Humidity Sensors

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Introduction

Another sensor unique to the MetaEnvironment board is the humidity sensor (hygrometer). As the name implies, this sensor measures the air moisture in the atmosphere.

How They Work

Electronic sensors measure humidity by measuring the capacitance or resistance of air samples. For a capacitive hygrometer, the air flows in between two metal plates. The change in air moisture directly proportional to the change in capacitance between the plates. In a resistive hygrometer, a ceramic or conductive polymer absorbs moisture which then affects its resistivity. is connected to a circuit where the moisture affects the resistance of the material. The relative humidity is then determined based on the change in current.

BME280 block diagram

BME280 block diagram

Hygrometers are often outfitted with a temperature sensor and a barometer which combine the temperature and pressure data along with the capacitance/resistance change to measure the humidity.

MetaEnvironment Hygrometer

The MetaEnvironment board comes with a Bosch® BME280 integrated environment unit. This sensor measures humidity along with barometric pressure and temperature. Below is a table of some of the BME280 operational parameters for the humidity sensing portion of the chip.

Parameter Bosch® BME280
Temperature Range -40C – 85C
Supply Voltage (VDD) 1.71V – 3.6V
Measurement Range 0 – 100 %RH
Resolution 0.008 %RH
Active Current Consumption 350µA

The BME280 reports relative humidity which is a percentage that relates the current vapor density to the amount of moisture needed to saturate the air at the current temperature.

Applications

Hygrometers are often found where humidity control is required. For example, a house can use them in a humidity control system, monitoring different areas of the house to prevent mildew growth. In a similar vein, greenhouses, saunas, museums and incubators also make use of hygrometers to ensure the amount of air moisture is at the appropriate levels for the plants, people, artifacts, and eggs in the enclosed area. A more complex use case is combining hygrometers with thermometers and barometers for use in HVAC systems to keep a building at comfortable temperatures and air quality.

On the research side, hygrometers, along with other environmental sensors, are used in weather stations where meteorologists collect environmental data to study weather/climate and make weather forecasts.