基于可调谐吸收光谱的畜禽舍氨气浓度检测

为开发一种基于可调谐吸收光谱(Tunable Diode Laser Absorption Spectroscopy,TDLAS)技术的畜禽舍NH_3浓度实时在线监测装置,以满足畜禽舍环境监测与控制的需要。该研究基于TDLAS技术,采用气室式封闭光程,搭建了一套畜禽舍NH_3浓度检测系统。该系统采用波长为1 512 nm蝶形激光器作为光源,根据分子吸收光谱理论,采用波长调制技术,实现了对畜禽舍NH_3浓度检测。为优化检测系统性能,通过改变锯齿扫描信号、调制正弦信号的幅值与频率以及输入信号与参考信号相位差,确定了系统最佳的调制参数,并通过系统优化试验确定了系统最佳的气室加热温度、系统响应时间与二次谐波平均次数等关键参数。最后,通过浓度标定试验与性能试验对检测系统进行了测试。试验结果表明,检测系统调制参数在正弦调制信号频率为9 kHz、正弦调制信号幅值为30 mV、锯齿扫描信号频率为1 Hz、锯齿扫描范围为170～215 mV、谐波分析中输入信号与参考信号相位差为50°参数下对应的二次谐波形状与幅值最佳;不同浓度NH_3与二次谐波幅值之间具有良好的线性关系(拟合方程相关系数r=0.995 8);检测系统的进气响应时间约为42 s(气室自充气达到目标浓度99%);气室加热温度为403K时,NH_3在气室吸附作用最小;Allan方差分析表明,检测系统在积分时间为10 s时达到探测限,探测限为0.038 mg/m~3。在最优系统参数下对系统进行性能试验,得到检测系统综合线性误差为1.00%,定量测量综合重复误差为0.51%,可满足畜禽舍内NH_3浓度长期持续监测的需求。

Detection of ammonia concentration in livestock poultry houses based on tunable diode laser absorption spectroscopy

This study aims to develop an NH3 concentration monitoring system based on tunable diode laser absorption spectroscopy (TDLAS) for real-time and in-situ control the environment of livestock and poultry houses. In an air chamber, an optical path was attached to a butterfly laser with a wavelength of 1512 nm as the light source. NH3 concentration was then detected in livestock poultry houses based on the molecular absorption spectroscopy and wavelength modulation technology. To optimize the performance of TDLAS system, the optimal modulation parameters were determined via tailoring the amplitude and frequency of sawtooth scanning signal and the sinusoidal modulation signal, as well as the phase difference between the input and the reference signals. Moreover, the optimal parameters included the heating temperature in an air chamber, system response time, and the average number of second harmonics. Finally, the concentration calibration experiments were used to verify the performance of TDLAS system. The results showed that the optimal second harmonic shape and amplitude were obtained when the frequency and amplitude of sinusoidal modulation signal were set as 9 kHz and 30 mV, respectively, while the scanning frequency and range of sawtooth scanning signal as 1 Hz and 170~215 mV, respectively, as well the phase difference between the input signal and the reference signal as 50°. The concentration calibration test achieved for the standard gas (N2, NH3). There was a good linear relationship between the different concentrations of NH3 and the amplitude of second harmonic (the correlation coefficient of fitting equation r2= 0.9958). The response time of detection system was about 42 s, from the start of air chamber self-inflation to the time when the target concentration of 99% was reached. In the response test, the results showed that the appropriate increase in the flow rate of gas can effectively improve the detection efficiency of system. According to the temperature test, too high heating temperature has led to ammonia oxidation, whereas too low heating temperature cannot effectively limit ammonia adsorption in the gas chamber. The adsorption of NH3 reached the lowest level, when the heating temperature was set as 403 K in the chamber. According to Allan variance analysis, the TDLAS system reached the detection limit of 0.038 mg/m3 when the integration time was 10 s. The average number was set to 5 times, indicating a high accuracy during the stable period of detection system. Performance tests were performed on the system under the optimal system parameters, indicating a comprehensive linear error of 1.00%, and a quantitative comprehensive repeated error of 0.51%. In four concentration conditions, the accuracy and stability of detection system were close to that of the acousto-optic spectrum detection, and better than that of the electrochemical detection. The findings demonstrated that the developed system can present highly accurate detection and stable performance, and thereby to serve as the needs of long-term continuous monitoring of NH3 concentration in livestock houses.