|
|
|
 |
|
  |
January, 2004 What’s wrong with this picture? Riffle splitters (riffles, rifflers) are used for mixing loose, solid material and also for sampling this material. Rifflers come in different sizes and with different features. Those used on the plant floor for large volumes of material can be two or three feet high. Those used in labs are smaller. Rifflers are fairly easy to use and perform well if built and used properly. Gy** (reference below) gives guidelines for riffler design and use. How they work: Material is poured from a feeder pan into chutes which angle down alternately in opposite directions. The two fractions of the material are collected in two receiving bins. For mixing, this process is repeated several times with the same material. For sampling, one of the two receiving bins is chosen at random, and the material in the chosen bin is the sample. The process is repeated if a smaller sample size is required.A flawed variation of a Jones riffle splitter is shown below. Can you find three problems with this riffler? What are some potential problems?
Many sampling systems are "incorrect" in the Gy sense. Correct sampling means that all constituent elements of the lot have an equal opportunity of being in the sample, and the integrity of the sample is preserved both during and after sampling. Correct sampling is a generalization to bulk materials (solids, liquids, gases) of statistical random sampling. Without correctness, sampling bias is introduced and sampling variation is increased, sometimes substantially, beyond the unavoidable statistical sampling variation. How good are your sampling systems (procedures, equipment, practices)? Sampling procedures may be vague, and sampling practices may not follow correct procedures. Sampling equipment may not perform well in practice even though it is designed correctly. Operators (those taking the samples) may not be properly trained in correct sampling. Since your samples are only as good as the sampling systems that generate them, incorrect sampling will remain undetected without an audit and assessment of your sampling systems. An examination of the riffler above reveals the following problems. Are there other things we should be aware of? Let me know what you think. Problems with riffler...
Potential Problems Since the chute ends are not enclosed, the following may occur.
Moral: A correctly designed riffler that is also used correctly will reduce sampling bias and sampling variation. More information about sampling equipment and procedures for solids, liquids, and gases is available in the Short Course March 6-7, 2004, in Chicago at PittCon and in the short introductory book A Primer for Sampling Solids, Liquids, and Gases, by Dr. Patricia L. Smith. ** Gy, Pierre M. (1992). Sampling of heterogeneous and dynamic material systems: theories of heterogeneity, sampling and homogenizing, Amsterdam: Elsevier, 653 pages. The next conference will be held in Brisbane, Australia, in 2005, and will be organized by the newly elected board members. I will be part of a working group to draft a document for Good Sampling Practices based on Gy's theory. More details on the Esbjerg conference and follow up activities can be found at http://www.isbf.info/index.asp. More information about sampling equipment and procedures for solids, liquids, and gases is available in the Short Course and in the short introductory book A Primer for Sampling Solids, Liquids, and Gases, by Dr. Patricia L. Smith.
Send This Information To a Friend "Your decisions are
only as good as your samples." Newsletters | About
the Owner | Bulk
Sampling Short Course
© 2001 All Rights Reserved
|
