A sensor device and method for tightening and positioning the sensor device with a wristband or strap (62) for a range of wrist sizes and shapes or other body portions, such that the optimal sensor position is obtained and maintained during ambulatory use, which provides two-sided fastening of the strap (62). The strap (62) has a marker (67) that is positioned by the wearer at a predetermined measuring position, e.g., on the middle of the volar side of the wrist. The wrist is then put on the table to fix its position, and the strap (62) is tightened on both sides of the wrist. This ensures a precision of at least 5 mm of the position of the sensors (66) in the strap.
