The idea is simple: wait for the water to boil, then start the timer. Making a machine agree with you about what "boiling" means turned out to be the interesting part.
This is the fact everyone half-remembers from school and nobody accounts for in the kitchen. A liquid boils when its vapour pressure equals the pressure of the air pushing down on it. Change the air pressure and you change the boiling point.
100 °C is only correct at sea level, on an average day. Go up a mountain and there is less air above you, so water boils cooler. Even staying put, a passing weather system moves the boiling point by a few tenths of a degree.
This is why high-altitude cooking instructions exist, and why an egg takes longer in Denver than in Dublin: the water is simply not as hot, however furiously it bubbles.
A device that waits for 100 °C in a kitchen where water boils at 97.6 °C will wait forever — the temperature never arrives, because the water is already turning to steam. Set the threshold too low instead, and the countdown starts while the water is merely hot. Either way the food is wrong, and the failure is silent.
PotWatch carries a barometric pressure sensor alongside the temperature probe. Near 100 °C the relationship between pressure and boiling point is almost a straight line, and the slope comes out of the Clausius–Clapeyron relation at roughly 0.03 °C per hectopascal. So the device stores one calibrated reference point and slides it up and down as the weather moves:
The linear approximation is deliberately only trusted over a small range. Weather shifts pressure by perhaps ±30 hPa, which is well inside where the straight line holds. Altitude differences of kilometres are not — and they do not need to be, because that part is handled by calibration instead.
Calibrating means boiling a pot of water once, in your own kitchen, and telling the device the number it sees. From that moment the reference point is correct for your altitude, your pot and your particular sensor, and the barometer only has to handle the daily drift.
Everything above describes finding the real boiling point, which is the obvious thing to want. But the calibration screen does not insist on it. What you are really setting is the temperature at which the countdown starts, anywhere between 85 and 102 °C, and there is a good argument for putting it lower.
Water does not jump from still to rolling. Well before a full boil it is already working hard — bubbles streaming off the base, the surface moving, plenty of heat entering the food. On the author's hob that point arrives near 88 °C, and it is a fine moment to start timing and turn the heat down. Gentler than a rolling boil, and for most things a better way to cook.
So read the feature as "start the timer when the water reaches this temperature". The true boiling point is the sensible default. A few degrees below it is a deliberate choice, not a mistake.
The probe is a DS18B20 — a digital sensor that does its own analogue-to-digital conversion and sends the result down a single wire, so there is no analogue signal to pick up noise on the way to the microcontroller. Resolution is set to twelve bits, about 0.06 °C, and it is read once per second.
Even so, a single reading is a bad basis for a decision. Water near a boil is a violent, uneven place: bubbles collapse against the probe, cold spots circulate, steam wraps the sleeve, and someone stirs the pot. Any of these can push one sample across the threshold.
So PotWatch requires five consecutive readings at or above the boiling point before it accepts the result. One reading below and the count resets to zero. In practice this costs about five seconds of delay and removes essentially every false start.
// photographed on an induction hob, one minute of Benedict, start to finish
Once the water is gone the probe temperature climbs past anything water can reach. Four consecutive samples above 105 °C and PotWatch stops the countdown and raises a NO WATER alarm.
A disconnected DS18B20 returns a specific error value, and an unconfigured one returns exactly 85.0 °C. Both are recognised as meaningless. After five bad samples the screen says CHECK PROBE and shows dashes rather than inventing a number.
During the ready alarm, a temperature well below boiling means the probe is in the air. The device treats that as acknowledgement, silences itself and returns to the menu — which is the gesture you were making anyway.
The principle behind all three: a device that quietly does nothing is worse than one that complains. If PotWatch cannot measure, it says so. If the situation is dangerous, it makes noise. The only thing it will never do is sit there pretending everything is fine.