A VPD corridor instead of degrees on the display
Temperature and humidity alone say little about the plant. We hold a value derived from them, vapour pressure deficit, in a 0.4–1.6 kPa corridor.
Most greenhouse control panels show two numbers: temperature and humidity. Both look clear, both are easy to set, and on their own neither answers the main question: how hard is the plant transpiring right now.
A plant does not read a thermometer. It responds to the difference between how much moisture the air can hold and how much it already holds. This difference is called vapour pressure deficit, VPD. At 24 degrees and 65 percent humidity VPD is one value; at 24 degrees and 45 percent it is quite another. On the display only the second number changes, and to the eye it looks like a detail.
What we hold
The climate setpoints in the greenhouse are 24 degrees and 65 percent humidity. But what gets checked is not the setpoints but the calculated VPD, and the corridor is set from 0.4 to 1.6 kilopascals. If the calculation leaves the corridor, the system does not stay silent and does not start adjusting the climate on its own. It reports that the setpoints need to be reviewed.
The limits were not chosen because they look neat. Below 0.4 kPa the air is too saturated: transpiration almost stops, the leaf stays wet, and that leads straight to fungal disease. Above 1.6 kPa the plant loses water faster than the root can supply it, the stomata close and photosynthesis stalls. Formally the greenhouse is “warm and dry”, and growth has stopped.
Why this is not automatic control
The temptation is understandable: since we can calculate VPD, let the system adjust ventilation and humidification to stay in the corridor. We do not do that, and not out of laziness.
The same VPD can be reached in different ways: by raising the temperature, by lowering humidity, by increasing ventilation. Each has its own cost in kilowatt-hours and its own side effects on neighbouring crops in the same volume. The right choice depends on what is growing, at what stage, and what electricity costs this month. The system does not know these things about your site.
So the calculation is advisory. It says “you have left the corridor” and shows by how much, and the decision stays with the agronomist. This is duller than an autopilot. It also avoids the situation where the system heroically holds a nice number while ventilation burns through a month’s budget.
What this changes in practice
Changing the measure changes the conversation about the greenhouse. The question “what temperature is it” becomes “are we in the corridor, and which way are we moving”. The first answer reassures. The second one informs.
There is another consequence: VPD links climate with irrigation. If the plant transpires more, it needs more water, and the other way round. Keeping VPD in the corridor also makes solution consumption predictable, so sizing the fertigation unit stops being guesswork.
All of these rules are covered by test scenarios, not just declared in documentation. This is a matter of principle: a safety rule that nobody has ever run is not a rule, it is a wish.