My wife and I just bought a lovely offset smoker, just the ticket for cooking up brisket, ribs, chicken, and so forth. One of the critical components of the smoking process involves maintaining a consistent temperature in the smoke box. Typical smoking projects take 6-24+ hours to complete, so maintaining temperature can be a challenge.
By coincidence, I work for an engineering company that specializes in platinum RTDs... temperature sensors. RTDs work because some metals (especially platinum) have predictable, linear resistances depending on the temperature of the alloy. In the case of this formulation of platinum, it has a 100-ohm resistance at 0 degrees C.
I got a great sample RTD from work. The platinum element is potted inside a magnetic coupling, with the sensor inside a spring-loaded button to press it against the surface I'm measuring. The plan is to create a rig using a microcontroller with wifi or wireless capability in conjunction with an Adafruit RTD amp.
Adafruit's RTD amp looks pretty cool. It accommodates 2, 3, and 4-wire RTDs. The one I am using is 3-wire.
I got Adafruit's RTD amp and got it wired up. I tested it just for a couple of minutes and it wasn't reading. I think I have the wiring reversed--there should be 2 grounds and 1 lead, but I think I got it reversed. Adafruit's instructions say to put the two matched wires on the + terminals but for this RTD I'm suspecting I need to connect them to GND. In any case, I won't have more time to play with this for a few days.
I was a little surprised that they had one. I had never even heard of RTDs before I started working at my current job. Adafruit even has RTDs to sell, though mine is better for my project because it has a magnetic coupling.
Adafruit's amp has ports for 2, 3, or 4-wire RTDs. So RTDs detect temperature based on the varying resistance of a specific alloy of platinum. This is great, but we can't forget that the lead wires have resistance too. Imagine if the room you wanted to monitor was 100 yards away--all that wire would dramatically change your readings.
There are two solutions. The first solution involves those third/fourth wires. 3-wire RTDs are quite common and use the resistance differential between the sensor+wire and just the wire to offset this bogus resistance. The 4-wire RTDs are even more ninja-like, creating a bridge circuit that provides a very accurate reading. The sensor I'm using is 3-wire, and I anticipate that will work perfectly for my needs.
The second solution simply involves using very short leadwires (= low resistance) in conjunction with a transmitter. The transmitter changes the variable resistance into a variable current--usually 4-20mA-- allowing clear transmission of data across any length of wire.
For the purposes of this project, the 3-wire configuration should provide all of the accuracy I'd need -- BBQ ain't rocket science!
I had the thought that, could a CO2 sensor--or more accurately a smog sensor--be somehow utilized to check the status of the smoke box? The PCB-mounted sensor, like the Adafruit component seen in the picture, might be too frail to get anywhere near the smoker, lest the solder melt.
Also, this is definitely a Version 2.0 feature... but it's something to think about. If the cookin' smoke is too thin, something might be wrong, and the system could send the chef a text.
I found the Pelican case I intend to use for this project. I recall buying it on Amazon because it was dirt cheap, but I had no immediate use for it and it ended up on a basement shelf. I'm mostly using the Pelican because I already own one, but it has a lot going for it. Crushproof, waterproof, and just the right size to hold both the electronics and the RTD itself, which is about the size of a C-cell battery. I also like how it has a robust carrying handle. Despite the fact that it's likely waterproof, the Smoker Controller doesn't live outside! It will need to be carried to and from the house.
My first thought for mounting the hardware inside is to not mount it at all. I'm thinking a 1/8" plywood box that fits snugly inside, secured only with foam tape. Everything gets mounted to the box so I don't have to pierce the Pelican's exterior to screw things into place. A primary concern would be ensuring the wooden box doesn't interfere with the waterproof rubber gasket or prevent the lid from closing.
The RTD also has a longish twisted-trio leadwire (maybe a meter?) that would have to be coiled up and stored inside the Pelican case as well. I had an idea for a laser-cut spool that accommodates the RTD's wire, mounted on a 1/8" plywood board that would be foam-taped to the inside of the lid.
My current idea for the project enclosure involves a Pelican 1120 case, which is about small enough for a kindergartner's lunchbox. I think it would be the perfect size for an Arduino, 9V battery, a few breakout boards, and the RTD itself.
The Pelican is also waterproof, so I could leave it outside during rain (for instance) without fear that it would get damaged. I'm thinking all buttons and switches would be inside the enclosure and I would open it up to make any changes to the settings.
Imagine there is a beautiful brisket inside the smoker. How long would it take to cook, and at what temperatures? For the purposes of this project the specifics of the recipe are irrelevant. What is important is that the system knows the temperature of the surface it's in contact with, and is able to communicate that to the chef via wifi.
1. Chef sets the Smoker Controller's temperature. The Smoker Controller will have a simple interface for setting the temperature. Maybe a button and potentiometer?
2. Once the temperature is set, the RTD is attached to the smoker and begins to monitor the temperature.
3. If the temperature drops below the set point, a text is sent alerting the chef to add more fuel.
I had some ideas for ways to improve the way the Smoker Controller works and to add to its capabilities. For instance, maybe the smoker could literally make adjustments to the smoker on its own?
* Vent control. A motor could control vent position, opening them up to allow in more air or closing them down to cool down the fire and slow the rate of the burn.
* Blow in air to increase temperature. Forget vents; If the Smoker Controller noted the temperature trending down, a fan could blow air into the firebox to increase the burn temperature.