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A multiple frame-of-reference scheme for assigning appropriate rotameter correction factors.

Hoover M
American Industrial Hygiene Conference and Exposition, June 1-6, 2002, San Diego, California. Fairfax, VA: American Industrial Hygiene Association, 2002 Jun; :86
Air sampling errors continue to occur when rotameters are used to monitor and control the sampling flow rate. Most errors can be traced to confusion over use of the rotameter equation and the ideal gas law to determine the flow rate associated with a given scale reading in relation to the calibrated flow rate for that scale reading. Most texts and tutorials refer to "actual" conditions and "calibration" conditions. Operators frequently assumed that "actual" conditions refer to the temperature and pressure at the sampling location, rather than inside the rotameter. Rotameters are typically located downstream of a filter or other sampling device, which results in a lower pressure than the ambient atmosphere from which the sample is being drawn. Depending on the pressure drop conditions (perhaps 1 psi for a filter and several psi for a cascade impactor) the errors can exceed the 5% level recommended for making a correction. Confusion can be eliminated by defining and using a multiple-frame-of-reference scheme involving the following conditions of temperature and pressure: (1) calibration, (2) operation (inside the rotameter), and (3) ambient (typically the actual conditions where the worker is located and the sample is being taken), as well as two types of reference conditions, (4) normal or standard ambient (760 mm Hg and either 20 degrees C or 25 degrees C), and (5) standard (760 mm Hg and either 00 C for chemists or 25 degrees C for ventilation specialists). This system clarifies that the rotameter equation is only to be used for the correction between calibration and the operation, and that the ideal gas law is to be used for corrections among all other combinations of the frames of reference. An Excel spreadsheet with detailed examples and calculations has been developed to demonstrate and apply this approach
Air-sampling; Monitors; Sampling; Temperature-effects; Analytical-instruments
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Research Tools and Approaches: Exposure Assessment Methods
Source Name
American Industrial Hygiene Conference and Exposition, June 1-6, 2002, San Diego, California
Page last reviewed: September 2, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division