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

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:

where

T is the temperature of the sample,
Tcal is the calibration temperature,
σT is the electrical conductivity at the temperature T,
σTcal is the electrical conductivity at the calibration temperature Tcal,
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 and 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

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