Salt water pool, salt measure result is much lower then TDS result what to do?

[Alanso]

Can somebody help me out please.
I have a new and just calibrated salt meter.

The meter measures in the salt water pool SALT 2.68 ppt but the TDS is 4.4 ppt

I thought those readings should be almost the same?

The question is now, do I need to add salt as that would be nessecary with 2.68ppt?

 

[domct203]

TDS (total dissolved solids) is a measure of all dissolved solids in the pool (salt, calcium, cya, etc). It would be expected to be higher than just a sodium reading.

 

[Alanso]

Thanks Dom,

but as a newbie I see everywhere that TDS meters are used to maesure the salt level.
I read that TDS is Total Dissolved Solids so how can you know what the salt level is as it reads also other chemicals?
It is confusing me.
So do I have to ignore the TDS and look at the salt measuring and add salt?

 

[domct203]

Ignore TDS, as I already explained it measures ALL solids in the water.

The big question here is if your meters have been properly calibrated recently.

Is it possible to get the Taylor K1766 Sodium Chloride test kit in your country?

 

[Alanso]

Hi Don,




I bought myself a great meter and I calibrated it.
The Taylor kits are not affordable here as shipping costs is too high.

So I will add salt then.

 

[Jimrahbe]

a,

Does your salt system display a salt reading? If so, I would compare your salt meter with what the salt system says.. To make the salt cells work, you will have to add enough salt to make them "Happy". (Within reason, of course).

Jim R.

 

[Alanso]

They installed two Emaux SSC mini . It has no display at all.
Bad choice.

 

[JamesW]

Let's suppose that you have two separate bottles of water. Each is one liter.

To one, you add 3.5 grams of sodium chloride. This is usually referred to as Salinity. It is a specific type of TDS.


To the other bottle, you add 3.5 grams of a mixture containing 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride. This is usually referred to as "TDS" (442 Natural water).

There are many different types of solids that will dissolve in water.

Both have 3.5 grams per liter (3,500 ppm) TDS.

If you use a conductivity meter on each solution, you will get a different conductivity.

That conductivity can be converted into TDS, but only if you use the right conversion chart.

So, when you use a meter, you have to have it set to the correct TDS makeup setting.

When testing the solution with sodium chloride, the meter needs to be set to Salinity. If set to TDS, the assumed makeup is incorrect and will result in an incorrect reading.

When testing the other sample, the setting needs to be set to TDS.

The misunderstanding comes from the use of term TDS. It should say 442 Natural water.

For pool water, the proper setting is Salinity.

The TDS setting is not an appropriate setting because it assumes an incorrect makeup, uses the wrong conversion and results in the wrong result.

When testing any sample, you can only use one setting, which uses a specific conversion chart. The chart is specific to each type of dissolved solid.

Here is a dual use meter and a chart that shows graphs of salinity vs conductivity and TDS (442) vs conductivity.

As you can see, the graphs show different values for conductivity vs TDS for both dissolved solids.

PoolMeter™

Standard Solutions and Buffers

In my opinion, the MyronL PoolMeter should not be dual use because the only appropriate setting is Salinity. The TDS setting is not appropriate for pool water and only causes confusion. Pool stores pour in the water and set the meter to Salinity and take a measurement then set it to TDS and take another measurement thinking that the salinity setting measures only salt and the TDS setting measures everything, which isn't accurate.

A conductivity meter measures the conductivity of the water. The conductivity is affected by all charged ions, such as sodium, chloride, calcium, sulfate, bicarbonate etc.

So, a meter set to Salinity does not just measure salt (sodium chloride).

Because the mixture is not 100% sodium chloride, the conversion won't be exact, but it's close enough because most of the TDS is salt (sodium chloride).

So, it's not accurate to say that the meter measures only salt when set to salinity and measures all TDS when set to TDS.

TDS is not a setting that should be used unless the TDS makeup is a 442 mixture.

If the TDS is something other than salt (sodium chloride) or 442, the correct setting is conductivity in microsiemens and then you use the appropriate chart to convert the conductivity into "TDS".

If you don't know what the composition of the TDS is, conductivity can only give you a rough idea of the TDS.

You would need to conduct further tests to identify the components of the TDS.

You could evaporate the water and weigh the leftover solids. That will tell you the TDS, but not the composition of the solids.

You could do ion specific tests to determine if specific ions are present.

When dealing with an unknown sample, you typically would need to do much more sophisticated tests if you really had to know the composition of the various ingredients of the TDS.

The K-1766 test kit is specific to chloride. As long as most of the TDS is salt (sodium chloride), the K-1766 should match a conductivity meter set to Salinity.

Many SWGs use conductivity to measure salinity.

