Temperature Compensation; Ec/Tds Coversion Factor - Hanna Instruments HI 9812 Instruction Manual

TEMPERATURE COMPENSATION

TEMPERATURE COMPENSATION TEMPERATURE COMPENSATION TEMPERATURE COMPENSATION TEMPERATURE COMPENSATION
The conductivity of an aqueous solution is the measure of its ability to
carry an electrical current by means of ionic motion.
The conductivity invariably increases with increasing temperature.
It is affected by the type and number of ions in the solution and by the
viscosity of the solution itself. Both parameters are temperature depen-
dent. The dependency of conductivity on temperature is expressed as a
relative change per degree Celsius at a particular temperature, com-
monly as percent per °C.
For common ionic solution, this value is about 2%/°C. Acids, alkalis
and concentrated salt solutions have somewhat lower value, typically
1.5%/°C.
Since a small difference in temperature causes a large change in
conductivity, it is necessary to compensate for conductivity readings at
high and low temperature. The readings are usually normalized at
25°C.
HI 9812 and HI9813 automatically compensate for temperature
differences with a built-in temperature sensor circuitry. With this
compensation, the display shows the readings at 25°C (77°F).
EC/TDS CONVERSION FACTOR EC/TDS CONVERSION FACTOR
EC/TDS CONVERSION FACTOR EC/TDS CONVERSION FACTOR EC/TDS CONVERSION FACTOR
The TDS value in aqueous solutions is directly proportional to conduc-
tivity. The ratio between the two parameters depends on the solution.
HI 9812 has a fixed conversion factor set to 0.5 (corresponding to
a solution of CaCO ). This means that 1 µS/cm is equal to 0.5 ppm
3
(mg/L) of TDS.
HI 9813 has a variable conversion factor form 0.56 to 0.72 accord-
ing to TDS442 curve (see fig. below). TDS442 standard curve
represents the best approximation for the TDS measurement in the
field of hydroponics, farming and ground water, where many ingre-
dients in fertilizer mixtures and ground water are present.
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