Snow height - ultra sonic sensor
Parameter to be measured:
Snow height
Principle and Accuracy of Measurement:
Ultra sonic sensor
The sensor’s principle of measurement deals with the transit-time measurement of an ultrasonic pulse. The sensor transmits several energy-charged pulses to the snow cover and subsequently receives its reflected signals. Based upon the required transit time of the ultrasonic signals, the sensor calculates the current snow depth. The processing time of the ultrasonic pulses here is enormously influenced by the air temperature. For this reason, the snow depth sensor also possesses integrated temperature compensation. This takes into account the influence of the air temperature on the processing time of the ultrasonic signals, in the calculation of the snow depth. The calculation takes place in the sensor or in the data logger.
Basis of Snow Depth Measurement
The measurement of snow depth is an important parameter for the measurement of the snow cover in the terrain. Its formation is subject to strong variance, based upon time and space factors. Influencing factors here are the development of snow cover, under the influence of further climatic events (air temperature, wind, global radiation), the characteristics of the land, as well as the spatial distribution of precipitation in an area. Because of these factors, the selection of a suitable location for measurement is decisively responsible for whether this is representative for an area or not.
Equipment:
Ultra sonic distance sensor: e.g. Campbell Scientific SR50, Judd Communications depth sensor, or USH-8 from company Sommer
Logger: Campbell recommend their loggers. But it can be connected to other loggers, e.g. a DL2E from Delta-T
Advantages:
- Continous measurement
- Low cost relative to more sophisticated scanning laser systems.
Disadvantages:
Measurement principle: The velocity of an ultrasonic signal in air depeding on the air temperature. For an accurate measurement of the snow height the puls length wich is proportinal to the distance has to be calibrated by the airtemperature. Sensor Campell SR50 use an external temperatur sensor and calculate the temperature compensation in the Campell data logger. The Sommer USH-8 use an internal temperature compensation and outputs the already compensated snow height to any data logger.
Measuring during snow fall is quite dificult because every snow flake produce additional reflected ultrasonic pulses. High-end sensors like, Sommer or Campell, can handle such additional reflections and measure reliable also during snow fall.
- Expensive high end sensors
- While ultrasonic sensors can be cheap (about 10-150$ for robotics), connecting it to a logger requires quite some knowledge about digital electronics and programming; the signal from the sensor is usually a puls with a length proportional to the distance. In addition, higher end sensors with a narrow field of view, large maximum distance, and wide operating temperature range are typically required for scientific work. Therefore, the more expensive (650$ to >1000$) sensors from scientific instrumentation companies are usually used.
Problems/Questions:
Starting a measuring cicle in the SR50 from the DL2E was difficult, because the pulse generated in the DL2E was too long. In the end a solution using a micro controller was found.
The measurement of snow depth is an important parameter for the measurement of the snow cover in the terrain. Its formation is subject to strong variance, based upon time and space factors. Influencing factors here are the development of snow cover, under the influence of further climatic events (air temperature, wind, global radiation), the characteristics of the land, as well as the spatial distribution of precipitation in an area. Because of these factors, the selection of a suitable location for measurement is decisively responsible for whether this is representative for an area or not.
Selection of the Measuring Location:
The right selection and positioning of the measuring location is of very great importance. The field of measurement to be recorded should be most preferably flat, protected from wind and secured against avalanches. Steep hillsides, hollows, terrain edges or large rocks in the immediate vicinity of the measuring location should be avoided. Steeply sloping fields of snow harbour the danger that the snow cover could begin to slide, thereby falsifying the measuring results. To ensure a problem-free measurement, the sensor must be aligned parallel to the ground. A minor incline of the ground can be compensated for by a correct alignment of the sensor, using the holding fixture provided. Furthermore, the expected snow depth at the location should be estimated, in order to assure a suitable mounting height for the sensor.
However, all of these concerns must be weighed against the need for unbiased results. Measurements made at the bottom of a valley will be different from those higher on a ridge for multiple reasons. In particular, snow fall is likely to be greater at higher elevations, and wind redistribution is likely to play a major role, especially in alpine environments.
Links
Projects that used the above equipment
Other related web sites