Checking the signal quality
60 Snow depth sensor SHM 31, V2.1
A basic measurement setup is shown in Figure 31.
Figure 31: Checking the signal quality with the white card 8365.KWK-WS
T
he measurements shown in Figure 30 were obtained with the measurement setup shown in Figure 31.
The domain for the signal intensity (signal normalised) is between 0 and 255. The sensors are set to achieve
a value of approximately 50 for the black plate and 200 for the white plate. The exact values for the
calibration are documented in the factory certificate.
In this sample measurement, these values were compared with the measured values on the concrete slab in
dry and wet conditions. Another card with a 50% reflectance was also used.
T
he SHM 31 sensors’ tolerances in signal intensity are in the order of 20%. This allows a rough distinction to
be made between a dark surface (grass, asphalt) and snow coverage. The threshold is stored in the sensor
at 130 as the default value and can be adjusted if necessary. The ‘snow flag’ = 1 is set above the threshold.
It is zero below the threshold.
If the user suspects that the sensor is no longer measuring adequately due to ageing or for other reasons,
the procedure described here provides a test option that is easy to carry out.
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