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Sunday, April 09, 2006

How many firkins in a hogshead?

While teaching mixture design for optimal formulation at two chemical companies the last two weeks, I was surprised to see that they still specify ingredients as pounds per hundred (pph)-- bsed on their main component. This is really old-fashioned, not only for the use of English units of measures,* but because it does not account for the dilution effect. For example, a Stat-Ease client making a construction caulk ran a central composite design, a response surface method (RSM), that varied the filler from 100 to 250 pph and plasticizer from 50 to 100 pph based on 100 pounds of polymer. They kept the other 57 pounds in their recipe constant in terms of relative proportions for the ingredients. We did some simple calculations with a spreadsheet to make the bargraph shown on a 100 percent weight basis. Seeing how everything (depicted by colored segments) varied by concentration (not just the two ingredients they intended to change), it now became clear to the client that they would best re-design their experiment as a mixture.

*(Hired into the petroleum industry as chemical engineer in 1975, I quickly learned that there are 42 gallons in a barrel. Further research into English units of volume revealed that some number of firkins would fit into hogshead -- I forget now. Just this last week I discovered a new measure of volume at a brew pub -- they sold a "growler" of beer (64 fluid ounces) -- a grrrreat deal!)


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