If your school weather station worked reliably for its first term and now reports 99% or 100% relative humidity every day regardless of the weather, your sensor has suffered saturation drift and surface contamination. It is the single most common failure mode in educational IoT weather stations, and it almost always comes down to using indoor-rated sensing elements without adequate condensation management.

The physics of the 99% lock-up

Most entry-level school robotics projects start with basic sensors like the blue DHT11 or the white DHT22 (AM2302). These units measure relative humidity using either a resistive polymer substrate or a basic open capacitive film. As ambient humidity rises, the polymer absorbs water vapour, altering its electrical impedance, which the internal microcontroller converts into a percentage reading.

These sensors are designed for indoor appliances such as air conditioners, incubators, and domestic humidifiers. In an outdoor environment, especially during South African winter mornings in the Western Cape or clear, high-radiation nights on the Highveld, the ambient temperature regularly drops to the dew point. Water vapour transitions into liquid dew directly onto the exposed sensor element.

Once liquid water condenses on a basic sensor substrate, three things happen:

• Capillary entrapment: Liquid water becomes trapped within the porous sensing layer. While water vapour can desorb quickly when temperatures rise, liquid droplets evaporate slowly, especially if trapped within an unheated plastic enclosure.
• Ionic contamination: Airborne dust, pollen, and coastal salt aerosols dissolve into the droplet film on the sensor's surface. When the water eventually evaporates, it leaves behind microscopic conductive mineral deposits. These bridge the sensing electrodes, permanently lowering electrical resistance and locking the reading near maximum.
• Substrate degradation: Prolonged saturation causes the moisture-absorbing polymer to swell beyond its elastic limit, permanently altering its baseline calibration.

The enclosure trap: Rain protection vs ventilation

When a school team discovers that weather is damaging their hardware, the instinctive reaction is to seal the enclosure more tightly. Teachers and learners frequently house microcontrollers and sensors inside waterproof junction boxes or solid 3D-printed containers with only small drilled holes at the bottom.

This creates a microclimate that accelerates sensor failure. During a sunny afternoon, internal air expands and pushes out. As temperatures fall at night, the enclosure draws in cool, moist air. Dew condenses inside the box onto cold plastic surfaces and circuit boards, but because there is no cross-ventilation, that moisture cannot escape when the sun rises. The sensor effectively sits in a warm, stagnant terrarium at 100% humidity.

Outdoor meteorological sensors must never be sealed in an unventilated box. They require a multi-plate louvred radiation shield (a mini Stevenson screen). A proper radiation shield provides three non-negotiable functions:

• It blocks direct solar radiation, preventing solar heating from distorting temperature and relative humidity calculations.
• It deflects vertical and wind-driven rain away from the electronics.
• It permits continuous, ambient airflow across the sensing element so that the air being measured is genuinely representative of the surrounding atmosphere.

The solution: Capacitive I2C sensors with built-in recovery

To build an outdoor weather station that survives multiple academic years without maintenance drift, switch from entry-level single-wire sensors to industrial-grade capacitive sensors operating over an I2C bus, such as the Sensirion SHT3x series (SHT30, SHT31, SHT35) or the Bosch BME280.

Modern capacitive I2C sensors solve the saturation problem through two key engineering features:

1. Hydrophobic PTFE filter caps

Sensors fitted with a microporous PTFE (polytetrafluoroethylene) membrane allow water vapour molecules to pass through freely while physically blocking liquid droplets, dust, and coastal salt particles from ever touching the sensing die. Even during driving rain or thick morning fog, liquid water cannot reach the active polymer layer.

2. On-chip diagnostic heaters

Sensors like the SHT31 include a built-in resistive heating element that can be toggled on and off via software over the I2C bus. If your microcontroller detects prolonged readings above 95% around dawn, it can activate the internal heater for 5 to 10 seconds. This gently raises the temperature of the silicon die a few degrees above ambient, burning off micro-condensation before taking a measurement.

A 4-step checklist to fix your school station

1. Retire the DHT series: Replace DHT11 and DHT22 modules with a conformal-coated BME280 or a filter-capped SHT31 module. For classroom weather and environmental monitoring kits designed specifically for outdoor resilience, explore the hardware configurations available at Sheen IoT.
2. Mount in a louvred shelter: Print or purchase a multi-tier solar radiation shield with open air channels. Mount it at least 1.5 metres above natural ground, away from hot asphalt or reflective metal roof sheeting.
3. Protect the PCB: Apply silicone conformal coating or acrylic lacquer to all exposed solder joints, pull-up resistors, and microcontrollers, leaving only the active sensing aperture of the humidity chip uncovered.
4. Implement pre-read heating in code: If using a sensor with an on-chip heater, add a short warm-up routine to your microcontroller's measurement loop during early-morning logging intervals.

Educational weather monitoring does not require research-grade meteorological budgets, but it does require sensors built to withstand actual atmospheric chemistry. Moving from indoor consumer modules to vented, membrane-protected capacitive sensors turns a fragile single-term science fair project into permanent, year-round school infrastructure.