Total Dissolved Solids (TDS)?

[tomfrh]

We have an SWG pool. I’m attempting to make a hydrometer so need to know the TDS in the pool, ie how much of extra density is salt and how much is other substances.

aside from salt, what are the other important TDS?

There’s calcium, Total alkalinity, and CYA. what other large ones?

i had my pool water tested at pool shop and they listed salt 4570 (which agrees with myclear choice salt titration test) and a TDS 5470

im not sure where the missing 900 that they measured comes from.

My calcium is 450 (pool shop measured it as 310), cya 70 (pool shop 53), TA 70 (pool shop 48), which leaves a shortfall however I measure it.

 

[cfherrman]

Do a search on how to calculate it on here, it will list all sources

pH down or sodium bisulfate is one of them if you ever used that

 

[AUSpool]

It’s good to see CCLabs have the salt detection kit. It measures the chloride ion directly with a silver nitrate titrating solution and a chromate indicator. It’s accurate to +/-200ppm which is more than adequate for a SWG pool. Use rubber gloves for this test and don’t do it anywhere your not happy to have stained for good.

TDS is a bit too arbitrary for a pool, there are too many substances that can be read at once. Trying to work backwards by removing ‘known’ substances to arrive at salt is near impossible. They are to many unknown substances, as a starting point my tap water has a TDS of around ~150ppm.

”Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.”

A hydrometer doesn’t work well at a pools relatively low salinity level, they are very temperature sensitive.

Your missing 900ppm is very likely poor shop testing. In Aussie the pool shops all tend to use the same TDS meter which uses a conversion factor to extrapolate ppm of salt. It’s an expensive bit of kit with to many features, many operators simply don’t know how to use or calibrate it properly. Ive found several store salt test to be out by up to 1000ppm.

Having said that the chlorinator ‘low salt’ light is a rudimentary TDS meter. A chlorinator has a very broad ideal salt range, you can easily manage salt concentration by using the titrating salt test as a starting point and put a bag of salt in when the ‘low salt’ light goes on.

 

[JamesW]

i had my pool water tested at pool shop and they listed salt 4570 (which agrees with myclear choice salt titration test) and a TDS 5470

im not sure where the missing 900 that they measured comes from.

For pools, tds and salinity are basically the same thing.

The only time it might be relevant would be if you could establish that the TDS was not almost completely salt.

For example, if you tested for salinity using the K-1766 salt test and compared it to a conductivity salt test and the conductivity test was way higher than could be accounted for with the other known components of TDS, like calcium etc.

That would indicate that some unknown solid was in the water that may or may not be a problem.

If someone is using a dual meter like the Myron L Poolmeter and they set it to TDS, they are using it incorrectly because the TDS setting is for use when a different TDS mix is used.

Almost definitely, the shop is using a conductivity meter with dual settings of salt and TDS and testing the same water on one setting and then the other setting.

This is a fundamental error because it assumes a different TDS makeup for each test.

The TDS measurement is not valid.

PoolMeter™

 

[AUSpool]

In Australia every pool shop Ive been in all use the T11 conductivity meter by corrosion electronics. It has an operators manual 19 pages long, 6 operating modes and user selectable or adjustable TDS factor. Ive seen operators calibrate with tap water, use it on the wrong mode and with an incorrect TDS factor.


T11 Conductivity Meter Corrosion Electronics

 

[tomfrh]

Thanks for the information. I think I’ve got my head around if now.