YOKOGAWA SC72 - Page 56

IM 12D03D02-01E
7-3
7. Measuring Principles of this Instrument
As shown in the above table, liquid conductivity changes with liquid temperature. So,
in order to compare conductivities, conductivity values at a xed liquid temperature
are needed. This SC72 meter includes standard temperature conversion functions,
to display a liquid conductivity value converted to 25°C, so this meter can be used
for such conductivity measurement comparisons. Temperature compensation
coe󰀩cients of an NaCl solution are stored in the SC72 meter. No other temperature
coe󰀩cient setting is required (for NaCl solutions).
Table 7.2 shows temperature coe󰀩cients of various electrolytic solutions.
Table 7.2 Temperature Coe󰀩cients of Electrolytic Solutions
Solutions
Temperature coe󰀩cient (%/°C)
1mol/l 1/10mol/l 1/1000mol/l
Lithium benzoate solution --- 2.28 2.28
Sodium acetate solution 2.20 2.20 2.20
Potassium chloride solution 1.74 --- 1.98
Sodium hydroxide solution 1.74 --- 1.87
Sulfuric acid 1.07 --- 1.38
• Finding Temperature Coe󰀩cient
If temperature coe󰀩cient tables containing the liquid to be measured can not be
found, measure conductivity twice at two liquid temperatures between 10 and 30°C
with temperature coe󰀩cient set to 0.00 and use the equation shown below to nd
approximate temperature coe󰀩cient (α).
α = Temp. coef. x 100 (%/°C)
K
2
– K
1
K
1
(t
2
– 25) – K
2
(t
1
– 25)
where t
1
, t
2
: liquid temperature (°C)
K
1
: conductivity at t
1
K
2
: conductivity at t
2
(Calculation example)
To nd the temperature coe󰀩cient of liquid with conductivity - 124.5 (S/cm at liquid
temperature 18.0°C and 147.6 (S/cm at liquid temperature 31.0°C, substitute t
1
=
18.0, t
2
= 31.0, K
1
= 124.5, K
2
= 147.6 in the equation above, thus we can obtain:
α = x 100
147.6 – 124.5
124.5 x (31.0 – 25) – 147.6 x (18.0 – 25)
= x 100
23.1
747.0 – (–1033.2)
= 1.298
Set the temperature coe󰀩cient 1.30 in the SC72 meter (as meter allows three digits to
be entered).