# TDS Conversion Factor



## Luvlyjub (27 Sep 2021)

I use a Apera PH and TDS reader and have been confused on the conversion factor from conductivity to TDS. With this model you can set the conversion factor that I understand is anywhere between 0.75 and 0.5 that seems to be on the type of water you are testing? My first unit was default calibrated to 0.71 (that had a fault and was returned) and my replacement set at 0.5 - so what is correct for ratio for testing tap water or aquarium water?

I note that TDS should be used as a base point to monitor and many may not bother testing but my tap water is very hard. If my conductivity is reading 780uS (in one of my tanks) then my TDS is either 585ppm or 390 ppm as a reference point. Looking at many tables online and that I know I have hard water then would I be correct to use 0.55?

My reasoning for knowing my TDS more accurately is using a sterilizer. As the Chihiros Doctor version has a table based on TDS and tank volume. I have seen a thread on this asking advice between this and the Twinstar and I will add a comment there. Basically it states do not use above 500 TDS and reading the table if you have a small tank of 60 litres this reduces to 150 TDS.


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## MichaelJ (27 Sep 2021)

Luvlyjub said:


> I use a Apera PH and TDS reader and have been confused on the conversion factor from conductivity to TDS. With this model you can set the conversion factor that I understand is anywhere between 0.75 and 0.5 that seems to be on the type of water you are testing? My first unit was default calibrated to 0.71 (that had a fault and was returned) and my replacement set at 0.5 - so what is correct for ratio for testing tap water or aquarium water?
> 
> I note that TDS should be used as a base point to monitor and many may not bother testing but my tap water is very hard. If my conductivity is reading 780uS (in one of my tanks) then my TDS is either 585ppm or 390 ppm as a reference point. Looking at many tables online and that I know I have hard water then would I be correct to use 0.55?
> 
> My reasoning for knowing my TDS more accurately is using a sterilizer. As the Chihiros Doctor version has a table based on TDS and tank volume. I have seen a thread on this asking advice between this and the Twinstar and I will add a comment there. Basically it states do not use above 500 TDS and reading the table if you have a small tank of 60 litres this reduces to 150 TDS.



@Luvlyjub   Yes, the correlation is nonlinear...  The factor for distilled water is close to 0.5 (0 ppm) whereas for seawater its around 0.75 (sea water is in the +35000 ppm range).  I've seen people using the average between the two endpoints (0.63), 0.5 and 0.55 as the conversion factor (I think its just so much easier to work with TDS as most literature, products, discussions etc. are mostly referring TDS rather than EC/uS).  Your TDS is way closer to 390 ppm than 585 ppm. 0.55 sounds like a meaningful factor to me. Of course, you can still have a relatively high TDS even in moderately hard or slightly soft water (say lower Calcium / Magnesium contents), but high contents of  Potassium, Nitrate, Phosphate etc. but thats probably not the case for your tap water.

Cheers,
Michael


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## Luvlyjub (27 Sep 2021)

Thanks Michael,

I have seen an article referring to using a average of 0.64 for freshwater and for solutions containing mostly sodium and chloride ions then between 0.49 and 0.56. So I thought tap water was in this range and hence why I thought of using 0.55 as I am sure I saw this in a table somewhere?


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## MichaelJ (27 Sep 2021)

Luvlyjub said:


> Thanks Michael,
> 
> I have seen an article referring to using a average of 0.64 for freshwater and for solutions containing mostly sodium and chloride ions then between 0.49 and 0.56. So I thought tap water was in this range and hence why I thought of using 0.55 as I am sure I saw this in a table somewhere?


@Luvlyjub  Yep I've seen 0.55 as well. Freshwater, EC = 300 − 800 μS⁄cm = 0.55




Luvlyjub said:


> My reasoning for knowing my TDS more accurately is using a sterilizer. As the Chihiros Doctor version has a table based on TDS and tank volume. I have seen a thread on this asking advice between this and the Twinstar and I will add a comment there. Basically it states do not use above 500 TDS and reading the table if you have a small tank of 60 litres this reduces to 150 TDS.


Ok, I am not familiar with this product, but there is a discussion about using the Twinstar at high TDS here

Cheers,
Michael


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## Luvlyjub (27 Sep 2021)

Thanks again - just seen your link as added a conversation to another thread with this question ....doh!


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## dw1305 (28 Sep 2021)

Hi all,


MichaelJ said:


> using the average between the two endpoints (0.63),





Luvlyjub said:


> I have seen an article referring to using a average of 0.64 for freshwater and for solutions


0.64 is the usual conversion factor for fresh water, and it assumes that the majority of ions are calcium (Ca++) and (bi)carbonate (2HCO3-). Using 0.64 (100 microS = 64 ppm TDS) would be appropriate for hard water in the UK.

