horiba U-51, Инструкция по эксплуатации

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6 Reference
111
Let us assume that the current, I(A), flows in a sample of conductivity L

under automatic
control of the voltage-applying electrodes

so that the voltage at the voltage-detecting-
electrodes, E(V), remains constant at all times.
Then, the resistance of the sample, R(

), across the voltage-detecting electrodes is
represented as R=E/I. The resistance, R, of the sample is inversely proportional to its
conductivity, L. Accordingly, a measurement of current, Is,
of a standard solution of known conductivity, Ls, enables calculation of conductivity of a
sample according to the formula L = Ls (I/Is) from the ratio L : Ls = I : Is.
Even in the 4-electrode method, polarization occurs, since AC current flows in the voltage-
applying electrodes. The voltage-detecting electrodes are, however, free from the effects of
polarization, since they are separated from the voltage-applying electrodes, and furthermore,
current flow is negligible. Therefore, the 4-electrode method is an excellent method to enable
measurement of conductivity covering a very high range.
6.5.2 SI units
New measurement units, called SI units, have been in use from 1996. Accordingly, the U-50
series also uses SI units. The following conversion table is provided for people who use the
conventional kind of conductivity meter.
Note that along with the change in unit systems, the measurement values and cell counts
have also changed.
6.5.3 Temperature coefficient
In general, the conductivity of a solution varies largely with its temperature.
The conductivity of a solution depends on the ionic conductivity, described earlier. As the
temperature rises, conductivity becomes higher since the movement of the ions becomes
more active.
The temperature coefficient shows the change in % of conductivity per °C, with a certain
temperature taken as the reference temperature. This is expressed in units of %/°C. The
temperature coefficient assumes the premise that the conductivity of a sample changes
linearly according to temperature.
Strictly speaking, with actual samples, however, conductivity changes along a curve.
Furthermore, the curve varies with the type of sample. In the ranges of smaller temperature
changes, however, samples are said to have the temperature coefficient of 2%/°C (at
reference temperature 25°C); this holds for most samples, except in certain special cases.
(The temperature coefficients for various types of solutions are listed on the next page.)
The U-50 series uses an automatic temperature conversion function to calculate conductivity
at 25°C at a temperature
coefficient of 2 %/°C based on the measured value of the temperature. Results are displayed
on the readout.
The U-50 series
temperature conversion function is based on the following formula.
L
25
= L
t
/ { 1 +K (t - 25) }
L
25
: Conductivity of solution converted to 25°C
t : Temperature of solution at time of measurement (°C)
L
t
: Conductivity of solution at t (°C)
K : Temperature coefficient (%/°C)
Former units
→
SI unit
Measurement
value
0.1 mS/c
ｍ
1 mS/c ｍ
100 mS/c ｍ
→
→
→
0.01 S/m
0.1 S/m
10 S/m