The correlation factor ranges between 0.5 and 0.8 and must be determined empirically. If the main ions are not known, a common correlation factor used is 0.67.
Temperature Compensation for Conductivity Measurements
Conductivity increases as temperature increases, which is automatically compensated for by many instruments. This standardizes the output to 25°C to allow for measurement comparison. A problem arises because the amount that conductivity changes per degree is influenced by the composition of the solution. The automatic temperature compensation (ATC) typically used is 2% increase per degree Celsius to standardize each reading to 25 °C, but can introduce errors in measurements of extremely low or high conductivity.
For extremely low conductivity, such as in ultrapure water, each degree Celsius change induce a 5% change in conductivity, whereas extremely concentrated ionic solutions require temperature compensation of only 1.5% per degree Celsius. The 1.5% compensation also applies to certain acidic and alkaline solutions and empirical testing is required to determine the most accurate compensation method for your application.
There are instruments that offer programmable temperature compensation, but that functionality comes with a price. It is relatively easy to compensate for as a consumer by following the equation below if the automatic temperature compensation is known and a different compensation value is desired. Many industrial automation programs such as PLCs will allow for inclusion of these equations to modify the output values to whatever is most suitable for the application.
Below is the equation for manual calculation of temperature compensation along with examples. By knowing the ATC percentage used, it is possible to work backwards in circumstances where a more accurate temperature compensation value is known. This equation can also find the most accurate temperature compensation value in a controlled environment.