There is a more complete explanation <"here">.

cheers Darrel


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## MichaelJ (28 Sep 2021)

dw1305 said:


> There is a more complete explanation <"here">.



Hi @dw1305 ... Interesting read. That factor is really all over the place... Any idea why the hobby (literature, products, forums etc.) is almost always referring to TDS ppm instead of uS/cm ?






Cheers,
Michael


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## dw1305 (28 Sep 2021)

Hi all,


MichaelJ said:


> Yes, the correlation is nonlinear....


It is <"pretty much linear"> at conductivity values below ~1000 microS. That is the "442" conductivity standard is nearest to hard fresh water (the 4 : 4 : 2 is 40% sodium sulphate, 40% sodium bicarbonate, 20% sodium chloride).


MichaelJ said:


> is almost always referring to TDS ppm instead of uS/cm ?


I think it must just be tradition. I much prefer microS. as a measurement, you can only really measure mg/L (ppm) TDS by  <"evaporating a known weight of water to dryness and weighing the residue">, which is why TDS meters all actually measure conductivity.

cheers Darrel


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## Luvlyjub (28 Sep 2021)

Well that means tap water is definitely 500 TDS. Wondering if I should look at RO or even mixing 50/50? 

Once my Cichlid tank is established then embarking on first proper aquascape and wanting to add shrimp but concerned if TDS would be too high?


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## dw1305 (28 Sep 2021)

Hi all,


Luvlyjub said:


> and wanting to add shrimp but concerned if TDS would be too high?


Cherry Shrimps like hard water, it would be an issue with Crystal Red Shrimps etc.

cheers Darrel


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## MichaelJ (28 Sep 2021)

dw1305 said:


> Hi all,
> 
> It is <"pretty much linear"> at conductivity values below ~1000 microS.


True. very much so,  but the factor is still depending on the known (or assumed) constituent of the water as you mentiond.


dw1305 said:


> I much prefer microS.


Yes, and now I understand why.  I wish my expensive Hanna TDS meter would readout the "raw" Microsiemens instead a possibly erroneous TDS value.

Cheers,
Michael


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## dw1305 (28 Sep 2021)

Hi all,


MichaelJ said:


> but factor is still depending on the known (or assumed) constituent of the water as you mentiond.


Yes, entirely based on an assumption of the salt composition. I don't know the answer, but I'd guess that you would need a different conversion factor for Lake Tanganyika or brackish water etc.

It is partially why I suggest a <"conductivity range"> for your <"datum value">,  it only needs a <"very small addition of salts"> (ions) to raise the conductivity appreciably and there is also the temperature effect (<"conductivity rises with increasing temperature">).

cheers Darrel


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## MichaelJ (28 Sep 2021)

Luvlyjub said:


> Wondering if I should look at RO or even mixing 50/50?


That would probably be the easiest way to take that TDS down. Perhaps getting it down to the lower 300 range with 2/3rd tap and 1/3rd RO will suffice.



Luvlyjub said:


> Once my Cichlid tank is established then embarking on first proper aquascape and wanting to add shrimp but concerned if TDS would be too high?


Cherry Shrimps do prefer moderately hard water (say 7-10 GH. In particular, they need a good amount of Calcium to build their exoskeleton).  but they also prefer water with lower conductivity in order not to cause them stress and interfere with the molting process, metabolism etc.  The breeder ( Aquaticarts out of Indiana, US) where got all my shrimps from told me to aim for 7-8 GH and keep the TDS down at about 260-270 ppm. This part has taken a bit of experimentation with various minerals, as I also wan't to keep nutrients in abundance available for my plants. It all worked out and the shrimps are seemingly happy and very colorful (and they are breeding).  All that said, plenty of people around here keep seemingly happy cherry shrimps at much higher TDS.

What Chiclids are you putting in?  Keep in mind that even Dwarf Chiclids (such as Rams) will pick small Cherry shrimps if they get the chance, so make sure you get nicely sized shrimps 

Cheers,
Michael


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## Luvlyjub (28 Sep 2021)

Darren you are truly a font of knowledge on water chemistry 👨‍🎓

I did come across that article on my search and decided it was way above my level of understanding at this juncture......and more likely any in the future.

So why do we use TDS and not Conductivity readings?


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## Luvlyjub (28 Sep 2021)

MichaelJ said:


> That would probably be the easiest way to take that TDS down. Perhaps getting it down to the lower 300 range with 2/3rd tap and 1/3rd RO will suffice.


I was looking at around 200 TDS for an aquascape so thought 50/50 would be an easy measure. I know that a number of hobbyists only use tap water for planted aquariums but is 500TDS too high? Also if I do mix with RO will I need to add salts still and/or will I need to know more precisely the content of my tap water in either case?


