Definition
An inline digital refractometer is an industrial process analysis instrument designed based on the principle of optical refraction, which is used to continuously measure the physical quantity of dissolved solids in liquids in real time. It directly outputs parameters such as refractive index, Brix, and concentration through digital display, and is widely used in online quality control and process optimization processes in food and beverage, chemical industry, environmental monitoring, and other fields.
Measurement principle
The core principle of the online digital refractometer is based on Snell's law of refraction. When light is obliquely injected from one medium into another, its propagation direction will be deflected at the interface, and the refractive index is related to the ratio of the sine value of the angle of incidence and the sine value of the refractive angle, which can be expressed as:
n = sin(θ₁) / sin(θ₂)
where n is the relative refractive index, θ₁ is the angle of incidence, and θ₂ is the angle of refraction. The instrument calculates the refractive index of the sample by measuring the critical angle or the change in the intensity of the reflected light, and then converts the concentration value of the component to be measured according to the pre-established correspondence model between the refractive index and the concentration.
Measurement method
Inline digital refractometers are usually installed in a flow-through type so that the liquid under test flows continuously through the measuring prism surface of the instrument. The light emitted by the light source inside the instrument is collimated by the lens and hits the interface between the prism and the sample at a specific angle. The detector array receives the reflected light signal, analyzes the light intensity distribution through the signal processing system, and determines the critical angle position. The microprocessor converts the refractive index data into a concentration reading based on the calibration curve and outputs a standard signal to the control system. The measurement process enables continuous and uninterrupted monitoring, with response times typically within seconds.
Influencing factors
Measurement accuracy is affected by several factors. Temperature changes can change the liquid density and molecular arrangement, which can affect the refractive index, so instruments are often equipped with temperature sensors and automatic compensation functions. Air bubbles or solid particles in the sample can interfere with the optical path, requiring proper filtration or degassing pretreatment. The stability of the flow rate affects the uniformity of the liquid film on the surface of the prism, and it is recommended to keep the flow path stable. Different substances may have similar refractive indices, and specificity issues need to be paid attention to in complex mixtures. Contamination of the optical window can degrade signal quality, and regular cleaning and maintenance can help maintain measurement reliability.
Applications
In the food industry, the instrument is used to monitor juice concentration, syrup concentration, dairy solids content. In the process of chemical production, solvent concentration, pH changes, and reaction processes can be tracked. In the field of environmental protection, it is used to monitor the total amount of dissolved solids in wastewater and the circulating concentration of cooling water. In addition, it is also of practical value in scenarios such as cleaning process verification, evaporation crystallization control, and mixing ratio automation. Its non-contact or micro-intrusive measurement characteristics make it suitable for applications with high hygiene requirements or corrosive media.
Selection considerations
When selecting a model, it is necessary to clarify the measurement range and accuracy requirements, and different models correspond to different refractive index ranges and resolutions. Consider the temperature and pressure conditions of the process to confirm the environmental adaptability of the instrument. The interface material needs to be compatible with the medium under test, and the common ones are stainless steel, Hastel alloy or coated sapphire prisms. The output signal type should match the existing control system, such as analog current signals or digital communication protocols. The installation method should be combined with the pipe size and flow direction design to ensure representative sampling. Ease of maintenance, calibration intervals, and long-term stability are also important points for evaluation. It is recommended to refer to relevant industry standards, such as food machinery safety requirements or chemical explosion-proof specifications, for compliance selection.