Near-Infrared Moisture Measurement Principle
The principle of infrared light absorption: The molecular structure inside a substance, such as the oxygen-hydrogen bonds in water and the carbon-hydrogen bonds in organic matter, absorbs near-infrared light at specific wavelengths. At a specific wavelength, the energy of the reflected near-infrared light is inversely proportional to the number of molecules that absorb near-infrared light. Moisture meters analyze the change in near-infrared energy at a specific wavelength based on the principle that near-infrared wavelengths are absorbed by water molecules.
Water molecules are not static: when they encounter a specific energy band, they vibrate. The bonds between the two hydrogen atoms and the oxygen atom in a water molecule stretch, contract, or twist in other ways. External energy is needed to cause these vibrations, and the required energy spans specific bands across the entire electromagnetic spectrum.
In different parts of the spectrum, some absorption bands are very strong, while others are very weak. In the near-infrared part of the spectrum, this band is particularly strong for water molecules, and the instrument is easier to implement in terms of emitting, filtering, and receiving this energy. Specific wavelengths of near-infrared light energy are used to provide the appropriate amount of energy to the moisture in the tested product. The wavelengths generally used for moisture measurement are in the range of 1 to 2.5 micrometers. The amount of energy absorbed at a specific wavelength depends on the number of water molecules encountered by the near-infrared energy beam and the absorption intensity at that specific wavelength. The number of water molecules encountered by the energy beam is directly proportional to the moisture content in the measured substance. Since the moisture meter uses a reflectance ratio, the measured light beam is also affected by the reflection and absorption characteristics of the measured substance. Near-infrared moisture measurement technology is a non-destructive, non-contact, real-time moisture detection technology.
The online infrared moisture meter focuses multiple wavelengths of near-infrared light beams onto the surface of the measured object. The near-infrared light beams reflected from the surface are received and processed by an advanced infrared optical detection system. This advanced infrared optical detection system can process data quickly. An embedded high-performance miniature digital signal processing chip is used to process, store, and display the required data, with outstanding accuracy and stability.
The online infrared moisture meter uses precision infrared filters mounted on a rotating wheel. This setup allows the reference light and the measurement light pulses to pass alternately through a filter. The retained light beams are focused onto the sample under test. First, the reference light is projected onto the sample, and then the measurement light is projected onto the sample. These two temporally separated light energy pulses are reflected back to a detector, which converts them into two electrical signals. The reference channel represents the expected amount of reflected energy. The measurement channel is partially absorbed by water molecules, so its energy will be attenuated. These two signals are combined to form a ratio. The difference between this ratio and other ratios obtained from samples with different water content is directly proportional to the difference in water content between the two.
The signal intensities of the two channels are combined into the following equation: R / M x M1 / R1 = Ratio
Where R represents the reference channel of the product beam, M represents the measurement channel of the product beam, M1 represents the measurement channel of the compensation beam, and R1 represents the reference channel of the compensation beam.
If a trigger drift occurs, such as a change in the intensity of the light at the measurement wavelength, the M and M1 values will be affected proportionally, and the result will be canceled out in the ratio. However, if the amount of water in the sample changes, only the M value is affected, resulting in a significant change in the moisture content.
Instrument Features
1. Near-infrared non-contact testing, real-time display of moisture content changes on the production line, no pre-treatment of samples required. Samples in enclosed spaces can also be tested through colorless glass (transmittance greater than 95%), ensuring the integrity of the sample and compliance with testing environment requirements.
2. Simple instrument operation. After installation at the required testing location, take a sample near the testing point and perform a standard laboratory method test for moisture content. Simply compare the standard method test data with the instrument data, adjust the offset and sensitivity parameters to complete the calibration, and then start operation.
3. Strong anti-interference. The probe has an automatic temperature compensation function, so changes in ambient temperature do not significantly affect measurement accuracy; the instrument performance is not affected by various changes in ambient light, and no light shield is required on the probe during use; a standard sintered filter dust cover is included, and connecting an air source can eliminate interference from trace dust and water vapor.
4. The probe features a built-in MCU and utilizes fully digital processing, resulting in strong optical signal processing capabilities and improved detection of light-absorbing materials such as black objects. The addition of visible light signal measurement significantly reduces the impact of color variations, making it more adaptable to complex testing environments.
5. Diverse input and output interfaces: Free protocol/MODBUS-RTU (read/write, supports remote parameter modification and calibration); Rs485 (three channels), DC 4-20mA (one channel), IO input/output, 2 inputs (input stop signal to halt measurement), 2 x 24V outputs, with optional alarm light.
6. Stable and reliable: Features a waterproof power supply module, aluminum alloy casing, IP67 rating, and AC 90-220V 50/60Hz global voltage adaptability. The probe is powered by DC 24V, providing safer and more stable low-voltage power, effectively reducing failure rates. The light source is powered by a regulated DC 5V supply, with a gradual voltage increase upon power-on and protection for the light source during power-on and power-off.
Application Scope
The instrument is widely used in the textile industry, tobacco industry, chemical industry, coal industry, timber industry, paper industry, food processing, feed industry, ceramic industry, glass industry, cement industry, and many others.
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