The Insulation Control and Low-E Coating Detection of Insulating Glass.

Abstract: Modern insulating glass technology, that is, low-emissivity coating and inert gas filling technology, has been able to fully improve the heat insulation function of doors and windows. These advanced ingredients or materials are difficult to detect with the naked eye, especially when it comes to detecting what kind of noble gas and how much of that kind of noble gas is. In addition, it is difficult to determine whether there is a low-emissivity coating on the insulating glass, the position of the coating, and the thickness of each layer of glass.

Both manufacturers and their customers should be able to test their purchases to ensure they meet their expectations and needs.

At present, there are several ways to detect the gas content in the gas-filled insulating glass and the structure of the insulating glass. Likewise, instruments are being developed to detect the presence and location of low-E coatings on insulating glass, so that the overall quality of the insulating glass can be assessed. This article is intended to give the reader a basic understanding of several different methods available in the market.

Keywords and Terms: 

Low-E Coating, 

Inert Gas, 

Argon, 

Air Cavity Layer, 

U Value, 

Gas Content, 

Gas Retention

I. The overview

With the development of glass curtain wall buildings, the demand for high-quality insulating glass is increasing day by day. Many countries have made international or domestic commitments to reduce carbon dioxide emissions. Energy demand is constantly growing driven by the world's growing population and economic growth. The basic goals of energy production and environmental effects are to reduce the rate of global warming and to produce energy in a safe manner. In the early 21st century, the increase in energy consumption has shown the fastest growth rate at an average of 3% per year, and this phenomenon comes from rapid economic growth, such as in India and China. The production of energy on a global scale is based on fossil materials. Carbon dioxide emissions are considered to be the most important cause of the so-called greenhouse effect, and energy is its largest source. Carbon emissions are projected to decrease on an average yearly basis, especially for industry, the largest source, which is set very high.

Achieving goals in the construction sector will require changes to established practices and regulations. In the building sector, the goal is divided into two phases, the first is to reduce the energy use of buildings, and the second is to replace the reduced energy demand with renewable energy. Investing in energy-efficient buildings is a wise choice, as evidenced by several international construction projects. Although the construction cost of low-energy buildings is 2 to 5 percent higher than that of ordinary buildings, the long-term cost is still low because the cost of energy is low. To reduce carbon emissions from building construction, government agencies have introduced formal regulations to control the energy efficiency of the insulation and building materials. Glass is usually the weakest link when it comes to building insulation, so improving the overall energy efficiency of a building has become a powerful driving force for improving the quality of insulating glass. Electricity and heating in buildings account for about 34% of total energy consumption in Finland.

These regulations also set requirements for doors and windows and the glass used to make them, including the materials and construction used for insulating glass.

When studying the energy-saving effect of insulating glass, observers will pay attention to its thermal insulation effect and the emissivity of the glass. This article will focus on the glass part used for insulating glass. Unless otherwise defined, the U value referred to in this article is the thermal conductivity coefficient. It describes the transfer of heat from the air on one side of a unit area of glass assembly to the air on the other side per unit of time under standard conditions. That is, U = 1/R = Qa/dT(W/m2K) (unit: watts per square meter per Kelvin). It is also known as the derivative of the R-value, which is the coefficient of thermal resistivity. The lower the U-value, the better the insulating properties of the glass.

Table 1: Effect of Glazed Parts on Window Performance

2. Low-emissivity coated insulating glass

The term "selective radiation" or "low radiation" glass refers to glass with one or more transparent films of metal oxides. Selective radiation means that the transmission and reflection of radiation depend on the optical wavelength of the radiation. The significance of the low-emissivity coating is to reduce the heat radiation between the glass sheets of the insulating glass to improve its heat insulation performance.

