Ohaus CARAT PLUS PRECISION JEWELRY BALANCES Density Determination Kit Manual User Manual
Page 7
7
IMPROVING THE ACCURACY OF THE RESULT
The following tips should help you improve the accuracy of the results in the density
determination of solids.
Temperature
Solids are generally so insensitive to temperature fluctuations that the corresponding
density changes are of no consequence. However, as work is performed with an
auxiliary liquid in the density determination of solids following Archimedes' principle,
their temperature must be taken into account as the temperature has a greater effect with
liquids and causes density changes in the order of magnitude 0.1 to 1% per °C. This
effect is already apparent in the third decimal place of the result.
To obtain accurate results, we recommend that you always take the temperature of the
auxiliary liquid into account an all density determinations. You can take the appropriate
values from a book of density tables when using all other balances except for Voyager
or Voyager Pro which have built-in density tables for water and ethanol.
Air Buoyancy
1 cm
3
air weighs approximately 1.2 mg (depending on the physical condition). As a
consequence, in the weighing in air, each solid experiences a buoyancy of this
magnitude (the so-called "air buoyancy") per cm
3
of its volume.
However, the air buoyancy must be taken into account only when a result is required with
an accuracy of 3 to 4 decimal places. To correct for this, the air buoyancy (0.0012 g per
cm
3
volume of the body) is added to the calculated result:
Calculated density + 0.0012 g/cm
3
air buoyancy = effective density
Surface Tension of the Auxiliary Liquid
Adhesion of the liquid to the Weigh Below Hook causes an apparent weight increase
of up 3 mg.
As the Weigh Below Hook is immersed in the auxiliary liquid in both weighings of the
solid (in air and in the auxiliary liquid) and the balance is tared before every measure-
ment, the influence of the apparent weight increase can be neglected.
To reduce the effect of air bubbles and to ensure the greatest possible accuracy, use a
few drops of a wetting agent (not supplied) and add them to the auxiliary liquid.
density kit.pmd
8/9/2005, 1:12 PM
7