The temperatures in the biomeiler pile are measured with thermocouples buried in several locations. Nine of them are found at three heights (1’, 2’, and 3’ above the ground) in the inner core, outer core, and mantle of the pile. The graph in today’s post shows the temperatures measured by thermocouples at 2’ height. These sensors are buried 2’ below the upper surface of the pile. A sheet of Reflectix, a tarp, and some snow lie above the top of the pile.
The 2-week cycle of temperature change, seen in past months, seems not to have continued in March. I am now inclined to consider it a coincidence. The aeration rate, kept fairly constant for the month, is probably what most helped resolve that concern. Previously, there were multiple variables changing at once, which makes interpretation of the temperature trends more difficult. In a curious turn, a past trend has reappeared. Once again, the general trend of the biomeiler temperatures was the inverse of the ambient air temperature.
The most plausible explanation is that the pile is behaving like a lake in the winter. When the air temperature drops, more ice forms at the surface of the lake, which releases its latent heat into the water below.
The main problem I have with that explanation is the speed with which the temperature changes take place. The mantle, being the first barrier to the cold air, should change temperature first, followed by the outer core. The inner core, being best protected, should change temperature last. In contradiction, my readings show that the layers often change temperature at the same time.
The woody material in the pile should act as an insulator, impeding the flow of heat and slowing the temperature change rate. The water in the pile would act as a conductor to allow for quicker temperature changes. I would not have thought that the effect of the moisture in the pile would predominate. I am left to wonder if there is a more obscure cause for the observation.
In the coming month, I will alter my aeration practice. The pile will be treated during morning and evening chores, with the goal of 6-7 minutes twice per day. My goal is to discover if that change improves the environment for aerobic bacteria. While the pile temperatures have been sufficient for heating the coop this winter, I still wish to see if they improve with shorter intervals between aeration times. The pile never reached a temperature at which it seemed sensible to attempt to extract heat by circulating water through the PEX tubing.