Heat transfer is achieved by gas flow, conduction, and radiation. Radiation is divided into long-wave and short-wave radiation. Short-wave radiation such as visible sunlight, part of the long-wave heat radiation entering the glass will be filtered by the Low-E coating. This improves the insulation and overall thermal performance of the glass. Likewise, frosting on interior surfaces is reduced due to the low-E coating. The frosting is caused by temperature and humidity differences between the different glass sheets. If the surface temperature of the insulating glass is lower than the temperature of the air inside, it will also cause cold air to flow around the window, but if the insulating glass used has good heat insulation performance, there will be no problem, because the temperature of the hollow surface will be relatively high Some, there will not be such a large temperature difference with the air inside.

For a standard window, nearly half of the heat loss is due to radiation between the glass sheets, while the other half is due to heat conduction and convection between the inert gases.

Table 2: Examples of U-values for different window constructions: windows used in Finland since 1978

^{Figure 1: Effect of Low-E coating in insulating glass}

3. Inert gas

3.1 Why inflate?

Rare gases such as argon and krypton, and even xenon are generally filled in insulating glass as inert gases because they have much smaller heat transfer and conduction than standard air. This is why an inert gas is used as the filling gas. The biggest improvement method can be achieved by coating it with Low-E film and filling it with argon. Argon is the most widely used noble gas because it is cheap and easy to extract. Argon is non-toxic and inactive, about 30% lower in thermal conductivity than air (0,0179 W/mk vs. 0,0262 W/mK), it is extracted from the air in the process of making liquid nitrogen and oxygen. As a non-reactive gas, argon can also protect other valuable materials inside the insulating glass. More expensive options than argon are krypton and xenon, which are more stable and inactive than argon. Sulfur hexafluoride has also been used for insulation due to its acoustic properties, but is now considered environmentally polluting and is rarely used.

The heat loss of the insulating glass can also be reduced by optimizing the width of the spacer between the glass sheets. Commonly used spacer widths are 16mm or 12mm, and spacer bars exceeding 18mm are rarely used. Gas convection can be reduced by selecting an optimal inert gas with a density higher than air. Spacers are usually fabricated from thermally conductive materials, such as aluminum or steel, creating a sort of linear thermal bridge. The advanced spacer material can reduce heat loss through its thermal conductivity so that the inside of the insulating glass can maintain the same temperature as the heated building when it is cold.

There are indirect or direct different standards for the requirements of the U value of the building, and according to these standards, the requirements for the argon gas filling rate are also different. Different standards may vary depending on the market area, but most of them set high requirements for production, that is, the air content is 90%, and the annual leakage rate is less than 1%.

^{Figure 2: The relationship between the U value and the emissivity of the third surface of the glass when the spacer width is 12 mm under different gases and different contents}

3.2 Problems with Inflation and gas retention

Some potential problems can occur during the filling process of insulating glass. Inflation can be performed by automatic box inflation or manual inflation methods, but both have their own risks and quality control issues. When using an automatic inflation line, the operator needs to verify that both the mechanism and the inflation process are working properly. Because noble gases are invisible and odorless, they can be difficult to detect without the right tools. Manual inflation may also be prone to leakage due to human error during operation, such as when filling a refill hole or sealing a second sealant. Also, inflation results in turbulence, or problems with the gas not being evenly distributed. Although the inflation process may have been successful, problems can still occur. The inert gas may escape slowly due to poor materials or workmanship used in the production process. Therefore, the gas content should be checked several times during the production process to ensure that the entire production stage is functioning as it is set.

4. Detection and detection of inert gas content and low-radiation coating in insulating glass

4.1 Detection of Low-E coatings

The coatings of selective radiation glass are generally metallic and conductive coatings. In addition, the coated glass will reflect more light than white glass, which can be used to detect whether the glass has a low-emissivity coating.

For capacitive detectors, an alternating current is directed across the glass surface through electrodes. Detectors can be designed for low-E coating detection or moisture detection. Such detectors will be able to detect the strength of the current. A piece of selective radiation glass can generate a stronger current than white glass. This method is used to detect whether Low-E coated glass really has a low-e coating. But it cannot be used for low-e coating inspection of multilayer glass structures.

