Electrical conductivity meter

An electrical conductivity meter.

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

Principle

The common laboratory conductivity meters employ a potentiometric method and four electrodes. Often, the electrodes are cylindrical and arranged concentrically. 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. 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 effects the 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
• A webpage by Horiba telling about temperature compensation
• A book which includes a discussion of temperature compensation
• Milli-Q System High Precision Coaxial Resistivity Cell
• ASTM D1125 - 95(2009) Standard Test Methods for Electrical Conductivity and Resistivity of Water
• ASTM D5682
• DIN 55667