Thermodynamics is bookkeeping on a substance's state: the handful of numbers that say what condition it is in right now. Before any equation earns its keep, you have to read those numbers correctly — and on a plant log sheet that is harder than it sounds, because two different quantities can wear the same unit.

Pressure is force per unit area, in pascals; plants work in kilopascals. It comes in two flavours that differ only in where they start counting. A gauge pressure reads zero when the tap is open to the room. An absolute pressure reads zero only in a perfect vacuum. They are linked by $P_{abs} = P_{gauge} + P_{atm}$ — read aloud P-absolute equals P-gauge plus P-atmosphere — where $P_{abs}$ is the absolute pressure, $P_{gauge}$ is what the instrument shows, and $P_{atm}$ is the local barometric pressure, all three in the same unit. Write kPa(g) or kPa(a) every time; the bracket is not decoration.

Temperature has the same split. A temperature in °C is one point on a scale whose zero is a puddle freezing. A temperature in kelvin counts from absolute zero, where molecular motion stops. And a temperature difference is a third thing again: a span between two points, and because the two scales share a degree size, a span of 1 K and a span of 1 C° are identical. That is why $\Delta T$ — delta T — may be quoted either way without harm, while an absolute temperature may not.

Finally, heat is energy in transit, in joules: $Q = mc\Delta T$, which the next lessons unpack. Note the word transit. A body does not contain heat; it contains internal energy, and heat is what crosses the boundary because of a temperature difference. Engineers say “the heat in the tank” and know better. You will too.