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Basic measurement knowledge
3.7
Basic measurement knowledge
Conductivity of metals is commonly measured in two
sets of units. In the SI system the unit of conductivity
is the MegaSiemens/meter (reciprocal ohm-meter).
Conductivities of metals at ambient temperature are
typically in the range of 1 to 60 MegaSiemens/meter.
For convenience metal conductivities are often ex-
pressed as a percentage of the conductivity of a stan-
dard sample of copper – the International Annealed
Copper Standard (IACS). 100 % IACS corresponds
to 58 MegaSiemens/meter. Note that since this "pure"
copper standard was established in 1913 pure copper
now often has a conductivity greater than 100 % IACS.
The AutoSigma 3000 measures the conductivity of
non-magnetic metals and alloys in the range 0.8 to
110.0 % IACS. It uses the Eddy Current technique for
measuring the conductivity of materials in % IACS, or
MegaSiemens/meter (set from the instruments menu
selections).
AutoSigma 3000
It is important to understand that eddy current mea-
surement is essentially a "skin" effect. The eddy cur-
rent fi eld intensity is greatest at the surface and de-
creases exponentially with depth. The depth at which
the fi eld strength reduces to 1/e (37 %) of its surface
value is referred to as the "standard depth of penetra-
tion". This depends primarily on the operating frequen-
cy and the conductivity of the metal.
It is generally considered that materials of thickness
greater than three standard depths of penetration can
be measured without any correction factors being re-
quired.
For example at 60 kHz this fi gure (the "effective depth
of penetration") is around 0.05" (1.25 mm) in Alumini-
um Alloys (conductivity approximately 35 % IACS) and
0.32" (8 mm) in Titanium alloy (conductivity approxi-
mately 1 % IACS). At 500 kHz the corresponding val-
ues are 0.02" (0.5 mm) and 0.11" (2.8 mm).
Care must also be taken when measuring non-homo-
geneous materials, for example materials which have
been surface heat-treated, clad or plated, or where the
surface is rough or corroded. Measurements at dif-
ferent frequencies will give different results due to the
Issue 04, 06/2007
Preparation for operation
3 - 9
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