# TDS Puzzles!



## MattT34 (23 Jul 2020)

Hi All!

Having a little TDS trouble, I bought a TDs meter to check my tank water as I am keeping shrimp, as it was a cheap TDs I decided to buy some 342 solution and my meter reads way high, around 422.







Decided I would buy a more expensive calibratable TDS meter to be sure, few days later it arrives supposedly calibrated so I test the 342 solution again and it reads high again around 405, I re calibrate the meter with the supplied solutions and test again, now it reads around 530!




Do I have two faulty meters or faulty test solution? The solution was brand new and only used for these tests, now I have no idea which way to go with my water.

Hope you can help me

Thanks
Matt


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## Nick72 (23 Jul 2020)

Can you take your TDS meter to a LFS and compare your results with their results on one of their tanks?


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## MattT34 (23 Jul 2020)

Nick72 said:


> Can you take your TDS meter to a LFS and compare your results with their results on one of their tanks?


Yes I can do this tomorrow 👍


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## dw1305 (23 Jul 2020)

Hi all, 





Nick72 said:


> Can you take your TDS meter to a LFS


I think that they won't be much help, but I may be pleasantly surprised.

My guess it is back to the fact that the TDS meter isn't actually measuring <"Total Dissolved Solids">, it is measuring electrical conductivity in microS (a measure of the ions in solution) and then using a conversion factor to estimate the ppm TDS from that. 

The usual conversion factor used for fresh water is 0.64 (64 ppm TDS ~ 100 microS) which assumes that the majority of the TDS is from calcium carbonate (CaCO3), as Ca++ and HCO3- ions.

If the major salt is suspected to be sodium chloride (NaCl)  then the conversion factor used is 0.5 (100 microS ~ 50 ppm TDS).  This is because ions differ as electrical conductors, it is just like a wire, where silver (Ag) is a better conductor than copper (Cu) etc.

If you look at the label on the standard it says: "*362 (ppm) as KCl*", <"*478 as 442*"> and "*700 microS*", this just means that K+ ions are a slightly less good electrical conductor than Na+ ions etc..

The important bit is that this *standard solution should read 700 microS at 25oC*.

So here goes for your meters: 422/0.64 = *659 microS* and 530/0.64 = *828 microS*.

You may be able to set your calibratable meter to "conductivity in microS" and then adjust it to 700 microS.

You have to be really careful with conductivity standards, <"minute amounts of salts (or RO water)"> effect them.

cheers Darrel


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## MattT34 (23 Jul 2020)

dw1305 said:


> Hi all, My guess is that they won't be much help, but I may be pleasantly surprised.
> 
> My guess it is back to the fact that the TDS meter isn't actually measuring <"Total Dissolved Solids">, it is measuring electrical conductivity in microS (a measure of the ions in solution) and then using a conversion factor to estimate the ppm TDS from that.
> 
> ...


Thanks Darrell that's gave me a hide insight!

Bizarrely my calibrated TDs will not read in ms/cm
But in calibration mode it does, so I put the meter in calibration mode as read the 342 solution at 730 so it's a bit out and there is also a setting for the TDS factor currently set to 0.71 I wonder if that can be adjusted to make my meter read 700 in the 342 solution then re calibrate and test again


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## MattT34 (23 Jul 2020)

dw1305 said:


> Hi all, My guess is that they won't be much help, but I may be pleasantly surprised.
> 
> My guess it is back to the fact that the TDS meter isn't actually measuring <"Total Dissolved Solids">, it is measuring electrical conductivity in microS (a measure of the ions in solution) and then using a conversion factor to estimate the ppm TDS from that.
> 
> ...


My non calibratable meter does do ms/cm and reads 824 which is dam close to your math in theory I should be able to set my other meter to a factor of 0.63-64 and then be right.


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## jaypeecee (23 Jul 2020)

Hi @MattT34

Below is a nice little conversion chart that I use. It will download from the link. Then double-click to open.

JPC


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## MattT34 (23 Jul 2020)

jaypeecee said:


> Hi @MattT34
> 
> Here's a nice little conversion chart that I use:
> 
> JPC


Awesome thanks 👍


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## dw1305 (23 Jul 2020)

Hi all, 





MattT34 said:


> also a setting for the TDS factor currently set to 0.71


That helps. 

So 530/0.71 = 746 microS. 

What is the range of the meter? Hydroponic solutions tend to be a lot saltier than aquarium water, so the meters read up to 30 milliS (30,00 microS). They don't tend to be as accurate in low conductivity solutions.

cheers Darrel


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## dw1305 (23 Jul 2020)

Hi all, 





jaypeecee said:


> Here's a nice little conversion chart that I use:


Thank you that would have saved a lot of typing.

When I make calibration solutions I use the  1411 µS  solution, made up with KCl.

cheers Darrel


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## MattT34 (23 Jul 2020)

dw1305 said:


> Hi all, That helps.
> 
> So 530/0.71 = 746 microS.
> 
> ...


0-1000 ppm not sure on the mscm


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## dw1305 (23 Jul 2020)

Hi all, 





MattT34 said:


> 0-1000 ppm not sure on the mscm


About 1500 microS, so a low range meter.

cheers Darrel


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## jaypeecee (23 Jul 2020)

Hi Folks,

One of the things I like about the table is that it shows how the conversion factor increases as conductivity increases.

For Matt's benefit, the 'S' of microS is Siemens. Strictly speaking, electrical conductivity is measured in microSiemens/cm but it is often abbreviated to microS.

JPC


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## hypnogogia (23 Jul 2020)

There is an overall upward trend, but in a couple of instances, it also decreases.  How is that explained?


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## jaypeecee (23 Jul 2020)

Hi @hypnogogia


hypnogogia said:


> There is an overall upward trend, but in a couple of instances, it also decreases. How is that explained?



That's where the typist spilled his coffee on the original and couldn't read his writing!

JPC


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## LondonDragon (23 Jul 2020)

hypnogogia said:


> There is an overall upward trend, but in a couple of instances, it also decreases.  How is that explained?


I do find this with my TDS meter, if I perform a few concurrent tests it tends to increase slightly! but if I let it dry then it goes back to the original value again! so I put it in the cupboard and its been there since


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## dw1305 (24 Jul 2020)

Hi all, 





hypnogogia said:


> There is an overall upward trend, but in a couple of instances, it also decreases. How is that explained?


I think it is a  pretty much linear relationship  between ions and conductivity until you get well into the millisiemens range. 





LondonDragon said:


> I do find this with my TDS meter, if I perform a few concurrent tests it tends to increase slightly!


You should really give them a good swirl in DI water between each use. 

The meters we have have a remote probe, so I normally turn the meter on, turn the tank lights, filter etc off, dump the probe in, go away for ten minutes, take the reading. 

cheers Darrel


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## dw1305 (24 Jul 2020)

Hi all, 





dw1305 said:


> I think it is a pretty much linear relationship between ions and conductivity until you get well into the millisiemens range.


This is the conductivity / TDS ppm relationship for potassium chloride (KCl).



cheers Darrel


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