Brigadoon - An Occasional Corner on the Internet
Ground Temperature (Overview)
Designed by Mark Little
Most domestic weather stations do not normally come with the provision for measure the temperature of the ground. Ground temperature can be an important measurement for the home gardener as it can determine when the soil is warm enough to trigger the germination of seeds after the cold of winter.

Traditionally, the temperature of the soil at various depths was done using special mercury thermometers that were designed to sit at the required depths and then pulled out to read the temperature was required. These thermometers have two disadvantages. Firstly, they can be quite expensive and secondly, they need to be read manually.

If the soil temperature is being measured near the surface, an alternative to the thermometers dropped down a tube is to use a special thermometer with a near 90° bend in the stem of the thermometer.
Some soil thermometers come with either a mechanical or electronic display of the soil temperature. Unlike the mercury thermometers above, they are normally inserted at the point where the temperature reading is required, rather than being permanently in place. However, like the thermometers above, the reading are manually recorded by the operator.
To record the temperatures continuously, the output of the temperature sensor needs to be readable by the Raspberry Pi.This can be achieved by using a digital sensor that communicates with the Raspberry Pi via an interface such as I2C or SPI. As an alternative, an temperature sensor with an analogue output can be connected to an Analogue to Digital Converter (ADC) and the temperature derived from the output of the ADC. Each approach has both advantages and disadvantages, depending on the measuring environment.
The next page will examine both options and then select the method to be used for the measurement of soil temperature in a suburban backyard. Below is  the sort of measurements that can be expected from ground temperature measurements. The image below shows the changes in soil temepratures at ground level and 50cm in Salisbury Downs, Adelaide during April 2001.
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