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'''Hydrostatic weighing''', also referred to as "underwater weighing," "hydrostatic body composition analysis," and "hydrodensitometry," is a technique for measuring the [[density|mass per unit volume]] of a patient's body.

==Method==
The following method has the advantage of needing no volume information of the body (see the note about the residual volume below). The procedure is based on [[Archimedes' principle]], using the following three measurable values: The weight of the body outside the water, the weight of the completely immersed body (a weight that may be negative if the body in question is less dense than water) and the density of the water:
:<math> \frac { \mbox{density of body}} { \mbox {density of water} } = \frac { \mbox{weight of body}} { \mbox{weight of body} - \mbox{weight of immersed body}}\,</math>
Multiplying by the density of water gives
:<math> \mbox{density of body} = \mbox {density of water} \frac { \mbox{weight of body}} { \mbox{weight of body} - \mbox{weight of immersed body}}\,</math>.
Correcting for the residual volume of air in the lungs upon maximal exhalation, and the approximately 100 cc of air trapped in the intestines gives:
:<math> \mbox{density of body} = \frac {  \mbox {density of water} * \mbox{weight of body}} {( \mbox{weight of body} - \mbox{weight of immersed body}) -  \mbox {density of water} * ( \mbox{residual lung volume} + \mbox{100 cc})}\,</math>.
Residual [[lung volume]] can be estimated as a proportion of [[vital capacity]] (0.24 for men and 0.28 for women).<ref>{{cite journal | author = Wilmore, J. H. | title = The use of actual predicted and constant residual volumes in the assessment of body composition by underwater weighing | journal = Med Sci Sports | year = 1969 | volume = 1 | pages = 87–90}}</ref>

==Application==
From this density information  [[body composition]] and percentage [[body fat]] may be estimated. This is occasionally used for assessment of [[obesity]]. The [[residual volume]] in the lungs can add error if not measured directly. Hydrostatic weights gives body density, and percent body fat can be estimated using the Siri<ref name="siri">{{citation | author = Siri, SE | chapter = Body composition from fluid spaces and density: analysis of methods | editors = Brozek J, Henschel A | title = Techniques for measuring body composition | location = Washington, DC | publisher = [[United States National Academy of Sciences|National Academy of Sciences]], [[United States National Research Council|National Research Council]] | year = 1961 | pages = 223–34 }}</ref> or Brozek<ref name="brozek">{{citation | author = Brozek J, Grande F, Anderson JT, Keys A | title = Densitometric Analysis of Body Composition: Revision of some Quantitative Assumptions | journal = Ann. N. Y. Acad. Sci. | volume = 110 | pages = 113–40 |date=September 1963 | pmid = 14062375 | url = http://www.blackwell-synergy.com/openurl?genre=article&sid=nlm:pubmed&issn=0077-8923&date=1963&volume=110&spage=113 | doi = 10.1111/j.1749-6632.1963.tb17079.x}}</ref> formula.

== References ==
{{reflist}}

==See also==
*[[Body composition]]
*[[Composition of the human body]]

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[[Category:Obesity]]

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