OHAUS Voyager V00640, Инструкция по эксплуатации

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3.14 Density (Cont.)
3.14.3 Improving the Accuracy of the Result of Solid Density Determinations
3.14.4 Liquid Density Determinations
The density of a liquid can be made using either a sinker of known
volume
or a Pycnometer. When using the sinker,
the sinker is weighed in air and then in the liquid whose density is to be determined. The density
Q can be deter-
mined from the two weighings as follows:
Q = Density of the liquid
A = Weight of the sinker in air
B = Weight of the sinker in liquid
V = Volume of the sinker
P = Buoyancy of the sinker in the liquid ( P= A-B )
V
A - B
Q =
When the Pycnometer is used, it is filled with a known volume of a liquid. The density is arrived at as follows:
Density = + { }
NOTE
: pycnometer may be obtained at laboratory supply firms.
Weight of full pycnometer - Weight of pycnometer
Volume of pycnometer
Air
density
• Follow the same procedure for solid density determi-
nation except select LIQUID under the Select material
display. The balance is prepared in the same manner.
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 0.1 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.
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 measurement, 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.