Tracking Environments with Digital Humidity Sensors

Whether gathering trace gas measurements, creating an ideal environment or sensitive chemical reactions, or configuring a feedback loop for a system in a harsh environment, a humidity sensor will play a critical role in your system. Digital and analog humidity sensors are available, although digital humidity sensors simplify communication with any digital electronics system.

Nowadays, most humidity sensors are combined with a pressure sensor, temperature sensor, or both. Some humidity sensors are even combined with gas sensors for atmospheric measurements as the presence of water vapor acts as a catalyst in certain atmospheric chemical reactions. This gives systems designers significant flexibility to adapt their system to different environments where gas monitoring is a real concern.

Digital humidity and combined sensors come in a variety of form factors for different applications. If you’re building a confined environmental system, you can use a temperature probe that connects to a PCB or DAQ module outside of your environmental chamber. Alternatively, these sensors are available as ICs that can be placed directly on a custom PCB or an evaluation board.

Response and Signal Output

Digital humidity sensors packaged on ICs are generally accurate to within 3% relative humidity, although this accuracy range can increase by 1-2% at very high or low temperatures. If you need extremely accurate, analytical grade measurements, you would be advised to use a probe-type humidity sensor. These sensors can have a longer response time as they internally measure a large number of measurements in order to provide a stable humidity reading.

The simplest humidity sensors will output an analog signal that can be ready with an ADC, or a digital signal that is output from an internal ADC. If you need to interface with a microcontroller, many humidity sensor ICs will send data via I2C or SPI. The option to use SPI allows these devices to be easily integrated into sensor networks while preventing address collisions.

A set of humidity, gas, position, and CO2 sensors

Parameters to Consider When Choosing a Humidity Sensor

Environmental sensors may need to go places that other electronic systems would never dare. As such, you’ll need to consider issues like operating temperature and pressure. Humidity and combined humidity/temperature sensors packaged as ICs that are placed on PCBs should be placed in packaging that prevents condensation on sensitive areas of the board, especially if this sensor will be used in extremely humid environments.

Another issue is the presence of corrosive gases in the testing environment. Some environments will contain corrosive gases, and water can act as a catalyst for corrosion on metal components. If this is the case, you might want to use a probe-type humidity sensor rather than placing an IC on a PCB as it is easier to isolate the probe from the controller board or DAQ module.

If you are building a system that operates with several electronic components or sensors, the right probe-type digital humidity sensor is easier to integrate with LabVIEW as it does not require a separate microcontroller to process data. The output from these sensors is generally just a DC voltage from an ADC with a linear response over the specific humidity range, allowing easy conversion between the output signal and the real value of humidity.

Sensiron, SHTC1

The SHTC1 is a combined humidity/temperature sensor comes packaged as a surface-mounted IC with a compact footprint. Although it is not the right choice for extremely accurate humidity measurements, its price is low enough that it can appear in high volume consumer applications. This sensor supports fast I2C digital communication with up to 400 kHz SCL clock frequency. Make sure to decouple the power supply and ground pins with a 100 nF capacitor. Sensiron also sells an evaluation board for this component.

The SHTC1...is strictly designed to overcome conventional limits for size, power consumption, and performance to price ratio in order to fulfill current and future requirements. Sensirion’s CMOSens^® technology offers a complete sensor system on a single chip, consisting of a capacitive humidity sensor, a bandgap temperature sensor, analog and digital signal processing, A/D converter, calibration data memory, and a digital communication interface supporting I2C fast mode.

Typical application circuit from page 6 of the Sensiron SHTC1 datasheet

Texas Instruments, HDC1080

The HDC1080 provides highly accurate humidity and temperature measurements with low power consumption in a 6-pin IC package. If power consumption in your system is a concern, this sensor will run at about 100 uA while active and from 100 to 200 nA in sleep mode. With 8, 11, or 14-bit serial communication via I2C, this sensor provides high resolution measurements. However, there is a risk of address collisions if this sensor is integrated into a large network.

One of the key features of the HDC1080 is its low power consumption, which makes the device suitable in battery or power harvesting applications. In these applications the HDC1080 spends most of the time in sleep mode: with a typical 100nA of current consumption in sleep mode, the averaged current consumption is minimal. Its low consumption in measurement mode minimizes any self-heating.

Found on page 9 of HDC1080 datasheet

Adafruit Industries, BME680

One extremely versatile combination sensor is the Adafruit BME680, which combines humidity, temperature, VOC gases, and barometric pressure sensing on a single board. This board has built-in I2C and SPI communication options, small form factor, and can be wired to any microcontroller, including Arduino. You can use I2C communication up to 3.4 MHz or SPI (3 or 4 wire) up to 10 MHz to avoid address collisions in systems with a large number of sensors.

Adafruit has made it easy to control this board in Python or with the CircuitPython BME680 module. This module allows you to easily write Python code that reads the humidity, temperature, VOC concentration, and pressure from the sensor. You can even use pressure readings to calculate altitude by comparing the measured pressure with that at sea level. This sensor can be used with any CircuitPython microcontroller board.

The BME680...contains a small MOX sensor. The heated metal oxide changes resistance based on the volatile organic compounds (VOC) in the air, so it can be used to detect gasses & alcohols such as Ethanol, Alcohol and Carbon Monoxide and perform air quality measurements. Note it will give you one resistance value, with overall VOC content, it cannot differentiate gasses or alcohols.

Found on page 16 of the 2652 datasheet

Using a digital humidity sensor that is combined with other features can provide accurate temperature and gas measurements with a minimal footprint. Whether you are incorporating a small digital sensor on a breakout board or you want to use an environmental probe that provides many measurements with a single package, you’ll find plenty of options on Octopart.

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