All About Moisture Analyzers

mark3-2012
Image: Sartorius

A moisture analyzer is a great tool for any plastic processor to have. It will tell you for sure that you are drying the material adequately. It is also a great tool for material suppliers to have so that they can aid customers who are trying to troubleshoot problems.

How exactly a moisture analyzer works seems like a bit of witchcraft. You put a bit of material in, you choose some type of program and hit the start button. Ten to twenty minutes later, the readout shows a percentage. The process is a little bit more complex than just heating the material and burning off the moisture weighing the sample before and after. You don’t need a moisture analyzer to do that, you can do that with a lab dryer and lab balance. How does it work?

The best way to determine the moisture content in any material (gas, liquid or solid) is to use a method called Karl Fischer titration. The test method is named after the German scientist that invented it in 1935. The method is based on the very specific way in which iodine reacts with water. By measuring the amount of iodine that is used by the chemical reaction, the exact amount of water can be determined. The iodine does not typically react with other chemicals so it can reliable be assumed that you can measure only water with this method.

The method works best with substances that give up their moisture easily, like liquids and gases. With solids like plastics, the sample will be put in an oven and heated to a high enough temperature to burn any moisture out of the material. The oven is sealed and all of the extracted gases are then analyzed for moisture content. Again, because the iodine only reacts with the water, any other chemicals that burn off will not be counted.

The problem with just heating a sample of plastic up in an oven, weighing it before and after, is that other additives can burn off in the process skewing your results.

The Karl Fischer titration method is too complex for the average process tech to perform. It requires very sensitive equipment and scrupulous methods to avoid sample contamination. In addition to that, it just takes too long to perform the test. This is where the moisture analyzer comes in.

The moisture analyzer is a relatively simple piece of equipment. It is really just a very sensitive scale enclosed in an oven. It does in fact heat up the material and measure how much moisture is burned off. However, it uses a very carefully crafted program to hit just the right temperatures at the just the right time and weighs the sample at very carefully chosen times in order the get an accurate moisture content result.

The temperature/time parameters are the real magic of the moisture analyzer. These programs are carefully written by the moisture analyzer manufacturers based on correlation to Karl Fischer titration (you wondered why I told you about Karl Fischer titration). The manufacturers use the Karl Fischer titration method to test a plastic material for moisture content numerous times and then devise a program for the moisture analyzer that yields the same result. Extensive repeatability and reproducibility studies are carried out to assure the accuracy of the measurements. When you buy one of these machines you tell them what materials that you will be testing and they will provide you with the programs. Some of the new machines have the ability to download new programs from the internet which is pretty cool.

The programming of these machines is where the real magic happens and is the reason for the cost of these machines. The machines that I see used the most are made by Sartorius since they purchased Omnimark Instruments in 2005. I am sure that there are other manufacturers as well.

When you see these machines that seem to perform magic in the future, you will have a better understanding of what is going on inside them.

 

For more information on Karl Fischer titration for which I barely skimmed the surface, there is a Wikipedia page on the method as well as a slightly more simple explanation on the Applied Technical Services website.

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