Rice Lake Discovery Series Semi-Micro and Analytical, Ohaus - Manual (Page 24)

EN-20 Ohaus Discovery
3.5.9 Density (Cont.)
Solid Density Determinations for items Less Dense Than Water
For density determination of solids with a density less than
1 g/CM3, the bottom of the Weigh Below Hook for solids must be used as it holds the solid body below the surface of the auxiliary liquid. If the
buoyancy of the solid is greater than the weight of the Weigh Below Hook, the Weigh Below Hook must be weighted by placing an additional
mass on the submerged part of the Weigh Below Hook as shown.
Weigh the sample in air first as explained in the previous procedure.
After loading the additional mass, tare the balance and start the weighing again. Wait until the balance has reached stability and note the dis-
played weight
P (buoyancy of the solid).
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 determina-
tions.
Air Buoyancy
1 CM3 of air weighs approximately 1.2 mg (depending on the physical condition). As a consequence, in the weighing in air, each solid experi-
ences a buoyancy of this magnitude (the so-called “air buoyancy”) per cm3 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 cm3 volume of the body) is added to the calculated result:
Calculated density + 0.0012 g/cm3 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.
80104130.indd 20 11/7/2006 9:22:36 AM