Methods of Measuring Blood Velocity
The hot-film anemometer is based on heat transfer from a small body placed in the fluid flow. This is a very small metal film which is sputtered or burned on a substrate or a mount. This small film is connected to a bridge circuit (Fig. 13.1) for the generation of heat by an electrical current. A servoamplifier feeds back the error voltage of the bridge circuit, so that the electrical resistance, which is a function of the film temperature, is kept constant. This type of electrical equipment is called a constant temperature anemometer (CTA). If heat loss from the film increases, due to the high velocity of the surrounding fluid, the film temperature falls and the electrical resistance decreases, which leads to the increase of error voltage of the bridge. In this situation, the feedback current or output voltage of the servoamplifier becomes a nonlinear function of the fluid velocity. The metal film should be covered by a thin layer of insulating material, usually quartz, which does not interfere significantly with the heat transfer.
KeywordsBlood Flow Velocity Doppler Frequency Blood Velocity Very Large Scale Integrate Electrical Output
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