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Abstract: Modern insulating glass technology, that is, low-emissivity coating and inert gas filling technology, can fully improve the thermal insulation function of doors and windows. These advanced components or materials are difficult to detect with the naked eye, especially when it is almost impossible to detect which noble gas is and how much the noble gas is present. In addition, it is difficult to determine whether the insulating glass is coated with low-emissivity film, the position of the coating, and the thickness of each layer of glass.

Both glass processing manufacturers and their customers should be able to inspect the insulating glass they produce or purchase to ensure that these products meet their expectations and needs.

At present, there are several ways to detect the gas content and the structure of the insulating glass in gas-filled insulating glass. Likewise, instruments to detect the presence and location of a Low-E coating on insulating glass are also being investigated, 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: Low-E Coating, Inert Gas, Argon, Cavity Layer, U-Value, Gas Content, Gas Retention

I. Overview

No matter where in the world today, the demand for high-quality insulating glass is increasing day by day. Many countries have made international or local commitments to reduce carbon dioxide emissions. Energy demand is 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 safely. In the early 2000s, the increase in energy consumption was already present at the fastest average annual growth rate of 3%, a phenomenon that came 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 leading cause of the so-called greenhouse effect, and energy is its largest source. Carbon emissions are expected to fall on average on an annual basis, especially set very high for the industrial sector, which is the largest source. Achieving goals in construction will require changes to established practices and regulations. In the building sector, the goal is to have 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. According to several international construction projects, it is a wise choice to invest in energy-efficient buildings. Although the cost of low-energy buildings is 2 to 5 percent higher than that of ordinary buildings, the long-term cost is still lower because of lower energy costs. To reduce carbon emissions from building construction, government agencies have issued formal statutes and regulations to control the energy efficiency of thermal insulation and building materials. When it comes to building thermal insulation, glass is usually the weakest link, so improving the overall energy efficiency of a building has become a powerful driving force for improving the quality of insulating glass. In Northern Europe or North America developed countries and regions, electricity and heating in buildings account for approximately 34% of total energy consumption. These regulations also set requirements for doors and windows and the glass used to make them, including the materials and structures used for insulating glass.

When studying the energy-saving effect of insulating glass, Jinan LIJIANG Glass will focus on 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 thermal conductivity. It describes the heat transfer per unit of time from the air on one side of a unit area of glass assembly to the air on the other side under standard conditions. That is U = 1/R = Qa/dT(W/m² K) (unit: watts per square meter per Kelvin). It is also known as the derivative of the R value, which is the coefficient of thermal resistance. The lower the U value, the better the insulating properties of the glass.

Table 1: Effects of glass sections on window performance

FactorInfluence of thermal insulation effect
The glass thicknesssmall
The distance between glass piecesmedium
The number of glass pieceslarge
The inert gas medium
The different Low-E coated glasslarge

2. Low-E coated insulating glass

The terms "selective radiation" or "low-E" glass refer to glass with one or more layers of metal oxides having a transparent film. Selective radiation means that the transmission and reflection of radiation are dependent on the wavelength of light of the radiation. The significance of the low-emissivity coating is to reduce the thermal radiation between the glass sheets of the insulating glass to improve its thermal insulation performance.

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

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

Table 2: Examples of U-values for different window constructions: Windows used in developed countries and regions in Northern Europe or North America since 1978.

YearTypeStructureU-value of the glass 
(W/m² K)
 19781+1+14+50+4+50+41.8
2003 1+21+100+2g (4-12-E4)1.2
20101+24K+100+2g (4-15ar-e4)0.8
 20122+24-18ar-e4+50+4-18ar-e40.5
Figure 1 The effect of Low-E coating on insulating glass

Figure 1 The effect of Low-E coating on insulating glass

3. Inert gas for insulating glass

3.1 Why should the insulating glass be inflated between the two pieces of glass?

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

The heat loss of the insulating glass can also be reduced by optimizing the width of the spacers between the glass sheets. Commonly used - spacers are 16mm or 12mm wide, rarely more than 18mm. Gas convection can be reduced by choosing an optimal inert gas with a density higher than that of air. Spacers are usually fabricated from thermally conductive materials, such as aluminum or steel, creating a kind of linear thermal bridge. Advanced spacer material reduces heat loss through its thermal conductivity, allowing the inside of the insulating glass to maintain the same temperature as the heated building when it is cold.

