GENERAL
Electric conductivity is defined as the ability of a solution to conduct an electric current and it is the reciprocal of electrical resistivity. The unit of measurement commonly used is the micro mhos or mili mhos.
In aqueous solutions conductivity is directly proportional to the concentration of dissolved solids. Therefore higher the concentrations of solids, greater the conductivity. The relation between conductivity and Total dissolved solids (TDS) is expressed, depending on the application and with a good approximation by :-
TDS = .63 conductivity approx.
[where PPM i.e. mg/l is the measuring unit for dissolved solids]
The conductivity of a solution is determined by molecular motion. Temperature affects molecular motion and it is therefore important to compensate for temperature when accurate and precise measurement are necessary. For comparative measurements, the standard temperature is normally 200 C or 250 C [680 F or 770 F].
LABMED digital conductivity / TDS meter is accurate, rugged and reliable instruments. Two models are available.
These are : -
(a) Hand Held Model LM 301 – 9 V DC operated only (TDS Mode).
(b) Table Model LM 401 – Mains / 9 V DC operated (TDS / Conductivity mode).
It is possible to differentiate conductivity meter according to the measurement method they use i.e. amperometric or potentiometric. The potentiometric system applies a known potential difference (V) to the electrode and measure the current (I) that passes through them. According to ohms law I = V / R.
Where R is resistance, V is the known voltage and I is the current going from the electrode to the other. It follows that the higher the current obtained, the greater the conductivity. The resistance however depends on the distance between the two electrodes and their surfaces, which can vary due to deposits of salts or other materials (electrolysis). The using potentiometric method is based on the principle of induction and eliminates common problem of effect of polarization. An alternating voltage is applied to the outer rings and induce a current loop in the solutions.
It is possible (approximation only) to obtain the value of hardness of water in French degree [0F]. The main cause of hard water is the presence of dissolved calcium and magnesium ions. The most common unit of hardness is French degree [0F]. Dividing the PPM measurement of TDS by 10 gives only approximate value of hardness with an error of 4 – 5 0F. [10 F = 10 PPM of CaCO3].