Continuous Body Monitoring - Ambient Intelligence for Scientific Discovery - page 209

Fig. 5. Sensor positions on the back and side of SenseWear. The GSR sensors are a
pair positioned on the underside of the unit, in contact with the skin. The heat flux
sensor has a skin contact component on the underside and a cover component on the
side of the unit.
between the 20 and 40 ◦ C range (68 ◦ F-104 ◦ F). At room temperature the reso-
lution is 0.02 ◦ C. Typically minute averages in temperature show small changes
from minute to minute. Subtle physiological effects can be seen in temperature
at higher sampling rates. The temperature sensor is recording the skin tempera-
ture, thus changes in the local thermal environment can also cause this reading
to change indirectly by causing changes in skin temperature.
3.2 Derived Measures - Synthetic Sensors
The SenseWear armband samples all of the raw sensors at 32 Hz. From these sig-
nals, many other signals are calculated and derived. In many SenseWear systems
a set of derived channels are then recorded at a much lower rate, such as once per
minute. These synthetic sensors can include simple metrics such as averages and
summations, but can also include complex computations such as activity classi-
fication and energy expenditure. In SenseWear systems these synthetic sensors
appear as additional sensor data streams.
Energy Expenditure (EE). The medical standard for measuring caloric ex-
penditure is the VO2 metabolic cart. But these machines tend to be large,
lab-restricted devices that require a technician's supervision. The SenseWear
armband was designed to provide results comparable to a metabolic cart but
in a small wearable form factor, invisible to observers, usable in a free-living
environment, easier to put on and as comfortable as a wristwatch (see Fig. 6).