SW Testing with Digital TDS Meter

setsailsoon

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TFP Guide
Oct 25, 2015
1,573
Stuart/FL
Folks,

I plan to convert to swg next month so I'm looking up things I need to learn a little more about. One of these is salt testing. Looks to me there are several commonly used methods to verify the swg's internal probe reading. The Taylor test, test strips, or a pretty expensive digital probe test. I'm curious if a digital TDS meter could work? Seems like you could get a pretty accurate reading by just testing the water and test the source water then subtract the two. Has anybody done this and do you find it reliable/accurate? In my case the source water varies only +/- 10 ppm over years of testing and it's only about 230 ppm so I could almost just subtract 230 and be pretty accurate.

I've been using RO systems for years and find the digital TDS meters very reliable (way better than the pH meters).

Chris
 

Bama Rambler

Mod Squad
TFP Expert
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Jun 22, 2009
23,628
SouthWest Alabama
As you know there's more to TDS than just salt, however if you want to do the math you could use a TDS meter and subtract CYA, CH, etc.. Everything except salt.

Keep in mind that if the TDS of the fill water contains salt, you don't want to subtract that number. So you may not be subtracting all that 230 ppm number.
 

setsailsoon

LifeTime Supporter
TFP Guide
Oct 25, 2015
1,573
Stuart/FL
As you know there's more to TDS than just salt, however if you want to do the math you could use a TDS meter and subtract CYA, CH, etc.. Everything except salt.

Keep in mind that if the TDS of the fill water contains salt, you don't want to subtract that number. So you may not be subtracting all that 230 ppm number.
Dave,

I was subtracting because I was thinking most of the source TDS was calcium and magnesium chlorides and/or carbonates since I have pretty hard water. But then I realized I added a water softener and it replaces virtually all of the hardness with sodium salts. I also use a Hach total hardness kit once every quarter or so to make sure the softener is working.

This methodology will definitely work better incorporating your comments. Thanks!!

Chris
 

JamesW

TFP Expert
Mar 2, 2011
16,550
A digital meter is just a conductivity meter.

To get TDS or salinity, you have to use a conversion chart that converts the conductivity into TDS or salinity.

A meter usually has the conversion chart built in and it does the conversion for you.

To get the right reading, you have to use the right chart.

There are different charts for different types of TDS.

There are charts for individual salts, like sodium chloride or lithium iodide.

There are also charts for specific mixtures of TDS components.

A TDS meter would need to have a Salinity setting to get a salinity reading consistent with a SWG that uses a conductivity sensor.

AquaPure, AutoPilot and Intellichlor have conductivity sensors.

I recommend the K-1766 salt test kit to check the salinity.

There's no reason to subtract the fill water salinity. Just measure the pool water salinity.

Here is a conductivity meter with dual range for salinity and TDS (defined as a "442" mix).

PoolMeter™

You have to set the meter correctly to measure salinity. Below, you can see a chart with conductivity vs ppm for salinity and TDS 442.

Standard Solutions and Buffers

A TDS meter won't give you a valid reading unless you're using the correct conversion chart.

A TDS meter should be set to salinity.

The dual meter referenced above only measures conductivity. It does not have any ability to determine what solids are in the water.

For example, some people will take a reading set to TDS and then a reading set to salinity and subtract the salinity from the TDS to get the tds other than salinity.

However, this is an error and not a valid test.

For a salt pool, only the salinity reading is valid. The TDS reading is not valid.
 
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AUSpool

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Sep 23, 2015
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I’ve been using a salt meter for a while now. As mentioned, it is a glorified conductivity meter and does exactly what my SWCG does. It wasn’t the cheapest but not to expensive at just under $100 and came with a nice blue plastic case that matches my K2006 case.

Any meter is only as good as the last calibration, I calibrate it regularly with a 6440ppm solution and reference it against a 6000ppm solution every time its used. It tends to slowly read lower over time between calibration. The last time I compared my calibrated salt reading was very close to what I got from my Taylor K1766 salt test.

My old SWCG required a salt level of 6000ppm so its calibration point was ideal but my new SWCG uses 3000ppm so its not as ideal any more although if my SWCG indicates that it wants more salt I can use it to guess weather I need a full bag or half a bag of salt and still stay just under the manufacturers recomended level. Conductivity, TDS and salt meters are single point calibration meters so selecting a meter and calibration fluid closest to the ideal range is important for the best possible accuracy.

I test FC, pH and salt every time and can use the salt level as an indicator for my CYA on occasions that I don’t test my CYA.
 
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Brad_C

Well-known member
Nov 15, 2018
130
Perth, Western Australia
I started using a cheap E-bay TDS meter a while ago. I also have a Clear Choice Labs salt/chloride titration test. The meter was _way_ off out of the box, but by fiddling with it I found it had an undocumented calibration mode. I knocked up a ~6000ppm salt standard and calibrated the meter to that. I then did a high resolution chloride test with the test kit and compared that to the TDS reading. They were within a couple of hundred ppm. Certainly within the uncertainty of both the titration test and my "standard".

In reality as long as the reading is within 500-700ppm it's a non issue for my SWG and I always aim ~500ppm high, so I use the TDS meter to keep tabs and if it looks like the reading is off I'll back it up with a chloride test. I periodically test the meter against a 6000ppm solution and haven't had to adjust it yet, so I'm actually quite impressed at the stability and accuracy of a $15 meter.
 

JoyfulNoise

TFP Expert
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May 23, 2015
15,866
Tucson, AZ
The K-1766 is a chloride ion test and it is only sensitive to chloride ion, nothing else (unless you regularly add cyanide to your pool). I only recommend using that for salt testing. It will always be more accurate than any meter or strip test. The reagents, if stored properly, are extremely stable and can have long shelf lives. The test is simple to perform. No need to worry about drifting electronics or calibration standards.
 
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setsailsoon

LifeTime Supporter
TFP Guide
Oct 25, 2015
1,573
Stuart/FL
Folks,

Thanks for the great input. Sounds like the titration method makes the most sense. Especially since I essentially get a digital meter with the swg display. Now I just need to decide if I go with zero warranty Jandy but easy automation or Circupool and more difficult automation I have to figure out a way to rig up myself.

Chris