TDS for calculating CSI

[jonesty]

Hi all, we've had our new pool for a few months now and it's been going great, largely due to the great advice and guidance found on TFP (thanks!). I've been using the PoolMath app to keep the water balanced but wanted to check whether I should be using a specific TDS value rather than just salinity due to the type of salt we use. We've been using Zodiac Mineral Elements, as this is what our installer provided. Its composition is 30% magnesium and 70% sodium. Our current TDS is 2000 ppm and salinity is 1400 ppm. Note our SWG is an ultra-low salt system.

I know the PoolMath app must derive TDS somehow but there are other apps (e.g. Orenda) that accept actual TDS as input and yield quite different results for SI with all other parameters equal. So, my question is what should I be using to get the most accurate CSI?

Thanks in advance.

Current readings below for reference.

FC: 3.2
CC: 0
pH: 7.6
TA: 90
CH: 275
CYA: 50
TDS: 2000
Salinity: 1400
Temp: 26°C

 

[casiokid]

Welcome to TFP!

TDS is a fairly useless measurement since it measures all the solids in the water and doesn’t really give you any specifics. It’s like telling you how many marbles are in a bag when you really need to know the number of marbles per color.

PoolMath doesn’t derive anything related to TDS. Even if it used TDS, it would be a number you would have to input as it would have no idea what your TDS would be just from the test numbers.

A salt test kit is the best way to measure your salinity, and using a recommended test kit is the only way to get an accurate CSI. Are those results from a recommended test kit?

I think there are some other concerns with using magnesium. I believe it throws off some other test results and can be an issue in higher concentrations. Someone else will be able to comment with more details. Is there a reason for not using salt without additives?

Your CYA is just a little low for having a SWCG, but the main concern is your FC. You’re on the lowest end for your CYA level. You probably want to keep it closer to the 6 - 8 ppm range.

 

[AUSpool]

Geday jonesty and welcome to TFP and the forum.

Congratulations on the K2006 it’s a good kit. Think about the magnetic stirrer or Speedstir, they are worth their weight in gold.

Use a 10ml sample for the FC test, it will save on reagent and a resolution of 0.5ppm is easily good enough.

How are you testing salt and TDS? It’s one or the other. After the salt is added to tap water the majority of the TDS is the salt. If you’re using a handheld it needs switched to ppm salt and ideally calibrated with something close. Hanna have a 3g/L (3000ppm NaCl) solution.

The ideal salt level for the XLS 900 SWG is ~1000ppm salt with a range of 900-2000ppm salt.

Just enter the salt level into PoolMath for the CSI.

Edit: Zodiac give a conversion factor of 0.68 for TDS to salt. ie. TDS x 0.68 = salt; 2000 x 0.68 = 1360ppm salt which I would round up to 1400ppm. Put 1400 into PoolMath.

 

[JamesW]

Our current TDS is 2000 ppm and salinity is 1400 ppm.

You cannot use a meter set to "TDS" because it assumes a TDS composition of 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride, which is not accurate for pool water.

The meter just measures conductivity and converts it into a reading based on the assumed composition of the TDS.

Below shows a graph of conductivity vs. TDS and conductivity vs. Sodium Chloride.

Standard Solutions & Buffers

Ensure specified water testing instrument accuracy. See a list of our Standards Solutions and Buffers (custom solutions available).

www.myronl.com

PoolMeter™

The Myron L® PoolMeter™ is designed specifically for the Pool and SPA Industry with “DUAL” Ranges, accurate and reliable results and is simple to use.

www.myronl.com

442 Natural Water™ Standard Solution
442 Natural Water™ Standard Solution is used in calibrating many Myron L® Instruments. It is the best choice when measuring boiler and cooling water samples, city water supply, lakes, wells, etc. “442” refers to the combination of salts mixed with deionized water to comprise this standard: 40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride.

 

[JamesW]

Every type of TDS has its own graph vs. conductivity.

So, you only measure conductivity and then you have to know the correct graph to convert to the correct TDS reading.

One conductivity reading can be converted into 100 different TDS readings depending on the actual makeup of the TDS.

If you do not know the makeup of the TDS, then the conductivity conversion is only an approximation with a wide range of possible values.

Pool water TDS is mostly sodium and chloride.

 

[jonesty]

Thanks a lot for all of your replies. That is the type of salt our installer used to "start" the pool, plus we still have some bags leftover that they supplied. After handover the salinity was 1700 ppm, so I've just been letting that fall naturally with a target of 1200 in mind.

I'm using a Taylor K-2006 kit for most of my testing and an Apera EC60 for TDS and salinity, which has been calibrated with the solutions provided by the manufacturer.

I guess I'll ignore TDS and keep using salinity with PoolMath. I just wasn't sure if the other (non-sodium) dissolved solids were significant or not, particularly with the additional magnesium.

Thanks again for taking the time here guys, it's much appreciated. I never imagined there would be so much to know about pool water! The pool was the wife's idea. Buy a pool, she said. It'll be fun, she said.

 

[AUSpool]

Pools are fun, you can make it as technical as you want but most just want to jump in and enjoy.