Another way to detect whether the glass surface has a low-e coating is to pass a direct current through one electrode to the other electrode and then to the glass surface. At this point, the coated surface conducts the current and the uncoated side behaves as an insulator since glass is not a conductor but an insulator. Operators using this solution need to measure from a known coated surface.

Optical measuring instruments are also used for low-e coating detection of glass and glass spacer thickness detection. These optical measuring instruments enable the operator to simultaneously detect the thickness of the glass spacer when detecting the thickness of each glass sheet of the insulating glass. An example of the instrument is shown in Figure 2. The technology is based on a beam of laser light that shoots out at a 45-degree angle and is reflected in the instrument sensor. The thickness of the glass sheet and the spacer is determined by the distance from the reflected laser column to the sensor. The technique also requires prior knowledge of the refractive parameters of glass and air.

There are many such instruments on the market for probing the thickness of low-E coatings and spacers. Its series of functions and prices depend on the needs of end users and range widely. For some instruments, the operator must summarize based on reflections in certain ranges. Similarly, some more complicated but user-friendly instruments can tell users the test results of insulating glass in a graphical way. Such an instrument can be used to inspect the entire configuration, check that the low-E coating is in the correct position, and check that the glass sheets and spacer widths are within tolerances. Conclusions about potential excess pressure are also measurable with appropriate instrumentation.

^{Figure 3: An optical inspection instrument for measuring glass and spacer thickness}

4.2 Why is it important to verify the inert gas content in insulating glass?

Unlike others, many factors that affect the gas permeability of insulating glass are very important, such as the elasticity and firmness of the sealant, the aging of the sealant, the material of the desiccant, the thickness and width of the sealant coating, and the permeability Diffusion speed of spacer and sealant, etc. When it comes to the production process, design, workmanship, and inflation techniques, it should all be controlled as much as possible.

The performance of insulating glass is affected on the one hand by the diffusion of moisture into its unit, and on the other hand by the diffusion of inert gas out of its unit. These diffusion processes are caused by differences in the surrounding gas and the stabilization of the noble gas. This can lead to negative pressure entering the insulating glass unit and, among other things, deformation of the glass. A leak of inert gas will cause a visible distortion and the customer will be able to detect the gas leak as the insulating glass will start to burst inwards. The emergence of this phenomenon requires us to take necessary measures, such as reducing the space between the glass sheets, stressing the sealant/glass surface pressure, and reducing the thermal performance over time. The same customers are also able to detect potential bursts and intrinsic gas content of insulating glass with today's advanced instrumentation. This way, manufacturers will have time to properly design and produce high-quality insulating glass products with good argon retention before a warranty claim is made.

Checking the argon content of insulating glass also allows operators to check that their filling equipment（Including insulating glass inflating production line and inert gas inflator equipment）is functioning properly. The inflation process is also susceptible to mechanical wear and failure during production. Because the characteristics of argon cannot be seen, there is no way to judge whether the argon is filled successfully, unless the filling line or the entire process is adjusted through inspection.

4.3 What are the practical tools to detect the content of inert gas?

From the beginning of the application of inert gas, the glass industry has been researching the detection method of inert gas content. Different oxygen analysis methods have been used by various industries for quite some time. The influx of oxygen in industrial gases is tested every day around the world, as is the inspection of gases in the medical industry and diving.

In this regard, the analysis methods of gas content in insulating glass can be divided into three types: physical methods, chemical methods, and sampling methods. This section will examine these three different approaches.

4.3.1 Physical detection methods for detecting inert gas content

The physical method is based on determining the physical properties of certain elements, the gases being unique to each element. Such properties refer to eg thermal conductivity, sound velocity, molecular weight, molecular size, radiation absorption coefficient or radiation waves emitted by the plasma, etc.

The absorption of electromagnetic radiation waves by gas and the amount of radiation depends on the gas content in the sample. For example, gases absorb infrared radiation of certain wavelengths. This method is applied to the detection of carbon dioxide in fire alarm systems. However, it is difficult to detect the noble gas content in this way, because the radiation wave has to pass through at least two layers of glass, which already filter out the relevant wavelengths.