There are indirect or direct different standards through the requirements for 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 region, but most of them set high requirements for production, requiring a 90% aeration content and an annual leak rate of less than 1%.

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

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

3.2 The problem of insulating glass inflation and gas retention

Some potential problems can occur during the inflation of the insulating glass. Inflation can be performed using automatic box inflation or manual inflation, but both have their risks and quality control issues. When using an automatic inflation line, the operator needs to verify that both the machinery and the inflation process are working properly. To solve this problem, some manufacturers of insulating glass production lines will use the physical properties of inert gas to float upward, inflate upwards, and install gas detection sensors in the lamination box to display the inflation process through the operation panel. Because noble gases are invisible and odorless, they are difficult to detect without the right tools. Manual inflation can also be prone to leaks due to human error during operation, such as filling the inflation hole or sealing a second sealant. Also, aeration results in turbulent flow, or problems with the gas not being evenly distributed. Although the inflation process may be successful, problems can still occur. Inert gases can slowly leak out 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 running as it was set up.

4. The 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. 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, alternating current is conducted on the glass surface through electrodes. The detector 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 when testing whether Low-E coated glass has a Low-E coating. However, it cannot be used for Low-E coating inspection of multi-layer glass structures.

Another way to detect whether there is a Low-E coating on a glass surface is to conduct a direct current through one electrode to another electrode and then conduct it on the glass surface. In this case, the coated surface conducts current and the uncoated surface acts as insulation because glass is not a conductor but an insulator. When using this solution, the operator needs to take measurements from a known coated surface.

Optical measuring instruments are also used for Low-E coating inspection of glass and glass spacer thickness inspection. These optical measuring instruments allow the operator to detect the thickness of the glass space synchronously 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, which shoots out at a 45-degree angle and is then reflected by the instrument sensor. The glass and spacer thicknesses are determined by the distance from the reflected laser beam to the sensor. This 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 range of functions and prices is very wide, depending on the needs of the end user. For some instruments, the operator must make a summary based on reflections in certain ranges. Similarly, some more complex but user-friendly instruments can graphically inform the user of the inspection results of insulating glass. Such an instrument can 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 allowable tolerances. Conclusions about the potential excess pressure are also measurable by appropriate instrumentation.

Figure 3 An optical inspection instrument for glass and spacer thickness

Figure 3 An optical inspection instrument for glass and spacer thickness

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

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

The performance of insulating glass is affected by the diffusion of moisture into the insulating glass unit on the one hand, and the diffusion of inert gas out of the insulating glass unit on the other hand. These diffusion processes are caused by differences in surrounding gas and the stabilization of noble gases. This can lead to negative pressure entering the insulating glass unit and, in addition, glass deformation. Inert gas leaks will cause visible distortion and customers will be able to detect gas leaks as the insulating glass will begin 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 thermal performance over time. The same customers are also able to detect potential bursts and the intrinsic gas content of insulating glass with today's advanced instruments. This way, manufacturers will have time to properly design and produce high-quality insulating glass products with good argon retention before a warranty claim occurs.

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

4.3 What practical tools are there to detect inert gas content?

The glass industry has been researching detection methods for inert gas content since the beginning of inert gas applications. Different oxygen analysis methods have been used in various industries for quite some time. The influx of oxygen in industrial gases is tested every day all over 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 chapter will examine these three different approaches.

4.3.1 Physical testing methods for detecting noble gas content

Physical methods are based on the physical properties of certain elements determined, and gases are unique to each element. Such properties refer to properties such as thermal conductivity, speed of sound, molecular weight, molecular size, radiation absorption coefficient or radiation waves emitted by plasma, etc.

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

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

The study of the spectrum of radiation waves emitted by ionized gases was proposed at the end of the 20th century. The technology is based on activating the gas atoms of noble gases so that they emit light beams through the glass that can be analyzed. Each gas has its unique spectrum, and these spectra can be identified. The gas content in the sample affects the intensity of the radiation wave, which makes it possible to detect the gas content in the insulating glass.

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 detecting is argon or krypton. Because this method is based on the activation of gas atoms by a high-voltage discharge, there are some limitations. The spark must penetrate the air cavity layer to make the measurement possible. The spark cannot skip low-E coated glass or thick laminates, as this will make the measurement becomes difficult. The background light is also sensitive to the influence of the measurement results, so the measurement should always be carried out under standard conditions. It is recommended that the background be supported by a matching bracket covered with a black cloth, or similar background. To circumvent these limitations, manufacturers usually prepare measurable samples separately for testing. For example, when making three-glass and two-chamber hollows, they will place the low-emissivity-coated glass in the middle, so that The inert gas content of the two chambers can be measured separately.

