In order to comply with regulatory requirements and achieve accurate operation, the accuracy of the checkweigher needs to be tested. In this article, we try to use the simplest language to explain how users of checkweighers should accurately measure and determine the accuracy of checkweighers.
In any weighing equipment, there are two kinds of errors that can have a significant impact on the accuracy of the equipment. One is repeatability, and the other is accuracy, which is the accuracy of the weighing result compared to a known weight (such as a weight). These two parameters are also the only two comparable parameters for dynamic checkweighing (online automatic checkweighing) and static weighing (static scale).
First of all, we have to confirm an "unconfidence interval" (Zoi). "Untrustworthy" sounds like a bad word, but to put it bluntly, it is 6 times the standard deviation obtained by reading the weight of a product many times. Unconfidence interval is a globally recognized method for calculating the uncertainty of dynamic checkweighing. This test will confirm the repeatability of the checkweigher for a specific product. For a specific product, if there is no stable repeated readings, you will never be able to perform other subsequent tests.
The specific method is as follows: Pass the same product through the checkweigher 10 or 20 times to ensure that the direction of the product is consistent with your actual production direction during each pass.
1. Record each data, some checkweighers can automatically record and calculate the standard deviation. If the checkweigher cannot automatically calculate, you can use the STDEV formula in the Excel spreadsheet to calculate.
2. After having the standard deviation, multiply by 6 to get the unconfidence interval.
3. Retrieve TNE(-T1) in the table below. For example, if your product weight is 200g, then TNE(-T1) is 5% or 9g.
4. Now calculate a quarter of this 9g, which is 0.25 x 9g = 2.25g. This value is your repeatability limit.
5. If the unreliable interval you calculated at first is greater than 2.25 g, then this checkweigher is unavailable under this setting. Various types and weights of products that need to be checked on the checkweigher are all Need to pass this test. If you want to do better, you should repeat this test every once in a while, record and analyze the trend of each test, and see how the test results change over time. Based on this trend, you can determine how often you need to perform such a test. These records and trend analysis are also valid proofs for users of checkweighers to assess property insurance risks.
The second test we are going to introduce is also carried out using the tested product. In addition, a calibrated static scale is also required. The resolution of the static scale needs to be the same as or higher than that of the checkweigher. This inspection needs to be performed before, after and at the end of a batch. Similarly, this test can be used for trend analysis for further risk assessment.
1. Use an adjustment weight similar to the product to check whether the static scale can be weighed correctly.
2. Pass the product through a running checkweigher, and then place it on a static scale for weighing. Compare the weight value obtained on the checkweigher with the weight value obtained on the static scale. If the difference is greater than 1/5 of TNE, this checkweigher is not applicable. During the batch process, some checkweighers have a built-in weight check function, which will eliminate the product that is only used for testing and display its weight on the screen to facilitate testing.
3. Repeat this test at least once to check the repeatability of the checkweigher.
For example, if the rated weight of our product is 200 g, the weight displayed on the checkweigher is 200.4 g, and the same product is taken to the static scale, and the weighing display is 199.7 g. The difference is 0.7 g. We know that the TNE of a 200 g product is 9 g. One fifth is 1.8 g. 0.7g is less than the limit of 1.8g, and the checkweigher can accurately measure and be used normally.
Remember to write these two tests into your standard workflow. Repeat these two tests at a certain interval. As long as you pass the test, you can use your checkweigher at ease.
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