I ordered a Magnehelic gauge from Dwyer Instruments that measures plus or minus 2″ of water column. This allows you to take pressure or vacuum readings within a limited range around atmospheric pressure. Observe the nipple in the picture below. To test the burn chamber I took my plasma
torch and cut a hole in the upper section of the burn chamber where I didn’t think a ash or flame would be an issue and welded a nipple airtight into the chamber. As you can see I added a coupler, 3/4″-3/8″ reducer and a 1/4″ barbed nipple. With the upper exhaust fan running alone, the burn chamber tested at barely .1″ H2O vacuum. With the draft fan running the burn chamber vacuum fell to .05″ H2O vacuum. No wonder there is occasionally a puff of smoke back through the system.
After that initial test in an uncleaned condition the system was taken apart and the second pipe was blocked. After reassembly the testing was done again and the burn chamber vacuum rose. This indicates that the venturi effect we thought we were inducing was not working. By changing fans and cleaning the soot out, we were able to induce a vacuum of .85 in the burn chamber. At this vacuum the smoke observed was non existent, but the exhaust pipe was significantly hotter than in the previous configuration indicating more waste heat. So tuning the vacuum and monitoring the vacuum makes sense and may be an integral way to test the cleanliness of the pipes as well as the required O2 of the burn process. To that end I am going to order a pressure gauge on a chip and see if I can get a reading to be monitored by the Arduino control system. After burning in the .85″ vacuum configuration it became apparent that smoldering in the auger pipe was taking place and the system was shut down safely to allow more work to be done. At this point it appears that .05″ H2o is insufficient and .7-.85″ is too much so a damper or variable fan drive is required to allow burn chamber vacuum adjustment.