A critical stage in the curing of vanilla is the “sweat,” where the enzymatic process that develops the vanillin takes place. The sweat box creates an environment that holds the beans at the optimal temperature for this process.
In the How to Cure Vanilla Beans article, I describe how to put together an ad hoc sweat box using a cooler and hot water bottles. In this article, I will describe how I built the electrically heated sweat box used to handle larger quantities of vanilla pods.
Not Just for Vanilla
For the fermentation do-it-yourselfer, a box like this has many uses. Basically, what this is is an incubator. You can set it up to hold any temperature above ambient, so any process that requires a constant warm temperature is something this box will be useful for.
In our house, this box gets used a lot for fermenting cacao. We have a few cacao trees, so we’re often fermenting small batches of beans for our own use. You could also use this box for making tempeh or yogurt.
A Brief Disclaimer
I am not a licensed electrician, the build instructions I provide here are offered on an at-your-own-risk basis if you decide to follow them. I’ve been using a box of this design, with the same electronics I describe for 4 years with no problems. My experience is that this is a safe design. Be aware of the risks of electrical shock and fire that are always present with a project like this.
This is a very detailed article, there’s a lot of instructions to follow. I strongly urge you to read it over first so you understand what you’re possibly getting yourself in to. There are a number of things in the project that can be or need to be improvised, so going over the article will help you get some ideas for what might work using stuff you may have just lying around.
Stuff You’ll Need to Get Started
The sweat box has 3 main components: the box itself, a rack for holding the beans, and the thermostatically-controlled heater.
For the box, I used a 50-quart Igloo MaxCold cooler. This cooler is the perfect size, and it’s well insulated. That will run you about $60 new, but I just used an old one I had sitting around. A trip to the thrift store is probably a good way to get a cheap one.
The internal dimensions are about 20″ x 10″ x 11″ deep, in case you’re looking for a similar-sized substitute. It’s no big deal to make a smaller one, we had a 25-quart one for a while at first.
The purpose of what I am calling the rack is to hold the beans away from the heating elements and allow air to circulate around the beans. The heating elements are hot enough (about 200℉) to burn if touched or melt plastic bags, so you will want to make sure there is some kind of barrier between the heater and the stuff you’re keeping warm.
Air circulation helps the whole setup be more efficient and prevents cold pockets.
I don’t go into a lot of detail on how I built the rack, it’s made out of stock aluminum of the kind easily found in a hardware store. It’s fastened together with pop rivets. I don’t really expect you to go to this much trouble…you can, with some imagination, come up with a ready-made alternative. A plastic rectangular storage basket that you can find in a hardware store, for example.
Anything that fits into the box and keeps the beans away from the heating element while allowing air circulation will work fine. The size you’re looking for is about 15″ x 9 1/2″ x 10″ deep or smaller.
This is undoubtedly the most complex part of the whole thing, and maybe a little daunting for those who haven’t worked with electronics before, but I’ll try to make it clear how it all goes together.
These are the parts you’ll need to build the heater (these link to a product page so you can see exactly what it is and possibly where to get it):
- Digital PID Temperature Controller
- 2 — 250 Watt Ceramic Heating Elements
- K‑Type Thermocouple Temperature Sensor
- 25 Amp Solid-State Relay
- 2- or 3‑prong 10′ Outdoor Extension Cord
- 6′ Extension Cord
- 6 Orange Wire Nuts
- Plastic Food Container
- Some popsicle sticks or tongue depressors.
- 8″ Zip Ties
- Aluminum Repair Tape
The extension cords can be salvaged from broken/unused electrical items: this is what I have done. The popsicle sticks/tongue depressors are to provide a thin insulator for mounting things that might get warm.
For tools, you’ll need a sharp utility knife, wire cutter and wire stripper, and a power drill with 3/8″ and 1/8″ bits.
The plastic food container is going to be our electronic project box. The container needs to have internal dimensions of at least 5″ x 5″ x 4″ deep. The kind I like have snap-on sides to the lid, but any one with a secure top will do. Don’t try to use a “disposable” one, you need the heavier weight ones.
What we’re going to do first is drill a hole on one side of the container, and 2 on the opposite side. These will be for the power cord, the heater cable, and the temperature probe. On the 3rd side, we need to drill 2 1/8″ holes, 2″ apart for mounting the relay.
