The SIMV coating thickness gauge is a near-infrared photoelectroanalytical instrument that uses a fixed near-infrared wavelength to measure the water content of different products. It is an integrated instrument in which all the components necessary for continuous monitoring are placed and two components can be tested simultaneously (for example, the coating thickness of water glue and oil glue) depending on the wavelength.

【 Detection principle 】

　　The O-H bonds of water molecules absorb near-infrared radiation at several specific wavelengths. The absorption and non-absorption wavelengths generated by the three filters on the rotating filter disc in the Thermerfield SIMV tester are alternately reflected from the surface of the product to the internal optical components, and the electrical pulses converted from the light signal are calculated by a ratio algorithm to produce a proportional moisture content, which is then corrected by bias and span into a direct moisture content. The specially designed filter reflection signal algorithm eliminates the deviation caused by optical component aging and different material reflection characteristics.

　　Most substances have different NIR absorption peaks due to their composition, so that the appropriate wavelength can be selected to test different substances. For example, all organic matter absorbs 2300-2350nm and 1670-1750nm while water 【 absorbs 1940nm, as shown in the following table:

Chemical Structure	Typical Material	Absorption wavelength nm
O-H (Moisture)	Moisture	1450,1940
O-H (Ethanol)	Alcohol, Butanol, Carbohydrates	2100
C-H (Aliphatic)	Coatings, Plastics, Solvents	1720,2300
C-H (Aromatic)	Coatings, Plastics, Solvents	2150

NIR can only test substances with special absorption peaks, porcelain powder and whitening agents do not absorb infrared rays, there is no special absorption peak, so NIR technology is not applicable.

【 Coating type 】

　　There are usually 3 types of coatings:

　　1. Water-based coating: the concentration of the solution is between 10% and 60%.

　　2. Oily coating: the solvent is an organic solution, which is similar to the application of water-based coating, but it should be noted that there are certain dangerous components.

　　3. Extrusion and hot sol coating: coated on the substrate is a molten substance, the type of coating head with hot press die, heating tank and dipping tank.

【 Test method and advantages and disadvantages 】

　　Nir moisture and coating testers can be used in a variety of ways and can generally be classified as wet and dry testing.

　　Wet test: The test is located near the coating head, for water coating, the water content of the coating is tested, for oil coating, the solvent content of the coating is tested. For wet measurement, the coating is not directly tested but calculated from known concentrations.

　　Dry test: The test is located where water or other solvents have been dried and is the direct final coating test. This test can only be performed if the coating contains infrared absorbers, which are almost always C-H bonds containing organic matter.

Wet test	Advantage	1. The control speed is faster because the test position is close to the coating head; 2. The accuracy is usually higher because the water and solvent content of the solution is usually higher and fewer coating changes can be tested.
Wet test	Shortcoming	1. The concentration of the solution needs to be maintained to ensure the accuracy of the test. A little difference between batches will not affect the test, but the tester is required to calibrate each different concentration. 2. Oily solvents need to consider that the test head is suitable for hazardous environments, which increases the size and cost of the test head.
Dry test	Advantage	1. The actual coating amount is directly tested; 2. Less exposure to dangerous goods.
Dry test	Shortcoming	1. The coating must be infrared absorbent; 2. The test head is installed far away from the coating head, causing a delay to the control system.

【 Typical Application 】

Application Field	Coating Type	Test Mode	Detection Accuracy
Film substrate	Water-based adhesive coatin	Wet or dry test	Accuracy is ±0.2gm (0.12lb/rm)
Film substrate	Water-based emulsion coating	Wet test	Accuracy is ±0.15gm（0.09lb/rm）
Metal substrate	plastic coating	dry test	Accuracy is 0.1gm（0.06lb/rm）
Metal substrate	Wax coating	dry test	Accuracy is ±0.15gm（0.09lb/rm）
Paper substrate	Water-based and oil-based adhesive coatings	Wet or dry test	Accuracy is ±0.15gm(0.09lb/rm)
	Hot melt adhesive coating	dry test	Accuracy is ±0.2gm/(0.12lb/rm)
	Plastic emulsion coating	Wet or dry test	Accuracy is ±0.2gm（0.12lb/rm）
	Wax coating	dry test	Accuracy is ±0.2gm（0.12lb/rm）