The Apera EC60 comes with two calibration fluids, a 1413uS/cm (~692ppm salt) and a 12.88mS/cm (~6440ppm salt). Given your low salt target use the 1413uS/cm to calibrate as per the instructions and set the meter to salt. The range is 0-10.0ppt with a resolution of 0.01ppt (parts per thousand). Multiply by 1000 for ppm (parts per million). 0-10,000ppm with a resolution of 10ppm but in reality just record the result to the nearest 100ppm.

After calibration I would suggest making a 1000ppm home reference solution using two liters of distilled water and a tiny amount of your pool salt. Poor off about 20-30ml of distilled and add a little salt. Then add that drop wise to the distilled until it reaches 1000ppm using your meter. Store in a dark cool cupboard and use regularly to check you calibration. It will save on the more expensive calibration fluid.

It’s not often said but remember to store your test kit inside in a cool dry cupboard. Never leave it outside or in direct sunlight. Dry your hands after taking a sample. The K2006C comes with 2 R0870 bottles, I’ve stored one in a vacuum sealed bag with a dissident pack from medication and put it in the back of the cupboard till I need it later. The R0870 is affected by moisture more then any of the other reagents.

Just as a fun fact and I didn’t realise this before but I’ve never had need to bother, the Orenda calculator wants you to enter a TDS/salt as TDS and they suggest TDS = salt + CH + TA + CyA which is not very accurate. Stick with TFP’s PoolMath using your salt for the input. Orenda do have so good information but they are heavily geared toward commercial operations.

 

[jonesty]

Thanks mate. Good idea about making up some of my own calibration solution.

Is there anywhere you can recommend for buying Taylor reagents to refill the kit? They seem to cost an absolute bomb to buy individually from Australian stores, so much so it's cheaper to buy a whole new kit from the US, but that seems wasteful.

 

[proavia]

It may be more cost effective to use test kits from

Clear Choice Labs – Simple. Accurate. Fast.

The Taylor kits and reagent refills can get quite costly in AU.

 

[JamesW]

Orenda calculator wants you to enter a TDS/salt as TDS and they suggest TDS = salt + CH + TA + CyA which is not very accurate.

If you use a conductivity meter, all ions contribute towards the conductivity, so the reading is the total.

If you use a K-1766 salt test, you can add all other types of TDS but Calcium and TA are reported in units of calcium carbonate.

Calcium should be reported in units of calcium ion.

TA should be reported in units of bicarbonate.

Sodium Chloride + CYA + Calcium ion + Bicarbonate ion + Magnesium ion + Sulfate ion etc.

Even then, you are mixing reporting units.

For the TDS reading, it assumes a single reporting unit, which is probably sodium chloride.

So, you would convert all units into sodium chloride.

 

[AUSpool]

Thanks mate. Good idea about making up some of my own calibration solution.

Is there anywhere you can recommend for buying Taylor reagents to refill the kit? They seem to cost an absolute bomb to buy individually from Australian stores, so much so it's cheaper to buy a whole new kit from the US, but that seems wasteful.

The homemade is more of a reference solution, you wont use it for calibration. It’s just to keep track of your meters calibration and because it’s cheep you don’t mind using it quite often. It doesn’t need to be super accurate, anywhere close to where your normal salt level is. My meter is now reading about 40ppm low so I’ll probably recalibrate the next time I use it. I always keep a bottle of demineralised water on hand for diluting and rinsing.

I got my current K2006C from Mr Poolman who do keep some of the Taylor reagents. I got a 1pt (0.47L) r0013 from Amazon and a few things from eBay. There’s a couple of online shops in AU but as Gene said above Clear Choice Labs (CCL) is the stand out for refills, the salt kit and they keep the magnetic stirrer. You have to be a bit careful, particularly with the titrating reagents that are formulated for the CCL bottles with different drop sizes. I will top up my kit as needed to extend its life and then totally replace it after a few years. I’ve imported a few things over the years but I think they can’t ship some things any more.

 

[AUSpool]

If you use a conductivity meter, all ions contribute towards the conductivity, so the reading is the total.

If you use a K-1766 salt test, you can add all other types of TDS but Calcium and TA are reported in units of calcium carbonate.

Calcium should be reported in units of calcium ion.

TA should be reported in units of bicarbonate.

Sodium Chloride + CYA + Calcium ion + Bicarbonate ion + Magnesium ion + Sulfate ion etc.

Even then, you are mixing reporting units.

For the TDS reading, it assumes a single reporting unit, which is probably sodium chloride.

So, you would convert all units into sodium chloride.

Using TDS has too many variables and I’m not sure there is a cost effective TDS calibration fluid at anything close to the average SWG preferred salt levels.

Given jonesty’s low salt level I think the Apera EC60 calibrated to 1413uS/cm will work quite well. If it’s still close to accurate in the 12.88mS/cm solution then it would be good enough in the 3000-5000ppm range too. I think both of those standards are made up with KCl.

In real world testing with my meter calibrated with a good lab grade 5000ppm NaCl standard I can get within a bees whisker of a chromate silver nitrate test on the same pool sample.