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noninvasive sensing requires that the sensor be separated from the target of interest and remain in the measurement volume for a time in order to adequately collect data. heart rate monitoring is an example of an application requiring both sensor placement and separation. noninvasive sensors have a lower degree of invasiveness than invasive sensors, but more time is required for signal collection and, if not properly designed, the signal may require some sort of amplification. infrasound and ultrasound rely on sound pressure waves to travel through air or tissue. sound pressure waves may be propagated through the medium with or without coupling to the surface. pulse oximeters create a waveform from arterial blood signals to measure oxygen saturation levels, which are then displayed on a monitor. electrocardiographic monitors measure the electrical signals that result from changes to the heart's ion channels. electromyographic (emg) monitors use electrodes to measure the electrical signal present in muscle. infrared light pulses are often used in this technology to measure body temperature or blood flow changes that may accompany a fall. light pulses may be used to measure oxygen saturation, blood flow, or artery blood pressure as well, using emissive or reflective technologies. strain gauge sensors are widely used in pressure measurement devices, such as foot-to-floor balance meters and the mosfet array that is part of the accelerometer inside many smartphones.
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mechanical sensors can measure strain, pressure, force, displacement, speed, or position through changes in stress and material response. unlike pressure sensors, displacement sensors may have no moving parts. electronic technologies are used to read the sensor signal, convert it to a physical change (a voltage or a measurable electrical component such as resistance or inductance), and convey the signal to a computer or other device. in many applications, a sensor acts as a switch that modifies the electrical circuit that it is part of. the signal can be processed through multiple stages of filtering to reduce noise and improve accuracy.