Heating Element Selection and Assembly
Editor’s Note: Confused? Read Part 1 of Building Your Own Sous Vide Machine for the intro, parts list, and blog post that sparked the inspiration for this project.
Instead of the three immersion heaters that Scott linked in parallel, I chose to use a 1500 watt 120v water heater element. This part will set you back less than $10 at a Home Depot or a Lowes. I know because I bought one at each throughout the course of this adventure. The one at Lowes is about an inch longer, but other than that, they’re pretty much the same.
I wanted more power than what was offered by the coffee cup heaters, and I thought that because the element was threaded I could just screw it into the bottom of my enclosure. The heating element selection turned out to be the direct cause of the failures of both versions 1 and 2.
In version 1 I did as I had planned and screwed the heating element into the bottom of the acrylic enclosure. As I was testing the machine for the first time, the PID was controlling the element, and the water was heating, but I hadn’t had the machine on for very long before I smelled melting plastic. I shut the machine off before there was any damage, but I knew at that point, that I needed something to mount the element on besides the plastic.
It was back to the hardware store to find some way that I could mount the element without it touching plastic. At the hardware store I happened upon a steel floor flange. The flange had a center hole that was threaded for a 1″ pipe. It also had four bolt holes spaced around the edge. I thought that this would be just what I needed. So in version 2, instead of mounting the heating element directly against the plastic, I mounted the flange up on rubber grommets within the enclosure, and then threaded the element into the flange. This left the element so that only about half the element was submerged in water once the water was filled and the circulator was mounted over the water.
Version 2 was a disaster. In the next test, I left the machine on long enough for the water to come up to 130 (Fahrenheit…come on this is America people) and hold temperature. At first, it seemed like everything was working, but then I realized that the flange was melting through the bottom of the enclosure. By the time I noticed and shut the machine off, not only had it melted partially through the bottom, but it had turned the inside of the enclosure into an oven, partially melting some of my other components. The heating element itself was actually beginning to melt at the top as it entered the first stage of burning out. I learned two lessons out of this:
- Lesson 1: Water heater elements must be submerged almost entirely when in use. Anything less and you’re flirting with a meltdown.
- Lesson 2: Never ever put a heat sink (e.g. steel floor flange) inside an airtight plastic case. Man and I even had a class on thermodynamics in college – you would think I would know better. To be fair to my thermodynamics teacher, I did sleep a lot in that class.
This led me to the third and final version of the heating element mounting assembly (yes, I know there’s a version 4 of the circulator, but it’s still using the same heating element mounting assembly from version 3, if it ain’t broke, don’t fix it). In version 3, I bought a sheet of aluminum from the hardware store and 4 2 inch long nylon bolts with 3 times as many matching nuts and a set of nylon washers. I also reused the rubber grommets that I purchased for version 2. Following are the instructions to make the mounting assembly for the heating element:
- On the sheet aluminum using a carpenter’s square, draw a 3’’ x 3’’ square.
- In the center of the square draw a 1 inch circle.
- Evenly spaced on the four corners of the square make marks for 4 bolt holes spaced far enough from the edge that the holes can be drilled safely. What you end up with should be a square of aluminum that looks something like this…only bigger.
- Once you have the design traced into the aluminum you’ll need to cut it out. I used a Dremel and with a circle cutting attachment for the one inch hole. For the bolt holes you should just be able to use the normal metal cutting bit and eyeball it.
- With your square cut out, attach your rubber grommets to the bolt holes. If you cut your holes correctly you should be able to pinch and slide them into place.
- Slide the four nylon bolts through the grommets and tighten one nylon nut onto each bolt all the way until the nut and bolt head are snug against the rubber grommet. You should end up with something like a tiny table with a huge hole in the top.
- Next, thread your heating element into your assembly. The heating element should be coming out opposite the side that the bolts are facing. Back to the table metaphor, the heating element should be coming out of your table top like a giant center piece.
- That’s it. Your heating element assembly is complete. On your enclosure, you’ll need to cut four corresponding bolt holes to mount the assembly with one small center hole to thread wires to power the element. These 4 bolt holes and one center hole will take the place of the three holes that are cut for the immersion heaters in Scott’s original design.
Next time, relays!