Connecting VPD readings to a moisture-removal specification
Which measurements and assumptions are needed to move from a humidity target to an indoor climate design?
Scope of this review
A technical desk review covering vapour-pressure calculations, environmental measurement and the distinction between a moisture load and equipment electrical demand. The scope is facility scoping and commissioning, rather than prescribing climate targets for a crop.
How we assessed the evidence
We checked definitions against FAO’s equations, used university guidance for the growing context and consulted a peer-reviewed measurement guide. We then calculated an explicit temperature example and a latent-load conversion. All example inputs are assumed. No farm dataset, dehumidifier performance curve or crop-response experiment was analysed.
What the evidence supports
The reference temperature must be explicit
The worked example uses the FAO equation with 24 °C air and 65% RH. Changing only the reference to a cooler leaf changes the estimated vapour-pressure difference. State whether a dashboard reports air VPD or an estimate using measured leaf temperature.
A target needs a crop context
MSU’s guidance informed the distinction between growth stages. Our design recommendation is to attach a crop, stage, sensor position and permitted duration of excursions to a target, rather than publishing an unqualified number.
A capacity needs a time and a boundary
The 10 kg/h example is a mass-to-thermal-load conversion, not an equipment selection. For procurement, Hamfy proposes an hourly room balance and a manufacturer duty point. For verification, the measurement guidance supports recording how the environment was measured.
What this review does not establish
- The leaf estimate assumes saturation at the leaf surface and does not resolve boundary-layer resistance or stomatal behaviour.
- The latent-load calculation does not include sensible cooling, compressor efficiency, ventilation energy or reheat.
- A room-average trend can hide spatial variation and condensation on a cooler surface.
Hamfy’s proposed next step
Prepare a climate design sheet with crop conditions, measured or estimated hourly moisture release, equipment duty points and a sensor map. Have the designer reconcile water removal with heat rejection. At handover, compare the stated design scenario with observed trends and record any untested conditions.
Evidence to collect for a real project
- Zone-level temperature and humidity traces across a full light cycle.
- Leaf and surface temperatures at representative and cool locations.
- Water-balance observations for the expected largest canopy.
- Equipment performance data at the requested temperature and humidity.
Source notes
- University extension · 2015Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?Michigan State University Extension · Wollaeger & Runkle
Explains the growing context for VPD. Its greenhouse examples are not a universal indoor crop specification.
- Technical reference · 1998FAO Irrigation and Drainage Paper 56: Meteorological dataFood and Agriculture Organization
Source of the saturation-pressure equation and the latent-heat value used in our worked calculations.
- Peer-reviewed guidance · 2015Guidelines for measuring and reporting environmental parameters for experiments in greenhousesBoth et al. · Plant Methods
Supports calibration, sensor placement records and independent environmental measurements.