Set the cycled water, then read the equilibrium verdict at the pipe wall. Supporting factors and the temperature curve wait until the current chemistry is established.
Model 04 · Open to everyone
Operating verdictScale forming+2.02 LSI at the wall
Concentration5.0×1,500 ppm cycled TDS
Surface temperature90 °FPipe consequence responds
Live saturation verdictScale forming
Diagram controlsInspect a live quantity
Supporting readoutsSeven measurementsOpen
Cycled calcium500ppm
Cycled alkalinity500ppm
VerdictScale forming±0.3 display bands
Operating curveLangelier index versus temperature90 °F selected
LSI vs temperature
Drag the plot to set the water temperature. The hottest surface in the loop sits to the right of the bulk reading.
Reference
How the saturation index model works
The Langelier Saturation Index compares the water's operating pH with the pH at which it would be exactly saturated with calcium carbonate. Positive means the water tends to scale, negative means it tends to corrode. In a cooling tower the water that touches the pipe wall is the cycled water, not the makeup, so the analysis is concentrated first.
The instrument above runs the chemistry live. The governing relationships are written out below.
Cooling tower water concentrates as it cycles. Each makeup analysis (calcium, alkalinity, TDS) is multiplied by the cycles of concentration to get the value the recirculating water actually sees.
Makeup analysis
Makeup calcium, M alkalinity, or TDS (each as CaCO3 where applicable) (ppm)
Cycles
Cycles of concentration
Assumes
Conservative concentration: every analysis scales linearly with cycles.
Saturation pH
pHs=(9.3+A+B)−(C+D)
The saturation pH: the pH at which the cycled water is exactly in balance with calcium carbonate. A = (log₁₀ TDS − 1) / 10, B = −13.12 × log₁₀(°C + 273) + 34.55, C = log₁₀(Ca) − 0.4, D = log₁₀(M alk).
TDS
Cycled total dissolved solids (sets A) (ppm)
Temperature
Recirculating temperature (sets B) (°F)
Ca as CaCO₃
Cycled calcium hardness (sets C) (ppm)
M alk as CaCO₃
Cycled M alkalinity (sets D) (ppm)
Assumes
Calcium carbonate saturation reference.
Temperature converted to °C internally.
Langelier Saturation Index
LSI=pH−pHs
The Langelier Saturation Index. Positive means the water tends to scale, negative means it tends to corrode, and most programs aim for roughly 0 to +1.
pH
Recirculating pH (measured or Caplan estimate)
pHs
Saturation pH
Assumes
Index only; not a rate.
Ryznar Stability Index
RSI=2×pHs−pH
The Ryznar Stability Index, a less sign-sensitive companion to LSI. Under 6 scales, 6 to 7 is roughly stable, and over 7 turns corrosive.
pHs
Saturation pH
pH
Recirculating pH
Assumes
Index only; reported without PSI.
Worked example
Take the room's default case: makeup at 100 ppm calcium, 100 ppm alkalinity, and 300 ppm TDS, run at 5 cycles and 90°F.
Cycled calcium
500 ppm
Cycled alkalinity
500 ppm
Cycled TDS
1,500 ppm
Estimated pH
8.49
Saturation pH
6.47
LSI
+2.02
RSI
4.45
Read the index against the bands: positive tends to scale, negative tends to corrode, and most programs aim for roughly 0 to +1.
Common questions
What does a positive LSI mean?
Positive means the water tends to deposit calcium carbonate scale, and negative means it tends to dissolve it and corrode. The index states a tendency, not a rate; most programs aim for roughly 0 to +1.
Why is the makeup analysis cycled first?
The recirculating water, not the makeup, touches the heat exchange surfaces. Each analysis value is multiplied by the cycles of concentration so the index describes the water the pipe wall actually sees.
How do LSI and RSI differ?
Both compare the operating pH with the saturation pH. LSI is pH minus saturation pH. RSI is twice the saturation pH minus the operating pH, read on its own bands: under 6 scales, 6 to 7 is roughly stable, and over 7 turns corrosive.