Monitoring Temperature

Monitoring the gasifier temperature will tell you a lot about how it's operating. There are several areas to watch, some are informative than others. Here's a list of the most important temperature points, and what to watch for.

Hopper

This will tell you when you're low on fuel, or if the fuel is bridging. A probe inserted 2" into the hopper wall as high as you can get it will be the most effective. When the hopper is full of wood, you will read temps from 100-200 degrees. If the temperature rises above 250, you know it is running out of wood, and you need to refill. An alarm set to this temperature will be useful. There will be a sharp increase in temperature as you run out. This temperature may vary somewhat depending on your probe placement. Keep in mind that tar boils at 230 degrees. If your hopper gets far above this temperature the hopper will lose any protective coating of tar until you refuel and drive a while. Bridging will also cause the hopper temperature to rise, to the point you may think you're running out of wood - if you know there's still wood left, you are experiencing bridging. Sometimes it's enough to keep driving, look for a pothole or rough patch and try to knock the wood down. Other times, especially if you're not sure of the wood level, you'd be better off to stop and poke down the wood by hand. You'll know the bridge has collapsed when temperatures return to normal.

Gas, near grate

The gas will be at its highest temperature here. Some folks are monitoring this in the crossover pipe between the gasifier and heat exchanger. This indicates the amount of reduction that is happening vs the oxygen supply. The more heat in this area, the further into the char bed your oxygen lobe is penetrating. If oxygen gets below the grate, you'll see a spike in temperature, almost instantly. This indicates an overpull. Alternatively, it will spike if there is an air leak developing in that area. Oxygen mixing with hot gas makes a blowtorch effect, and can damage the gasifier. Watch for this spiking, and react quickly by shutting things down. If you mount your probe in the crossover pipe, about halfway up the side, you will also get an indication of char laying in the pipe. When the probe is covered in char, it will stay cooler and react very slowly to changing temperatures. This char is blocking the full flow of gas to the engine, and should be cleaned out at the next opportunity.

Gas, post exchanger

This is where Wayne has traditionally monitored the gas temperatures, the last point before cooling the gases to ambient temps. Comparing this temperature to the crossover pipe will give you an idea how effective your heat exchanger is. Temperatures will normally range from 100-700 degrees depending on the gasifier output. If you only monitor one gas temperature, this is a good overall picture of your gasifier. Be sure the probe is inserted far enough into the gas stream to get an accurate reading. The best position is directly in the heat exchanger exit; for analog direct-mount gauges, it may be necessary to move the probe down to be visible in the window. Expect slightly lower temperature readings as a result.

Thermocouples vs Analog

For most temperature locations, you will need a thermocouple, as there is no direct line of sight to the monitoring location. However for the hopper and post-exchanger you have the option of using an analog gauge. This is what Wayne has traditionally used, and it works quite well. You will need to position analog gauges to be visible in the truck's rear window, ideally through the rear view mirror. There are pros and cons to this approach; these gauges will be hard to see at night, and trying to read them reversed in the mirror is another skill to learn. However you won't have to install anything in your dashboard, and there aren't any electronics to fail. Probes: Thermocouples themselves are very simple devices, a bimetal junction in a protective metal sheath. High temperatures generate a small voltage that can be measured by any thermocouple display, or even a voltmeter. For gasification we want a type K thermocouple, the most common type. For high temperature areas and long life probes, go for an inconel sheath and weatherhead connection. Omega is a quality brand; for less heat critical areas, or more easily replaced, you can use less expensive stainless probes from Ebay. Note that extension wire must be the proper thermocouple type and polarity; choose one that's waterproof (PVC or teflon), or plan to run it inside a protective sheath. If the wire gets wet it will ground out the temperature reading. Only get UNgrounded thermocouples; grounded ones will not function correctly with a body ground wiring system.

Displays

Thermocouple displays come in several flavors, from basic chinese readouts up to hundreds of dollars for scientific precision. We don't need particularly accurate temperatures (within 10-20 degrees is fine). Many folks are using this gauge, about $12 on Ebay. For a little more functionality, there's this gauge at $40. Any type K gauge will work, look for a 12v gauge with a Fahrenheit or Celsius readout (whichever you prefer). Alarm functions for the hopper gauge would be a bonus. Gauges do take up valuable dashboard space, which is a concern for some vehicles. There are aircraft displays available which will display multiple readouts in one gauge, but they are prohibitively expensive.