TDS, IRON, COPPER

mas985

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May 3, 2007
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Pleasanton, CA
They indicate that the meter can differentiate between salinity and TDS, which is nonsense.
Not entirely. I think they are just defining TDS differently. Each soluble has a different conductivity so you need to calibrate for the solution you are working with. I think they are just allowing the user to use that meter for two different solutions. One for 442 and one for NaCl. So they can switch back and forth without re-calibrating. I think the real issue is labeling.

I am using one similar to this: Water Quality Tester, MagicPro 2016 Newest Released Premium TDS-2 Handheld TDS Quality Water Tester Meter 0-9990 ppm TDS Measurement: Amazon.com: Home Improvement

I bought it a long time ago and they don't have the exact model anymore. But it does do temp compensation which is a must. When calibrated properly it does match the SWG fairly closely. At least until the cell is near end of life.
 

JamesW

TFP Expert
Mar 2, 2011
17,339
  • Displays in both TDS and NaCl values​
  • Great for making differential readings in HIGH salt pools and spas utilizing electrolysis for bio control.​

I think that they are definitely indicating that the meter can differentiate between salinity and TDS.

A meter for pool use should be specifically designed to measure salinity because it uses the correct graph to convert the conductivity reading into a salinity reading.

If the meter is designed to use the TDS 442 graph to convert the conductivity reading into a salinity reading the result will not be very accurate except maybe right near the calibration solution's reading.
 

mas985

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May 3, 2007
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You are assuming they are using a single multiplier. They can easily capture the relative separation across both curves in a table or even an equation. You would only need a single calibration point, 442 or NaCl calibration at any concentration to shift both curves but the relative separation would remain the same. What they are calibrating is really just the conductivity.
 
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JamesW

TFP Expert
Mar 2, 2011
17,339
My point is that they are implying that the meter can measure salinity and then measure all TDS including salt and everything else in the water.

For example, set to salinity, the reading is 3,200 ppm and set to TDS, the reading is 4,900 ppm.

This implies that there must be 1,700 ppm of something other than salt in the water, which is not a valid conclusion.

The TDS setting or a TDS meter uses the TDS chart for converting the conductivity reading into a ppm reading.

The TDS chart is only valid for water where the constituent makeup is consistent with a 442 mixture.

When measuring pool water, the correct setting is salinity, which uses a NaCl conversion chart.

If you use a salt standard solution to calibrate a TDS meter, you're essentially forcing the TDS graph to intersect the salinity graph at the reference point.

Using the TDS meter in that way will still be using the wrong graph and will give the wrong result at any point not equal to the reference point.

Any reading close to the reference point will be close enough that it won't make much difference. So, you can use a TDS meter to measure salinity if you calibrate it it with a salt standard solution.

The correct way is to use a salt standard solution to calibrate a salinity meter that uses the salt NaCl graph for maximum accuracy.
 
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JamesW

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Mar 2, 2011
17,339
Measuring salt and TDS gets pretty complicated when you get into the chemistry.

One way to get a TDS reading is to weigh the water to see how much solids are dissolved by comparing the weight to pure water with nothing dissolved or to evaporate the water and weigh the leftover solids.

A salt drop test only measures chloride and assumes that the molarity of the sodium is roughly equivalent to the molarity of the chloride, which is usually true but not necessarily.

The chloride could have been introduced by lithium chloride, magnesium chloride, calcium chloride etc.

Adding up the various components of the TDS is not so simple.

For example, calcium is measured by measuring calcium, but the amount is reported in units of calcium carbonate.

Total alkalinity is also reported in units of calcium carbonate.

When doing a conductivity test, you have to have a graph that converts the conductivity reading into a ppm reading.

A salinity meter assumes that all of the TDS is sodium chloride even though that's not strictly accurate.

If you take all of the various components of the TDS and add them up, they should be relatively close to a salinity meter reading.

The bottom line for the OP is that, in my opinion, the TDS readings of 4,500 ppm and 5,000 ppm are probably based on a conductivity test that was set to the wrong setting.

In my opinion, the TDS readings are inaccurate and should be ignored.

