Ryznar Stability Index (RSI) Calculator
Use this tool to estimate the indice de Ryznar from key water chemistry values. Enter all values in the requested units and click Calculate.
Formula basis: pHs = (9.3 + A + B) − (C + D), RSI = 2 × pHs − pH
What is the Ryznar Index?
The Ryznar Stability Index (RSI), also called indice de Ryznar, is a widely used indicator for estimating whether water will tend to form calcium carbonate scale or become corrosive. It is common in boiler treatment, cooling towers, potable systems, pool chemistry, and industrial process water programs.
Unlike a simple pH check, RSI combines multiple chemistry factors. This gives a better practical picture of scale or corrosion tendency under real operating conditions.
How the calculator works
This calculator computes the saturation pH (pHs) first, then derives RSI:
B = −13.12 × log10(°C + 273) + 34.55
C = log10(Ca hardness as CaCO3) − 0.4
D = log10(alkalinity as CaCO3)
pHs = (9.3 + A + B) − (C + D)
RSI = 2 × pHs − pH
The lower the RSI value, the more likely scaling becomes. The higher the value, the more likely corrosion becomes.
Typical interpretation ranges
| RSI Range | General Tendency | Operational Meaning |
|---|---|---|
| < 6.0 | Strongly scale-forming | High likelihood of calcium carbonate deposition |
| 6.0 – 7.0 | Slightly scale-forming to stable | Often manageable with routine control |
| 7.0 – 7.5 | Near balanced to mildly corrosive | Watch metal surfaces and monitor trends |
| 7.5 – 8.5 | Corrosive tendency | Increased risk for metal loss and pinhole failures |
| > 8.5 | Highly corrosive | Prompt treatment review is usually needed |
Input tips for more accurate results
- Use fresh lab or field readings collected at the same time and location.
- Confirm all units are mg/L as CaCO3 for hardness and alkalinity.
- Use representative operating temperature, not ambient room temperature.
- If your water contains unusual species (phosphates, silicates, organics), treat RSI as a screening estimate.
What to do if your RSI is outside target
If RSI is too low (scaling tendency)
- Review hardness and alkalinity control strategy.
- Lower pH carefully within your safe operating envelope.
- Check concentration cycles and blowdown in recirculating systems.
- Consider antiscalant dosing where appropriate.
If RSI is too high (corrosive tendency)
- Check if pH is too low for the material of construction.
- Verify alkalinity reserve and buffering capacity.
- Evaluate corrosion inhibitor program and residual levels.
- Inspect high-risk zones (heat exchangers, dead legs, low-flow sections).
Important limitations
RSI is a strong first-pass index, but it is not the only decision tool. Corrosion and scale are affected by flow velocity, microbiology, oxygen, chloride/sulfate level, metallurgy, and treatment chemistry. For critical systems, always combine RSI with direct monitoring, inspection, and professional water treatment guidance.
Quick FAQ
Is RSI the same as LSI?
No. They are related but not identical. Both are based on saturation concepts, yet interpreted differently. Many operators track both for context.
Can I use this for pools, cooling towers, and boilers?
Yes, as a general indicator. However, each system has different target ranges and control priorities.
Why does temperature matter so much?
Temperature significantly shifts calcium carbonate solubility and therefore changes pHs and RSI.