Current mainstream products in the moisture meter industry: chemical moisture analyzer (Karl Fischer moisture analyzer) and physical moisture analyzer (infrared moisture analyzer). Karl Fischer moisture meter is divided into Karl Fischer volume method and Karl Fischer coulometric method; infrared moisture meter is divided into infrared incandescent lamp moisture meter and halogen ring lamp moisture meter, the following are the characteristics of each moisture meter:
A: Chemical moisture analyzer
1) The principle of Karl Fischer volumetric moisture determination
The Karl Fischer volumetric method is mainly based on the electrochemical reaction when determining the moisture content:
I2 + 2eó2I-
When I2 and I- coexist in the solution of the reaction cell, the reaction proceeds simultaneously at the positive and negative ends of the electrode, that is, I2 is reduced on one electrode and I- is oxidized on the other electrode, so Current flows between the electrodes. If there is only I- and no I2 in the solution, no current flows between the two electrodes.
The Karl Fischer reagent contains substances such as pyridine and iodine, which can be metered into the reaction cell and react with the water in the solution to be tested as follows:
H2O + SO2 + I2 + 3C5H5N → 2C5H5N • HI + C5H5N • SO3
C5H5N • SO3 CH3OH → C5H5N • HSO4CH3
C5H5N • HI → C5H5N • HI-
The reaction continues and consumes water continuously to generate I-, which is consumed until the end of the reaction titration. At this time, only a small amount of unreacted Karl Fischer reagent exists in the solution, and then I2 and I-exist at the same time. The solution between the two platinum electrodes begins to conduct electricity, and the end point is indicated by the current, and the titration is stopped. Therefore, the water content in the solution is calibrated by measuring the volume (capacity) of the Karl Fischer reagent consumed.
(2) Measuring principle of Karl Fischer Coulomb method (electricity method)
The galvanometric method is based on dissolving the sample in an electrolyte containing a special solvent containing iodine. Water consumes iodine, but the required iodine is no longer titrated with calibrated iodine-containing reagents, but through electrolysis The process of oxidizing the iodide ions in the solution to iodine:
2I¯—2e─ → I2
The generated iodine reacts with the water in the sample. The end point is indicated by double platinum electrodes. When the concentration of iodine in the electrolyte returns to the original concentration, the electrolysis is stopped. Then according to Faraday's law of electrolysis:
CMi • t
m = --------- × ----- = ---------
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Calculate the moisture content of the sample to be tested.
B: Physical moisture analyzer
[About infrared bulb] Infrared bulb is composed of fused quartz tube and radiating element. Infrared bulb is mainly used for industrial heating or drying, such as glass products, metal parts, circuit board packaging, film, etc., and other drying and drying , Heating, quartz near-infrared and far-infrared lamps use transparent or translucent quartz glass as the lamp shell to produce near-infrared radiation lines or far-infrared radiation lines. It is a typical ancient heating method. We also use ultraviolet lamps to treat certain For some skin diseases or indoors, you can get bronze skin that can only be obtained by exposure to the sun.
In addition, the main component of the physical moisture analyzer is the weighing sensor. The term of the weighing sensor is the heart of the moisture meter. The accuracy of the weighing sensor is directly related to the stability and accuracy of the measurement of the moisture meter.
[Strain sensor] Strain sensor is a sensor based on measuring the strain generated by the deformation of the object under force. Attach the strain gauge to the cantilever beam (elastic element). When the measured object moves, the measuring rod moves, and the tension spring stretches to deform the cantilever beam, which causes the resistance of the strain gauge to change. This method can be used to measure the force. , Displacement, pressure, acceleration and other physical parameters. Its characteristics: simple structure, low cost, light weight, but limited accuracy, it can not achieve very high precision at present;
[Electromagnetic balance sensor] The working principle is to measure the mass of the measured object without measuring the weight of the weight, but by using the principle that the electromagnetic force is balanced with the gravity of the measured object. The weighing pan is connected to the coil through the bracket connecting rod. Within the weighing range, when the weight of the measured weight acts on the coil through the connecting rod bracket, the direction is upward. At this time, if a current passes through the magnetic field, the coil will generate an electromagnetic force
It is characterized by accurate weighing, reliable accuracy, stable performance, strong repeatability, fast and clear display, and has an automatic detection system, a simple automatic calibration device and overload protection devices.
The ring-shaped halogen lamp ensures that the sample is heated evenly, with easy operation and accurate measurement. The biggest difference between the halogen ring lamp and the infrared incandescent lamp is that the halogen lamp glass is filled with some halogen gas (usually iodine or bromine). Its advantages are clean, no accessories, easy installation, and temperature control. . Its working principle is: when the filament heats, the tungsten atoms are evaporated and move to the direction of the glass tube wall. When approaching the glass tube wall, the tungsten vapor is cooled to about 800 ℃ and combined with halogen atoms to form tungsten halide (iodine) Tungsten or tungsten bromide). The tungsten halide continues to move toward the center of the glass tube, and then returns to the oxidized filament. When it is heated, it will be decomposed into halogen vapor and tungsten again, so that tungsten is deposited on the filament to make up for the evaporated part. Through this regeneration cycle, the service life of the filament is not only greatly extended (almost 4 times that of infrared incandescent lamps)
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