Investigating the abnormal combustion temperature, I wanted to eliminate the issue of a faulty regulator, which was the original from 2004. I also wanted to double check the gas valve mechanism to make sure that it was actually allowing the flow of gas. I purchased new regulator and tank fitting parts. It was important to get a regulator that could be adjusted somewhat to match the original. Almost all low pressure regulators are set to 11 inWC (inches of water column), and it looks like they cannot be adjusted. The high pressure regulators go from 0-40 psi, which would not be expected to do well in the 1/3 psi range used by low pressure appliances.
But first, I needed a way to measure the existing regulators, so that I might match the output propane pressure, necessary for correct operation as originally designed. I didn’t want to build a water column manometer, plus I wanted something wireless, so I could analyze the pressure as the temperature fluctuated. See the previous post.
It’s hard to see, but there is a right-angle Schrader extension (yellow) connecting to the quick clear Schrader value. The extension releases the trap valve to allow gas to flow. The other end, with the Schrader core removed, is screwed into an empty plastic eye drop bottle, machined to just fit over the valve thread for a tight fit when screwed together. A slot sawed through half of it’s side on the bottom allowed opening a flap enough to insert a BMP180 barometric breakout board, connectors, and wires. The flap and the connecting wires are sealed to the container with two rounds of casting plastic applied freehand. As usual, it is a bit sloppy and not the best job, but it seems quite solid. The wires plug into a backup NodeMCU running the trap software with the BMP180 temperature/pressure sensor driver substituted for the HTU21D temperature/humidity driver. The orange cable connects the NodeMCU to a high-capacity charger brick for power. It should run for quite a while, and is easier and safer than running more AC power to the trap. I am fairly confident that the bottle does not leak appreciably. Of course, I should have attached it to a bicycle tire to measure the pressure long term, but I was in a hurry for results.
And results I got.
Temp=67C (-0.4) at 3:0 F=1 I=1 G=0 S=1 E=0 T=35.8C H=24.2% M=11232 (651/1013:33851) R=70~140 B=1.0 V=3.1
Temp=-43.7C (0) at 0:109 F=0 I=0 G=0 S=0 E=0 T=20.6C H=30.48 inHg, 413.87 inWC% M=20872 (9/9:18804706) R=40~140 B=0.0 V=3.2
Temp=67.1C (0.1) at 3:1 F=1 I=1 G=1 S=1 E=0 T=35.8C H=24.3% M=11232 (648/1010:33695) R=70~140 B=1.0 V=3.1
Temp=-43.7C (0) at 0:110 F=0 I=0 G=0 S=0 E=0 T=20.6C H=31.19 inHg, 423.53 inWC% M=20872 (17/16:18804706) R=40~140 B=0.0 V=3.2
Two data streams are interleaved as the trap just turned on the Gas. With the propane off (G=0), the pressure is 413.87 inWC. With it on (G=1), the pressure rises to 423.53. The pressure from the regulator is the difference, about 9.5 inWC, which is close to what is expected from a gas grill regulator. This particular regulator is from my old Liberty. I will later replace it with the decrepit original Defender regulator and measure it.
In this run, the temperature went up to 115°C, then dropped again down to the 70°C range. During the run-up, the pressure was 423.30 inWC, and at the peak (when the combustion changed to abnormal) it was 423.41 inWC, not much difference.
So I didn’t see any fluctuation in the pressure that would trigger a temperature drop, but I did notice something else. When operating normally, the trap noise almost all comes from the fan. But there are times when there is a very pronounced, rough burning sound from inside, a like a blow torch or jet engine. A couple of times, it matched the combustion mode change. Normal, quiet. Abnormal, noticeable rough sound. I conclude that this rough sound is an uneven, chaotic burning and is a related to the abnormal temperature issue.
So what could cause this to occur occasionally (although right now, almost every time)? Perhaps the pressure is just wrong, stable as it seems when measured every second. Perhaps a higher frequency pressure disturbance not caught by the gauge reporting at 1 sample/second is the problem (the BMP180 runs at about 128 samples per second). Perhaps the atmospheric pressure is changing and the regulator is not keeping up quickly enough. Perhaps the nozzle is somehow clogging and shooting off a long, uneven stream. Perhaps the air intake via the fan is too strong and the mixture too lean, or the gas flow and flame is too large for the chamber. The system seems so fussy. Also unexplained is yesterday’s gas interruption for 30 seconds setting off a very quick temperature rise, abnormal the other direction, although a small, brief temperature rise and peak is observed before the fall in nearly all cases.
The igniter on for 120 seconds command resulted in a temporary temperature rose from 78.6°C to 80.7 °C that fell back after the igniter went off. This, plus previous attempts, indicates we cannot recover using the igniter by itself.
Turning the gas off for 25 seconds, then 10 seconds later (to allow 15 second warm up), turning the igniter on for 50 seconds, restarted normal combustion.
This sequence could be added to the controller program, but I am concerned by the wear on the igniter if this keeps cycling several times per hour or day. I would rather look for other methods to control this issue. Perhaps the new nozzles are somehow misbehaving. Perhaps the fan might be used to bring the combustion under control. With the additional memory available with the new NodeMCU LFS firmware, it might be time to implement full range pulse width modulation (PWM) for the fan control, rather than the simple 10 ms duty cycle startup algorithm in the main loop (matching the original controller).
Substituting a full propane tank for the 85% empty (3# remaining) tank resulted in a pressure of about 11 inWC, exactly what the fixed regulators typically provide. On restart, the trap climbed to 120°C and has stayed there for a while, unlike with the other tank. It is hard to believe such a small pressure difference between 9.5 and 11 inWC would make such a difference, but I suppose anything is possible. According to Arthur Conan Doyle, “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”
Oh, by the way, today with the trap running mostly at 70°C and an old attractant, there were still 3-5 live mosquitos at noon in the catch basket. So maybe none of this abnormal combustion stuff really matters.
This is a work in progress. Stay tuned