I’ve just “upgraded” my compost thermometer.  Previously it consisted of a digital cable-probe thermometer taped to a length of reinforcing rod.  Simple, effective, and awkward to use, and risking breakage of the thermometer and the probe.

The new model is made almost completely from bits and pieces lying around in the workshop, things from Reverse Garbage in Brisbane, or cheap “off the shelf” parts from plumbing supply shops, supermarkets, or eBay.  The shaft length is 880mm, plenty long enough for reaching into a 1 cu.m. or larger compost heap.

The thermometer itself is housed in a plastic kitchen container from the local supermarket – cost about $2.  It’s held onto a short length of threaded 25mm PVC water pipe (cost about $3.50 for a 300mm length from Masters Hardware, and I’ve still got the remainder with a threaded end to use for another project) with a 25mm conduit saddle clamp (cost $0.65 from hardware). This screws into a threaded PVC “T” (from my plumbing spares box – cost maybe $2.50) that is closed off on the other side with a 25mm bung (also from my plumbing spares box and not really necessary).  The T joins the shaft via a 25mm nipple joiner (only used in order to achieve a join with the blue part of the shaft – cost maybe $2.00).  The internal diameter of the nipple didn’t quite match the blue part of the shaft, so I cut off a short section of rural polypipe (from a huge heap of off-cuts) and hit it with the heat gun to slip it over the blue shaft.  No glue needed.  The piece of blue PVC conduit came from Reverse Garbage in Brisbane (cost maybe $0.70 – Reverse Garbage is a fantastic place to browse.  I always come away with things I didn’t even know I needed until I saw them).

The main thermometer unit with the digital screen on the front is made to be inserted into an instrument panel, so the black “flashing” around the screen is a few mm wider than the main housing, to hide the edges of the cut-out.  On each end of the main housing there is a “lug” which pops past the end of the cut-out to hold the unit in place. I carefully measured the main part of the thermometer housing and marked the outline onto the top of the case, then cut it out using a drill and a hacksaw blade.

Using this waterproof kitchen box helps to keep moisture and dirt from getting onto the back of the thermometer when using it around compost piles.

I got the thermometer from Jaycar in Brisbane years ago for $14.95.  They haven’t had them in stock for quite a while, but you can buy these on the internet or on eBay.  Make sure the cable length is long enough, and that there is an on/off switch (if you want one).  Good idea to check the range too, though the range we are interested in for compost-making is usually within the range of this type of thermometer.  Oven/barbecue thermometers are generally way overpriced and not easily adapted to something like this.

The case is held onto the threaded pipe with a 25mm saddle clamp made for clamping conduit or water pipe onto a wall. The short bolts I had in my “odd nuts & bolts” box needed large washers because of the large holes in the clamps.  On the other side I used large washers inside the case to spread the force so as not to crack the case.

Because the clamp isn’t tight around the pipe I put a Tek screw (wood thread, not metal) through a hole drilled in the base of the case and into the wall of the pipe.  This needed a largeish washer under the head of the Tek screw so that the pressure of tightening the screw didn’t crack the case.  The hole where the cable exits the case could be sealed with silicone, but I’d be wary of causing condensation inside the case.

It’s really easy to pop the lid off the case to press the on/off button on the thermometer.  This prolongs the life of the battery, but I’ve had a couple of similar digital probe thermometers that don’t have on/off switches lying around for months, with the display constantly “on” and their batteries haven’t gone flat yet – you could always take the battery out between uses (store it in the case) if battery life is an issue.  Note the possibility of leaving the probe in the compost heap and recording the maximum and minimum temperatures over a period.

Depending on how you mount the thermometer case on top of the shaft there are many ways of getting the probe on its cable to the other end of the shaft.  I chose to drill a hole in the “T” above the shaft and drop the probe down there.  You need to make sure that if there are any joins in the shaft they do not stop the probe from passing.  I could have (and probably will) put some silicone over this hole – mostly so that if anything snags the cable it doesn’t transfer the strain to the cable-probe join.

