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tips & techniques a Processor's advice on how to get Moisture testing right By Joel Lischefski and Alison Maicke, Teel Analytical Laboratories gravimetric moisture analyzers are the most commonly used instrument for determining the percentage of moisture in resin. the eDitOR'S nOte: about a year ago (see Processor strategies, Jan. 2012), we reported on how a 2010 decision to invest in a state-of-the-art laboratory was paying dividends for custom profile extruder teel Plastics in baraboo, Wis. here, two of teel's key lab personnel share their insights on gravimetric moisture analysis and how to ensure your testing procedures are appropriate. instrument is essentially a sensitive balance contained within an oven. The sample is placed inside and the mass is compared before and after the heating cycle to determine the percent moisture. Performance specifcations for these instruments are often derived in tightly controlled laboratory environments rather than in a production or plant setting. As a result the values displayed on product brochures may not represent the instrument's performance in a plant setting using your methods and materials. It is important to verify your instrument's performance where it's actually used and with your methods and materials. SenSitivity iS Key It is helpful to get a sense of scale for what it is that you're measuring with a moisture balance. If you assume that your target moisture for a resin is no greater than 0.02% and you use a 40-g sample, than you need to be able to accurately measure a difference in weight of 8 mg (0.008 g). That amount of water is roughly the size of a pin head. If your limits are even lower than 0.02%, the amount you must measure reliably is even tinier. When dealing with measurements of such small quantities, minor issues can cause large effects in results. A variety of interferences can affect the instrument reading, including these six : •Air currents, •Vibrations, •Electromagnetic àelds/static, •Whether the unit is level, •The materials used, •The method used. All of these factors indicate that you should read carefully the manual that comes with your instrument and place it in the best possible location. In addition, take the following fve steps to characterize the performance of your moisture balance: Step 1: Ensure that your moisture balance does not have a bias or interferences. Run fve replicate tests with a weight placed in the center of an empty pan. The weight simulates the mass of your sample, but will not give off any mass when heated. The results of all the tests should ideally be 0%, but there are many reasons it may be otherwise. 34 february 2013 Plastics technology Instrument sensitivity is crucial when measuring moisture, as sometimes the amount of water is the size of a pin head. Gravimetric moisture analyzers are commonly used to determine the percentage of moisture in resin. The instrument is essentially a sensitive balance contained within an oven.