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Instrument Accuracy?  Is it only an illusion?

Posted by Art Gatenby on Aug 28, 2019 12:05:00 PM

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 you ask about the accuracy of something like weight or temperature the answers are straight forward and relate to the instruments' precision. For example 25 grams or 50 degrees plus or minus (+/-) 0.1 gram or 1/2 degree.  

Target % Moisture

Percent is Relative

When you ask the question of how accurate is my percentage reading, the answer becomes complicated. It’s complicated because percentage (%) is a relative term. For example, the answer for the result of a Loss on Drying (LOD) moisture test, is based on the size of the sample you use.

Using the example of a LOD moisture test we can understand the concept. The result of this type of test is calculated by subtracting the weight of the sample at the end of the test from the weight of the sample at the beginning of the test. This difference is divided by the initial weight of the sample. This procedure produces a % moisture result.

Sample Size

In the following analysis, the implication s of sample size and instrument precision will be illustrated. This will be based on a sample that has 23.5% moisture and an electronic LOD moisture balance that has a sensitivity of 0.001 grams and repeatability of ± .005 grams.

Sample Analysis

Effects on Percent % Accuracy of Changes in Sample Size

Using a Ten (10) gram sample the results are as follows:

Beginning Weight Ending Weight Difference % Moisture

In the Middle of Range 10.000g

7.650g

2.350g

23.50%

Start Wt. High – End Wt. Low 10.005g

7.645g

2.360g

23.59%

Start Wt. Low – end Wt. High 9.995g

7.655g

2.340g

23.41%

Difference between High & Low = 0.18%

Using a five (5) gram sample the results are as follows:

Beginning Weight Ending Weight Difference % Moisture

In the Middle of Range 5.000g

3.820g

1.180g

23.50%

Start Wt. High – End Wt. Low 5.005g

3.815g

1.190g

23.78%

Start Wt. Low – end Wt. High 4.995g

3.825g

1.170g

23.42%

Difference between High & Low = 0.35%

Using a One (1) gram sample the results are as follows:

Beginning Weight Ending Weight Difference % Moisture

In the Middle of Range 1.000g

0.765g

0.235g

23.50%

Start Wt. High – End Wt. Low 1.005g

0.760g

0.245g

24.38%

Start Wt. Low – end Wt. High 0.995g

0.770g

0.225g

22.61%

Difference between High & Low 1.17%

6.5 Times Change in Accuracy

This example illustrates that accuracy in terms of % moisture is an illusion unless the sample size is considered. In this example, the accuracy in percent moisture ranges from 0.18% (plus or minus 0.09%) at 10 grams of sample to 1.17% (plus or minus 0.56%) at one gram of sample. This shows that the accuracy in % moisture can be improved 6.5 times when going from a 1 gram sample to a 10 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 originally published in 2008.

Art

P.S. Did you know that you can subscribe to these exposés, rants, raves and ramblings? All you have to do is insert your email address into the box just to the right of the title.

P.P.S. Check out the features of the CSC Digital Moisture Balance.

One of the Rugged LOD Moisture Balances

Topics: Moisture Analysis, Moisture Testing, CSC Digital Moisture Balance, Loss-On-Drying, Moisture, moisture balance, instrument accuracy

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