User manual

PBS-A03/A04-BA-e-1213 17
6.2 Influence Factors for Measurement Errors
6.2.1 Air bubbles
A small bubble of, for example, 1mm
3
will have a considerable influence on the
measurement if the sample is small. Buoyancy will be increased by approximately 1mg
resulting immediately in an error of 2 digits. Hence, it has to be ensured that no air
bubbles cling to the solid immersed in the fluid. The same applies to the sinker that is
immersed in the test fluid.
Take great care when removing air bubbles by swirling, to prevent the fluid from spurting
out and splashing onto the sifting bowl or from water splashing. Moisture on the
suspension bracket of the sifting bowl results in increased weight.
Do not touch the solid sample or sinker with bare fingers. An oily surface causes air
bubbles when immersing the specimen in fluids.
Do not place solid samples (in particular flat objects) in the sifting bowls outside the liquid
as this would result in air bubbles when immersed together. For this reason examine the
bottom of the sifting bowl for air bubbles after the specimen had been immersed in fluid.
6.2.2 Solid body sample
A sample possessing too great a volume that is immersed in fluid will result in an increase
in fluid level inside the glass pitcher. As a result, part of the suspension bracket of the
sifting bowl will also be immersed causing buoyancy to increase. As a consequence the
weight of the specimen in the fluid will drop.
Samples that change the volume or assimilate fluid are unsuitable for measurement.
6.2.3 Liquids
Water temperature is another factor to be taken into consideration. The density of water
changes by c. 0.01% per degree Celsius. A temperature measurement showing an error
of 1 degree Celsius results in an inaccurate fourth decimal place.
6.2.4 Surface
The suspension bracket of the sifting bowl penetrates the surface of the fluid. This state
undergoes continuous change. If the sample or the sinker is relatively small, the surface
tension will impair repeatability. The addition of a small amount of detergent makes the
surface tension negligible and increases repeatability.
6.2.5 Sinker for liquid density measurement
To save precious specimen liquid, a small beaker and a sinker that fits such a beaker
are usually used to measure liquid density. However, note that a larger sinker will
bring higher accuracy.
Both the buoyancy and the sinker volume are desired to have as many significant
figures as possible for precise results. Because they are the numerator and
denominator of the expression to compute the liquid. Seeking accurate volume of the
sinker is also important.