The end of the shaft is a metal section that looks like it was once a short, snap-on leg for a piece of equipment (from Reverse Garbage).  The “snap-on” end is crimped to just the right size to fit into one of those disposable applicators that come with silicone tubes (or glue, or gap sealant, etc. etc.) for use in caulking guns.  I cut off the tip of the applicator so that the opening was just wide enough for the probe to be squeezed through, then filled the space behind it with wall panel glue, putting some around the end of the shaft to hold the applicator on.  In theory this should hold the probe securely in place when it is pushed into the compost heap.  I forgot that the glue I used needs contact with air to harden, and putting the applicator straight onto the shaft before the glue was dry meant that it dried very slowly.  I gave the probe a testing “wiggle” a few hours after I’d put the glue in, but the glue was far from dry so that there is now some movement in the probe.  Dumb!  That doesn’t really matter, since I’ve always used a length of conduit to make a hole in the compost heap to insert the probe, so I’ll just keep doing that.  The applicator was free, and the glue was already in the workshop.

It’s not shown in any photos, but I’ve found a bit of communication cable conduit that has just the right internal diameter to slip over the shaft, and fit firmly around the rural polypipe at the top.  This is longer than the shaft, so it protects the probe when the thermometer isn’t being used.

The thermometer takes longer (1-1.5 minutes) to reach a steady reading than it did previously, probably because part of the probe is inside the end of the shaft.  This compost heap was reading 54 degC before I gave it its second turning yesterday (about two weeks after the heap was made).  It’s already back up to 46.2 degC, and might get into the 50s though it has already had a high temperature phase in the first week.  I’ll be happy if it sits in the 40+ range for a while.  You can see details of the making of this heap here.

So there it is.  A compost thermometer for less than $30, depending on how much you can make use of “found” and recycled items.

How does it compare to other compost thermometers?

First, it can measure compost temperature, probably at least as accurately as any other compost thermometer.

Second, with the digital thermometer I used it is possible to stick it in the heap, turn the readout on and leave it there for days or weeks, giving a constant reading of temperature as well as recording maximum and minimum temperatures (I haven’t tried this yet, but the function buttons are there on the back of the case).  I haven’t seen a commercial unit which has these functions.

Third it probably isn’t as robust as commercially available compost thermometers, but if it is treated carefully it should last well.  And, at this price I wouldn’t be too unhappy if I had to replace the thermometer unit or a part of the housing or shaft.  If I’d bought a commercial compost thermometer and it broke I’d be very unhappy because of what they cost.

And that brings me to the fourth consideration – cost.  You can get a compost thermometer in Australia from Agricultural Solutions for AUD$160.  A friend of mine bought one recently and he is happy with it, and I’d assume there are others out there for around the same price.

Until at least the middle of 2013 you used to be able to buy a compost thermometer from the Permaculture Research Institute (Geoff Lawton’s organisation) for AUD$294.72 via the online shop on the Permaculture News website.  However that unit no longer appears in the products listing.  It was built like a truck, with an analogue dial like an old fashioned oven thermometer and surrounded by a steel “wheel” grip, but functionally it was still a thermocouple in a shaft probe with a readout – that’s all.  I had some discussion via email with Craig Mackintosh (the PRI website editor) about the price/value issues with that unit, after he rejected a comment I’d submitted to a posting which promoted it and linked to their shop.  I’d described the unit I was using at the time (an early prototype of this one) and it’s very low cost, and in the email exchange Craig asserted that “There is no comparison between the two probes you’re speaking about.”  Well, no, at least not in terms of price.

So there are other units out there, and you can buy one of them off the shelf much easier than making a unit like the one described above, if that suits your available time and resources.  But the option does exist to make a fully functional compost thermometer for a very low cash outlay and at the same time recycle some resources.