Psychrometrics for Plant Growth Chambers

Psychrometrics helps researchers understand and control the relationship between temperature, humidity, and moisture in plant growth environments. By visualizing key factors like vapor pressure deficit (VPD), Conviron’s psychrometric chart helps optimize plant health, improve chamber performance, and support more consistent, reproducible research results.

What is Psychrometrics?

‘Psychrometric’ comes from the Greek ‘psykhros’ meaning cold and ‘metron’ meaning measure. Psychrometrics is the science of measuring and analyzing the thermodynamic properties of air and water vapor mixtures. It helps us understand how temperature, humidity, moisture content, and enthalpy interact within an environment.

The psychrometric chart was first published in 1904 and since then has been expanded and amended to improve its usefulness and accuracy. The development of psychrometric charts was motivated by the invention of air conditioning in 1903 and has become the standard for human thermal comfort systems and design [1].

Psychrometric charts were not solely used for indoor thermal comfort; they also contributed significantly to the development and advancement of modern controlled environments for plant growth. Psychrometric conditions influence these processes through altered stomatal behavior, water uptake, nutrient transport, and plant stress levels. Measuring, reporting and understanding the relationship between air temperature, relative humidity, and vapor pressure from a psychrometric chart helps researchers create environments that support healthy and predictable plant growth.

It's More Than Just Temperature

Early protected growing environments were designed primarily to shield plants from cold temperatures. Temperature is easily controlled in environmental growth chambers and growth rooms. However, plants constantly exchange water and gases with their environment through photosynthesis and transpiration. Photosynthesis affects the gas composition of the air and transpiration causes increases in relative humidity (RH). It is important to control both temperature and RH, so these important plant physiological processes run smoothly. For instance, there is an inverse correlation between and RH and transpiration, when RH is high transpiration decreases and vice versa [2].

In addition, there is an inverse correlation between temperature and RH, when the temperature is high RH decreases and vice versa. This understanding was a
breakthrough for CEA. These environmental parameters work together and need to be measured and adjusted in unison. This is what makes psychrometric charts useful for designing and maintaining controlled environments for plant growth.

Psychrometrics, Plant Physiology & VPD

Tiny pores known as stomata are the gateway for photosynthesis and transpiration. Both processes are temperature and RH dependent as are stomata. During the day the stomata are open, CO2 is taken in and O2 is released through the process of photosynthesis. At night O2 is taken in and CO2 is released through respiration. Transpiration is the process of water movement through the plant from the roots to leaves where it is released to the air through the stomata. Approximately 95-99% of the water taken up by plants is released through transpiration and this occurs day and night, with nocturnal transpiration being between 5-30% of the daytime transpiration rates [3].

Maintaining consistent RH levels between 60-80% is optimum for all stages of plant growth. Psychrometric charts report the dry bulb temperature (°C) and relative humidity (%) which allows for the calculation of vapor pressure deficit (VPD). VPD (kPa) measures the drying power of the air, the difference between the amount of moisture air can hold and the amount it actually contains. VPD is an important metric as it strongly influences plant transpiration rates. When the VPD is too high (dry air), plant yields may decrease, water stress occurs and decreases in localized calcium result in tip burn. Low VPD (humid air) inhibits transpiration, reduces plant growth and the moisture trapped on leaves provides a breeding ground for pathogens. The recommended VPD for controlled environment chambers or growth rooms is between 0.7-1.5 for actively growing plants [4].

From Psychrometric Data to Research Insights

Visualizing psychrometric data allows researchers and growers to understand how environmental variables interact. Rather than reviewing temperature and humidity separately, a psychrometric chart reveals their combined effects on moisture, VPD, and the potential plant response. This makes it easier to identify trends, diagnose problems, and optimize environmental control strategies.

Psychrometrics for More Reliable Results

Psychrometric charts provide a standardized way to assess and document environmental conditions. Researchers can:

• Verify that chambers are operating within target parameters.
• Identify environmental deviations that may affect experimental results.
• Replicate environmental conditions across multiple chambers, locations, or studies.
• Improve consistency and repeatability of research outcomes.

Together, these benefits strengthen scientific rigor by ensuring environmental conditions are consistently measured, documented, and reproduced across experiments.

Real-World Applications of Environmental Precision

Examples include:

• Plant phenotyping studies, where consistent environmental conditions are critical for comparing genotypic or phenotypic traits.
• Vertical farming operations, where VPD management improves crop quality, yield, and water-use efficiency.
• Propagation and tissue culture environments, where maintaining appropriate environmental conditions improves rooting success by reducing stress.
• Climate stress research, where precise control of humidity and temperature enables scientists to simulate drought, heat, or disease conditions.
  

In each case, psychrometric information helps ensure that environmental conditions are accurately understood and controlled.

Endnotes

1. Teitelbaum, E., Miller, C., and Meggers, F. (2023) Highway to the Comfort Zone: History of the Psychrometric Chart. Buildings. 13(3):797. https://doi.org/10.3390/buildi...

2. Shelford, T and Both, A.J. (2021) Plant production in controlled environments. In: Introduction to Biosystems Engineering. Holden, N.M., Wolfe, M.L., Ogego, J.A. and Cummins, E.J. ASABE in association with Virginia Tech Publishing. https://doi.org/10.21061/Intro....

3. Wang, Y. et. al. (2021). Optimization theory explains nighttime stomata responses. New Phytologist 230: 1551-1561. https://doi.org/10.1111/nph.17...

4. Runkle, E. Water vapor-pressure deficit. Michigan State University Extension