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Freezer kiln for wood drying

I'm making a small freezer into a kiln for drying wood bowl blanks.

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Wood changes when it dries.  Some bits stay the same, some bits shrink a bit, and some bits shrink a lot.  As well as distorting the wood, this causes stresses which can pull the wood apart, resulting in cracks.

The worst time for this to happen is after you've made something - your beautiful bowl, or piece of furniture, or trinket box develops an unsightly crack somewhere obvious.  To prevent this, the final working of a piece of wood should be when it is completely dry.

Unfortunately, wood dries very slowly, and often unevenly - it dries more from the ends of the grain than it does across the grain.  To speed up the drying, especially when in a changeable and unpredictable climate, it helps to have some form of heated dry box to store the wood in, often called a kiln.  Whilst the word "kiln" evokes pictures of super-hot clay firing structures, it can be as simple as a lightbulb in a cupboard heating it up by a few Kelvin.

Inspired by a setup I saw on Jim Sprague's YouTube channel (see below), I decided to make a small unused freezer into my own little kiln.

  • 1 × ESP8266 / NodeMCU v3 Running the Lua firmware, with relevant modules.
  • 1 × Relay board (3.3v input controlling 240v switches) As many channels as you have heaters; I have 2
  • 2 × Vivarium heater lamps I have a 50W and a 100W lamp
  • 3 × DHT22 temperature / humidity sensors
  • 1 × PWM-controlled 3.3v fan I have a Noctua, which is controllable down to 0 RPM

View all 10 components

  • 30 days of temperature and humidity readings

    Kevin07/25/2021 at 16:28 0 comments


    30 days of temperature and humidity readings, on the same window as the "weight" graph in the other log entry.  The blue line here is the ambient sensor from outside of the kiln, whereas the green and yellow are at 2 different places inside the kiln.

  • 30 days of "weight" measurements

    Kevin07/25/2021 at 11:04 0 comments

    30 days of "weight" measurements.  This is actually taken from a strain gauge attached to a thin bridge across an aluminium block, where the block is supported at one end and the platform that the wood is on is attached to the other end.  The problem with this approach is that as aluminium heats up it expands, causing more strain, therefore the reading is only representative of weight as long as the temperature stays constant.

    The graph should be read not as an absolute value, but more as a "shape" - as it tends towards horizontal, then the weight of the wood is changing less and the wood is therefore dry.

    There are periods on the graph shown here where the heater is not running (the two large dips and the later shorter dip) and where the fan speed is reduced (causing the smaller increases in the middle of the graph, as the overall temperature increases slightly due to the reduced fan speed).

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