ELMETRON CC- 411, User Manual, Page: 21 / 52

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7. BASIC INFORMATION ABOUT THE CONDUCTIVITY MEASUREMENT
The conductivity measurement is based on applying electric current with
proper voltage and frequency between two electrodes and measuring it. In
CC-411 the frequency depends on the measuring range and may vary from 1
kHz up to 12 kHz. The voltage is dependent from the kind of the measured
liquid, its concentration and temperature. The conductivity result informs
indirectly about the salt concentration in the measured liquid. The greater it
is, the higher conductivity (KCl, NaCl). This dependence doesn’t concern all
of the solutions. In some cases, after exceeding certain salinity value
conductivity starts decreasing. Temperature has also a great influence on the
results of conductivity measurement, which increases together with the
temperature rise.
The electrodes’ surface and the distance between them have decisive
influence on the K constant value of the cell. This value has a great influence
on the accuracy of the measurement. Depending on the measured
conductivity value cells with K constant = 0.05 cm
-.1
up to 1.500 cm
-.1
are
used. During measurement the meter multiplies the measured value by the K
constant introduced to the meter’s memory and displays the result in units of
conductivity
(
µ
S/cm or mS/cm). Shortened symbol of the unit is displayed next
to the result (
µ
S or mS). Conductivity is affected by the temperature and
salts concentration. In order to enable comparing of the results, measured
value is counted by the meter to the value which corresponds to measurement
in 25
0
C. Measurement in this temperature is the most accurate. In other
temperatures there is temperature compensation used, which means that the
meter takes currently measured temperature and
α
coefficient into
consideration. This coefficient describes how much the result will change
when the temperature change is 1
0
C. The
α
coefficient in this meter is
constant and equal 2% /
0
C. This is the closest value to that of most of the
salts in 25°C. For example if measurements are made in liquid of temperature
coming to 30
0
C, the result change will total up to 5 x 2% = 10%.
The conversion is made automatically and this change is counted into the
result. The conductivity measurement should always be treated as burdened
with a certain error, which depends on the conductivity cell (its linearity) and
temperature. When measurements are not made in the reference
temperature, the error is dependent mainly on the
α
coefficient, which is
affected by the temperature and concentration changes.