1. Introduction
Insulating glass is a glass product that is evenly separated by two or more pieces of glass with effective support and sealed around the perimeter, so that a dry gas space is formed between the glass layers. With the development of modern architecture and curtain wall glass, Low-E insulating glass has been widely used in buildings, vehicles, refrigerators, and places that can create constant temperature, constant humidity, quiet and comfortable conditions for the interior. According to relevant statistics, the energy consumption of building doors, windows and curtain walls accounts for about 50% of the energy consumption of building envelope structures and about 25% of the total energy consumption of buildings. The heat transfer coefficient of insulating glass is an important basis for measuring whether the thermal performance of the building's external envelope meets the standard specifications, and the emissivity is an important factor affecting the heat transfer coefficient of insulating glass. How to accurately detect the emissivity of glass with the correct method plays a vital role in the calculation of the heat transfer coefficient of insulating glass.
2. The principle of emissivity detection equipment
According to "Coated Glass Part 2: Low-E Coated Glass", emissivity is the ratio of the radiant emittance of a thermal radiator to the radiant emittance of a Planck radiator at the same temperature. The emissivity of low-emissivity coated glass refers to the hemispherical emissivity of the film surface within the range of 293K and 4.5-25μm wavelength. There are three commonly used devices for detecting the emissivity of coated glass, namely Fourier infrared spectrometer, emissivity meter and surface resistance meter.
2.1 The principle of Fourier infrared spectrometer detection
The basic principle of Fourier infrared spectrometer is that the light emitted by the light source is divided into two beams by a beam splitter (similar to a semi-transparent and semi-reflective mirror), one beam reaches the moving mirror by transmission, and the other beam reaches the fixed mirror by reflection. The two beams of light are reflected by the fixed mirror and the moving mirror respectively and then return to the beam splitter. The moving mirror moves in a straight line at a constant speed, so the two beams of light after being split by the beam splitter form an optical path difference and produce interference. The interference light passes through the sample pool after the beam splitter meets. After the light passes through the sample, the interference light containing the sample information reaches the detector, and then the signal is processed by Fourier transform, and finally the infrared absorption spectrum of transmittance or absorbance with wave number or wavelength is obtained.
2.2 The principle of detection of radiation meter
The radiation meter is a compact handheld analytical instrument that uses a non-destructive technology to measure thermal emissivity in seconds. The device can be used for a variety of materials, from low emissivity to high emissivity, from smooth to textured surfaces. The sample is subjected to thermal radiation at a temperature of 100°C, and the sample is encapsulated by a black body hemispherical radiator to ensure uniform irradiation. The reflected infrared radiation is shot into the thermopile infrared sensor through the infrared lens at a certain angle and converted into a numerical value. Then, the emissivity value is plotted according to the calibration table of high emissivity and low emissivity reference standards.
2.3 The principle of detection of sheet resistivity meter
At present, there are two kinds of sheet resistance meters prepared by principles for detecting the emissivity of materials, namely electromagnetic induction non-contact sheet resistance meter and four-probe contact sheet resistance meter. The detection principle of the electromagnetic induction non-contact method is that when the coil carrying a sinusoidal current excitation is close to the measured surface, the alternating magnetic field around the coil induces current on the measured surface, generating a magnetic field of the same frequency in the opposite direction of the original magnetic field, which is reflected to the probe coil. The different surface resistances lead to changes in the resistance and inductance of the detection coil impedance, which changes the current size and phase of the coil. By measuring the change, the resistance of the measured surface can be obtained. The detection principle of the four-probe contact method is to apply a known constant current to the measured surface through the two outer probes, and measure the voltage of the two middle probes to obtain the resistance of the measured surface.
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3. Detection methods of emissivity in various standards
According to the national standard "Coated Glass Part 2: Low-E Coated Glass", the emissivity is determined according to the method specified in "Determination of Visible Light Transmittance, Direct Sunlight Transmittance, Total Solar Energy Transmittance, Ultraviolet Transmittance and Related Window Glass Parameters of Architectural Glass". For vertically incident thermal radiation, the thermal radiation absorption rate αh is defined as the vertical emissivity, calculated according to formula (1) and (2):
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In the formula, αh is the thermal radiation absorption rate of the specimen, that is, the vertical emissivity, %; ρh is the thermal radiation reflectivity of the specimen, %; ρ(λ) is the measured thermal radiation spectral reflectivity of the specimen, %; Gλ is the relative spectral temperature of thermal radiation at an absolute temperature of 293K.
