Plant Temperature camera = IR Temperature sensor
A Plant Temperature (PT) camera is used to measure the leaf temperature which, along with the ambient temperature and humidity, can then be used to determine the Vapour Pressure Deficit (VPD). When used in combination with the plant temperature (PT) this is a measure which shows how easy it is for the plants to transpire. How to use these measurements is explained a bit later.
The PT camera is basically simply an IR temperature sensor with an I2C interface. I broke out the center of the glass in a dummy security camera and installed the IR temp sensor through it. Was quite a good fit and with some extra padding around the sensor it fit rally snugly. I hooked up the sensor to an USB cable and this plugs into the Arduino.
You can see the sensor peeking out through the hole in the middle:
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The sensor mounted to the front bit of the dummy camera housing and the simple connection i set up to the USB cable:
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The PT camera in action monitoring the ladies:
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How to calculate the VPD?
The VPD is the difference between the saturation vapor pressure at the plant temperature and the measured ambient vapor pressure. I’m using this formula:
VPD = 0,6108*EXP((17,27*PT)/(PT+237,3)) - 0,6108*EXP((17,27*AT)/(AT+237,3)))*RH/100
The VPD calculation is based on the following parameters:
- Plant temperature (PT in degrees Celsius)
- Ambient temperature (AT in degrees Celsius)
- Relative Humidity (RH percentage)
In general for most plants the VPD should be slightly below 1kPa, but I also read that for cannabis it should be between 1.0 and 1.5kPa. When the VPD is too high the plants will be transpiring too much so you should either lower the temperature (dim the lights) or raise the humidity. When the VPD is too low, the plants could grow more if their transpiration was higher and if very low the plants might get wet becaue they can;t lose their moisture through transpiration.
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When a plant is in vegetative phase it can still adapt to it’s surroundings. So the VPD is not so much an issue then. You shouldn’t change the climate based on that. Also when the plants are smaller you will usually measure not just the plants but also the walls or floor behind the plants. When the grow room is nicely filled with plants which are flowering then the VPD becomes more interesting.
I noticed the VPD has some sort of cycle where it starts off high when the “day’ starts, then it drops during the “day” and before the lights go off it goes up again. The VPD is the orange line at the bottom of this image:
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I’m still trying to figure out if this actually means the plants could use more light in the middle of the day or that this is just the result of something else. My idea was to dim the lights on a curve similar to how more sunlight reaches the plants at noon and less in the morning and evening. Overall the plants would get the same amount of light energy, but then perhaps they can use it more efficiently this way.
Dimlux sells a controller that protects the plants from overheating based on the same VPD principle. Although they only use it as a protection when the system is unable to keep the climate within the proper range.
The plants should always be slightly colder than the ambient temperature because the plants are cooling themselves by transpiring water. The Dimlux controller will dim or switch off the lights when the PT goes above the air temperature or when the VPD goes too high. When the plants are no longer transpiring something is seriously wrong. The plants will then have closed their stomata to protect themselves against drying out. That means no more photosynthesis is going on either. That’s damaging to the plants and a massive waste of energy in lighting. You can see the same occur with plants outside when they receive too much sun and heat.
That'll come down to how loud they are.
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