Ohaus Voyager Balances - Manual (Page 113)

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6.9.4 Improving the accuracy of the result of solid density
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, 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.I 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 on
all density determinations.
Air buoyancy
1 CM 3 of air weighs approximately 1.2 mg (depending on the physical condition). As a consequence, in the weigh-
ing 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 calcu-
lated 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), the influence of the apparent weight increase can be neglected because the balance is tared before every
measurement.
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.
Balance modes