This standard clearly states that for vertically incident thermal radiation, the thermal radiation absorption rate αh is defined as the vertical emissivity, and the hemispherical emissivity is equal to the vertical emissivity multiplied by the coefficient of the corresponding glass surface. However, the coefficient of the glass surface coated with a metal film or a multi-layer coating containing a metal film is 1.0. Obviously, this standard is no longer suitable for detecting the emissivity of coated glass.
The emissivity detection method is also mentioned in "Calculation and Determination of Steady-State U Value (Heat Transfer Coefficient) of Insulating Glass". The emissivity is the corrected emissivity ε of the glass surface in the spacer layer, which is used to calculate the radiation heat transfer coefficient hr. For ordinary glass surfaces, the corrected emissivity value is selected as 0.837. For the coated glass surface, the standard emissivity εn is measured by an infrared spectrometer. The corrected emissivity is calculated according to Table 1. The standard emissivity εn is determined by measuring the reflectance curve of the spectrum of the coated glass surface under near normal incidence conditions using an infrared spectrometer. According to the 30 wavelength values given in Table 2, the corresponding reflectivity Rn (λi) curve is measured at a temperature of 283K±0.5K, and the arithmetic average is taken to obtain the conventional reflectance curve at a temperature of 283K. It is calculated according to formulas (3) and (4):
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《Calculation and Determination of Steady-State U Value (Heat Transfer Coefficient) of Insulating Glass》further clarifies the calculation formula of the standard emissivity, and for the first time establishes a table of the coefficient relationship between the corrected emissivity and the standard emissivity, which is convenient for technical personnel to use. The most important thing about this standard is that "standard emissivity" replaces "vertical emissivity" and "corrected emissivity" replaces "hemispherical emissivity". The determination of surface emissivity is also stipulated in the "Code for Thermal Calculation of Glass Curtain Walls for Building Doors and Windows". The calculation of the standard emissivity εn of the far-infrared opaque coating surface should be done by measuring the reflectivity curve of its spectrum line with an infrared spectrometer under a condition close to normal incidence. According to the 30 wavelength values given in Table 2, the corresponding reflectivity Rn (λi) curve is measured, and the mathematical average is taken to obtain the conventional reflectivity at a temperature of 283K. The formulas and tables in this standard are the same as those in GB/T 22476-2008 and will not be repeated. The difference is that this standard uses "standard emissivity" instead of "standard radiance" and "corrected emissivity" instead of "corrected radiance". The "Technical Specifications for Application of Architectural Glass" also stipulates the method for determining the standard emissivity εn of coated glass. The glass reflection curve should be tested with an infrared spectrometer under near normal incidence conditions. On the reflection curve, the corresponding reflection curve Rn (λi) can be determined according to the 30 wavelength values given in Table 1. The wavelength table for determining the standard reflectivity Rn at 283K and the table for the relationship between the corrected emissivity and the standard emissivity are the same as the previous standards and will not be repeated. This standard uses the standard emissivity and corrected emissivity of JGJ/T 151-2008.
