The core components of the online near-infrared analysis system include probes, optical fibers, and flow cells. Its design and selection need to be closely combined with the requirements of the application scenario. The following analysis is conducted from three aspects: functional characteristics, selection principles, and maintenance points:
1、 Probe design: "perception terminal" adapted to the scene
The probe needs to be customized according to the material state (solid/liquid/gas) and process conditions (temperature/pressure/corrosiveness). For example, in grain and oil processing, contact probes can be installed on the mixer housing or conveying pipeline to monitor the protein content of flour through the principle of diffuse reflection. The samples passing through the optical window are averaged within the integration time to improve data representativeness; Non contact probes are suitable for high temperature and corrosive environments, such as monitoring semiconductor cleaning solutions. In situ analysis is achieved through polytetrafluoroethylene tubes to avoid probe contamination. In addition, the probe needs to be matched with the light source power, fiber coupling efficiency, and receiving angle. For example, an integrated light source probe design can be used to achieve a 1200W/m ² irradiance at a detection distance of 2 meters, ensuring signal strength.
2、 Fiber selection: the "high-speed channel" for signal transmission
Fiber optic cables need to meet the requirements of long-distance and low loss transmission. Quartz fiber has excellent transmittance in the 700-2500nm wavelength range, and the selection of core diameter needs to balance resolution and signal capture capability: a small core diameter (such as 50 μ m) improves spatial resolution and is suitable for high-precision detection; A large core diameter (such as 400 μ m) enhances signal strength and is suitable for analysis of low concentration samples. The numerical aperture (NA) affects the range of optical signal acquisition, and high NA (0.37) optical fibers can receive light at larger angles, improving detection sensitivity. In addition, optical fibers need to have anti bending properties, with a minimum bending radius of ≥ 10 times the diameter of the fiber to avoid optical transmission distortion.
3、 Flow pool design: a "precision container" for liquid detection
The flow cell is suitable for online monitoring of liquid samples, and its design needs to take into account optical path length, material, and anti-interference ability. The selection of optical path should ensure that the sample absorbance is within the linear range of the instrument (0.3-1.0A). For example, for high concentration syrup, a short optical path (0.5-2mm) is used to avoid saturation, while for low concentration drinking water, a long optical path (5-10mm) is used to enhance the signal. In terms of material, quartz or sapphire flow cells can withstand high temperatures and pressures, making them suitable for monitoring chemical slurries; The PTFE flow cell is used for the analysis of highly corrosive liquids, such as alkaline solutions in alkaline washing towers. In addition, the circulation pool needs to integrate anti bubble mechanisms and constant temperature sleeves to avoid interference from bubble scattering and temperature fluctuations, such as controlling temperature fluctuations within ± 1 ℃ through a PID temperature control system.
4、 Core principles of selection and maintenance
Scene adaptability: Select probe type (contact/non-contact), fiber parameters (core diameter/NA), and flow cell structure (optical path/material) based on material status, process conditions, and detection requirements.
System compatibility: Ensure that the light source power, detector sensitivity, and software algorithm of the probe, fiber, and spectrometer are matched, such as using a symmetrical C-T optical path structure grating spectrometer to improve integration time and signal stability.
Maintenance convenience: Design self-cleaning function (such as air blowing cleaning) or easy to disassemble structure to reduce "paste powder" pollution; Regularly check the coupling degree of the fiber optic interface and align it with the optical path to avoid signal attenuation.
By accurately matching the core parameters of probes, fibers, and flow cells, the data accuracy and stability of online near-infrared analysis systems can be significantly improved, providing reliable support for industrial process control.