Regularly I’m asked the question; “How accurate is this moisture balance?”
The questioner asks for the answer in terms of percent (%) moisture. When I answer “it depends”, I always get the unspoken response, “Why did I ask this stupid idiot?”
Absolute Measurement
When inquiring about the precision of measurements such as weight or temperature, the responses are clear-cut and tied to the accuracy of the instruments. For instance, 25 grams or 50 degrees with a margin of plus or minus (+/-) 0.1 gram or 1/2 degree.
Percent is A Relative Measurement
When questioning the accuracy of your percentage reading, the answer can be quite complex. This complexity arises from the fact that percentage (%) is a relative measurement. Take, for example, the outcome of a Loss on Drying (LOD) moisture test, which is heavily influenced by the size of the sample used.
To better grasp this concept, consider how the result of an LOD moisture test is derived. It involves subtracting the weight of the sample at the end of the test from its initial weight, and then dividing this difference by the original weight of the sample. This calculation ultimately yields a % moisture reading.
Sample Size
The upcoming analysis will shed light on the impact of both sample size and instrument precision. We will delve into a sample containing 23.5% moisture, examined using an electronic LOD moisture balance with a sensitivity of 0.001 grams and a repeatability of +/- 0.005 grams.
Sample Analysis
Effects on Percent % Accuracy of Changes in Sample Size
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Using a Ten (10) gram sample the results are as follows: |
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Beginning Weight Ending Weight Difference % Moisture |
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In the Middle of Range 10.000g |
7.650g |
2.350g |
23.50% |
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Start Wt. High – End Wt. Low 10.005g |
7.645g |
2.360g |
23.59% |
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Start Wt. Low – end Wt. High 9.995g |
7.655g |
2.340g |
23.41% |
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Difference between High & Low = 0.18% |
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Using a five (5) gram sample the results are as follows: |
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Beginning Weight Ending Weight Difference % Moisture |
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In the Middle of Range 5.000g |
3.820g |
1.180g |
23.50% |
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Start Wt. High – End Wt. Low 5.005g |
3.815g |
1.190g |
23.78% |
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Start Wt. Low – end Wt. High 4.995g |
3.825g |
1.170g |
23.42% |
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Difference between High & Low = 0.35% |
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Using a One (1) gram sample the results are as follows: |
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Beginning Weight Ending Weight Difference % Moisture |
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In the Middle of Range 1.000g |
0.765g |
0.235g |
23.50% |
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Start Wt. High – End Wt. Low 1.005g |
0.760g |
0.245g |
24.38% |
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Start Wt. Low – end Wt. High 0.995g |
0.770g |
0.225g |
22.61% |
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Difference between High & Low 1.17% |
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In this demonstration, it becomes evident that the precision of % moisture readings is intricately linked to the size of the sample used. The accuracy can vary significantly, from 0.18% (with a margin of +/- 0.09%) when working with a 10-gram sample, to 1.17% (with a margin of +/- 0.56%) when using just one gram. This highlights a remarkable 6.5-fold enhancement in % moisture accuracy when transitioning from a one-gram sample to a ten-gram sample.
In Summary
I hope you conclude from this treatise the importance of sample size. This component of a test parameter governs the accuracy when the result is in terms of a %.
Please share this with colleagues who may find it useful.
This has been a remake of an article by Art Gatenby, originally published in 2008.
Art
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P.P.S. Check out the features of the CSC Digital Moisture Balance.
