Why your controller shows a different VPD than Meridian

The short answer

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.

What the offset is

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.

• Leaf VPD uses the leaf's temperature. This is what the plant responds to, and what Meridian shows.
• Air VPD ignores the leaf and uses air temperature for both halves of the calculation.

The numbers

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.

What to do

1. Find the offset setting on your controller. It is usually called leaf temperature offset, leaf VPD offset, or similar.
2. Set it to 2 C (about 3.6 F) to match Meridian's default. Your controller and this site will then agree.
3. Better: measure it. Point an infrared thermometer at a healthy, well-lit leaf and compare to your air temperature reading. Under strong direct light the leaf may be less than 2 C cooler; with vigorous airflow or a cool canopy it may be more. Enter the real difference.

If you would rather compare against air VPD, set Meridian's offset to zero in the calculator. The two will then match exactly.

Which should I target?

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.

Related

• Vapor Pressure Deficit: the full guide
• VPD Calculator