Is it possible to guess at ppm Ca and Mg from only ppm TDS?

Silver_Is_Money

Active Member
Lifetime Member
Established Member
Joined
Dec 21, 2016
Messages
319
Reaction score
234
Points
43
Location
N/E Ohio
This is a participatory experiment, and all friendly participation is welcome. I'm attempting to see how well an educated guess based solely upon the ppm data derived via a budget hand held TDS meter might do at ballpark predicting ones Ca++ and Mg++.

Apply this method and provide feedback as to how close it gets you to "known" values for your waters calcium and magnesium:

Total Hardness (as CaCO3) ~= 0.78 x ppm_TDS (as read via the TDS meter)
Total Hardness (as CaCO3) = 2.5(Ca++) + 4.12(Mg++) (this, with the constants rounded, is a truism)

~70% of Total Hardness comes from Ca++ (on loose average/approximation for fresh water)
~30% of Total Hardness comes from Mg++ (on loose average/approximation for fresh water)

Example: Lets say your budget hand held meter says your TDS is 400 ppm

400 x 0.78 ~= 312 ppm Total Hardness (as CaCO3)

0.70 x 312 = 218.4 = the ~hardness that comes from Ca++
0.30 x 312 = 93.6 = the ~hardness that comes from Mg++

218.4/2.5 = 87.4 ppm Ca++ (calcium ion)
93.6/4.12 = 22.7 ppm Mg++ (magnesium ion)

This presents an admittedly loose ballpark stab at deriving a source waters calcium and magnesium ppm from only a TDS meter. It's akin to educated guessing, but that is sometimes better than uneducated guessing. There is clearly room for refinement, but data feedback is my only means of garnering the data from which to refine this approach.
 
Last edited:
Can a water sample have only Ca++ and absolutely no Mg++?
 
Can a water sample have only Ca++ and absolutely no Mg++?

Sure, why not? But as to Alkalinity, it is a 'contrived' conflation of all extant "basic" ion species under the umbrella of CaCO3. Not that this is right, or the best way, but rather that is is universally accepted, and entrenched in 100+ years of use as such. Did you perhaps intend to post this question within my RA/Alkalinity thread?
 
No, this thread. So if I had a bunch of Ca in a water sample but no Mg, would I have a TDS where my estimate of Mg would be completely incorrect? And vice-versa, all Mg and no Ca?

I guess my point is you cannot estimate the ratio of Ca to Mg from TDS. I don't think.

Now, is there some other measurement we can make to estimate this ratio?
 
No, this thread. So if I had a bunch of Ca in a water sample but no Mg, would I have a TDS where my estimate of Mg would be completely incorrect? And vice-versa, all Mg and no Ca?

I guess my point is you cannot estimate the ratio of Ca to Mg from TDS. I don't think.

Now, is there some other measurement we can make to estimate this ratio?

Ah, I may see where your coming from now. But this is strictly a method for ballpark determining natural fresh water sources CA and Mg concentrations via nothing more than a budget hand held TDS meter. And I assuredly do believe that it will have some nominal potential for statistical relevance. Albeit that it may need some tweaking. I'm in no way inferring that it will be "ballpark" valid for all natural fresh waters, but merely for some sufficient percentage of them whereby to indicate relevance as opposed to pure randomness. Or in other words, to justify my contention that sometimes an educated guess is better than an uneducated guess.
 
Last edited:
It makes me wonder if there is a particular ratio that is more common in nature than some other ratios. That would add some validity to these guesses.
 
It makes me wonder if there is a particular ratio that is more common in nature than some other ratios. That would add some validity to these guesses.

That (on average for fresh water) 70% of Total Hardness comes from Ca, and 30% of Total Hardness comes from Mg was noticed about a decade and a half or so ago by Kai Troester, and incorporated into his 'Kaiser Water Calculator', which to my knowledge was the first of the mash pH prediction assistant software solutions to materialize within the public domain. I've played with this 70/30 relationship by matching it against many a randomly selected fresh water analysis report and confirmed a fair degree of nominal validity for it.

It is the uniquely new "TDS x 0.78 ~= Total Hardness (as CaCO3)" part that needs more tweaking and more comparisons by which to hone and refine it, and to see if it has any level of statistical validity. And that's why I've openly asked for people to kick in and give it a try.

"Total Hardness (as CaCO3)" as seen on water analytical reports already magically conflates both calcium and magnesium water hardness into merely calcium carbonate related hardness alone, so even if Ca and Mg are not pegged individually, the conflation of their educated guess derived values still has usefulness, as Kai showed us long ago.
 
Last edited:
OK, so there is a ratio. Mine is 50:8 (Ca:Mg), so outside the norm but not wildly so.

Where do I get a TDS meter? Got a recommendation?
 
OK, so there is a ratio. Mine is 50:8 (Ca:Mg), so outside the norm but not wildly so.

Where do I get a TDS meter? Got a recommendation?

Amazon.com or Walmart online. Recommendation is that if it costs $10 you are paying too much.

----------------------------------------------------------------------------------------------------------------

If your Ca++ is 50 ppm and your Mg++ is 8 ppm, then your Total Hardness (TH) is:

TH = 2.5(50) + 4.12(8)
TH = 158

70% x 158 = 110.6
110.6/2.5 = 44.24 ppm of "estimated" Ca++ (vs. 50 ppm actual)

30% x 158 = 47.4
47.4/4.12 = 11.5 ppm of "estimated" Mg++ (vs. 8 ppm actual)

Not all that far off for educated guesses....
 
