Mettler Toledo 33360 Mode D'emploi page 7

3.2 Air Buoyancy
Depending on its physical conditions, each cubic centimeter (cm ) of air weighs 1 ... 1.2 mg. Thus, any object
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that is being weighed in air is subject to this kind of buoyancy for each cm of its volume. This means that – with
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a density of 1 g/ cm 3 – an error of approximately 0.1 % would occur if the air buoyancy is not taken into
consideration. If a result with 3 or 4 decimal places is required, the result will have to be corrected for air buoyancy.
The true density is about 0.001 g/ cm more than the calculated density.
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When the density of liquids is determined, the operating procedure (division by 10 ml, corresponding to the
volume of the sinker), provides a fourth-place result, even if the balance display shows only a three-place
indication. It is therefore advisable – in this case – to make a general buoyancy correction.
3.3 Volume tolerance of the float
German Weights and Measures Regulation EO 13-4 paragraph 9.21
The volume of the float, together with the lower portion of the suspension wire, must be adjusted so that a 30 g
float arrangement does not produce a measuring error exceeding ± 0.00059 g/ cm when determining the water
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density at a temperature of 20 °C.
3.4 Immersion depth of the sinker or of the gem holder
The sinker (immersion body) is suspended from a platinum wire which has a diameter of 0.2 mm. In the water,
the wire thus experiences a buoyancy of 0.3 mg when 10 mm of the wire are immersed.
If the fluid level stands 10 mm above the eylet of the sinker, about 20 mm of the wire are immersed. With a density
(of liquid) of about 1, this would result in a buoyancy of 0.6 mg. But since this would again have to be divided by
10 ml, this influence can be disregarded.
The submersible part of the gem holder is made of a wire that has a diameter of 0.8 mm. With a liquid density
of approx. 1, this results in a buoyancy of about 5 mg for each 10 mm that is submerged. But since the gem holder
remains submerged while the solid body is weighed in air, and since – with electronic balances – the immersion
depth does not change from one weighing to the next (in spite of the difference in weight), the buoyancy of the
gem holder remains constant and can thus be disregarded. Condition: Do not change the liquid level. (The change
of the liquid level caused by submerging the solid body can – in most cases – be disregarded. A solid body with
a volume of 1 cm causes the liquid to rise by about 0.5 mm. This is equivalent to a buoyancy of about 0.15 mg,
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i.e. a density error of 0.15 mg/ cm ).
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3.5 Surface tension of liquid
Since the liquid adheres to the suspension wire, an apparent weight increase occurs. In the case of the sinker
(wire diameter 0.2 mm), and when water is used as the liquid, this force amounts to about 1 mg. By using wetting
agents or organic liquids, this force can be reduced to 0.3 mg. However, this value will also be divided by 10 ml
so that the resulting density error amounts to no more than 0.0001 g/ ml.
Because of the larger diameter of the wire submerged in the water, a force of up to 3 mg acts upon the gem holder.
Here, too, similar to what is described in Section 3.4, any influence on the result will be virtually eliminated by
having the gem basket submerged during both weighings (A and B). For very high accuracy requirements,
reducing the surface tension would constitute an additional precautionary measure (see also Section 3.6).
METTLER TOLEDO 33360 + 210260 5