Here are some basic instructions on how to build a $2 compost thermometer using readily available supplies. It looks crappy (in a good rustic kind of way), but works great and uses few materials.
You really don't need to read this... I like this design because it's pretty functional, reaches the correct temperature faster, and looks cool, but you could just duct tape everything together in a minute and see if it works for you first.
I chose to use a dry fallen tree branch because they are super abundant, eco friendly, and have reasonably low thermal conductivity. This is good because you don't want your stick to be conducting much heat to or from the thermometer probe tip as this will increase the amount of time it takes to get a stable reading. My first attempt took me a minute to duct tape the probe to a piece of steel rebar, but I found that the rebar was not only cooling down my probe, but also my compost pile! Steel is roughly 200 times more conductive than wood (54 vs 0.28).
- $1.77 ebay fridge thermometer (free shipping!)
- a bit of galvanized strapping (all tinkerers need this anyway!)
- a small screw
- a small dry fallen branch
- any old drill
- 1/8" drill bit
- screw driver
- hack saw or any thin blade hand saw
- vise or bandages
- tin snips or something to cut strapping with
The build consists of two parts: securing the probe, and mounting the display.
Step 1: Securing the Probe - Part 1/2
Cut the branch off roughly square then mount in a vise so that you can drill into the face of the branch. Drill down about 0.75".
Raise the branch up a bit in the vice and then use the hacksaw to cut through the center of the hole. Continue cutting until you've cut down about 1.25 inches (this is 0.5 inches past the depth of your initial drill). I chose a hacksaw because it has a thin blade and won't erase the hole we just drilled. We still want the round sides of the hole to be present to help hold the temperature probe in place.
Loosen the vice so that it is just barely securing the branch and rotate the piece so that you can cut out a wedge on one side. We are intentionally making the cut on one side wider so that we can run the temperature probe's wire down it. You want the vice tight enough that it prevents the two halves of the branch from breaking or moving, but not so tight that it pinches the blade while cutting. You could try skipping this, but then you'll have to pry the two halves apart quite a ways to insert the cable and the branch may split. If you do just try prying it apart, I'd recommend making your initial cut maybe 2" to 3" (instead of 1.25") deep to reduce the risk of splitting the branch. The down side of making the initial cut deeper is that the two branch halves won't hold the probe quite as tight, but you could try wrapping some duct tape around the cut halves tightly once the probe is inserted.
Step 2: Securing the Probe - Part 2/2
Next we want to drill a second hole for the temperature probe cable to escape. Clamp the piece as shown below and drill a hole half way through the piece so that this new hole meets up with the first drilled hole. Locate the new hole about 0.75 inches away from the end of the piece. If the two holes don't meet up, you can drill the first hole down a bit more.
Now run the cable down the wider cut and out the second drilled hole as shown below.
Gently pry the two branch halves apart and push the probe down into the first drilled hole. There will be some resistance as the hole is undersized, but this friction will help keep the probe secure. I chose to leave a little over half of the probe exposed to help increase surface area of the probe touching the compost, but found that it eventually pushed much further in when I tried pushing it hard down deep into a pile of of compacted grass. If you find yourself needing to push hard, use a different stick to make a pilot hole in the compost pile first and then reinsert your compost thermometer. This will help keep the probe tip from pushing further into the branch. I also tried whittling the end of the branch into a point to help make it more staby, but it only improved things a bit - make a pilot hole.
Part one is done! Now time to start on mounting the display.
Step 3: Mounting the Display
You could make the following a lot easier with some duct tape, adhesive, or zip ties to simply mount the display to a straight piece of galvanized strapping. I chose to make a friction fit for the display by bending the galvanized strapping into the shape shown below because I thought it worked well, would last a long time, and makes it easy to pop out the display for when you need to change the battery. Note: my thermometer has been on for over a year now so I'm not sure how often you'll actually need to change the battery. Another good option would be to use releasable zip ties which are amazing!
Cut off a piece of galvanized strapping as shown below. If you don't have tin snips, you can also bend the strapping back and forth a bunch of times to fatigue the metal and break it. I'd actually leave another inch of strapping at the very bottom to give more mounting options. Use some pliers to bend back the sharp edges of the strapping where you cut it (click on the picture below to see a close up). Then take your handy dandy pliers and bend out the galvanized strapping as shown below. I would not use the LCD itself to bend against as it doesn't feel that strong. It may take a few tries, but make it just snug enough that friction holds the display in place. Also note how the strapping doesn't go all the way around the display. This makes it easier to bend the shorter side further down before inserting the display. This will cause the strapping to act like a spring and clamp down on the display. Why this shape? The battery compartment is on the back and I wanted to be able to rotate the display up and down to get the best angle for visibility on the LCD screen without having to hunch down.
Screw the display mount onto the branch wherever you like, but don't over tighten - you want to be able to rotate the display to get the best angle. It helps to bend the strapping a bit away from the screw and the branch so that the only place where the strapping touches the branch is by the screw. This makes rotating the display much more convenient.
Wrap the thermometer cable around the branch a few times over the length of the branch and then wrap up any excess and tie a gentle knot near the top and insert the display. I figure the few loose wraps near the bottom make the compost less likely to tug on the cable as the branch is being inserted into a pile. Angles and stuff.
Congrats! Go measure some compost piles!
Remember not to leave outside or in the rain. The display is surely not waterproof.
Picture 4 (NEW)
I added another photo showing the compost thermometer in action inside a prototype insulated hot composting bin. 54.1°C is near the ideal zone for hot composting. I've actually seen it peak above 65°C which is too hot. If things go well with the insulated bin, I'll make another instructable :)
Also, watch out for evaporating water damaging the display. Against my own advice, I left the thermometer in my insulated bin because it is normally tarped so I figured rain wouldn't be an issue. However, a lot of water evaporates out of my insulated hot composting bin and it nearly killed the display. Maybe I'll try wrapping it in some clear plastic someday to protect it.
- You can try whittling the end near the probe into a point to help increase its stabbing ability a bit.
- Use another stick to make a pilot hole if you feel you need to push hard into a pile.
- Add markings to the stick so that you know how deep you are measuring at.
- I wanted to be able to hang my compost thermometer on the garage wall so I bent a deep inverted "U" shape into the strapping near the branch so that it would wrap around a screw. You could also just drill another hole large enough to fit over a nail / screw near the top of the branch. I wouldn't hang it upside down as water droplets may run down the wire and into the display causing damage.
Please share your own ideas! I'm no expert wood worker.
I originally posted these build instructions here.