And the "educated guess" as to your waters predicted TDS via meter then becomes:

158/0.78 ~= 203 ppm TDS

Get a budget model TDS meter and see how close it comes to reading 203 ppm. (no guarantees)
 
personally I start out with as clean water as I can and make all the additions I need.

ezwater program has worked great so far.
 
@Minbari: Oh yeah, I use either Palmer's water calculator or the one here on Brewer's Friend. Whether they are correct, even slightly accurate, or way way off, it doesn't matter to me: My beers have been coming out just fine.

I'm always looking for new knowledge. It is a struggle to keep up with @Silver_Is_Money and his equations, but every time I go over it again, something new clicks. Is this something I need? No. But never know when it might be useful.
 
Well I'll join in as I have a TDS meter but not used it yet so maybe these examples might help your calculations. (I'm still trying to work out the pH equations ATM.

1) The bottled spring water I use for my beers. Reading of 156ppm (meter went red background from green before test).
upload_2021-4-10_14-33-25.png
upload_2021-4-10_14-33-48.png


2. An alternative bottled water I have in reserve (serves me right for buying 60 bottle of it). TDS = 170ppm.
upload_2021-4-10_14-36-2.png
upload_2021-4-10_14-36-23.png


3) my filtered tap water - not sure of spec yet but will update here later. TDS = 287ppm.
upload_2021-4-10_14-38-17.png
 
From above = Local water stats
upload_2021-4-10_14-42-47.png
 
Well I'll oin in as I have a TDS meter but not used it yet so maybe these examples might help your calculations. (I'm still trying to work out the pH equations ATM.

1) The bottled spring water I use for my beers. Reading of 156ppm (meter went red background from green before test).
View attachment 15162View attachment 15163

2. An alternative bottled water I have in reserve (serves me right for buying 60 bottle of it). TDS = 170ppm.
View attachment 15164View attachment 15165

3) my filtered tap water - not sure of spec yet but will update here later. TDS = 287ppm.
View attachment 15166
I use water from the machine at Wal-Mart. Its ed and RO.
Well I'll join in as I have a TDS meter but not used it yet so maybe these examples might help your calculations. (I'm still trying to work out the pH equations ATM.

1) The bottled spring water I use for my beers. Reading of 156ppm (meter went red background from green before test).
View attachment 15162View attachment 15163

2. An alternative bottled water I have in reserve (serves me right for buying 60 bottle of it). TDS = 170ppm.
View attachment 15164View attachment 15165

3) my filtered tap water - not sure of spec yet but will update here later. TDS = 287ppm.
View attachment 15166
I always usr the machine at Wal-Mart. Filtered and RO. Has a tds of 7
 
OK, lets tackle the @AHarper bottled spring water reading 156 ppm TDS first:

156 x 0.78 = 121.68 = Total Hardness

(0.70 x 121.68)/2.5 = 34.1 ppm of predicted Calcium (vs. 38 on the bottle)
(0.30 x 121.68)/4.12 = 8.9 ppm of predicted Magnesium (vs. 12 on the bottle)

Next the one reading 170 ppm TDS:

170 x 0.78 = 132.6 = Total Hardness

(0.70 x 132.6)/2.5 = 37.1 ppm of predicted Calcium (vs. 40 on the bottle)
(0.30 x 132.6)/4.12 = 9.7 ppm of predicted Magnesium (vs. 14 on the bottle)
 
Last edited:
I use water from the machine at Wal-Mart. Its ed and RO.

I always usr the machine at Wal-Mart. Filtered and RO. Has a tds of 7
Wall-Mart is a LONG way off for me to get it delivered. I'm in the UK and their equivalent store is ASDA - and they don't sell RO water.
 
A well accepted formula for deriving TDS = Sum(all ion ppms) + Bicarb/2

For the 156 TDS metered spring water we get from this:
TDS = 38 +12 + 2.5 + 8 + 12 + 14 + 5 + 135/2
TDS = 159

For the 170 TDS metered spring water we get:
TDS = 40 + 14 + 3 + 6 + 11 + 10 + 8 + 166/2
TDS = 175

@AHarper, Your TDS meter is quite accurate.
 
@AHarper, now lets try your TDS = 287 ppm tap water:

287 x 0.78 = 223.86 = Total Hardness

(0.70 x 223.86)/2.5 = 62.7 ppm of predicted Calcium
(0.30 x 223.86)/4.12 = 16.3 ppm of predicted Magnesium
 
A well accepted formula for deriving TDS = Sum(all ion ppms) + Bicarb/2

For the 156 TDS metered spring water we get from this:
TDS = 38 +12 + 2.5 + 8 + 12 + 14 + 5 + 135/2
TDS = 159

For the 170 TDS metered spring water we get:
TDS = 40 + 14 + 3 + 6 + 11 + 10 + 8 + 166/2
TDS = 175

@AHarper, Your TDS meter is quite accurate.

Thanks for that. It was quite cheap too. I got it with a pH meter and test samples for £17 (about $20) from Amazon.

Good information and thanks for the instruction.
 

Back
Top