25 October 2021
Adjusting the TA and pH won't address the need to add calcium. My suggestion would be to add calcium to the pool and raise the level to 200-400 ppm. Raising the calcium, maintaining the pH at 7.4-7.6, and adjusting TA to 80-120 will keep the LSI balanced.
We contacted Taylor Technologies, the manufacturer of test kits, and asked for their opinion. This was their response.
Maintaining appropriate calcium levels is definitely an issue and should not be ignored. Yes, adjusting the CH level for water balance (LSI) is a part of the overall water balance, it is easier to adjust TA and pH than CH.
The ideal level for CH in a standard gunite pool is 200-400 ppm. For a vinyl liner or fiberglass shell, whether as a spa or a pool is 150-250 ppm. You may not have much control of your incoming CH if high to begin with. In the Phoenix AZ area, for example, CH cam be 1500 ppm +. Where I live in the Mid Atlantic area, the CH out of the tap is about 50 ppm.
The LSI is always relevant. Assuming that the TA, pH, and CH levels are within ideal ranges, then the LSI is balanced. CYA affects the TA reading, therefore the TA must be adjusted to compensate for the CYA reading. In the example you sent, it is not clear if the TA has been adjusted. Also, not knowing what kind of pool (or spa) we are dealing with also helps make answering this difficult.
Ideal readings are…
Adjusted TA 80-120 ppm
CH 200-400 ppm (150-250 ppm vinyl/fiberglass)
The other two LSI factors, temperature and TDS, you have no control over.