On the lid of the container, we’re going to cut a 1 3/4″ x 1 3/4″ square hole for the controller. Use the bottom of the controller module to draw the square on the lid with a sharpie and then cut it out with the utility knife. Use light pressure with multiple passes to safely and accurately cut through the plastic. Be sure to cut on the inside of the line you drew, you don’t want the hole too big! A metal straight edge is not a bad idea here.
Unbox the controller unit and insert it into the square hole so the face is on the outside. There are two plastic spring clips to hold it in place.
Wiring it Up
OK, you have your container drilled and the controller unit is mounted into the lid. Next, we wire it up. This first diagram gives you a big-picture view of how the whole thing is wired up, so you know where all this is going.
Wiring the Control Box
Start by cutting an 12″ piece of wire off the 6′ extension cord. Cut the receptacle end off, you won’t be using it.
In the wiring diagram (brown wire), this is the wire that connects the controller to the control inputs on the relay. Split and strip the ends and consulting the circuit diagram to see which terminals to use, connect the controller to the relay.
Now thread the thermocouple temperature probe wire through it’s hole in the side of the container. Connect the probe to the controller according to the circuit diagram below.
Wiring in the Heater Power
To wire in the power, take the 10′ outdoor extension cord and cut it so that the plug (male) end has 7′ of cord (for the power cord) and the receptacle (female) end has the remaining 3′ of cord…this is your heater cord.
Using the utility knife, carefully strip off about 10″ of the outer sheath of the end of each cord to expose the wires inside. Avoid cutting into the wires when stripping off the outer sheath. How I do this is I set the utility blade to just barely protrude from the handle, then make a cut along the length of the sheath, cutting just deep enough to go through it. Peel it back and cut it off.
Take the heater cord and cut about 6″ off of both wires, leaving 4″ of wire on the cord. This will give you the 2 wires you need to connect the power to the controller unit.
Thread the power cord and the heater cord through their respective holes in the container. Use the wire stripper to strip 1/2″ of insulation off all the wires you just exposed, then use the wiring diagram and circuit diagram to connect everything together. When inserting wires into the terminals, take care to get all the wire strands into the terminal. Use the wire nuts to connect the power to the controller unit as shown in the wiring diagram. Pull on all the wires you just connected to make sure they are not going to come out. Note: the power connections to the relay switch are not polarized, so it doesn’t matter which terminals you use.
Once you’ve got it wired, double-check all the connections against the circuit diagram.
Using zip ties, anchor the 3 cords that go through holes in the container: double-wrap and cinch a zip tie around the cord on the inside of the container right where it goes through the hole. This is to keep something pulling on the cord on the outside from pulling on anything on the inside. The zip tie should prevent the cord from pulling out.
The Circuit Diagram
Mounting the Relay
The relay is mounted to the side of the container. The popsicle sticks (or tongue depressors) are used to provide an insulating pad between the relay’s metal bottom and the side of the container.
The relay will build up some warmth, but not a lot, certainly not enough to ignite wood. The wood keeps the plastic from possibly melting.
Cut the wood down to the width of the relay. Place the wood on the side of the container, between the holes, then put the relay on top of that, lining up the holes. If the wood covers the holes, drill through the wood so the zip tie can go through the wood too.
Use the zip tie to go though the holes in the relay and the side of the container to anchor the relay in place with the wood sandwiched between the relay and the side of the container. You can see how it’s mounted in the photo below.
Finishing the Control Box
Now put the box together, arranging the wires so that everything has some room in there. The wires should be loose, but if you can’t tame them, you can use some tape to hold them so they’re out of the way. Do it so that there is still enough slack in the wires so you can still open and close the box. You’ll be glad you did that later. I like to put the little instruction booklet for the controller unit in there to so that I can find it later when I’ve forgotten how to program it.
Setting Up the Sweat Box
Now we’re going to set up the wiring in the sweat box itself.
You’ll need 4 wire nuts and the rest of the 6′ extension cord for this. The cord will have more than enough length to connect the heating elements and then make it out to the control box.If it seems like too much, you can cut it down.
