After some experimentation with two original regulators, one from a Defender and the other from an older Liberty, it seemed like it made no difference in the abnormal combustion phenomena from which my Mosquito Magnet Defender has been suffering. A new high quality regulator that might support some range of pressure adjustment and an adapter for the existing hose was put in service.
The pressure NodeMCU described earlier measures the absolute ambient pressure with the gas valve shut. Opening the valve yields a higher reading, and the difference is the gas relative pressure.
The new regulator output was nearly exactly 11.0 inWC, a bit higher than the old Defender’s 10.5 and the older Liberty’s 9.5. It looked like there was no need for adjustment. Remarkably, the trap reached its operating temperature and stayed there without rapid increase or decline. Over night, the combustion temperature declined, but then rose the next day. The system seems stable.
I then realized something after re-reading the forum post from another contributor, who posted graphs showing combustion chamber exterior temperature vs ambient as offset by a more or less fixed amount. Perhaps there is no such thing as a fixed “operating temperature.”
The thermistor doesn’t actually measure combustion temperature, but rather exhaust temperature after the catalytic converter. At this point, all the unburned propane and CO and should have been combusted then converted to CO2, and the air stream should be fairly laminar. This implies that the temperature being probed by the rod is measuring how much heat energy is being added to the input air stream by the propane, and not the flame temperature. Using this principle, we would expect a more or less fixed temperature rise over the ambient air temperature, because a BTU’s worth of energy from propane adds that energy to a certain amount of air, and results in a certain amount of temperature rise. So, in a properly working trap, the measured exhaust temperature should equal the ambient temperature plus a more or less fixed amount depending on the air and propane flow, assuming complete combustion. If the temperature drops 20°C, then the exhaust temperature should go down the same amount, and there should be no worry when the exhaust temperature drops to 90°C or below.
The exhaust temperature rise seems to be about 100°C measured at the thermistor, which is connected to the exhaust through a metal rod with some unknown thermal impedance, but I doubt it is dropping 50°C along the rod. I would feel better about this explanation if my calculations of temperature rise were within an order of magnitude of measured, but no, they are not. Starting with propane flow BTUs per second, and In^3/Sec exhaust flow, it should be possible to calculate the temperature rise, but this analysis is not working today.
Nonetheless, this is an exciting development. Besides that, the most important thing is that the trap is stable for now. Perhaps the time has come to move the temperature/humidity sensor to the air & mosquito intake stream. Then the temperature differential can be accurately measured and this theory supported with experimental evidence.
Regrettably, the original Defender regulators are no longer available. The modern inexpensive regulators come with a fixed cable attached, and the fitting does not match the Defender valve, but requires an adapter that may not fit completely in the case. Of course, the heavy duty regulator with fittings works with the existing cable, but it is overall twice as expensive, a bit larger and awkward. It may not perform well long term because it is designed for a much higher flow rate. We shall see.
P.S. The Patriot uses an inexpensive ($10) IGT A300USL L.P. Gas 2.8 KPa (11.4 inWC) regulator. This, as all other modern replacement regulators, requires a fitting to match the 1/8″ NPT female on the valve inside the Defender.
The temperature/humidity sensor has been moved to the vacuum intake side of the trap to measure the incoming air. At 25°C 50% RH intake, the temperature differential is about 95°C. I expect the humidity to play some role here. I will measure and publish more data when it becomes available as the trap runs during various weather conditions.