Hayward aquarite is unique in that it uses the performance of the cell to calculate salinity. The cell performance is based on chloride levels and water temperature (assuming the cell is working correctly).

As long as most of the TDS in a pool is sodium chloride, a conductivity meter set to salinity, a K-1766 and a Hayward aquarite should match.

A mismatch could indicate that the TDS is not mostly sodium chloride.

For example, if you had pool water that matched the 442 profile, you would get a big difference between the conductivity meter, the K-1766 and the Hayward aquarite.

 

[tomfrh]

That’s a large discrepancy. a lot of non-salt

my pool has TDS 5100ppm and salt 4500ppm

Mine is:

salt 4500
calciumn 450
TA 80
etc


Test your salt in another way to gain confidence in your meter. I find electronic meters a bit dodgy....

 

[JamesW]

That’s a large discrepancy. a lot of non-salt

my pool has TDS 5100ppm and salt 4500ppm

Mine is:

salt 4500
calciumn 450
TA 80
etc


Test your salt in another way to gain confidence in your meter. I find electronic meters a bit dodgy....

How are you measuring salt and TDS?

There's no discrepancy. The TDS reading is not valid.

 

[JamesW]

Suppose you have 5 different bottles of water and 5 different types of TDS. Copper sulfate, lithium fluoride, sodium chloride, potassium iodide and calcium bromide.

You put only 1 type of TDS in each bottle. You put enough in each bottle so that the conductivity is the same.

Because each type of TDS has different conductivity at a specific concentration, the amount of each type of TDS will be different even though the conductivity is the same.

Someone with a conductivity meter can measure the conductivity of each solution and determine that the conductivity is the same for each, but they can't determine the TDS reading without knowing which conversion chart to use.

There is a different chart for each type of TDS. You can only determine the TDS amount if you know which chart to use for each sample.

Obviously, you can only use one chart per sample. Only the correct chart will give you the right conversion from conductivity to TDS.

Using the meter set to salinity tells the meter that the TDS is sodium chloride and to use the sodium chloride conversion chart.

Using the meter set to TDS tells the meter that the TDS is 442 mix and to use the 442 chart.

The TDS can't be both. You can only choose one conversion chart. The correct choice is salinity.

The TDS is not a 442 mix, so the TDS setting is not appropriate.

If you look at the chart in the second link that I previously posted, you can see that the conductivity is the same for the readings of salinity and TDS posted in the original post.

One way to check how much of TDS is not sodium chloride is to do a salt meter test set to salinity and a K-1766 test, which is specific to chloride.

Both assume that the TDS is all sodium chloride. If the TDS is not all sodium chloride, the tests will be significantly different.

The usual chemicals are calcium and maybe some cyanuric acid. Those can be checked to see if they make up the difference.

Even then, it's not that easy. Calcium can come with a lot of chloride, such as from calcium chloride.

Also, the numbers won't always add up because each type of TDS has its own reporting numbers. Calcium might be reported in units of calcium carbonate even though you're not measuring calcium carbonate.

If there's a big difference between a salt meter set to salinity and a K-1766 test that can't be explained, then there might be something in the water that warrants concern.

For example, if you put 3.5 grams per liter of copper sulfate in a liter of water, the conductivity meter set to salinity would register the conductivity and translate to a salt reading (not 3.5 grams per liter because the wrong chart is being used, but it would read whatever the conductivity converted to based on the chart).

A K-1766 test kit would get a reading of zero because there is no chloride.

So, if you gave the sample to someone and told them to check for salinity (sodium chloride concentration), they could tell that some type of TDS was in the water but it's not sodium chloride.

Tests for calcium would probably not work due to the copper and might point to the copper.

Most of the time, the components of TDS are fairly well known for pool water. If pool water ever tested high for conductivity but low on known components, it might be best to dump and refill or send out for lab tests.

 

[tomfrh]

How are you measuring salt and TDS?

There's no discrepancy. The TDS reading is not valid.

I measure TDS with hydrometer. I measure calcium, TA, etc, and substract that to get salt.

Ive verifies that with clear choice labs salt titration kit.

ive also had salt and tds measured at pool stores as a cross check. They use electronic method too, where they put a probe into the sample.

 

[JamesW]

Measuring the density of the liquid is similar to evaporating the water and weighing the remaining solids.

It can give you a relatively good idea of the actual TDS.

Calcium and Total Alkalinity are usually reported in units of calcium carbonate.

To get a more accurate weight, you could use the actual weight of calcium and bicarbonate.

Salt and cyanuric acid are reported in their own units (sodium chloride and cyanuric acid respectively).

Borates are measured in units of boron.

For all of the test, there is always going to be some inaccuracy. Typically, up to about 400 ppm for salt (sodium chloride).

I don't know how accurate your hydrometer is but it's not going to be exact.