If you want to compare readings, add up all of the TDS components and compare the total to a salinity reading from a salinity meter. The numbers won't be exactly the same due to the inherent inaccuracies in the process but they should be within about 800 ppm.

Also, note that the Aquarite salinity reading is based on the performance of the cell, which is specific to chloride and is not a conductivity reading.
 

mas985

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May 3, 2007
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Pleasanton, CA
Also, note that the Aquarite salinity reading is based on the performance of the cell, which is specific to chloride and is not a conductivity reading.
The only cell performance metrics that are available in the SWG are amps, volts and temperature everything else is calculated from these three values including conductivity (amps/volts). Hayward techs have confirmed this for me but I also was able to come to the same conclusion myself by coming up with a conductivity and temperature to salinity formula that fits the readings of SWG with less than 50 ppm of RMS error (display only shows to the nearest 100 ppm so this is consistent). Plus I traced the lines from the cell to the Aqualogic board and you can see where the voltage, current and temperature are measured on the board. There are no other parameters that can be measured and that is why the display only shows volts, amps, temperature and salinity with the later being calculated from the other three. The SWG cell is basically a TDS/salinity meter. The plates are being used in the same way as the probes of a TDS meter. However, the formulas for the conversion are different because of the application and implementation.
 

JamesW

TFP Expert
Mar 2, 2011
17,339
The performance of the cell is related to the chloride concentration.

If you removed all of the sodium, a regular conductivity meter would read that half of the "salt" was removed but the Aquarite reading would stay the same.

If you tripled the amount of sodium, a conductivity meter would register that the "salt" had doubled but the Aquarite reading would remain the same.

The Aquarite doesn't measure the conductivity of the water. It calculates the amount of "salt" based on the chloride concentration only like a K-1766 drop test does. The Aquarite calculates the amount of chloride from the amps, volts and water temperature. So, it not conductivity of the water, it's more the conductivity of chloride. So, you can consider it a type of conductivity but it's distinctly different than a conductivity meter.

A conductivity meter reacts to all ions in the water, sodium, chloride, calcium, magnesium etc. Current passes from one electrode to the other through the water.

In a cell, current is not traveling from one electrode to the other. Electrons are being removed from chloride ions and pushed onto hydrogen ions. So, it looks like current is flowing from one side to the other but it's not.
 
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mas985

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May 3, 2007
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Pleasanton, CA
If you removed all of the sodium, a regular conductivity meter would read that half of the "salt" was removed but the Aquarite reading would stay the same.
First, according to the principle of electroneutrality, the total charge of an aqueous solution must be zero so one can't remove just one ion. But even if you could, I would argue that the TDS meter would measure zero current and therefore zero TDS/salt. There must be both positive and negative ions for current to move through the water. The SWG is bit more complicated but this is all theoretical anyway.

The Aquarite doesn't measure the conductivity of the water. It calculates the amount of "salt" based on the chloride concentration only like a K-1766 drop test does. The Aquarite calculates the amount of chloride from the amps, volts and water temperature. So, it not conductivity of the water, it's more the conductivity of chloride. So, you can consider it a type of conductivity but it's distinctly different than a conductivity meter.
I don't think I ever said the Aquarite measures conductivity. In fact, no meter does. A multi-meter measures volts and amps and then calculates conductivity/resistance. This is exactly the same for a TDS meter. The relationship of volts to amps in a resistive circuit is conductivity by definition. To determine conductivity/resistance, a voltage is applied to plates or probes and the current is measured. The two values can then be used to calculate conductivity. The same principles apply to a TDS meter and SWG electrolysis. The only difference is one uses AC and the other DC mainly to avoid an electrolytic reaction in the TDS meter. However, in both cases, the current traveling through the water is proportional to the ion concentration and voltage across the plates.

If you look at the relationship of volts, amps and salt level, salt level is proportional to the ratio of amps/volts which is conductivity. I believe this is how the Aquarite determines the salt level. In fact, here is a formula you can use to calculate salt level from the amps, voltage and temperature:

Salt (ppm) = 17000 * amps/volts - 42 * Temp (F) + 2323

As I said before this formula is pretty accurate over a large range of salt levels (2500-4500 ppm).
 
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