I love smoked fish! This blog covers the build of my Arduino-controlled fish smoker. This smoker is built out of an oak wine barrel and uses a DIY pellet-to-burnpot feeder system using a motor-driven auger, commonly referred to as a pellet smoker. The Arduino microcontroller runs a multi-hour program that monitors the temperature of the barrel and internal fish temperature to smoke fish to perfection!
For example, the Smoked Salmon program runs over a period of about 11 hours. During this time the program goes through the following stages:
- 3 hours - Air dry the fish*
- 1 hour - Smoke at 100°F
- 1 hour - Smoke at 120°F
- 1 hour - Smoke at 140°F
- 1 hour - Smoke at 160°F
- 1 hour - Smoke at 180°F or an internal fish temperature of 145°F
* Air drying the fish is done by running only the fan at high speed to form a pellicle, or shiny skin, on the fish as a protective layer to retain moisture. This layer also allows smoke to better adhere to the surface of the fish for the best flavor.
The diagram below provides an overview of how this smoker works. Pellets are fed from the hopper (funnel) on the right into the auger screw which is driven by an electric motor. The auger delivers the pellets to the burn pot. This is also how a commercially available Traeger pellet smoker operates. A ceramic pizza stone sits above the burn pot to dissipate heat from the fire. Smoke rises around the ceramic pizza stone, over the fish placed on two grill grates at the top of the barrel, and then out through a tall flue, or chimney, at the top of the barrel. A small fan is used to supply air to the burn pot.
This is sometimes called the fire triangle.
Wood pellets are the fuel in this case. An auger connected to a pellet hopper is driven by a motor to slowly add more pellets to the burn pot when the motor is engaged. The ignition source is a high wattage ignitor rod that sits at the bottom of the burn pot. When temperature is detected as being very low electricity is supplied to the ignitor, making it extremely hot, and that starts the pellets burning. The variable speed fan located on the back of the smoker feeds air into the fire in the burn pot through holes in the pot.
Increasing air speed is the primary method of raising temperature, and if air speed alone fails to raise the temperature then more fuel (pellets) are added. Conversely reducing air speed is the primary method of lowering temperature. One major topic has not yet been discussed, but related to reducing air speed, which is the smoke! What is smoke anyways? It is essentially a collection of tiny unburnt particles** from the combustion process mixing with air. Fires with low availability of oxygen burn tend to have more unburnt particles and therefore more smoke, so constantly lowering temperature by reducing air speed keeps the smoke flowing.
** Getting much more technical, unburnt particles of wood smoke are an oil known as syringol and a substance known as guaiacol. Both syringol and guaiacol are produced when a plant polymer present in wood, called lignin, is burned. These substances are predominantly responsible for that good smoky taste.
I also built a simple user interface for starting a program and performing other tasks like pre-feeding pellets.
The graph below shows a smoking session in progress with various temperatures graphed, as read by probes in the oak barrel or inserted into fish, as well as the air speed of the fan, whether pellets are being fed, and if the ignitor is on. How to read the graph is described in more detail on the Display and UI page.
Another interesting element of a smoker system is the flue, or chimney, which operates on the principle that hot air rises because it is less dense than cold air. Two factors affect the amount of draft produced by a chimney:
1. Heat: The hotter the gases in the chimney compared to the air outside, the stronger the draft
2. Height: The taller the chimney, the more draft it will produce at a given temperature difference
The column of warm air inside the chimney, being more buoyant, rises in the chimney, reducing the pressure at the bottom of the flue so that ambient air flows into the combustion zone and moves the gases up and out of the chimney. A taller chimney provides a greater accumulated pressure differential due to the taller column of warm air inside the chimney, each cubic foot of which is lighter than a cubic foot of cooler ambient air. As you can see in the photo below I have a rather tall chimney to provide a nice amount of draft.
Ahh.. the final product, and it is very tasty!
A few Q&As...
- Why use a wooden barrel instead of a metal barrel or drum?
I like the idea of the purity of the wood. No need to worry about some cheap metal barrel having a mystery chemical in the metal or in its finish that would give off toxic fumes.
- Isn't the idea of fire in a wooden barrel a bad one? How did you know you wouldn't find your smoker at the end of the day in a pile of ashes?
I only smoke fish in this barrel with a top temperature of about 180°F. The burn pot is suspended in a stainless steel housing and never touches wood, and additionally that housing sits on concrete that I poured in the bottom of the barrel. I have had no worries of the barrel catching fire. I can't say I would recommend this configuration if you want to smoke at a higher temperatures, such as with smoking meat which reaches temperatures upward of 250°F.
- Why didn't you just buy a Traeger, Yoder, Outlaw, or other commercial pellet smoker?
Good question! I asked myself this particularly when I was creating the burn box out of stainless steel sheets and rivets. What a pain that was! The main reason built instead of bought was I wanted to get very low smoking temperatures and control airflow more precisely. The fan on commercial smokers is often tied to the pellet auger, whereas I control them separately. Plus, I like a good project and this cost way less than buying one. If you go this route, I recommend you look for after market parts to save time on fabricating your own, but expect to spend much more time than a trip to Home Depot to buy one!
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