# TDS - what a mess!



## clansley (28 Aug 2020)

Thanks to Darrel I learnt about TDS using different conversion factors (0.5, 0.65, 0.7, etc).

I was using TDS as a signal as when to do a water change; I was trying to keep my tanks in the range of 200-300. Which was doable using my old meter.

But since my previous TDS meter (TDS-3; fake most likely) was almost random (dropping by 30 in a day), although not random enough for me to notice quickly. (I.e the same tank would read the same with tests minutes apart)
I had to replace it with something better!

Unknown to me at the time the old meter used a 0.5 conversion factor, my new meter (HM Digital COM-100) is switchable between factors. AFAICT a 0.65/0.7 conversion is more realistic for our tanks, but that now means it is completely impossible for me to keep my tanks within the range of 200-300, since my new water (RO, remineralised with Saltyshrimp GH/KH+) is around 303 TDS (0.7 conversion factor)!
I originally choose 200-300 based on the range preferred by Red Cherry Shrimp. The question is, are almost all TDS figure we see on the web using the 0.5 conversion factor?

Ultimately this mess, makes TDS a useless reading since most will not know what conversion figure was used. I'm wanting to switch to just using conductivity and avoid all this conversion factor mess, but I fear I will miss out on something. 
All that said, my new meter says that thermal correction when in conductivity mode, will be using the KCl correction, AFAICT this is the same as what the TDS 0.7 conversion factor would use, so in theory I would get a meaningful reading and avoid the conversion mess.

(sorry for the rambling, my first real post )


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## dw1305 (29 Aug 2020)

Hi all, 





clansley said:


> since my new water (RO, remineralised with Saltyshrimp GH/KH+) is around 303 TDS (0.7 conversion factor)!


Just add less "Salty Shrimp" to get your starting water at 200 TDS ppm. 

Personally I would just cut your RO with some tap water, it will be hard and you won't need to add very much water. Because of where you live your tap water will supply both dGH and dKH from the dissolved calcium carbonate (CaCO3), same applies just add enough tap to get you to your lower TDS datum.

If you want more control of your salts, there is a very useful page at <"James' Planted Tank">.

 You are going to add magnesium (Mg) and potassium (K) with your fertiliser addition so you don't need to worry about them if you go down the tap water route. 

cheers Darrel


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## dw1305 (29 Aug 2020)

Hi all, 





clansley said:


> I learnt about TDS using different conversion factors


That is partially why I like conductivity, in microSiemens, as a measure, rather than ppm TDS. Conductivity is affected by temperature, but standard readings are given at 25oC, so pretty much tank temperature.

Also added salts and conductivity have a <"linear relationship">, which makes them easier to interpret <"than pH values"> (a log^10) scale, and a ratio.

Conductivity isn't the parameter that we're most interested in, but it is definitely the easiest one to get <"an accurate reading">.

cheers Darrel


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