The controller's sensor is one spot in the room. Run the room by what the canopy actually sees: put independent sensors at canopy height in several places, check them against a reference handheld, measure leaf temperature, and judge the room on how it holds through a full day and night, not on the setpoint. For the setpoints themselves, start with our LED environmental guide (or the HPS guide) and Choosing Environmental Setpoints.
Measure the canopy, not the controller
- Reference: we check room temperature and RH with a Vaisala HM41 handheld.
- Room sensors: Govee H5179 Wi-Fi sensors hung at canopy height, shaded with a piece of cardboard so the lights don't heat them, in several spots including the corners and the far end from the cooling. Setup and calibration are in How We Use Govee Wi-Fi Sensors.
- Leaf temperature: read it with an IR thermometer. Then put air temperature, RH and leaf temperature into our VPD calculator and compare with the stage target. If your controller shows a different VPD, here's why.
Equipment: separate cooling and dehumidification
We run separate AC and Quest dehumidifiers instead of one combined unit. If one goes down, the other keeps working, and each can be serviced on its own. Size them for the room fully loaded with plants and running its irrigation, not for the empty room. Keep the dehumidifier drains clear and check them on your walk-through.
Circulation fans mix the air so the canopy doesn't develop damp or hot pockets, but fans don't remove moisture from a closed room.
Watch the transitions
Look at the sensor history around these times:
- Lights on and lights off. Expect a quick warm-up and a quick cool-down. As the room cools at lights-off, RH climbs and leaves get closer to the dew point. Keep lights-off humidity lower; don't raise night RH to chase a daytime VPD number. More in Why Humidity Spikes at Lights Off.
- Irrigations. Each feed usually puts a bump in the humidity graph. If it takes a long time to clear, look at the dehumidifiers.
- Equipment cycles. Check how long a humidity rise lasts, not just the peak. A short spike that clears is normal; a climb that sits for hours isn't.
Fix one cause at a time
| What you see | What to check |
|---|---|
| Controller and handheld disagree side by side | Calibration of the controller sensor |
| They agree side by side but read differently in their normal spots | Sensor placement and airflow |
| One corner stays damp | Redirect circulation and clear obstructions before changing the whole room's RH |
| RH spikes at lights-off | Dehumidifier operating band, drains and real removal capacity at that moment |
| Leaf VPD too high | Raise humidity and/or lower heat |
| Leaf VPD too low | Lower humidity and/or add warmth |
Make one small change, let the room settle, then measure the leaves again. A change worked when the canopy readings across the room improve and the equipment holds them through its normal cycles, day and night.
Climate changes water use
A hotter or drier room pulls more water through the plants, so pots dry back faster and runoff EC can climb on the same tank recipe. Before you change feed strength after a room change, check dryback and runoff against what they were (How Temperature and Humidity Change Your Watering Needs).
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