Air Mixing Sensor from Temperature, Humidity, and Barometric Pressure
Airflow is the movement of air molecules from one area to another. Indoor-grows typically have several fans positioned along the walls, as shown above, to move air around the plants. Additionally, they may employ vertical draft fans that pull the air from underneath the benches to avoid the denser CO₂ from pooling below the benches. Having good air movement inside the grow is crucial to prevent stagnant air pockets that can be potential zones of fungus and mold growth. It has also been shown that the flow of air over the topsoil helps to keep it dry, thereby slowing the reproductive cycle of pests that like to make this damp topsoil its preferred breeding ground [1].
An anemometer is the standard toolkit used by growers to measure airflow [2]. The figure below shows two commonly used classes of anemometers. Mechanical anemometer estimates airflow by measuring the rotational velocity of a propeller. In contrast, solid-state anemometers have no moving parts, and they infer airflow by measuring the extra current required to keep a thermal probe at a specific temperature due to the cooling effect produced by air moving over the heated probes.
Typically, the grower would use the above instruments to spot check air movement in regions where they suspect to have issues. However, indoor-grows have complicated air pathways where stagnation spots can be dynamic, and the only time grower suspects a region to have airflow issues is during post-analysis after an infection has been spotted in an area. Additionally, these instruments are limited to only measuring air movement in the direction of propeller or air-ducts and cannot be permanently installed in indoor-grows as they are prone to failure due to particulate clogging. Finally, the hindrance to daily operations and the overall cost of ownership when these devices are deployed at high density can be significant.
The objective of this post is to demonstrate how we were able to measure air mixing in an indoor-grow using our high-density deployment of environmental sensors that measure temperature, humidity, and barometric pressure.
The primary purpose of measuring airflow in a grow is to infer the extent of air mixing. As pointed out earlier, proper air mixing is critical to prevent pest infestation, but it also plays a crucial part in the plant’s energy production/consumption cycle. Plants, like any other living organism, rely on O₂ to breakdown the sugar molecules for energy. Unlike animals, plants have the unique ability to generate sugar from light, CO₂, and water. Consequently, they need to be fed a constant stream of O₂ rich air round the clock and during the photosynthetic period, a stream of CO₂, in addition to O₂, to help with sugar production.
Inlet vents in an indoor-grow continually bring in O₂ rich air at the desired temperature and humidity, and the exhaust fans are continuously removing the O₂ depleted air. The fans inside the grow help to distribute this fresh air throughout the grow. To mathematically understand this mixing process, let’s introduce a term called enthalpy, which is a measure of the total energy contained in a volume of air. Air with the same enthalpy (energy) could be colder or hotter, humid or dry. As long as the enthalpy is below the saturation (100% relative humidity) value, there is no condensation. This relationship between condensation and enthalpy can be understood from a psychrometric chart that is familiar with growers, as shown below.
Orange lines represent lines of constant enthalpy. In the above plot, enthalpy per unit mass called specific enthalpy is used instead of the raw enthalpy, which is a bulk property. Let’s consider an air mixing process where incoming air with specific enthalpy hᵢ and density ρᵢ (marked by position A in the above psychrometric chart) is mixing with stale air at specific enthalpy hₛ and density ρₛ (position B above). The resulting enthalpy hᵣ (position C) and density ρᵣ after mixing is derived from the ideal gas mixing equation [3] as
From the above equation, enthalpy hᵣ depends on the density of the mixing air (ρᵢ, ρₛ) and the final density ρᵣ is the average of the incoming ρᵢ and the stale gas ρₛ around the plants. As the mixing progresses, inlet vents bring in the fresh air, and the exhausts fans are continually pulling the stale air out of the grow resulting in ρᵣ → ρᵢ and ρₛ → 0. If the circulating fans in the grow are doing their job effectively, then every space within the grow has the same air density trend of ρᵣ → ρᵢ and ρₛ → 0. Therefore, a useful metric to evaluate air mixing is to measure similarity in air density between adjacent grow spaces in the indoor-grow over time.
For our airflow analysis, we collect air density samples from each environmental sensor every 30 seconds. A co-variance test that runs every half-hour provides an air density similarity metric between regions bordering neighboring sensors. Areas with thorough mixing have high co-variance when compared to regions where there is a relatively lower air mixture. Since we have a uniform placement of sensors inside the grow, we can infer a 3D vector of the similarity metric for air mixture. A heatmap plot with mixing directional arrows are then subsequently generated to help visualize this data.
A blue to red gradient color scale represents the extent of air mixing with blue representing areas of a lower air mixture, and the red regions have better air mixture. In the above grow, as expected, areas with plant canopy in the middle have lower air mixture compared to the aisles around the benches where air can move freely.
References
- https://www.epicgardening.com/grow-room-ventilation-101/
- https://www.agriculturesolutions.com/crop-soil-and-water-testing/anemometer-wind-speed-meters
- Cantwell, B.J., 1996. Fundamentals of compressible flow. Chapter 3.19, https://web.stanford.edu/~cantwell/AA103_Course_Material/AA210_Fundamentals_of_Compressible_Flow_BOOK_BJ_Cantwell%20copy.pdf
