KERN & SOHN EMB 500-1BE - Manual (Page 33)

EMB-V-BA-e-1422 33
ρ =
A
A-B
o
ρ
9 Preconditions for Precise Measurements
There are numerous error possibilities during density determination.
Accurate knowledge and caution are required to achieve precise results when
applying this density kit in combination with the balance.
9.1 Calculation of Results
The balance displays results for density determination by giving four decimal places.
However, this does not mean that the results are accurate down to the last decimal
place as this would be the case for a calculated value. Therefore all weighing results
used for calculations have to be examined closely.
Example for density determination of solids:
To ensure high-grade results, numerators as well as common denominators of the
formula below must show the desired accuracy. If either of them is instable or flawed,
the result, too, will be instable or flawed.
ρ = density of sample
A = weight of the sample in air
B = Weight of the sample in the aid liquid
ρ
o
= Density of the aid liquid
9.2 Influence Factors for Measurement Errors
9.2.1 Air bubbles
A small bubble with a diameter of 1 mm results in a raise of 0.5 mg, while those with
2 mm Ø already produce a raise of 4 mg.
Therefore, make sure that no air bubbles adhere on the fixed object or sinker that is
immersed in the liquid.
An oily surface causes air bubbles when immersing in the liquid, so
 degrease the solvent-resistant solid sample
 clean all parts that are immersed regularly and don't touch them with bare
fingers
Don't lay solid samples (particularly flat objects) outside of the liquid on the sample
dish, because this results in air bubbles by the joint immersion.
9.2.2 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.