Why Your VPD Controller and Calculator Disagree

Usually it's the leaf temperature. Most controllers assume a leaf offset, a fixed number of degrees below the air, instead of measuring the leaves. Our VPD calculator uses the leaf temperature you actually measured. If the controller's offset is wrong, the two numbers won't match even when they agree on air temperature and RH.

The gap can be bigger than people expect. Same room, 82°F and 60% RH, three different leaf temperatures:

A few degrees at the leaf is the difference between too dry and too wet for the stage.

Line up the three inputs

1. Hold a handheld you trust right next to the controller's sensor at canopy height and let both settle. We use a Vaisala HM41.
2. Measure several top leaves under the light with an IR thermometer.
3. Find the controller's leaf setting: measured leaf temperature, a manual offset, or a default.
4. Put the same air temperature, RH and leaf temperature into the calculator, in the same units.

If the handheld and the controller disagree on air temperature or RH in the same spot, that's a sensor problem; use the sensor's own calibration or offset procedure. Readings taken in different places or at different times don't prove a sensor is off.

Check placement and timing

• A probe near a humidifier, wall, light or supply vent doesn't see what the canopy sees.
• Sensors lag right after a dehumidifier cycles or a door opens. Let them settle.
• Check with the AC and dehumidifier both on and off. One good reading doesn't show the whole cycle.
• The leaf-to-air difference moves during the day and after a light change. A new fixture can shift it even when the room reads the same.

Which number to run the room on

Use our chart for the target and the calculator for your actual VPD. The stage targets are in VPD: Why Leaf Temperature Matters.

• Controller takes an offset: set it from your measured leaf-minus-air difference, not a permanent guess. Recheck it after any light change.
• Controller can't account for leaf temperature: use the calculator to find the air temperature and RH that hit the target at your measured leaf temperature, then run the controller on those temperature and RH setpoints.

Then correct in small steps: VPD too high, add humidity or take out heat; too low, drop humidity or add a little warmth. Measure the leaves again after each change. If pots start drying faster or slower, adjust the watering, not the CropSalt formula.