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It is almost always the leaf-temperature offset.
Meridian reports leaf VPD. Most controllers ship with their leaf offset set to zero, which makes them report air VPD. Air VPD always reads higher. Neither number is broken; they are answering slightly different questions.
VPD is the gap between the moisture the air holds and the most it could hold. But the plant does not experience the air's temperature: a transpiring leaf runs cooler than the air around it, usually by about 2 C.
That matters because saturation rises steeply with temperature. A cooler leaf has a lower saturation point, so the real gap at the leaf surface is smaller than the gap in the open air.
At 24 C air and 60 % relative humidity, with the leaf assumed 2 C cooler:
| Reading | Value |
|---|---|
| Leaf VPD (Meridian) | 0.85 kPa |
| Air VPD (offset unset) | 1.19 kPa |
| Difference | 0.34 kPa |
That gap is large enough to move you a whole growth stage on a VPD chart, which is why it is worth getting right.
If you would rather compare against air VPD, set Meridian's offset to zero in the calculator. The two will then match exactly.
Use leaf VPD, and make sure whatever you compare it against is also leaf VPD. The single most common mistake is targeting a leaf-VPD chart with an air-VPD readout, which leaves the room persistently more humid than intended.
Consistency matters more than the convention itself: pick one, apply it everywhere, and label it.
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