Measure where the plant grows
A thermometer across the room may not represent conditions under the light. Put the air sensor near the leaves, sheltered from direct lamp heating and splashes. Measure the solution separately: room temperature cannot stand in for reservoir temperature.
Record readings with the light on and after it switches off. Note the positions of the lamp, fan and sensor. If you move the sensor, log that too; a different measuring position can look like a change in climate.
Why humidity changes with temperature
Relative humidity describes how close the air is to saturation at its current temperature. Cooling can raise it without adding water. A steady daytime reading therefore does not rule out condensation after the light switches off.
Check the lid, walls and leaves in the morning. Drops on a cold surface show local condensation. Persistently wet foliage and still air call for a review of ventilation and plant spacing. Raising the light output to dry leaves also changes heat and water demand.
What airflow can do
Gentle movement helps reduce the layer of still, humid air around leaves. Start with indirect airflow and watch the plants. If leaves bend strongly all the time or their edges dry quickly, reconsider the direction and strength.
A fan mixes air inside a room but does not remove moisture from it. Venting or an extractor exchanges room air; the result depends on outdoor conditions. Check readings after changing the schedule, especially during cold or hot weather.
Check the reservoir separately
Lights, pumps and a warm room can heat the solution. As water gets warmer, it holds less dissolved oxygen while roots still need oxygen for respiration. A working diffuser does not remove every effect of overheating.
Keep the reservoir out of direct sunlight and check heat from equipment. Avoid sudden temperature changes from adding a large amount of icy water. Record measurements before a change and again after mixing.
Choose conditions for the crop
Use temperature recommendations for the crop and stage in your growing plan. Lettuce and basil may not suit the same climate. For a shared shelf, account for mature plant size, dense foliage and heat from lighting.
VPD describes the difference in water-vapour pressure between the leaf and air. An accurate calculation needs leaf temperature, so a chart based on air temperature is an approximation. Start by making reliable measurements, removing condensation and observing plants before trying to hold one VPD value.
Check the result of an adjustment
- Compare air temperature and humidity with the sensor in the same position.
- Check solution temperature and water use.
- Look for condensation after the dark period.
- Make sure airflow does not dry seedling plugs.
- Check new leaves and record equipment settings.
Changing the light and ventilation together makes their effects hard to separate. Outside an urgent problem, adjust one factor and record the result before making another change.