To enable us to measure skin temperatures in our subjects 24/7 for up to a year we currently make use of custom wristbands with Blue Maestro’s unique “Tempo Disc”. The Blue Maestro “Tempo Disc” is a smart temperature sensor and data logger based on the nRF51822 Bluetooth Smart chip. The Disc transmits both skin and environmental temperature information via Bluetooth Low Energy (BLE) to a custom App loaded on a user’s Android smartphone. The waterproof device has an IP67 rating and features a replaceable 2032 coin cell battery with an expected one-year operational life. The dimensions of the device are 33×9.5mm, it is accurate to +/- .05 degrees Celsius and has a range of up to 40 meters line of sight
Blue Maestro has taken several measures to guarantee the Disc’s long operational life: the Disc’s BLE update frequency is lowered to one broadcast per second to conserve energy and Blue Maestro has embedded the live online temperature data in its broadcast (Advertisement packet) message – a use of BLE far more efficient than the official standard allows for. In practice, this means an application can read out the temperature data without having to explicitly connect to the device. Not having to connect saves a lot of current (the Disc maintains an average run current of less than 10 uA), which enables the Disc to achieve long operational life on such a small battery.
Additionally, Blue Maestro has customized the Disc specifically for our purposes in several ways. Firstly, they by replaced its default cover for one made of a different polycarbonate plastic that is more responsive to the small changes in temperature that we are interested in. Secondly, they made some changes in the firmware to ameliorate the Disc’s sensitivity in 10-40 Celsius degree range. And thirdly, they activated an additional thermometer on top of device, as to allow us to measure environmental temperatures. We have furthermore been granted access to JAVA Android source code, enabling us to incorporate the device in our own custom build Android apps.