Page 809 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

764 Pre-Appendix Tables

TABLE CDQR.6
Miscellaneous Notes
Multiplicity of Units with Same Name
Frequently, in looking at unit conversions, there are several choices, for some units.
The normal quantities used in the United States are, according to Lindeburg (1993):
Barrel 31.5 gal (U.S. liquid)
Barrel 42 gal (petroleum)
Btu Traditional (thermochemical) value
Calories Thermochemical
Gallons US liquid
Ounces Avoirdupois
Pounds Avoirdupois
Tons Short tons of 2000 lb
Force and mass conversions
Conversions between different forces often requires going back to basics. To help, in the conversion task, force definitions from Wandmacher and Johnson
(1995) are,
. Newton is the force required to accelerate 1.0 kgm at the rate of 1.0 m=s 2
. Poundal is the force required to accelerate 1.0 Lbm at the rate of 1.0 ft=s 2
. kgf is the force required to accelerate 1.0 kgm at 9.80665 m=s 2
. Lbf is the force required to accelerate 1.0 Lbm at the rate of 32.1740 ft=s 2
The related definitions for the derived mass units are,
. The slug is that mass which when acted upon by one pound force will be accelerated at the rate of 1.0 ft =s
2
2
. The gravitational metric unit of mass is that unit of mass which when acted upon by 1.0 kgf will be accelerated at 1.0 m=s . The o inference is to the kgm.
. The kgm and the Lbf are base units, not derived units.
Rounding Off
The number of places in any conversion result should be the number of places in the source number. Rather than round the conversion factor, the result of the
conversion should be rounded. This can be done in a spreadsheet by setting the number of places in the calculated cell to that desired (Heausler, 1994).
Significant Figures
The ‘‘significant figures’’ represents the accuracy of a calculation. Its misleading to designate a result with more significant figures than warranted. The number
of significant digits should not be greater than the lowest number of significant digits in any one of the numbers in a calculation. On the other hand, including all
the significant figures up to, but not including, the final result helps to trace a sequence of calculation (which was the modus operandi for this text).
Example 1 Convert 8 in. to mm, i.e., 8 in. 25.4 mm=in. ¼ 203.2 mm. Only one significant digit is warranted, giving a result, 200 mm.
References
Heausler, T. F., Metric Conversion—Rounding Off to the Appropriate Tolerances, Technical Briefs, Burns & McDonnel, Kansas City, No. 4, 1994.
Lindeburg, M. R., Engineering Unit Conversions, 3rd edn., Professional Publications, Inc., Belmont, CA, 1993.
Wandmacher, C. and Johnson, A. I., Metric Units in Engineering—Going SI, ASCE Press, New York, 1995.
Elias, H. G., An Introduction to Polymer Science, VCH, Weinheim, New York, 1997.
Two Web sites found useful for conversions are given below. The respective, years of the last contacts are shown also.
http:==www.Imnoeng.com=units.htm, 2010.
http:==OnlineConversion.com, 2010.