All of your results are within the margin of error.

In any case, the TDS setting on a conductivity meter is not accurate and should not be used.

 

[tomfrh]

My hydrometer measures density to within 200ppm salinity fairly easily.

I use a bulk allowance of 600ppm for the other parts of TDS that the hydrometer is reading.

i never bothered doing the exact density numbers for carbonates etc, as salinity and calcium seemed the most important.

 

[JamesW]

Calcium measured by a pool test kit is usually reported in units of calcium carbonate. For more accuracy, you should only subtract the weight of the calcium, which is 40% of calcium carbonate.

So, if the test kit result is 450 ppm calcium, you only subtract 180 ppm as the actual weight of calcium.

How did you determine a bulk allowance of 600ppm for the other parts of TDS?

 

[tomfrh]

I added up the 450ppm calcium plus 80 TA plus a bit more to cover the rest, rounded up to 600ppm.

I added calcium as calcium chloride

 

[JamesW]

When subtracting the calcium from the TDS, you should multiply the test result by 0.40 because the calcium is only 40% of the result. 0.4 x 450 = 180.

When subtracting the TA, you should multiply the test result by 1.22 because the TA is also reported in units of calcium carbonate. 1 mole of calcium carbonate is equal to 2 moles of bicarbonate. Bicarbonate is 61% of the weight of calcium carbonate but you multiply by 2 because 1 calcium carbonate means 2 bicarbonate. So, the net result is that you multiply the TA test result by 1.22 before subtracting it from the TDS. 80 x 1.22 = 98.

180 + 98 = 278.

Cyanuric acid can be subtracted as tested because it is reported in units of cyanuric acid.

 

[tomfrh]

When subtracting the calcium from the TDS, you should multiply the test result by 0.40 because the calcium is only 40% of the result. 0.4 x 450 = 180.

I measure ~450, which is equivalent to ~500ppm Calcium Chloride (which I added). I've been taking it as 1:1 for simplicity. Are you saying I should only take 40%? What about the other 60% of chlorides I added? This is adding to TDS isn't it, same as with Sodium Chloride? Or should you be ignoring it?

When subtracting the TA, you should multiply the test result by 1.22 because the TA is also reported in units of calcium carbonate. 1 mole of calcium carbonate is equal to 2 moles of bicarbonate. Bicarbonate is 61% of the weight of calcium carbonate but you multiply by 2 because 1 calcium carbonate means 2 bicarbonate. So, the net result is that you multiply the TA test result by 1.22 before subtracting it from the TDS. 80 x 1.22 = 98.

180 + 98 = 278.

Cyanuric acid can be subtracted as tested because it is reported in units of cyanuric acid.

Again, I haven't bothered fussing over the unit weight conversions. The difference between 80 and 98 isn't significant at the end of the day. 18ppm is too fine to worry about when I'm measuring something 4000+. I just wanted to add a bulk allowance to offset everything that isn't salt.


Happy to stand corrected with the 40% calcium if that's the case, as that's substantial. I'd like to know if I should only be correcting 300ppm not 600ppm

 

[JamesW]

Calcium chloride is a type of salt. If you added 2.85 grams of calcium chloride to 1 liter of water, a K-1766 test kit would measure 3 grams per liter (3,000 ppm) salt because adding 2.85 grams per liter of calcium chloride adds the same number of chloride ions as 3 grams of sodium chloride.

A conductivity meter set to salinity would measure about 2.95 grams per liter because the conductivity of a 2.85 g/L solution of calcium chloride is about the same conductivity as a 2.95 g/L solution of sodium chloride.

A calcium test would measure 2,570 ppm reported in units of calcium carbonate.

Subtracting the calcium doesn't help, because calcium chloride is a salt. When measuring for salinity with the K-1766, only chloride is measured. The sodium is assumed to be there and is counted as part of the weight. It doesn't matter if the calcium is there instead of the sodium because they don't do anything.

If you want to verify that the TDS setting on the meter is not a valid setting, try this.

Get a 1 liter bottle of distilled water and add about 3 grams of salt to the water. Mix well and then test with the meter set to salinity. You should get about 3 g/L (same as 3 ppt or 3,000 ppm).

Then, set the meter to TDS and take a reading. Since you know that the TDS is 3 g/L, that's what you should get if the TDS measurement was valid.

But, it will show a value higher than 3 g/L because it's using the wrong conversion.

The meter will read 3,000 ppm when set to salinity but will read about 4,600 pm when set to TDS.

So, where did the "extra" 1,600 ppm of TDS come from? What is it? It's nothing because it's not there. You know that it's not there because you made the sample.

Only the reading set to salinity is valid. The reading set to TDS is not valid.

 

[tomfrh]

I’m not the OP.

sorry op for hijacking the thread...