First, you’ll need to remove the drain plug from the cooler. On the inside, there will be a big plastic nut. Loosen and remove that nut: you’ll probably need a medium sized pair of slip-jaw pliers to loosen it at first. Once the nut is off, hit the remaining part of the drain plug that is still protruding into the inside of the cooler with something like a hammer to pop it out. It’s just held in place with a rubber gasket, there’s no glue or anything like that.
Now run the cut end of the extension cord through the drain hole into the box so the plug end is on the outside. Also thread the sensor end of the temperature probe into the cooler.
Use the aluminum repair tape to tape the temperature probe to the inside wall of the cooler above and to one side of the drain hole. It should be about midway up the side. The idea is that the heater element will be on the other side so the probe is not directly above the heater element.
Wiring in the Heating Elements
Connect the heating elements according to the diagram below. You will need to make a tricky little cut into the extension cord about 2′ from the end: cut only one of the two wires, then tear them apart, giving you two 3″ wires on one side, with the other side uncut. This gives you a way to splice a heater element into the cord.
About the Heater Element Wiring
This is a side note about supplying power to the heating elements. In this circuit, the two heating elements are wired in series. If you look at similar circuits, you’ll often find the heating elements are wired in parallel. The key difference between the two is the voltage that the elements will get.
If the elements are wired in parallel, each will get the full line voltage. If you wire in series (you’d only do this with two identical elements!) the voltage will drop by half, so in this circuit, each heater will get half the voltage, or about 55 volts.
The reason we’re doing this is these heating elements will get up to 300℉ or more at full voltage. We don’t need that high a temperature, so for safety, we’re running the elements at a lower voltage. At that voltage, the elements will only get to about 200℉, which is pretty safe as far as a contact burn or fire hazard is concerned.
The downside is that if in the unlikely event one of the elements blows its fuse (these elements have a safety fuse), the other one will stop getting power. It’s a good idea to check on the box daily (you’ll be doing this anyway to cycle the beans through the dehydrator) and make sure it stays warm.
Attaching the Sweat Box Components
Once you have all the electrical parts wired up, they need to be mounted so they don’t move around inside the box. In my setup, the heating elements are attached to the rack I built, but you probably won’t be doing that.
A reasonable and easy alternative is to use 2″ metal “L” brackets of the kind found in a hardware store. Use zip ties to attach a pair of brackets to each of the heater elements so that they can be attached to the side of the box as though they were sitting on a shelf. Use the aluminum repair tape to tape the brackets to the wall of the cooler. This is secure enough if they don’t get bumped a lot. You could use screws they give you with the bracket if you wanted to make them more secure.
Now that the heaters are both affixed into place, use strips of the aluminum repair tape to tack the wires along the bottom/side corner of the box to keep them out of the way.
On the outside of the box, where the heater cord and temperature sensor cable come out, tape the hole up with aluminum tape. This will keep the wires from moving around and seal the box up so bugs won’t crawl in there.
Setting It Up, Turning It On
Now, you’re ready to go. The heater cord from the cooler should be plugged into the heater plug on the control box. Check the wiring again to make sure it’s correct, then plugin the control box in.
The temperature controller will go through a test cycle and then settle down. There are a lot of settings with this device, but for the most part, you can leave it as it is with the factory settings. The instructions booklet is written in broken English, but it can be followed. One of the settings you may want to change is the C/F setting depending on the units you’re comfortable with.
Setting the temperature is pretty easy, just press the blue AT button and one of the green numbers will flash. Use the AT button to select a digit, and the green up/down buttons to set the temperature. For sweating, we use 115℉.
Once you have the green number set to your target temperature, press the yellow “set” button to accept the change. The red “out1” light should come on, meaning the relay is energized. There should be a red light on the relay indicating it is allowing current to flow to the heaters. You should feel some warmth coming from the heater elements if you put your hand over one.
The top red numbers tell the current temperature reading from the probe.
These temperature controller modules (commonly known as a PID controller) can be a bit temperamental, but I won’t go into too much detail here. These units are widely used by hobbyist home brewers, so there is a lot of help out there if you do some internet searching using “TA4 PID” in your search terms. That’s what I do if things are not working as they should.
Have fun building this! If you’ve got questions, use the comment form below.