Electrical conductivity meter

An electrical conductivity meter.

An electrical conductivity meter (EC meter) measures the electrical conductivity in a solution. Commonly used in hydroponics, aquaculture and freshwater systems to monitor the amount of nutrients, salts or impurities in the water.

Principle of operation

The common laboratory conductivity meters employ a potentiometric method and four electrodes. Often, the electrodes are cylindrical and arranged concentrically^{[citation needed]}. The electrodes are usually made of platinum metal. An alternating current is applied to the outer pair of the electrodes. The potential between the inner pair is measured^{[citation needed]}. Conductivity could in principle be determined using the distance between the electrodes and their surface area using the Ohm's law but generally, for accuracy, a calibration is employed using electrolytes of well-known conductivity.

Industrial conductivity probes often employ an inductive method, which has the advantage that the fluid does not wet the electrical parts of the sensor. Here, two inductively-coupled coils are used. One is the driving coil producing a magnetic field and it is supplied with accurately-known voltage. The other forms a secondary coil of a transformer. The liquid passing through a channel in the sensor forms one turn in the secondary winding of the transformer. The induced current is the output of the sensor.

Temperature dependence

Main article: Electrical conductivity

The conductivity of a solution is highly temperature dependent, therefore it is important to either use a temperature compensated instrument, or calibrate the instrument at the same temperature as the solution being measured. Unlike metals, the conductivity of common electrolytes typically increases with increasing temperature.

Over a limited temperature range, the way temperature affect conductivity of a solution can be modeled linearly using the following formula:

$\sigma_T = {\sigma_{T_{cal}}[1 + \alpha (T - T_{cal})] }$

where

T is the temperature of the sample,

T_cal is the calibration temperature,

σ_T is the electrical conductivity at the temperature T,

σ_Tcal is the electrical conductivity at the calibration temperature T_cal,

α is the temperature compensation slope of the solution.

The temperature compensation slope for most naturally occurring waters is about 2 %/°C, however it can range between 1 to 3 %/°C. The compensation slope for some common water solutions are listed in the table below.

Aqueous solution at 25 °C	Concentration (mass percentage)	α (%/°C)
HCl	10	1.56
KCl	10	1.88
H2SO4	50	1.93
NaCl	10	2.14
HF	1.5	7.20
HNO3	31	31

See also

• Conductivity factor
• Electrical conductivity
• Salinometer
• Total dissolved solids
• TDS meter

External links

• IEEE paper telling how a four electrode meter works
• Sales literature telling how a four electrode meter works
• A webpage by Horiba telling about temperature compensation
• A book which includes a discussion of temperature compensation
• Cole–Parmer article on conductivity and conductivity meters
• Milli-Q System High Precision Coaxial Resistivity Cell
• ASTM D1125 - 95(2009) Standard Test Methods for Electrical Conductivity and Resistivity of Water