Radioactive radiation wave absorption is used in fire alarm systems to detect fire gases. Glass absorbs alpha radiation waves, but it cannot detect noble gases, while beta and ϒ radiation waves are not safe for use in mobile equipment under production conditions.

Research on the spectrum of radiation emitted by ionized gases was proposed in Finland at the end of the 20th century. The technique is based on activating the gas atoms of a noble gas so that they emit a beam of light through the glass that can be analyzed. Each gas has its own unique spectrum, and these spectra can all be identified. The gas content in the sample will affect the intensity of the radiation wave, which makes it possible to detect the gas content in the insulating glass. In existing applications, the atoms of the gas are activated by a high-voltage electric spark. We introduced these applications to the market in 2001, and today the third-generation technology is marketed under the Gasglass brand.
This technology can accurately and repeatedly detect the content of argon and krypton in insulating glass, which is a non-destructive detection. The operator must know whether the gas he/she is testing is argon or krypton. 

It is based on high-voltage discharge to activate gas atoms, so there are some limitations. The spark must penetrate the gas cavity layer to make the measurement possible. Sparks cannot jump over Low-E coated glass or thick laminations, as this would make the measurement difficult. The background light is also sensitive to the influence of the measurement results, so when measuring, it should always be carried out under standard conditions as far as possible. It is recommended that the background be supported by a matching bracket covered with black cloth, or a similar background. To circumvent these limitations, manufacturers usually prepare measurable samples separately for inspection.

For example: when making a three-glass two-cavity insulating glass, they will put the glass plate coated with Low-E film in the middle, so that The inert gas content of the two cavities can be measured respectively.

Table 3: Accuracy and Repeatability of Gasglass Technology

Table 3 is the accuracy and repeatability of the third generation of Gasglass instruments. As can be seen from the report, the accuracy limits for each aeration content are given in standard deviations, and the measurement average here refers to the repeatability of the equipment. It can be seen that the accuracy and repeatability of the technique are based on the inflation level. For example, when inflated to 94.9%, the accuracy is very high and the standard deviation is small. When the gas filling rate is about 70.9%, the measured average value is shifted down by 0.5%, and the sum of the standard deviation is 2.2. However, this can be used for the detection of inert gas content. As we can see from the relevant figures, each instrument has its unique calibration curve. Manufacturers of Gasglass instruments recommend that users calibrate their instruments annually. The calibration process of Gasglass is complex, it requires 8 different calibration points and must be calibrated by the manufacturer or a certified service provider. To date, the manufacturer has three service centers around the world, the United States, China, and Finland. Calibration for each calibration point is performed with a certified standard gas. Spectroscopy based on spark discharge or plasma emission techniques is now widely used in the glass industry around the world.

Physical methods for the analysis of other noble gases have also been investigated. These methods are based on different techniques such as tunable diode lasers, Raman spectroscopy, variation of sound velocity in materials, or variation of thermal conductivity in noble gases. Regardless of the method used, when it comes to the analysis of the gas content of insulating glass, all methods have their difficulties or limitations, so far there is no way to solve them, so they can be applied in the market. Instruments based on breakthrough voltage change technology are available in the market now, but not many, because many factors affect their measurement results.

4.3.2 Chemical Methods for Detection of noble gas content

Noble gases are used as inert gases and form compounds with other elements only under special conditions. Although it is possible to analyze the inert gas content by analyzing the gas component content of the air remaining in the air cavity layer after inflation. In this case, the method does not verify the inert gas itself but rather identifies other gas components of the air remaining in the air cavity layer.

Certain materials may change their color due to changes in the oxygen content of the gas mixture. This is like sticking paper that will change color after a long time. This method can be used, for example, to check the difference between a sample of insulating glass before and after a climate cycle.