MichaelJ said:


> Cherry Shrimps do prefer moderately hard water (say 7-10 GH. In particular, they need a good amount of Calcium to build their exoskeleton).  but they also prefer water with lower conductivity in order not to cause them stress and interfere with the molting process, metabolism etc.  The breeder ( Aquaticarts out of Indiana, US) where got all my shrimps from told me to aim for 7-8 GH and keep the TDS down at about 260-270 ppm. This part has taken a bit of experimentation with various minerals, as I also wan't to keep nutrients in abundance available for my plants. It all worked out and the shrimps are seemingly happy and very colorful (and they are breeding).  All that said, plenty of people around here keep seemingly happy cherry shrimps at much higher TDS.


 That's what I thought for shrimp around the 200/250 level seems a good balance and possibly better for the plants?. Not tested by GH but KH is 7/8 and with an RO mix I guess this will reduce and why I am not sure if I would need to add anything else?


MichaelJ said:


> What Chiclids are you putting in?  Keep in mind that even Dwarf Chiclids (such as Rams) will pick small Cherry shrimps if they get the chance, so make sure you get nicely sized shrimps
> 
> Cheers,
> Michael


The Shrimp are for a separate High-tech 55L nano tank I am setting up after Cichlids that will be a 200L low-tech and epiphyte planted


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## Geoffrey Rea (28 Sep 2021)

Luvlyjub said:


> I know that a number of hobbyists only use tap water for planted aquariums but is 500TDS too high?







Short answer… no


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## MichaelJ (28 Sep 2021)

Luvlyjub said:


> I was looking at around 200 TDS for an aquascape so thought 50/50 would be an easy measure. I know that a number of hobbyists only use tap water for planted aquariums but is 500TDS too high? Also if I do mix with RO will I need to add salts still and/or will I need to know more precisely the content of my tap water in either case?


It's always good to know a bit about what is making up the tap water. At least KH and GH and Nitrate contents, which for some households can be surprisingly high as well (up in the 50 ppm range).

The plants in general don't really care about high TDS... There is no milage from using RO water for the sake of the plants. If I only had plants I would just use straight tap and add the Macro ferts that my plants need.  The thing about RO is that it generates a lot of waste water and also can take quite a while to make. I strive to use as much tap water as I can and balance that with the minerals that my plants and livestock needs.  My situation is a little atypical because all my household water runs through a water softener using Potassium Chloride, so it essentially comes out at "0" GH with somewhere around 140-160 ppm of Potassium, a KH around 10 and a TDS around 300 ppm.  

Cheers,
Michael


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## tiger15 (28 Sep 2021)

Conductivity is a measurement of electric current that is dependent on ionic strength.  TDS is a gravimetric measurement by evaporating H2O and weighing the residue.  The two are correlated but there is no one right conversion factor from conductivity to TDS as it is ion specific as tabulated in Post #7.  The correlation is highly erratic from one person's tank to another as the source water and management practices (WC, food and chemicals added) can be highly variable.  However,  the correlation in one's tanks can be fairly consistent over time and so it can be used as a relative measurement to determine, for example,  when water change is over due.


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## MichaelJ (28 Sep 2021)

Geoffrey Rea said:


> Short answer… no


@Geoffrey Rea  What a beautiful tank!   ... well, there you go... 500 ppm.   I still think keeping the condictivity/TDS low for the sake of livestock (not just shrimp) arguably makes sense, but sure,  I know from experience  fish can be okay at even extremely high TDS.  And TDS creep is a sure sign that maintenance is insufficient.

Cheers,
Michael


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## Luvlyjub (28 Sep 2021)

@Geoffrey Rea is that 500TDS reading 'Total Dissolved Steroids'! WOW!


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## MichaelJ (28 Sep 2021)

Luvlyjub said:


> @Geoffrey Rea is that 500TDS reading 'Total Dissolved Steroids'! WOW!


Ha!   I had well over 1500 ppm at one point in both my tanks...only discovered this when I got a  TDS meter ... 1500+ that's almost considered brackish water territory... Surprisingly, fish and plants did well, but I don't think the situation would have been sustainable.  The short story is that I only did 20% WC weekly at that time with occasional lapses and I had the remineralization dosing wrong for my pure RO water I did at that time (was paying too much attention to KH and pH in particular). I was essentially piling on to my TDS every week...

Cheers,
Michael


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## Geoffrey Rea (28 Sep 2021)

Luvlyjub said:


> @Geoffrey Rea is that 500TDS reading 'Total Dissolved Steroids'! WOW!



The TDS monitoring with the tap water here is most useful in the summer. They scrub the nitrate sometimes and you’ll notice a significant drop with the incoming water supply. The 500ppm is a summer feature, probably due to agricultural run off. In the winter it will be 400-420ppm on average. Share the same water supply as Aquarium Gardens.

It’s sort of meaningless without knowing what composition you can expect from your water provider via a water report, or getting an ICP water analysis done.


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