Unlike LSI, RSI uses a database that contains different values of scale thickness. The databases are used to predict the effects of water chemistry, especially for companies supplying this commodity to major municipalities in the world. The index was developed after it was observed that water with a high scaling potential often caused films in steel pipes. In addition, the water was seen to corrode such pipes (Papavinasam, 2013).

The value of RSI is obtained using the following formula:

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RSI = 2 pHs- PH (Value of the pH measured).

For water to be at equilibrium with CaCO3, the value of RSI should range between 6.5 < RSI < 7 (RSI greater than 6.5, but less than 7). Within this range, no scaling will occur (Papavinasam, 2013). The CaCO3 contained in the water neither precipitates nor forms scales. When the value of RSI is greater than 8 (RSI > 8), it is said to be under saturated. The CaCO3 contained in the water at this point tends to dissolve as a result of its deficiency. There are lower chances of scaling at this RSI values. Water with an RSI value that is lower than 6.5 is said to be super- saturated with CaCO3. Such medium tends to precipitate the compound, leading to scaling (Papavinasam, 2013).