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Ground Temperature Sensors
Designed by Mark Little
A pre-assembled waterproof sensor based on the DS18B20 1-wire temperature sensor provides between 9 and 12-bit temperature readings over the range -55°C to +125°C. The 64-bit ROM means that only three (3) wires are required to connect a large number of temperature sensors to the Raspberry Pi. The waterproof cable reduces the requirement to protect the sensor in the ground.
An alternative to the digital temperature sensor is an analogue temperature sensor that provides a D.C. Level proportional to the temperature in Celsius - 10mV/°C is a common scale factor. Typical examples of this sort of sensor is the TMP36 ($1.50) and the LM335 ($0.80) The TMP36 is designed not to require calibration, while the LM335 can obtain higher accuracy with the addition of a calibration resistor.
Previous experimentation has shown that the need for a waterproof assembly can be overcome by using irrigation polypipe with endcaps and plumber's tape around the threads.
This project describes the construction of a ground temperature system using the analogue sensors and one with the waterproof digital sensors.
No software will be provided for the analogue thermometers are they were originally used with the a predecessor to Hot Chip Environmental Sensor connecting to a Windows program. Any project that uses an Analogue to Digital Converter (ADC) with the Raspberry Pi will be able to use these sensors. Software for the DS18B20 will be provided as it is the only project currently using the 1-Wire interface.