The emissivity detection methods in the above standards all use infrared spectrometers. However, since infrared spectrometers are very expensive, they are generally purchased by larger scientific research and testing institutions. Most glass manufacturers generally use emissivity meters and surface resistance meters to detect the emissivity of coated glass during process control. There are matters that need to be paid attention to when using each detection equipment. The correct use of emissivity meters and surface resistance meters plays a vital role in glass manufacturers accurately detecting the emissivity of coated glass. Taking the radiometer TIR100-2 as an example, special precautions need to be taken before preparing glass samples for testing the emissivity of coatings. The sample should be kept at a temperature of 23/-2℃ and a relative humidity of RH50%±20% for at least 2 hours. Ensure that the calibration standard plate, sample and instrument are at the same temperature and humidity. The measurement area must be kept closed and airtight without air convection. The instrument should be installed in a fixed position and must not be moved during measurement. During the test, first turn on the radiometer and wait for about an hour to heat up to 100℃ until "Continue" appears on the display. After clicking Continue, the equipment needs to be calibrated with a standard plate. The operator should wear clean white gloves. First, place the high-radiation side of the calibration standard plate close to the radiator. After the high-radiation side of the calibration plate completely covers the instrument detection port, immediately click Calibrate and repeat the above steps on the low-radiation side. The reference area of the standard plate cannot be touched during this process. After the calibration is completed, remove the standard plate from the detection port, and then measure the calibration standard plate again to confirm whether the calibration is accurate. When testing coated glass, a black background plate should be placed, and the coated glass surface should be close to the test port in a vertical direction. The black background plate should be close to the coated glass surface. The distance between the test port and the sample surface must be the same as that of the calibration standard plate. After the test port is completely covered, click to measure. After the measurement is completed, the sample should be quickly removed from the test port. In order to avoid temperature changes of the sample during the measurement process, the residence time of the sample at the measurement position must be reduced to a minimum. The time between sample positioning and the start of measurement should not exceed 1 second. In order to minimize the measurement error, the instrument should be recalibrated using two calibration standard plates after a maximum time interval of 1 hour. The emissivity of the coated glass sample measured by the emissivity meter is the vertical emissivity. The hemispherical emissivity needs to be obtained by multiplying the coefficient according to the relationship table given in EN673. The relationship table between vertical emissivity and hemispherical emissivity in standard EN673 is the same as Table 1. The coefficient table itself has certain limitations. For some vertical emissivity coefficients, linear interpolation or extrapolation should be used to calculate them. Regarding the coefficient between vertical emissivity and hemispherical emissivity, it is recommended to establish a formula through fitting, especially when the vertical emissivity is less than 0.05. Here, readers are provided with a formula (5) to calculate the coefficient C. For soda-lime-silica glass and its coated products, formula (6) can also be used to directly calculate.
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It is relatively simple to use a surface resistance meter to detect the surface resistance of coated glass. The detection principle of the surface resistance meter is the electromagnetic induction non-contact measurement method. First, the surface resistance meter is calibrated to zero, and then placed on the coated glass film surface. At this time, the reading shown by the surface resistance meter is the surface resistance value of the coated glass film surface. Then, according to formula (7), the hemispherical emissivity of the coated glass can be obtained by conversion.
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Example: Detection of the emissivity of a new type of Low-E glass (JN91) - After the new coated glass JN91 is combined with other Low-E glass products into insulating glass, the thermal insulation capacity of ordinary double-glass insulating glass is improved by changing the indoor convection heat transfer coefficient (the double-glass insulating heat transfer coefficient is less than 1.2W (/ m2·K)). Table 3 shows the test results of this new type of coated glass.
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Table 3 shows the test results of the hemispherical emissivity of 6mm JN91 samples using three emissivity test equipment. The hemispherical emissivity value is rounded to 2 decimal places. The test result using Fourier infrared spectrometer is 0.21; the test result using emissivity meter is 0.21; and the test result using surface resistance meter is 0.21. It can be seen from the test results that the test results of these three emissivity test equipment have very small errors and will not have a great impact on the calculation of the heat transfer coefficient of insulating glass.
4. Summary
4.1 It is recommended to unify the concept of emissivity of coated glass and use vertical emissivity and hemispherical emissivity.
4.2 It is recommended that glass manufacturers can use more than two types of equipment to test the emissivity of the same film system coated glass to increase the accuracy of the test results.
4.3 The direct test results of Fourier infrared spectrometer and emissivity meter are only vertical emissivity. The hemispherical emissivity needs to be multiplied by the vertical emissivity. After the surface resistance meter detects the surface resistance of the coated glass, the hemispherical emissivity should be calculated according to the formula.
4.4 When using the emissivity, the coated glass should be tested strictly according to the operating instructions. In addition, it should be noted that the calibration standard plate of the emissivity meter should be regularly sent back to the equipment manufacturer for calibration.