Table 3 Accuracy and repeatability of the Gasglass technique             

Argon content in the test pieceMean measuredMean 
deviation
Standard deviation
97,597,500,1
94,994,900,1
90,090,000,3
85,185,100,6
80,280,1-0,1 0,5
70,970,4-0,52,2
50,149,7 -0,42,4

 Table 3: Accuracy and repeatability of the Gasglass technique

As can be seen from the report, the accuracy limits for each aeration content are given as standard deviations, the mean of the measurements being referred to here as the repeatability of the equipment. It can be seen that the accuracy and repeatability of this technique are based on the inflation level. For example, when inflation reaches 94.9%, the accuracy is very high with a small standard deviation. When the aeration is about to reach 70.9%, the measurement average is shifted down by 0.5%, and the sum of the standard deviations is 2.2,  however, can be fully used for the detection of inert gas content. As we can see from the associated numbers, each instrument has its unique calibration curve. Manufacturers of Gasglass instruments recommend that users calibrate their instruments annually. The calibration process for Gasglass is complex, it requires 8 different calibration points and must be calibrated by the manufacturer or an accredited service provider. To date, the manufacturer has three service centers around the world. All calibration points are calibrated with certified standard gases. Spectrometry based on transient discharge or plasma emission techniques is now widely used in the glass industry around the world.Physical methods for other noble gas analyses have also been investigated. These methods are based on different techniques such as tunable diode lasers, Raman spectroscopes, changes in the speed of sound in materials, or changes in thermal conductivity in noble gases. Regardless of the method used, when it comes to analyzing the gas content of the insulating glass, all methods have their challenges or limitations that have not been solved so far, so they can be applied in the market. Instruments based on breakthrough voltage change technology are also available on the market, but not many, because many factors have an impact on their measurement results.

4.3.2 Chemical methods for detecting noble gas content

Noble gases are used as inert gases and only form compounds with other elements under special conditions. Although it is possible to analyze the inert gas content by analyzing the gas composition content of the air remaining in the air cavity layer after inflation. In this case, the method does not verify the noble gas itself, but rather identifies other gas components of the air that remain in the air cavity layer.Some materials may change their color due to changes in the oxygen content of the gas mixture. This makes the stickers discolor over time. This method can be used, for example, to examine the differen

4.3.3 Sampling analysis method

The sampling analysis method provides users with a reliable method of determining the type and content of inert gases. There are many techniques for gas analysis by sampling, which are also used by manufacturers in the gas industry. Sampling from insulating glass always destroys its integrity. Usually, it is necessary to punch holes in the spacer, and the sample can be taken out with a syringe for analysis or directly probed with a measuring instrument for analysis. Gas samples require different volumes, from µL to mL, depending on the technique used. Gas chromatographs require extremely small gas samples, so measurements can be repeated. Although the manufacturer explained how to seal the hole punched for the sampling well, the gas still leaks easily. If an oxygen analyzer is used to sample, the sample size of this gas will be relatively large, and there is no way to repeat the measurement.

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 drawn is also very small, about 25µL, allowing the operator to repeat the measurement many times. The gas chromatograph adopts the gas chromatography method, and the composition is gas, which is suitable for qualitative and quantitative analysis.

Figure 4 Schematic diagram of the operation of the gas chromatograph

Chromatography is the separation of the chemical components of a complex system. Chromatography utilizes a fluid phase passed through a narrow tube, called a column. The components of the sample travel 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 immobilization in the chromatographic column corresponds to this separation of the different components of the gas sample being analyzed. Likewise, the temperature within the column and the flow rate of the gas sample are controllable. Several different types of detectors are used in gas chromatographs. The most common types are flame ionization detectors and thermal conductivity detectors. A gas chromatograph can also be connected to a mass spectrometer that will operate as a detector.In the author's experience, gas chromatography is often used as a traceability instrument in glass industry laboratories around the world. 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 assays 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 of a gas sample, allowing the operator to determine the content of the gas being measured. This determination is based on assumptions about the content of various gases in the air, such as air containing about 78% nitrogen, 21% oxygen, 0,94% argon, and other gases. Based on these data, the oxygen analyzer calculates the oxygen content in the gas sample and obtains the total amount of air. 

