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Arduino Based Weather Station
Designed by Mark Little
ATMega328/P Datasheet Complete" by Atmel
"DS18B20 Programmable Resolution 1-Wire™ Digital Thermometer" by Maxim Integrated
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The selected sensor is a waterproof version of the Dallas Semiconductor DS18B20 temperature sensor which is provided with a lead slightly over 900 mm.

Because each DS18B20 sensor contains a unique silicon serial number, multiple DS18B20 can exist on the same 1-wire bus.
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Temperature Thermometers
The second sensor to be used in this weather station is a set of ground thermometers. Although the Australian standard depths for ground temperature is 50 mm, 100 mm, 250 mm, 500 mm and 1000 mm, this weather station will use 100 mm, 250 mm, 500 mm and 900 mm. These depths were selected as they allowed the use of existing pipe to be used to hold the thermometers.
This means that the leads simply need to be trimmed to length and paralleled into three wires for the five temperature sensors that are used for the Ground sensors or the Pool sensors.

The image below shows an example of how the sensors are connected. As can be seen from the example, the three wires from each sensor are paralleled. While prototyping this system, it was found that any more than nine (9) sensors in parallel did not operate.

As a result, the five (5) Ground Thermometers are driven by one pin on the Arduino and the five (5)  Pool Thermometers are driven by another pin on the Arduino.
Installing Sub-Surface Temperature Thermometers
Theoretically it is possible to simply dig a hole and place the waterproof sensors at the required depth, or weight them down and dangle them in a swimming pool. However, practically speaking,  for the ground thermometers, it can be a lot of work to dig a hole that is 900 mm depth and then put the sensors in place at the correct depth as the hole is filled in. Dangling thermometer cables in a swimming pool can also be an annoyance.

Alternatively, the thermometers can be placed at the correct places in a pipe and sealed.

For the ground thermometers, a hole to hold the pipe can be relatively easily drilled, using a garden hose to install the pipe using hydraulic digging. If the pipe is mounted vertically and buried to the correct depth, all the thermometers will be installed in the correct position in one step.

The same type of pipe construction can be used for the swimming pool thermometers, but the bottom of the pipe needs to be weighted down with lead shot of similar to ensure that the pipe will float vertically.