Turner M J, Culbert S
Department of Electrical Engineering, University of the Witwatersrand, South Africa.
Physiol Meas. 1993 Aug;14(3):317-26. doi: 10.1088/0967-3334/14/3/010.
The dynamic characteristics of gas analysers are often assessed by measuring the step response. It is difficult to generate a verifiable instantaneous step change in gas composition. We constructed a 0.06 ml measurement chamber connected via high-speed valves (0.5 ms response time) to two 31 reservoirs pressurized to 50 kPa with gases containing different concentrations of CO2. An electronic system opens the valves alternately depending on the polarity of a control voltage Vc. Two walls of the chamber contain narrow-band infra-red filters centered at 4.24 microns (50% transmission points at 4.16 and 4.32 microns) where CO2 absorption is high. A photoconductive infra-red sensor and an infra-red source are positioned on either side of the chamber. The output of the sensor is amplified by an instrumentation amplifier. Signal averaging of the sensor output in either the time or frequency domain was used to overcome the noise of the infra-red sensor. Step changes in Vc yielded exponentially changing outputs with a time constant of 1.1 ms. A quadrupole mass spectrometer's response to step changes in CO2 concentration generated in the measurement chamber fitted single exponential curves well with a maximum time constant of 37.7 ms and transport delay of 194 ms. The frequency response of the infra-red system, from Vc to the sensor output, fell by 0.7 dB with a phase lag of 30 degrees between 1 and 50 Hz. Using the infra-red system to measure the true input to the mass spectrometer, the frequency response of the mass spectrometer was found to fall by 35 dB with a phase lag of over 3000 degrees between 0.2 and 50 Hz. A first-order model with delay fitted to the step response predicted the mass spectrometer frequency response well below 10 Hz but overestimated the response above 10 Hz. A third-order model with delay fitted to the frequency response predicted the step response very well. Our results suggest that low-order models cannot predict the high-frequency performance of a mass spectrometer.
气体分析仪的动态特性通常通过测量阶跃响应来评估。在气体成分中产生可验证的瞬时阶跃变化是困难的。我们构建了一个0.06毫升的测量室，通过高速阀（响应时间为0.5毫秒）连接到两个31升的储气罐，储气罐中充有不同浓度二氧化碳的气体，压力为50千帕。一个电子系统根据控制电压Vc的极性交替打开阀门。测量室的两壁包含以4.24微米为中心的窄带红外滤光片（在4.16和4.32微米处的50%透过点），此处二氧化碳吸收很强。一个光电导红外传感器和一个红外源位于测量室的两侧。传感器的输出由一个仪表放大器放大。在时域或频域对传感器输出进行信号平均，以克服红外传感器的噪声。Vc的阶跃变化产生指数变化的输出，时间常数为1.1毫秒。四极质谱仪对测量室中二氧化碳浓度阶跃变化的响应，生成的单指数曲线拟合良好，最大时间常数为37.7毫秒，传输延迟为194毫秒。从Vc到传感器输出的红外系统的频率响应，在1至50赫兹之间下降了0.7分贝，相位滞后30度。使用红外系统测量质谱仪的真实输入，发现质谱仪的频率响应在0.2至50赫兹之间下降了35分贝，相位滞后超过3000度。拟合阶跃响应的带延迟的一阶模型在10赫兹以下能很好地预测质谱仪的频率响应，但在10赫兹以上高估了响应。拟合频率响应的带延迟的三阶模型能很好地预测阶跃响应。我们的结果表明，低阶模型无法预测质谱仪的高频性能。
