The Defender stops combusting, cools down, and eventually stops with an error when running at a high temperature for a period of time. I have so far no explanation for this failure. Apparently, combustion just stops, even though the propane solenoid and air flow fan is still on. Should be impossible. A voltmeter on the valve solenoid indicated that the solenoid was continuously powered even when the temperature hit extremes, which refuted an earlier theory. Also, once it stops, it will not restart until the trap completely cools down, regardless of the commands sent to the unit. The ambient case temperature at 40C is not that high to cause problems for the controller. It is possible that the conducted heat might mess up the solenoid.
We know that combustion requires fuel, oxygen, and, to start, ignition. Something must be interfering with the fuel or the air. Perhaps the gas valve is shutting down when its temperature (via heat conducted through the brass fittings) gets too high. Perhaps the air flow is closing down because of something swelling up and restricting the two passageways from the fan to the combustion chamber. I will have to disassemble to investigate (but not now). Fortunately, this problem occurs only during the heat of the day, when the sun is shining on the trap, and then it is too hot to catch mosquitos anyway. In the evening, when things have cooled down a bit, I can restart the trap. Actually, this failure saves propane and energy, but I would like the trap to run per spec.
Perhaps it would be wise to limit the temperature to, say 140C. But how to do this? The chamber temperature is a function of the gas, the air flow, and the ambient temperature.
We could reduce the gas flow by modulating the solenoid signal, e.g., one second off every X seconds. After the valve is shut, there is still sufficient gas to continue combustion for some period of time. Some experiments to see how long it takes closing to cause combustion to stop indicate it can tolerate several seconds of the propane solenoid off and still recover.
The air flow is a function of the restrictions from the net and ‘trumpet’ air intake. The installing the net box certainly reduces the air flow dramatically. Replacing the net box with a new one had no effect. We do not have the capability to increase the air flow, since the fan is on full blast, and we don’t want to fool with the vacuum apparatus.
Perhaps switching the gas off periodically would result in a lower operating temperature by causing a net pressure reduction and lower gas flow. But do we want to reduce the net gas flow? After all, we want the carbon dioxide to be emitted. … More later. For more information, see the wiki
If anyone can think of an explanation, please let us know.