Summary of "스마트팜 습도 제어, 식물은 수확으로 보답한다｜스마트팜 설명서 EP. 08"

Main ideas & lessons (humidity control in smart farms)

Humidity in air is water vapor, which has relatively small molecules; higher humidity makes the air “lighter.”
Practical implication for greenhouse ventilation:
- If you open a ventilation window on the side, the humid air will rise into/through that opening ineffectively, because the humid air is lighter and rises.
- Therefore, the ventilation opening should function like a skylight (roof opening) so that humid, rising air is vented effectively.
Lesson: Where you place actuators (ventilation windows) matters as much as how much you vent.

The video uses the Mollier (psychrometric) diagram to connect temperature, relative humidity, and absolute humidity.

Diagram interpretation

Left vertical axis: Temperature
Top horizontal axis: Relative humidity
From the intersection point (temperature + relative humidity):
- Drop vertically downward to the lower scale to read absolute humidity (amount of gaseous moisture).

Saturation vs. absolute humidity

At a given temperature, the saturation vapor content corresponds to relative humidity = 100%.
Example at 25°C:
- Saturation water vapor content ≈ 23 g
- If absolute humidity is 15 g, then relative humidity at 25°C is about 65%
- Margin to add moisture before saturation:
  - 23 g − 15 g ≈ 8 g

In winter:
- Outside air may already have high relative humidity (e.g., 80%) because the temperature is low.
If you heat the same air:
- Absolute humidity stays nearly the same (water vapor content doesn’t instantly change)
- But higher temperature increases the saturation capacity → relative humidity drops sharply
Result: heated rooms can become very dry (a key comfort and crop-risk issue).

Key agricultural concept

Tomato-specific danger mechanism

Visible droplets can be noticed, but condensation on leaves/stems may be hard to detect.
If a water film forms on plant surfaces (blocking normal gas/water exchange):
- Transpiration (covered earlier) doesn’t work properly
- Water still rises from roots
- Cells become water-rich → turgor pressure increases
- After transpiration resumes, turgor pressure drops again
- Repeated “swelling/shrinking” cycle can cause cell wall rupture
- Ruptured tissue allows bacteria, fungi, and pathogens to penetrate → disorders and diseases

How to determine dew point using Mollier diagram

Given greenhouse conditions:
- Temperature = 20°C
- Relative humidity = 80%
Steps described:
1. Find the intersection of 20°C and 80% RH on the Mollier diagram
2. From that intersection, go straight down to the saturation (100% RH) curve
3. Then go straight left to read the corresponding temperature at the dew-point line
Example result:
- Dew point ≈ 16°C
Lesson:
- If crop or surface temperature is 16°C or lower, condensation forms.
- If greenhouse temperature stays the same but humidity increases, the dew point rises → condensation becomes easier.

Humidity control principle

Prevent crops from reaching the dew point by:
- Lowering humidity
- Maintaining an appropriate humidity range

Most accurate measurement method (per video)

Analog method using a weather station with wet-bulb and dry-bulb thermometers.

Method logic using Mollier diagram

Dry-bulb temperature corresponds to the air temperature you feel.
Wet-bulb temperature represents cooling due to evaporation at a wetted bulb; at the wet-bulb point:
- Relative humidity is 100% (saturated at that local condition).

Enthalpy concept (explained as the key bridge)

Enthalpy is treated as the “energy content” combining:
- Sensible heat (temperature you feel)
- Hidden heat associated with moisture
On the Mollier diagram:
- Use the measured dry-bulb (e.g., 20°C) and wet-bulb (e.g., 15°C) to locate points where their enthalpy matches (via a diagonal/enthalpy line).

Example procedure

Dry-bulb = 20°C → locate the 20°C line
Wet-bulb = 15°C:
- At 15°C wet-bulb, RH is 100%
Move from the (15°C, 100%) point along the same enthalpy line to the 20°C dry-bulb line
Read the resulting relative humidity at that intersection
Result in example: RH ≈ 60%

Outcome

With dry-bulb + wet-bulb readings, relative humidity can be read from the Mollier diagram.

Problem with asking for a fixed RH percentage

Relative humidity depends on temperature—like changing the “size of a glass.”
Therefore, “What RH is best for tomatoes?” is incomplete without temperature context.

HD definition

New unit used starting in the Netherlands (as described):
- HD (Humidity Deficit) = saturated water vapor content − absolute humidity
It is described as being usable regardless of temperature.

Operational guidance

HD range for growth/control: approximately 2 to 8
Suggested optimal crop-growth target: HD ≈ 3.5
Implementation philosophy:
- Rather than calculating humidity concept-by-concept manually,
- The environment control system (using Mollier-based integrated logic) handles it.
- You only input target settings and the system controls automatically.

The next episode will discuss:
- Temperature control formulas
- Principles behind temperature control

In-kyu Lee — Global Smart Farm Research Institute (speaker)
Graph Smart Farm Greenhouse at Daehan Jaegang — location demonstrated (environmental context; not a person)
Mollier diagram — conceptual tool/source referenced (diagram/standard psychrometric representation)
Dictionary definition of enthalpy — referenced as a conceptual source (not a named author)