Why Your Temperature Sensor Is Lying to You

The Problem Nobody Talks About

High-intensity grow lights — whether LED or HPS — emit infrared radiation. This is the same type of radiation you feel as warmth when you stand in sunlight. It's not a problem for your plants, but it's a massive problem for temperature sensors.

When infrared radiation hits a sensor, it heats the sensor itself — not the air around it. The sensor reads its own elevated temperature and reports that as the room temperature. The result? Your readings are consistently higher than the actual air temperature.

This isn't a minor issue. In our testing, we've seen sensors read 1-2°C higher than actual air temperature when exposed to grow light radiation. That's enough to throw off your VPD calculations, trigger unnecessary cooling, and waste energy — all while you think everything's dialled in perfectly.

What Is a Stevenson Screen?

Meteorologists solved this problem over 150 years ago with the Stevenson screen — a white, louvred enclosure that houses temperature and humidity sensors.

A Proper Radiation Shield Does Three Things:

1. Blocks direct radiation — The sensor never "sees" the light source directly
2. Reflects infrared — White surfaces reflect rather than absorb radiant heat
3. Allows airflow — Louvres let air circulate freely so the sensor measures actual air temperature

Every weather station on the planet uses this design. It's the accepted standard for accurate temperature measurement in the presence of radiation sources. Yet somehow, the grow industry has completely ignored it.

Why Most Grow Room Sensors Fail

Take a look at the temperature sensors marketed to growers. Most fall into two categories:

Exposed Sensors

A bare sensor on a PCB with no housing at all. Direct line of sight to grow lights. Maximum radiation absorption. These can read 2°C+ too hot.

Solid Enclosures

A plastic box with the sensor inside. Blocks radiation but also blocks airflow. The enclosure heats up and so does the sensor. Slow response time, still inaccurate.

Neither design accounts for the unique conditions in a grow room: intense light sources, high radiant heat, and the need for accurate readings to calculate VPD and control climate.

What We Do Differently

The OMGROW temperature sensor uses a proper Stevenson screen style housing — the same fundamental design used by meteorological stations worldwide.

✓ Dual-tone louvre system — White outer louvres reflect infrared; black inner louvres absorb any that gets through
✓ No direct line of sight — The sensor is shielded from above, below, and all sides
✓ Passive ventilation — Air moves through the housing naturally, no fans to fail
✓ Swiss-made Sensirion sensor — Lab-grade accuracy inside a housing that preserves it

Why Two Colours?

Traditional Stevenson screens use white-painted wood. We use plastic for durability — but white plastic has a problem: it's never fully opaque to infrared. Some radiation always passes through. Our solution is a double-louvre system: white louvres on the outside to reflect the bulk of the radiation, and black louvres on the inside to absorb anything that gets through. Zero infrared reaches the sensor. The best of both materials.

The result is a sensor that reads actual air temperature — not the temperature of a piece of plastic being blasted by grow lights. Your VPD calculations are accurate. Your climate control responds to real conditions. Your plants get what they actually need.

We've done exhaustive testing on nearly every temperature sensor available for consumer hydroponics. So far, every single one gives inaccurate readings under grow lights. We're confident in saying that OMGROW is the only company offering sensors that give you truly accurate readings in a grow room environment.

Place Sensors Where They Matter

Your temperature sensor needs to measure what your plants are actually experiencing. That means placing it right in the canopy — not tucked away in a shaded corner, not mounted on a far wall, not hidden behind equipment.

The problem is obvious: the best location for accurate plant-level readings is also the worst location for an unshielded sensor. Right under the lights, right where the infrared radiation is strongest.

This is exactly why proper housing matters so much. A Stevenson screen lets you place your sensor anywhere in the grow room — including directly under your most intense lights — and still get an accurate reading. No compromises. No "close enough" placement. You measure exactly what your plants are experiencing, because your sensor can handle being exactly where it needs to be.

Without Proper Housing, You're Forced to Choose:

✗ Accurate placement, wrong readings — Sensor in the canopy but reading 2°C too hot
✗ Accurate readings, wrong location — Sensor in the shade measuring conditions your plants never see

With OMGROW sensors, you don't have to choose. Place them where they need to be and trust the readings.

Why This Matters for VPD

A plant's metabolism is intimately tied to temperature. A plant running at 25°C might have up to 30% less metabolic activity than one at 28°C. That's not a rounding error — that's a significant difference in growth rate, nutrient uptake, and yield potential.

VPD — Vapor Pressure Deficit — is calculated from temperature and humidity. If your temperature is off by 2°C, your VPD calculation is wrong. You might think you're running at 1.2 kPa when you're actually at 1.0 kPa.

That's the difference between optimal transpiration and stressed plants. It's the difference between hitting your targets and chasing phantom problems. Worse, if your sensor reads too hot, you might think your VPD is higher than it actually is — leading you to run more humidity than you should. That's how you create the perfect environment for mold, mildew, and other pathogens.

It all comes down to whether your sensor is measuring air temperature or measuring itself.

OMGROW VPD Is Calculated Correctly

Because our sensors measure actual air temperature, the VPD values displayed in your dashboard and app are accurate. We also apply a leaf temperature offset automatically based on your lights-on/lights-off cycle — so you're getting true leaf-level VPD without any guesswork.

Summary

✓ Grow lights emit infrared radiation that heats sensors directly
✓ Sensors without proper housing read 1-2°C too hot
✓ A Stevenson screen blocks radiation while allowing airflow
✓ This design has been the meteorological standard for 150+ years
✓ OMGROW sensors use Stevenson screen housing for accurate readings
✓ Accurate temperature means accurate VPD — no more guessing