4 things to check when glass optical & thermal test results are different from manufacturer’s specifications
It is common and inevitable that glass optical & thermal lab test results are different from manufacturer’s specifications. This article aims to explain the 4 important things that should be checked to understand the possible causes of the result deviation:
- Test method
- Glass manufacturing variation
- Accuracy of manufacturer’s specifications
- Accuracy of lab test results
- Result differences caused by different test methods
- Results differences caused by glass manufacturing variation and measurement errors
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1. Test method
The first thing to check is if the same test method is used in the lab test and the manufacturer’s specifications.
As explained in our NFRC/EN/ISO test method comparison article, the test results obtained with different test methods could be significantly different. It is obvious that, if different test methods are used, the lab test results and the manufacturer’s specifications would be different.
2. Glass manufacturing variation
The second thing to check is the glass manufacturing variation.
In the lab, we often observe the following glass performance variation issues:
- Sample-to-sample variation: result variation between multiple samples
- Sample non-uniformity: result variation at different measurement points of the same sample
Our lab test results are based on the test sample received and tested by us. The manufacturer’s specifications are based on another different glass. It is obvious that the glass manufacturing variation would cause our lab test results different from the manufacturer’s specifications.
3. Accuracy of manufacturer’s specifications
The third thing to check is the accuracy of manufacturer’s specifications.
The manufacturer’s specifications are with certain accuracy. The specification accuracy is highly dependent on the sampling procedures and testing procedures implemented in the manufacturer’s in-house lab.
4. Accuracy of lab test results
The forth thing to check is the accuracy of our lab test results.
Our lab test results are with certain accuracy, due to issues such as instrument accuracy and sample non-uniformity. The measurement accuracy of our glass optical & thermal test results are discussed in detailed in this article.
In summary, there are mainly 4 possible reasons causing the differences between the lab test results and the manufacturer’s specifications:
- Different test methods used
- Different samples used in the lab test and the specification test
- Errors with the manufacturer’s specifications
- Errors with the lab test results
Result differences caused by different test methods
In our article on the NFRC/EN/ISO test method differences, 5 examples were presented. It is clear that, for many glass types, the result differences caused by different test methods are significant.
Results differences caused by glass manufacturing variation and measurement errors
We can use glass shading coefficient (SC) as an example to further explain the concepts.
For a glass with average SC = 0.30 and manufacturing variation of ±0.01, the SCs of actual produced glasses range between 0.29 and 0.31.
If a glass with SC = 0.31 is sampled for the specification testing by the manufacturer, the specification test results would range between 0.30 and 0.32, assuming a specification test accuracy of ±0.01 for SC.
If a glass with SC = 0.29 is sampled for the lab testing by OTM, the lab test result would range between 0.28 and 0.30, assuming a lab test accuracy of ±0.01 for SC.
In the worst case, the maximum difference between the lab tested SC and specified SC is 0.04 (0.32 – 0.28).
In practice, the differences between the lab test results and the manufacturer’s specifications are not so large, but it is clear that the glass manufacturing variations and measurement errors cause results variations.
Last update: 03/01/2021
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