4.3.3 Sampling analysis method

The sampling analysis method provides a reliable method for users to determine the type and content of inert gas. There are many techniques for analyzing gases by sampling, which are also used by manufacturers in the gas industry. Sampling insulating glass always compromises its integrity. It is usually necessary to punch a hole in the spacer, and the sample can be taken out with a syringe for analysis or directly probed in with a measuring instrument for analysis. Gas samples require different volumes, from µL to mL, depending on the technique used. Gas chromatographs require an extremely small amount of gas sample, so repeatable measurements are possible. Although the manufacturer introduced how to seal the hole punched for sampling well, the gas still easy to leaks. If an oxygen analyzer is used to sample, the gas sample volume will be relatively large, so there is no way to perform repeated measurements.

4.3.3.1 Gas chromatography analyzer

Gas chromatographs with different types of sensors have been on the market for some time and have always been considered to be highly accurate and reliable. The amount of gas sample that needs to be extracted is also very small, about 25µL, so the operator can repeat the measurement many times. The gas chromatograph adopts gas chromatography, the composition is gas, which is suitable for qualitative and quantitative analysis.

^{Figure 4: Schematic diagram of gas chromatograph operation}

Chromatography is the separation of the chemical components of a complex system. Chromatography utilizes a fluid phase passing through narrow tubes, called columns. The components of the sample move through the column at different rates based on their unique chemical and physical properties, where the known and stationary phases interact. As chemical components leave the column, they are detected by various electronic detectors. The stationary phase in the column should separate the different components of the gas sample being analyzed. Likewise, the temperature inside the column and the flow rate of the gas sample are controlled. There are several different types of detectors used in a gas chromatograph. The most common types are flame ionization detectors and thermal conductivity detectors. The gas chromatograph can also be connected to a mass spectrometer which will operate as a detector.

According to the author's experience, laboratories in the glass industry around the world often use gas chromatography as an instrument for traceability. This approach would be too complex, time-consuming, and expensive to use in a production plant.

4.3.3.2 Oxygen analyzer

A wide variety of oxygen analyses can be found on the market. In addition to the glass industry, oxygen analyzers are widely used in the medical industry, and diving equipment is also used to detect oxygen levels. They detect the oxygen content in a gas sample, allowing the operator to determine the content of the gas being measured. And this judgment is based on the assumption of various gas content in the air, such as the air contains about 78% nitrogen, 21% oxygen, 0.94% argon, and other gases. The oxygen analyzer calculates the content of oxygen in the gas sample based on these data and obtains the total amount of air. Expect to end up with argon (or krypton) remaining in the gas sample.

Oxygen analyzers generally have a variety of sensors that can be used. The common ones are electrochemical cells and paramagnetic cells. Electrochemical cells, also known as fuel cells, measure the percent oxygen present in a gas or mixture of gases. The gas sample to be analyzed enters the sensor through a gas-permeable membrane. Oxygen in the gas sample is dissolved in the electrolyte after contacting the anode and cathode in the sensor. The flow of electrons and current generated from the cathode to the anode is proportional to the oxygen in the gas sample. These electrochemical cells will wear out over time because the lead anode used in the sensor is fragile in the presence of high oxygen levels. Once the lead anode is oxidized, the battery can no longer produce output and is useless except for a new lead anode. The operator only notices this when the instrument can no longer be calibrated or the calibration takes a very long time. Here, take the Sensoline handheld oxygen analyzer as an example. It uses an electrochemical battery and is easy to use because it has an integrated pump inside, so the operator does not need to manually extract and inject gas samples, because the measurement Results may be gas sample flow rate dependent. The operator can take some air as a gas sample and calibrate the instrument with a simple push of a button. Operators should take care when doing calibrations, as analyzers like this can drift over time. The manufacturer states that the technique allows an accuracy of 0.1% when calculating the argon content from the measured oxygen.

