熱拡散等価法による熱電血流計のゼロ設定理論

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  • A Theory on Setting Zero-flow Level of Thermoelectric Flowmeter by Means of Compensation for Thermal Diffusion
  • ネツ カクサン トウカホウ ニ ヨル ネツデンケツリュウケイ ノ ゼロ セッテイ リロン

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For measurements of blood velocity with a thermoelectric blood flowmeter, various sources of errors, which result in the zero drift of the flow level. These include difficulty of correcting the measured results mainly attributable to the nonlinear relationships between the blood velocity and the measured output such as originated from thermal shift of the hot junction and/or of the heater current, some degradations of the sensor during use, and noise and thermal drift in the amplifier system. Since the majority of these errors resulted in the unstable drift of the zero velocity level during measurements, it is important to facilitate detection of the zero velocity level during measurements without stopping the blood movement around the sensor by mechanical compression, which sometimes can not be done successfully. In order to eliminate this zero level setting during experiments, a new technique is proposed in this paper, i.e., that of setting the isothermal atmosphere around the hot junction located in the outer guide sleeve as a heat sink. This new method of setting zero velocity level has been developed by pulling back temporarily the thermal sensor into the guide sleeve, where thermal diffusion is maintained constant by a specially designed thermal compensation layer placed inside the guide tube. While the sensor is kept within this compensation layer, the thermal diffusion around the sensor may be simulated as the zero velocity atmosphere. Theoretical analysis related to this new technique has been considered from the standpoint of the thermal diffusion theory, and the theoretically computed results indicated fairly good agreement with the results obtained by the in vitro experiments.

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