Editing Relative density (section)

===Hydrostatic weighing===
Relative density is more easily and perhaps more accurately measured without measuring volume. Using a spring scale, the sample is weighed first in air and then in water. Relative density (with respect to water) can then be calculated using the following formula:
<math display="block">RD = \frac{W_\mathrm{air}}{W_\mathrm{air} - W_\mathrm{water}},</math>
where
*''W''<sub>air</sub> is the weight of the sample in air (measured in [[newton (unit)|newton]]s, [[pound-force|pounds-force]] or some other unit of force)
*''W''<sub>water</sub> is the weight of the sample in water (measured in the same units).

This technique cannot easily be used to measure relative densities less than one, because the sample will then float. ''W''<sub>water</sub> becomes a negative quantity, representing the force needed to keep the sample underwater.

Another practical method uses three measurements. The sample is weighed dry. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the displaced water has overflowed and been removed. Subtracting the last reading from the sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method allows the use of scales which cannot handle a suspended sample. A sample less dense than water can also be handled, but it has to be held down, and the error introduced by the fixing material must be considered.