# In the heat of the pile

by cfastie | | 125 views | 3 comments | 15 Jun 03:19

Above: The height of the compost pile dropped about a foot in a week of cooking. The temperature probes dropped less so they extend farther above the pile. This changed the relationship between the sensors and the center of the pile.

I left the Riffle in the compost pile for 5.5 additional days unattended and it logged every five minutes. That is a total of one week of logging without recharging the battery. I would have continued the logging, but the compost is now spread around the garden.

Above: In a week the pile was reduced to about 80% of its original volume (the wire cage was originally filled to above the wire).

Above: The temperature of the pile stayed high all week. The highest temperature reached was 69°C (156°F) on the third day. It's too bad the garden needed the compost because the pile would have cooked for a few more days.

Above: The temperature and humidity record from the DHT22 sensor in the jug above the compost pile. Some of the days were cool and rainy, so the diurnal pattern is not very tidy here.

Above: The battery voltage dropped steadily all week. The temporary recoveries might be when the Nalgene bottle holding the Riffle and battery got warmed in the sun.

The sketch running on the Riffle is attached to the previous note. It is based on Kina's original sketch with the buggy SD library.

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Yay, data!!

Neat observation about the battery voltage and the temperature. Makes me want to compare the rate of battery voltage drop to the temperature. Perhaps: smooth the battery voltage data (running average?) and take the derivative ... then plot that against temperature? If that makes any sense. If you attach the data from this experiment, I might try that out ...

In general, it'd be great to know how battery drain is impacted by temperature.

Also interesting re: not measuring the same point in the pile as it cooks. I guess another approach could be to set up temperature probes at several fixed heights relative to the ground, along a pole ...

This is all making me recall Ned's beehive temperature monitoring idea ...

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This is probably not the best data set to do calculus on. The air temperature data is from a sensor in a jug suspended above the pile, and the battery data is from inside the Nalgene bottle. The jug and the bottle were close, but got heated by the sun differently.

What we need is temperature data from the RTC on the Riffle which I was not logging here.That can be added to Kina's new sketch. Then if the battery is close to the Riffle you can have a good record of the temperature and any related changes in voltage.

With a few more MCP9808s we can definitely instrument a beehive for 3D temperature modelling.

I am also very curious how much temperature affects the batteries in very low current applications. I haven't found good documentation about that. If a device is pulling less than 100µA, or in some of my applications, less than 10µA would the cold be better? Isn't the self-discharge rate of the battery more than that? You can get temp from the RTC with these two functions:

rtc.convertTemperature();


and then

temp = rtc.getTemperature();


getTemperature() returns degrees C as a float.

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