Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation
Tensiometers and Stalagmometers
Q. Can you provide an overview of the pros and cons of tensiometers and stalagmometers in measuring surface tension? I ask from a small business perspective trying to comply with the NESHAP for chromium electroplating. Also, can you provide a range of costs for each of the two instruments?
A. I'm not the ultimate authority on this issue. With that in mind, here's what I can offer.
On January 25, 1995, the EPA promulgated national emission standards for chromium emissions from hard and decorative chromium electroplating and chromium anodizing tanks. On July 9, 2004 amendments were promulgated to add certain compliance provisions, including use of surface tension limits using either a stalagmometer or tensiometer.
Based on emission test data from on hard chromium electroplating tanks EPA determined that compliance with the 0.015 milligram per dry standard cubic meter (mg/ dscm) emission limit can be achieved when the surface tension of the electroplating tank bath is maintained below certain levels. Based on those data, EPA set two surface tension limits: . 45 dynes per centimeter (dynes/cm), when measured using a stalagmometer, and . 35 dynes/cm, when measured using a tensiometer.
The reason for the two limits is that EPA's research and other data show that, when used to measure the surface tension of chromium electroplating baths, tensiometers typically read about 20 percent lower than surface tension measurements of the same bath made using a stalagmometer.
Most chrome plating facilities use stalagmometers to measure surface tension. Stalagmometers cost about $250. Fewer facilities use tensiometers that cost several thousand dollars. Conventional wisdom says that stalagmometers are not be as accurate as tensiometers. I have not done any independent testing to confirm or dispute this conclusion.
A du Nouy tensiometer is an instrument that measures surface tension by increasing force to a platinum-iridium ring in contact with the surface of the liquid. The tensiometer pulls on the ring and measures the force it takes to break the ring from the surface. The stalagmometer is an instrument used to measure surface tension by determining the mass of a drop of liquid by weighing, a known number of drops or by counting the number of drops obtained from a known volume of liquid. Measuring surface tension with a stalagmometer is sometimes referred to as the 'drop weight' method.
During the past several years, the state of California conducted testing as part of their proposed rule making, including evaluating differences in surface tension readings based on two types of instruments: a stalagmometer and a tensiometer. There was a difference observed between measurements with the two instruments. While all stalagmometer surface tension readings were higher than the tensiometer readings, no uniform difference was seen (i.e. they did not consistently vary by the same amount of dynes/centimeter, which is different than EPA's conclusion.
I believe the variability of the two methods has more to do with the greater difficulty of using a stalagmometer as compared to tensiometer. However, if a person is well trained and adheres to a well established standard operating procedure, then either method should work. As such, and considering cost, I'd have to recommend the stalagmometer.