It is expected that argon (or krypton) will be left in the gas sample in the end.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 percentage of trace amounts of oxygen in a gas or gas mixture. The gas sample to be analyzed enters the sensor through a gas permeable membrane. The oxygen in the gas sample is dissolved in the electrolyte after contact with the anode and cathode in the sensor. The electron flow and current generated from the cathode to the anode are proportional to the oxygen in the gas sample. These electrochemical cells wear out over time because the lead anodes used in the sensors are 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 to replace it with 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 cell, which is convenient to use because it has an integrated pump so that the operator does not need to manually draw and inject gas samples, because the measurement The results may be dependent on the gas sample flow rate. The operator can take some air as a gas sample and then calibrate the instrument with a simple push of a button. Operators should take care when doing the calibration, as analyzers like this can drift over time. The manufacturer states that the technique allows the accuracy of 0.1% when calculating the argon content from the measurement of oxygen A paramagnetic sensor is combined with two glass spheres filled with a reference gas, typically nitrogen mounted on a suspended rotating device. The device, commonly known as a dumbbell, contains a powerful magnetic field. When a gas sample containing oxygen enters the sensor through the process, the oxygen molecules are attracted to the stronger of the two magnetic fields. This couple causes displacement within the dumbbell and then rotation. There is an optical system inside which is used to measure the rotation angle of the dumbbell. The effect of the reverse current is to restore the sub-bell to its original position. That current is proportional to the partial pressure of oxygen and can be converted into a readout of oxygen so the operator knows the oxygen content inside.Paramagnetic sensors are generally considered to be durable. The paramagnetic sensor is very sensitive to vibration, placement, and its application, and is generally of the "standalone" type, which means that in addition to the analyzer itself, it must be in a stable environment, including injecting gas samples and objects. The magnetic susceptibility exhibited by other gases may produce larger-scale measurement errors, and manufacturers should provide detailed information on such gases.  

Depending on the manufacturer's requirements, 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%, and also need to analyze the pure argon without any air. As an oxygen analyzer with a paramagnetic sensor, in addition to Helantec's Helix KVSN-F, the German company KIWA zemlabor GmbH can also provide instruments with a more or less approximate accuracy than the Sensoline oxygen analysis exemplified previously.There are also oxygen analyzers with other types of sensors on the market, such as thermal conductivity sensors, zirconia sensors, ambient temperature sensors, polarographic sensors, or a combination of many sensors.

5. Future research

As we have said, due to the global requirements for energy saving and building energy efficiency, from government departments down to the common people, more and more attention is paid to the solution of window energy saving. This also brings the insulating glass solution closer 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 piece will be higher. These can be achieved through high-quality materials, workmanship, and quality control.Because the major 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 Chinese equipment assembly manufacturer that can not destroy the gas analysis when the insulating glass is inflated online is Jinan LIJIANG Glass, which has installed a Gasglass device on its insulating glass production line to analyze argon.The addition of inspection devices to the production line enables manufacturers to inspect every 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 every single one of their products is tested during production and spot-checks before shipment.

6. Conclusion

The purpose of this article is to provide readers with an understanding of what techniques can be used to detect Low-E coatings on insulating glass and the content of inert gases in them.We describe the major existing techniques for detecting the presence and location of low-E coatings. 

There are many different types of products that can perform this task. Users need to set a feature requirement and product quality for themselves.

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

All methods have weaknesses in the detection process. Gas chromatography is widely considered the best option. It requires very expensive machinery, fragile sensors, and very experienced users. This technique is more commonly used in research laboratories in the glass industry. Oxygen analyzers are also widely used in the industry, but also require sampling. Plasma emission spectrometers are also widely used in the industry and are the only non-destructive noble gas detection equipment. The weakness of this technology is that it cannot penetrate low-emissivity-coated glass or thick laminates, requiring manufacturers to prepare 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 results obtained are trustworthy. The sampling test method 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 may be gas chromatography with a sample size of µL. Nondestructive plasma emission spectrometry can be tested repeatedly. 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 to check whether the inflator is adjusted to the best working condition.


For more information about Jinan LIJIANG Glass insulating glass processing equipment and insulating glass processing accessories, please click here to learn more.   

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