Tds And Si

[KAT_Irish]

I understand that Total Dissolved Solids (TDS) and Saturation Index (SI) provide good indicators of the overall health and quality of your spa water. What I would like to know is:

1-    Is there a way of measuring / determining TDS at home? I can't seem to find an affordable solution to accomplish this.

2-    Can you reduce TDS without draining your spa?

3-    Do TDS have a significant impact on your SI?

After approximately 2 weeks of modest use (1 hour per day on average) my readings are as follows:

TDS approx. 600ppm (according to water test lab at spa store)

SI approx. -0.05 (very happy with this

[chem geek]

The TDS is pretty meaningless. It is not an indicator of quality of the water. Add 3000 ppm to a saltwater chlorine generator pool and it's still good quality. The only thing TDS can tell you is a rough age of the water, but if you've just been adding chemicals and not using the spa then the water will be in good shape regardless of the rise in TDS which is mostly salt. It's what TDS is composed of that is relevant -- not the TDS number itself.

The saturation index likewise isn't important in a spa unless it is too high (> 0) since scaling in the heat exchanger can be very bad. A low saturation index is not a problem because there is no plaster or grout in tile in an acrylic spa so no calcium carbonate to dissolve.

Your near-zero SI is a bit risky if your pH were to rise. The SI will be higher in the gas heater where the pipe surface is around 30ºF higher. Just be careful.

[KAT_Irish]

Thanks for the reply. A quick question so. If I am to use SI as an indicator, what should I aim for -0.1? or do you recomend ignoring SI and just focus on TA and PH?.

I am under the impression that if all is well balanced in the hot tub the SI should be close to 0, however you state differently. How do you recomend I proceed, what should I monitor to ensure well balanced water. Currently:

CH 220

Alk 70

PH 7.5

CYA 40

Chl, typically between 1 and 3ppm.

Thanks again

[chem geek]

For a spa, don't worry about a negative SI.  It's only if it gets largely positive that you risk getting scaling.  If it's negative, it's not a problem because there is no calcium carbonate to dissolve since you don't have plaster or grout.

I would target 120-150 ppm for your CH.  No need to have it any higher than 150 ppm.  That should be enough to help prevent foaming.

The rest of your numbers are fine, especially if your pH is stable (if it tends to rise, then lower your TA further and if you need to lower it to 50 ppm, then consider also adding 50 ppm Borates).