A second device functionality that enables the simple, compact circuit design is the use of temperature readings to control device functions. As shown in the program flow diagram (Fig 3), data logging is started from the initial device sleep state when the measured thermistor voltage is above a reference value set programmatically. A reference corresponding to 50°C was used to initiate logging. Logging may be started by simply placing the device in a cup of hot water for approximately 1 minute. Water is used here as a convenience, and any method of raising the device temperature to 50°C or more would work equally well. Similarly, logging is stopped when the thermistor voltage is below a programmatically set “stop” threshold, easily achieved by exposing the device to a low temperature such as a cup of ice water or a refrigerator. A simpler method of stopping logging that may be more useful in a trial setting is to simply cut the IVR elastomer to sever the battery wires and remove device power. The stored data is retained in non-volatile memory in the unpowered device and may be retrieved through the program/data interface as described above.