If certain chemistry parameters are impossible, the LSI value becomes a red "(-infinity)" on the Orenda Calculator™.
On the Orenda Calculator™, (-infinity) represents an impossible LSI value. This usually occurs due to a carbonate alkalinity value less than zero. While such water chemistry is technically possible, it is impossible to calculate the LSI in such conditions, and therefore, the Orenda Calculator™ shows (-infinity) as an indication of an error.
There are several ways for the LSI value to be impossible (-infinity). Since the most common cause is having a carbonate alkalinity value less than zero, let's briefly cover how that can happen.
Carbonate alkalinity correction factors: CYA and Borate
Yes, the Orenda Calculator™ automatically corrects total alkalinity to factor in cyanuric acid (CYA), and borates (if used). As an aside, borates are not shown by default on the calculator, but you can enable borates by going to the main menu > App settings > borates > toggle ON.
Both CYA and borate corrections depend on the pH of the water. The higher the pH, the higher the CYA and borate correction factors. In other words, a higher pH means CYA and borate factors matter more. You can see this for yourself on the calculator. Tap "show" secondary readings at the top left of the calculator. Then put in, say, 30 ppm CYA. Watch the carbonate alkalinity level drop as you increase the pH, proving the CYA correction factor is increasing along with pH.
Yes, it is entirely possible to see the LSI actually go DOWN while the pH goes up. But only if you have enough CYA and/or borate in the water to more-than-offset the rising pH.
So that's the technical explanation of what's happening. In simplified terms, if you have high enough CYA and borates, they can eventually deduct enough from total alkalinity to make the corrected alkalinity (carbonate alkalinity) a number less than zero. You can see this in the graphic above. It is impossible to calculate an LSI with zero or less carbonate alkalinity.
Carbon Dioxide off-gassing, carbonic acid, and Henry's Law
As you may already know from our previous articles, pH naturally rises as the pool off-gasses CO2 to equalize with the CO2 in the air above the pool. This is due to Henry's Law. The point in which CO2 has equalized is the point where pH can no longer naturally rise (because the water cannot lose more CO2, so the pH stops rising). This is called the pH ceiling.
To get a negative carbonate alkalinity sometimes requires a high enough pH to make the CYA and borate correction factors large enough. But for this to occur from the natural off-gassing of CO2 would be virtually impossible. What IS possible, however, is when people abuse acid by overdosing it or not diluting it first. This can etch cement and pull out 12.6 pH calcium hydroxide (Ca(OH)2), which spikes the pH and can force a pH above the pH ceiling.
(-infinity) represents an impossible LSI to calculate. It is usually caused by having to much CYA, borate, or both. These factors subtract from total alkalinity and can eventually make the carbonate alkalinity level zero or less. When that happens, it indicates an impossible LSI to calculate.