8.D7 conductivity probe FYD741LF
8.1.Basic principles
The conductivity (unit S/m = Siemens/meter)
is a measure for the ion concentration in a
measuring solution. Conductivity is proportio-
nal to the amount of salt, acid, or base alkaline
content measured in the solution, High-purity
water has a conductivity of approx. 0.05 µS/
cm (at +25 °C), natural water approx. 100 to
1000 µS/cm, and some alkaline solutions (e.g.
potassium hydroxide solution) slightly more
than 1200 mS/cm. The diagram on the left
shows further relevant examples of aqueous
solutions.
Standardization
The method for determining the electrical con-
ductivity of water is defined in DIN EN 27
888.
Temperature compensation
Conductivity is a temperature-dependent variable. For most diluted, aqueous salt solu-
tions within a certain temperature range conductivity is an approximately linear functi-
on of temperature T.
κ
T
=
κ
25
(1+α(T-25°C)/100)
Conductivity, at reference temperature 25°C
κ25, is calculated as follows :
κ
25 =
κ
T
/ (1+α(T-25°C)/100)
The temperature coefficient α describes the
relative change in conductivity in % as the
temperature changes with respect to refe-
rence temperature 25 °C.
α
= (
κ
T
-
κ
25
) * 100% / κ
25
(T-25)
Temperature coefficient α depends on:
●
the chemical composition of the soluti-
on
●
the concentration of the electrolyte
●
the temperature, especially at low con-
ductivity levels of <1 µS and at very high
conductivity levels
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