Description of the device
Snow depth sensor SHM 31, V2.1 13
Figure 10: SHM 31 assembly, determining the
distance d
0
and angle α. h: height above ground.
Figure 11: Calculating the snow depth h
1
from the
difference Δd in distance and the tilt angle α.
T
ypes of communication:
Communication over the RS232 interface takes place using the UMB-ASCII 2.0 protocol. Data telegrams can
be requested using the ‘polling method’, or an automatic transmission mode can be set.
The configuration of the RS232 interface is: 9600 baud, 8N1 (8 bit, none parity, 1 stop bit, no handshaking).
The default configuration of the RS485 interface is: 19200 baud, 8N1. This interface can be used to
communicate either using the UMB-ASCII 2.0 protocol (query or automatic transmission mode) or using the
UMB binary protocol (pure query mode). Various tools are available here for the UMB binary protocol in
particular, such as the Lufft UMB ConfigTool.NET software, as well as connection to other Lufft-specific
communication and database solutions such as SmartView.
Modbus RTU and Modbus ASCII are also available over the RS485 interface from firmware version v16.
The specifications for the SDI-12 interface can be found in section 10.
3.3. Sensor heating
The SHM 31 has two integrated heating circuits. One circuit brings the laser to the right temperature and
prevents fogging of the optical window as far as possible. One heating circuit can be parameterised and is
used in ‘defrost mode’ to de-ice the pane. Significant warming of the inner housing compared to the outside
temperature corresponds to the sensor’s normal operating behaviour.
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