Paramagnetic batteries take advantage of the higher volume magnetic susceptibility of oxygen than ordinary gases. A paramagnetic sensor is combined with two glass spheres filled with a reference gas, usually nitrogen, mounted on a suspended rotating device. This device, commonly known as a dumbbell, contains a powerful magnetic field. When a gas sample containing oxygen passes through the process into the sensor, the oxygen molecules are attracted to the stronger of the two magnetic fields. This couple causes displacement and then rotation within the dumbbell. There is an optical system inside that is used to measure the rotation angle of the dumbbell. The function of the reverse current is to restore Yaling to its original position. That current is proportional to the partial pressure of oxygen and can be converted to a readout of oxygen so the operator knows how much oxygen is inside.

Paramagnetic sensors are generally considered to be persistent. Paramagnetic sensors are very sensitive to vibration, placement, and application, and are generally of the "stand-alone" type, that is to say, in addition to the analyzer itself being in a stable environment, including injected gas samples and objects. The magnetic susceptibility exhibited by other gases may cause relatively large-scale measurement errors, and the manufacturer should provide detailed information on such gases.

Depending on the requirements of the manufacturer, the operator may have to select the measurement range from the switch. At the beginning of the operation, the operator should analyze the air sample to ensure that the instrument shows the correct value of 20.9%. The same also needs to be analyzed for pure argon without any air in it. As an oxygen analyzer with a paramagnetic sensor, in addition to Helantec's Helox KVSN-F, Germany's KIWA zemlabor GmbH can also provide an instrument with more or less similar accuracy than the Sensoline oxygen analysis example previously exemplified.

There are also oxygen analyzers on the market with other types of sensors such as thermal conductivity sensors, zirconia sensors, ambient temperature sensors, and polarographic sensors, or a combination of many sensors.

5. Future research

As we said, due to the global requirements for energy saving and building energy efficiency, government departments and ordinary people are paying more and more attention to window energy-saving solutions. This also makes insulating glass solutions more closely linked to energy efficiency. From the history of many markets, we can see that the requirements for the U value of insulating glass have been tightened, and in the future, the expectations for this area will be higher. These can be achieved through high-quality materials, workmanship, and quality control.

Because the main gas analysis methods have their limitations, the industry needs to develop a new detection method that does not require a glass structure and does not damage the glass. As far as the author knows, the only online non-destructive gas analysis is only available in Cardinal IG in North America. The company has installed a Gasglass device on its insulating glass production line to analyze argon.

Installing detection devices on the production line enables manufacturers to detect each product produced and ensure the normal operation of the inflation process. This will also be a marketing advantage for manufacturers, as they can tell customers that each of their products is tested during production and spot-checked before shipment.

6. Conclusion

The purpose of this article is to let readers understand what technology can be used to detect Low-E coatings on insulating glass and the content of inert gases in them.

We describe the main existing techniques for detecting the presence and location of low-E coatings. Many different types of products can perform this task. Users need to set characteristic requirements and product quality for themselves.

Through our introduction of different analysis methods for noble gases, we hope readers can understand the different detection methods that can be used. The methods used in the glass industry are mainly sampling analysis done in the laboratory such as gas chromatographs and oxygen analyzers with different sensors. In addition to sample analysis, analyzers based on plasma emission spectroscopy are commonly used in the industry.

All methods have weaknesses in the detection process. Gas chromatography is widely regarded as the best solution. It requires very expensive machines, fragile sensors, and very experienced users. This technique is used more in research laboratories in the glass industry.

Oxygen analyzers are also widely used in the industry but also require sampling. The plasma emission spectrometer is also widely used in the industry and is the only non-destructive noble gas detection equipment. The weakness of this technology is that it cannot penetrate low-emissivity coated glass or thick laminated film, and the manufacturer needs to prepare some additional glass that can be used for testing. All detection methods can achieve the required accuracy. For reliable testing, operators should be familiar with the techniques they are using to ensure that the test results are trustworthy. The method of sampling test is not repeatable, because the inert gas in the sample will have a relatively large composition change after each test. The only reproducible sampling test possible is gas chromatography with a sample size of µL. Non-destructive plasma emission spectroscopy technology can repeat the test. This also allows the operator to pay attention to the leakage rate of the inert gas in the insulating glass, and at the same time check whether the inflator is adjusted to the best working condition.

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