ActiGraph wGT3X-BT and wActiSleep-BT devices contain a metallic plate on the back of the battery near the cover which can be used to detect monitor removal when worn against the skin. At this time, this technique works best for wrist-worn devices.
There is no need to activate the wear sensor. Data is gathered continuously during normal usage after initialization. The wear sensor data can be consumed alongside all of ActiLife's built-in wear time validation algorithms using the new wear sense preview graphs. The wear sensor can detect short device removals (e.g., 10 minute long shower) whereas the built-in algorithms struggle with detecting non-wear periods shorter than an hour in length.
ActiGraph sells special wrist straps for the wGT3X-BT and wActiSleep-BT devices which ensure good contact with the skin for wrist-worn studies. Visit http://www.actigraphcorp.com/product-category/accessories/ for more details.