The very first thing that needs to be understood, is how a cooker "cooks", i.e., does it cook (provide heat) from the top down, from the bottom up, reverse flow, etc. For all of the Yoder Smokers cookers, offset wood and pellet, the answer to this is; from the bottom up. This means that the heat source enters into the lower part of the cooker and "flows" or "radiates" upward and out of the chimney.
Since the heat comes from the bottom of the cooker, the lower grate will be the hottest spot where you will place food to cook, so the physical lower grate temperature is what we use to test our cookers. If we didn't, food would be cooking at a higher temperature on the grate than may be desired.
The rest of this article will relate mostly to the Yoder Smokers pellet cookers, but the principles outlined will also cover the offset wood cookers.
When we test pellet cookers at the factory, we use very expensive, and highly accurate, scientific temperature logging test equipment, in conjunction with our proprietary controller software monitoring tools. Using these two data sources allows us to monitor and tune the performance of the cookers via the firmware installed in the controller.
For testing we use 2 probes on the lower grates of the cooker. These probes are directly attached to the grates at specific locations, to measure the temperature of the metal of the grate surface. The locations are different for each model of pellet cooker depending on the size of the lower grates. We take the measurement of the lower grates from the hopper wall to the chimney wall, and divide this measurement by 3. We then use the resultant answer to place the two probes at equal distances from the hopper wall of the cooker. For a YS640, the grate measurement is 32 inches. 32 divided by 3 = 10.66, which we round up to 11. So we then measure 11 inches from the hopper wall and directly attach the first temperature probe to the grate, and then measure 11 inches from the first probe, which is 22 inches from the hopper wall, and directly attach the second probe to the grate, The following picture illustrates this in a YS640, with the addition of 2 probes placed on the upper grates directly above the probes on the lower grates, and also 2 additional probes which will be discussed next:
When we program the firmware for our pellet controller, we use the temperatures derived from the physical metal surface of the lower grates as described above. This means that when the cooker is running empty (no food in the cooker), the temperature displayed on the controller approximates the temperature at the grate surface, This can be confusing as most 3rd party temperature reading devices provide a holder which puts the probe some distance above the grate surface.
If you notice in the picture above, I have a red circle around the lower left probe at the 11 inch position. I used 3 probes at this position to show the difference between an expensive scientific probe, and the two probes from the normal $50 temperature reading kit: one lying directly on the grate to approximate the reading of the scientific probe and the other in the included probe holder putting the probe approximately 3/4 inch above the grate surface (all of the probes were tested in boiling water to verify their readings prior to taking measurements). The following pictures show the 3 locations of the probes from 2 different viewing angles,
So after allowing the cooker to normalize for about 45 minutes (cooker was empty and the lid was never opened), at a set temperature of 250 degrees on the controller, I took a picture of the readings of all three probes. Probe #1 is the expensive probe attached to the grate (the probes and logging unit cost well over $1000), probe #2 is one of the probes from the $50 kit lying directly on the grate, and probe #3 is the other from the $50 kit held above the grate. As you can see, there is a difference between #1 and #2, with #3 being completely different, The difference between #1 and #2 is mostly the accuracy of the probes (reflected in their cost) and potentially the fact that #1 is physically attached to the grate rather than passively lying on the grate like #2.
Now let's discuss probe #3, as it is a much lower temperature reading than the other two probes. Remember that the Yoder Smokers cookers all cook from the bottom up, so as the distance increases from the heat source (bottom of the cooker), the temperature decreases. This decrease in temperature compared to distance is not linear because of airflow within the cooker, and may be compounded depending on the shape, mass and temperature of food placed in the cooker. It then follows that any analog temperature gauges mounted in the door at a distance above the grate will read significantly lower temperatures than the controller displays.
You as the cook, are in total control of how you want to cook:
If you want to allow the cooker to do its thing and cook by the set temperature, which relates to physical grate temperature, set to your desired cooking temperature on the controller and walk away, This is cooking by grate temperature (trust the cooker).
If you decide that you want to cook at a specific temperature, at a specific distance above the heat source (grate), then place a probe at the desired distance above the grate (using a probe holder, potato, upper grate, etc.) and change the set temperature on the controller to achieve the desired temperature at that specific height above the lower grate. In the case I have shown above, to cook at 250 degrees at the height level of probe #3 (approximately 3/4" above the grate), I would need to start by raising the set temperature on the controller to 260 degrees. This is cooking by suspended air temperature.