Use the following formula to correct this error:
W (a) · [ρ (fl) – ρ (a)]
ρ =+ ρ (a)
0.99983 [W (a) – W (fl)]
Since the correction factor is determined exclusively by the geometry of the
measuring device setup, be sure to only use the large diameter beaker (76 mm)
from the kit when determining the specific gravity of a solid. The “Supplement”
to this manual shows how this correction factor is derived.
Adhesion of Liquid to the Wire
When the sample holder (or sieve) is immersed in liquid causing buoyancy,
liquid travels up the wire because of adhesion forces and generates an additional
weight in the range of a few milligrams.
Since the sample holder (or sieve) is in the liquid causing buoyancy during both
weighing in air and weighing in liquid, and the balance is tared at the beginning
of each measuring procedure, the effect of the meniscus can be disregarded.
To reduce the surface tension and the friction of liquid on the wire, add three
drops of a tenside (Mirasol Antistatic or an ordinary dishwashing detergent) to the
distilled water in the beaker.
Because of the liquid travelling up the wire, the weight may slowly change even
after the stability symbol “g” appears. Therefore, read off the weight immediately
after the “g” is displayed.
Air Bubbles
The measuring error caused by air bubbles adhering to the sample can be
estimated in the following manner. An air bubble with a diameter of 0.5 mm
causes an additional buoyancy of less than 0.1mg when a sample is weighed in
water. An air bubble diameter of 1mm causes additional buoyancy of 0.5 mg
and an air bubble diameter of 2 mm causes approx. 4.2 mg additional buoyancy.
Larger air bubbles must be removed with a fine brush or other utensil.
You can also wet the sample in a separate container before you weigh it.
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