Humidity and Temperature Sensors Widen Potential for Wearable Devices

Author: Pascal Gerner, Head of Product Management

Wearables are attracting increasing attention in the market, miniature computers that are worn as clothes or accessories, and what make them smart are the integrated sensors. With environmental sensing, these devices can also be made aware of their environment. Sensors are able to measure relative humidity and temperature (RH/T) to enable devices and their users to understand their ambient condition and what is going on around them. The main advantages of these sensors are that they are very energy efficient, do not require lots of computation power and are becoming smaller and smaller. Hence, they are ideal for wearables and offer a broad range of potentially valuable applications.

Integrating ambient sensors into wearable devices like Smartwatches is a non-trivial undertaking. This is especially true for ambient temperature sensors and all measures dependent on ambient temperature such as humidity. Temperature sensors built in a wearable or mobile device face three major challenges.

First, the electronic components in the tightly packed device generate heat and influence the sensor reading. This effect gets even worse as the heat dissipation of the different components is highly load dependent and therefore is changing all the time. Second, the sensor readings are influenced by the heat of the skin. Third, the device has a certain thermal mass which results in a slow thermal response. Similar to the fact that a hot cup of coffee needs about 30 minutes to cool down to room temperature a smart watch or phone needs about 30 minutes to adopt to temperature changes.

There are multiple measures to mitigate or even eliminate this obstacle. One of the most crucial parts is the placement of the sensors. It is important that they are very well decoupled from the main device-internal heat sources and the human skin. The placement of the sensor is highly device specific and has to be well considered for every product independently. An ideal sensor placement, however, is not enough as a complete decoupling is most likely not feasible. To compensate for the remaining influences, the influence factors have to be monitored and their impact on the temperature reading has to be determined. For example to compensate for the body heat, an additional sensor can be placed closely to the skin. A heat-propagation model can then be applied to estimate how big the temperature gain of the body heat on the sensor is. This information allows for compensation. This environmental sensor fusion software called Sensirion Engine is already actively used in several smart phones enabling accurate ambient temperature and humidity sensing. Further the Sensirion Engine allows to speed up the response of the temperature and humidity signal to ambient changes far beyond the physical limits. Which is necessary as no user wants to wait up to 30 minutes to get accurate readings. Combining all these approaches allow wearable devices an accurate measurement of ambient temperature and humidity by providing an experience users expect from wearable devices.