Evidence brief: separating cooling from moisture at lights-off
How can an operator distinguish a temperature-driven RH rise from inadequate moisture removal?
Scope of this review
Air psychrometrics, sensor placement and night-time control checks. No crop-specific disease threshold or equipment sizing calculation.
How we assessed the evidence
We reviewed the cited sources, separated published observations from our operating recommendations, and checked the units and assumptions of any calculations. This is a literature-based technical analysis, not an on-site experiment.
What the evidence supports
The calculation isolates one cause
We used es(T) = 0.6108 × exp(17.27T / (T + 237.3)), with pressure in kPa. Actual vapour pressure is 0.60 × es(24), held fixed for each step.
Dew point and RH answer different questions
RH changes as air temperature changes. Dew point in this fixed-vapour example stays constant and can be compared with measured surface temperatures.
Equipment needs operating context
A room average and a rated capacity do not establish conditions within the crop or the water removed during a particular night.
What this review does not establish
- The plotted time sequence is invented and is not a measured cooling curve.
- The calculation omits moisture release, ventilation, heat transfer and equipment behaviour.
- Condensation and disease risk cannot be inferred for every surface from one air sensor.
Hamfy’s proposed next step
Combine a time-aligned temperature/RH log with lights, irrigation and HVAC status, then verify moisture removal under representative night conditions.
Evidence to collect for a real project
- Measurements before and after lights-off at crop level
- Relevant surface temperatures and outdoor moisture conditions
- Equipment status, condensate drainage and night operating capacity
Source notes
- Technical source · 1998Crop evapotranspiration — Guidelines for computing crop water requirements · Chapter 3FAO · Irrigation and Drainage Paper 56
Basis for saturation vapour pressure, actual vapour pressure and the dew-point calculation. It is not a grow-room control recipe.
- Technical source · 2015Guidelines for measuring and reporting environmental parameters for experiments in greenhousesBoth et al. · Plant Methods
Supports careful sensor placement and reporting of environmental measurement conditions.
- Technical source · 2015Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?Michigan State University Extension
Explains why temperature and RH should be interpreted together through vapour-pressure deficit.