램프 온도 추적 기반의 NDIR형 Capnography
NDIR type capnography based on lamp temperature tracking method
원격 감지기술 램프 온도 NDIR형;
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In most hospitals, capnograph systems have been widely used for surgeons and anesthetists to monitor the variation of CO_(2) gas concentration in the patient's exhaled gas and get useful information to determine patients' status and various diseases of human respiratory organ, or to detect a failure of ventilation systems. In majority of conventional capnograph systems based on the non-dispersive infrared (NDIR) absorption method, the tungsten filament lamps have been used as IR sources and they are required to have a stable IR emission for the accurate measurement of CO_(2) concentration. However, the variation of lamp driving current causes the distortion of sensor output due to the unstable radiant intensity of the lamp. And the IR radiation intensity of the lamp, as activating the lamp for a long time, is changed and causes the unwanted variation of sensor output due to the lamp degradation. As a typical way of compensating the variation of IR radiation intensity, calculating the ratio of the sensor output reduced by CO_(2) gas to the reference sensor output with no reduction by CO_(2) gas is used. To get the reference sensor output signal, the several methods using optical filters on mechanical chopper wheel, dual channel sensors, or auxiliary lamps have been utilized. But the methods increase the complexity of a chamber structure and can cause the sensor output errors by the different degree of contamination among the multiple optical filters or lamps. In this paper, we propose a NDIR type capnography based on lamp temperature tracking for acquiring reference signal without any reference optical filter or auxiliary lamp. Temperature tracking of our proposed NDIR type capnography makes use of the characteristics between resistance and temperature of tungsten filament lamp. As the temperature of tungsten filament is increased by the supplied electrical power, the resistance of filament is also increased. By using the relationship, the temperature of lamp can be calculated by measuring the resistance of a filament. And the radiation intensity of 4.26 ㎛ wavelength can be calculated according to the Planck's radiation law using the computed lamp temperature. After the relationship between the computed radiation intensity and the sensor output signal is obtained for N_(2) gas or CO_(2) free gas in advance, the reference output signal can be generated by the relationship and the radiation intensity calculated from the lamp resistance. Finally, the ratio of measured sensor output signal to the generated reference output signal represents the compensated sensor output signal without the distortion due to the fluctuation of radiation intensity. The proposed method has the advantage of creating the reference output signal from the simple structure of chamber composed of one lamp and one sensor without a reference optical filter or an auxiliary lamp. Since the created reference signal is not affected by any other gases including CO_(2) gas, the reference signal is the function of only radiation intensity. Also, the aging process of a lamp can be detected by observing the resistance at operating points of lamp driving current. To verify the proposed method, NDIR type optical chamber and CO_(2) measuring circuit based on lamp temperature tracking have been implemented. The experiment for compensating the variation of lamp radiation intensity has been performed by changing a lamp driving current on purpose. When the measured sensor output for 5% CO_(2) concentration shows the error of 12.5% by the alternation of a lamp driving current, the ratio of the sensor output to the reference signal created by the proposed method shows the error of 2.1%. Hence, it is verified that the proposed NDIR type capnography on the basis of the lamp temperature tracking method is possible to measure CO_(2) gas concentration precisely with compensating the variation of a lamp radiation intensity and detecting a lamp aging without a mechanical rotating wheel, a dual channel sensor, or auxiliary lamps.