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[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

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,
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 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