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How to Measure and Set the VPD Leaf Temperature Offset
How to Measure and Set the VPD Leaf Temperature Offset

This short guide will teach you the correct way to measure, calculate, and set the VPD leaf temperature offset.

Chris Tosi avatar
Written by Chris Tosi
Updated over a week ago

The correct VPD helps plants grow their best. To accurately track the VPD that plants feel, you need to know the temperature at the leaf. Pulse can take that "leaf VPD" into account when displaying VPD in the app to guide you to better results.

What you'll need to get started

  1. IR thermometer gun

  2. Notepad

  3. Pulse App

  4. Calculator

Calculating the VPD leaf temperature offset

  1. Make sure lights are on and have been for a good amount of time ( > 1 hr)

  2. Get your IR thermometer gun

  3. Collect 4-6 temperature readings of leaves in the canopy at various different spots: top, bottom, middle, sides, etc. Write these down.

  4. Collect 4-6 temperature readings of non-plant material: supports, walls, pots, plastic, wood, etc. Write these down.

  5. Get the average of the canopy temperatures.

  6. Get the average of the non-plant temperatures.

  7. Subtract non-plant temperatures from canopy temperatures: (T Canopy) - (T Other) = Leaf Temperature Offset
    Ex: Canopy average temps 75, other average temps 80
    ​ 75 - 80 = -5
    Ex: Canopy average temps, 65, other average temps 60
    ​ 65 - 60 = 5

  8. Enter the leaf temperature offset into the app:

    1. Log in > go to the dashboard > tap / click on a device > click settings tab above the chart > scroll down until you see a section called "VPD Leaf Temperature Offset"
    2. Enter the offset you calculated in step 7, make sure to mind the negative sign if there is one.
    3. Tap / click the blue update button.

FAQ

How often should I do this?

  • Once a month is sufficient.

Why don't you have a separate VPD leaf temperature offset for night?

  • VPD doesn't matter at night since plants close their stomata.

Where should I measure the leaves?

  • All across the canopy - left to right, top to bottom.

Want to get an even more accurate leaf temp offset?

Different materials have different IR emissivities, but if you use a material that you know the emissivity of, you can really dial in the measurement!

  • Hang a piece of cardboard or plastic covered in black electrical tape at canopy height, shaded away from light, and use that as your measurement comparison standard.

  • Make sure to hold your IR thermometer right up to the electrical tape when taking a comparison measurement.

  • Black vinyl electrical tape has an emissivity of 0.95. Plant leaves have an emissivity of 0.98 [1]

  • If you have an adjustable emissivity IR thermometer, just use the correct settings for each thing when taking measurements.

  • If you have a normal IR thermometer, the emissivity setting is probably set for 0.95

  • You can do a correction of the leaf temp using this formula:

    • Leaf Temp corrected = ((0.95/.98 * (Leaf Temp from IR thermometer)^4))^1/4

    • This comes from the Stefan Boltzmann Law

    • Temperatures are in Kelvin

  • Or you can just subtract 4.1F (or 2.3C) from the leaf temperature value before calculating the offset

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