Correctly calibrate sensors in the grow room

Introduction

A stable climate can only be controlled as well as it is measured. Many climate problems in the grow room are actually measurement problems: Sensors drift, hang in the wrong place or are distorted by draughts, warm devices or direct radiation. This leads to incorrect decisions regarding exhaust air, dehumidification, heating or CO2 control.

This guide deals exclusively with climatic sensors in the grow room: temperature, relative humidity and CO2. The aim is to make your measured values reproducible, comparable and really useful for climate control.

Why calibration in the grow room is important

Even small measurement errors can have a noticeable negative impact on climate control:

• temperature errors of 1-2°C significantly change the assessment of heat stress or night setback.
• Humidity errors** of 5-10 % RH distort the assessment of the risk of mold and VPD.
• CO2 errors** lead to too little or unnecessarily high CO2 dosing.

It is particularly problematic that many inexpensive sensors do not suddenly fail, but slowly drift away. Without a comparison with a reference, this is often only noticed late.

Which sensors should be calibrated

Three sensor types are particularly relevant in the climate sector:

Temperature sensors

They should measure the air temperature in the relevant plant zone. Good devices are usually relatively stable, but can deliver highly distorted values due to poor positioning.

Hygrometers

Humidity sensors drift more frequently than temperature sensors. Accuracy often decreases over time, especially in humid environments or near condensation.

CO2 sensors

With CO2 sensors, it is particularly important to check whether it is an NDIR sensor. These are much more suitable for the grow room than simple, inaccurate estimation sensors. However, NDIR sensors also require regular testing and, depending on the model, calibration or a fresh air reference.

Positioning sensors correctly

Calibration is of little use if the sensor is in the wrong place. The following applies for usable climate data:

• Place temperature and humidity sensors at the height of the canopy
• Do not hang directly in front of supply or exhaust air
• Do not mount directly in front of recirculation fans
• Keep away from warm appliances, drivers, dehumidifiers and walls
• Do not measure in corners or directly under the ceiling

For CO2 sensors, also observe the following

• Do not install directly at the CO2 outlet
• Do not place in dead zones without air movement
• If possible, measure in a representative area of the plant population

How to test temperature and humidity sensors

The most practical way is to compare several devices under the same conditions.

Procedure

• place all the sensors to be tested in the same quiet room for several hours
• place them at some distance from each other, but without direct draughts.
• use a reference device that is as reliable as possible
• compare the values only when they have stabilized
• note the deviation of each sensor.

Important: Do not just look at a momentary value. A comparison over at least 30-60 minutes of stable conditions is better.

Salt test for hygrometers

A salt test is useful for many hygrometers. In an airtight container with a saturated saline solution, a relative humidity of approximately 75 % RH is achieved at around 20-25°C. After sufficient equalization, the deviation of the hygrometer can be noted.

This test is simple, but only useful if:

• the container is really tight
• the salt is moist but not completely liquid
• sufficient time is given for equalization

Check CO2 sensors correctly

CO2 sensors should be checked regularly with fresh outside air or according to the manufacturer's instructions. Outside air is typically roughly in the range of around 420 ppm, but varies slightly depending on the environment.

Important points:

• Allow the sensor to warm up before testing according to the instructions
• Do not test directly next to people, as exhaled air greatly increases CO2
• Do not reference roads, garages or combustion sources
• Observe the manufacturer's instructions for zero point or fresh air calibration

Automatic baseline calibration can be problematic in permanently closed rooms if the sensor never sees real fresh air. The reference point then shifts in the long term.

Calibration intervals in practice

Sensible intervals in practice are:

• Temperature/humidity: check every 3 to 6 months
• CO2 sensors: monthly visual inspection, functional according to manufacturer's specifications or in the event of abnormal values
• Immediate check after a fall, condensation, heavy soiling or unusual measurement jumps

Documentation instead of guesswork

Keep a small calibration list with you:

• Device and location
• test date
• reference value
• Measured deviation
• Correction carried out

This allows you to recognize whether a sensor remains stable or is increasingly drifting. This is exactly what turns individual measured values into a reliable climate system.

Conclusion

Good climate control does not start with the controller, but with correct measurement. If temperature, humidity and CO2 sensors are placed correctly, checked regularly and documented, you are responding to real climate data instead of measurement errors. This makes the grow room calmer, more predictable and much easier to control.

Pro Tips

• A good reference device saves many wrong decisions.
• Hygrometers usually drift more than temperature sensors.
• Never install CO2 sensors directly on diffusers.
• Always check sensors again after condensation.
• Documented deviations are more valuable than estimates.