the variety of data presented to include currents at three different depths, wave height and wave direction.

Comments on data presented

Wave height is referred to as the mean of the largest 1/3 of the waves during a certain time period. This is also known as the significant wave height. Occasionally you will see spikes in the wave data, and this is normally dueto wakes from passing ships.

Principles of operation

Below is a brief description of the AWAC instrument. A more thorough description of the principles and the technology can be found at www.nortek-as.com and www.nortekusa.com

The AWAC measures current profiles at discrete levels using the Doppler principle. In short, the instrument sends out acoustic pulses (“pings”) at a known frequency and measures the change in frequency from the returning echoes. These echoes come from scattering material in the water (biological, sediments, etc.), which are assumed to move passively with the water. The measured change in frequency is proportional to the speed of the scattering matter relative to the acoustic instrument (AWAC). By combining information from three acoustic beams oriented at different directions it is then possible to calculate three components of the flow.

The wave measurements are more complicated both in the measurement and the processing of the data. During the measurement phase the AWAC measures waves in three different ways; (a) pressure, (b) wave orbital velocities, and (c) Acoustic Surface Tracking (AST). The AST may be thought of as a simple inverted echo sounder from the AWAC to the surface. The position of the surface is tracked and the history of the track is used to estimate wave parameters. At the Steilene station, the AST is used as the primary means to estimate wave height.

Since waves are random in nature, we cannot simply measure a few wave cycles and estimate the wave parameters. Instead, waves are measured over a period for which the measurements are statistically significant or, in other words, the data record is sufficiently long to prevent deviations to bias the final estimates. With this in mind wave records are normally approximately 10 minutes in fjords, whereas open ocean wave measurements may require records as long as 30 minutes.

Once the measurement record (“wave burst”) is complete it must be reduced to more meaningful estimates of height, period, and direction. This post processing step is typically performed with special software packages. For online applications such as the Steilene station, the data is processed with similar algorithms in an embedded computer. Hence, all data coming from the AWAC is processed and formatted prior to data transmission.

Communication

Data from the met sensors and the AWAC are collected and transmitted through the so-called AOS (autonomous online system). The AOS offers online access to Nortek Instruments from virtually any location worldwide, where battery power and satellite communication ensures do-it-yourself and cost effective, real time data transfer from anywhere in the World. It is a complete data collection solution that includes local power supply, data transfer and WEB data display. It does not require significant engineering resources and once deployed, your system will be up and running in a matter of minutes.

Data Access

The webpage provides an easy and intuitive user interface. Key parameters are presented graphically and historical data are available for download to Excel. Built-in GPS and map functionality helps track the unit. The flexible user interface allows you to tailor a presentation according to your specific requirements and preferences.

The number of cells and transmit interval may be reconfigured from the client webpage. The webpage also includes a “cost calculator” showing estimated data cost for the current configuration.

Data from the connected instruments are transmitted via serial interface and cable to the AOS. The AOS Iridium satellite modem then communicates data to the AOS-server, which stores and provides data for presentation on the client website. The data volume and cost is kept to a minimum by focusing on vital data subsets. Full current and wave datasets are stored in the instrument memory.