# Help understanding TDS pen readings



## Adam E (20 Nov 2019)

Hi all. Got a little issue I'm struggling to get my head around. Every time I use a tds pen on my aquarium I get different reading(like 20 mins apart). Basically I let my tank slip whilst I was working 7 days a week and the tds when i checked was 867ppm. My tap water is anywhere between 250 and 350ppm( varies on how long i run the tap prior to filling the bucket) so I've been doing partial water changes every 3 days and the tank is now at roughly 450ppm. I went todo a water change today and when i tested the water it said 457ppm. I took 15ltrs out and put 15ltr of tap water into a bucket at 270 ppm with de-chlorinater which increased the reading to 301ppm. I let the bucket sit with a heater for 30 mins to get it to tank temp. I then tested the water again and it said 390ppm?? So I then tested the tank  before adding the 15ltrs back in and it's now saying 527ppm? So I cleaned the pen by running under water. Tested tap water and it cam up at 268ppm. Then tested the bucket and it said 315ppm, done another test on tank and it's now at 370ppm? Is there something I'm.not taking into account or is it just a dodgy pen? Sorry for the long winded post. Just trying to explain as good as I can


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## tam (21 Nov 2019)

So the tank changed from 457 to 527 whilst you were mixing water in a bucket and you didn't add anything to the tank in that time? I'd blame the pen, if your testing the same water without doing anything to it and getting massive differences something is off.


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## MJQMJQ (21 Nov 2019)

Its based on electrical conductivity basically ions.I believe dechlorinater breaks stuff down into ions which increases conductivity and hence TDS. TDS in tap water will vary. Also the amount of dechlorinator put in and how it reacts with the compounds also affects TDS.
https://www.wikihow.com/Calculate-Total-Dissolved-Solids


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## dw1305 (21 Nov 2019)

Hi all, 





Adam E said:


> Is there something I'm.not taking into account or is it just a dodgy pen?


Try it in some DI water in a beaker.   It should read less than 10. 

Add a very small amount of table salt to the water, the TDS value should rise pretty quickly.





MJQMJQ said:


> Its based on electrical conductivity basically ions


It is. Have a look at <"Is my tap water...>.

You can make <"conductivity standards"> from sodium (or potassium) chloride (NaCl/ KCl). Table salt will do at a pinch, but it has a desiccant added, so it is only ~98% NaCl.

cheers Darrel


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## sparkyweasel (21 Nov 2019)

Are the batteries in the pen fresh?


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## akwarium (21 Nov 2019)

is your TDS-pen temperature corrected?

 EC/TDS will increase around 2 % per degree celsius (more at high temperatures) so heating up a bucket of water will indeed increase TDS if your TDS-pen does not correct for it.


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## jaypeecee (21 Nov 2019)

Hi @Adam E

Another option is to buy a bottle of *still* water, not carbonated. The bottle label is likely to specify the TDS. Here's a useful starting point:

http://www.finewaters.com/bottled-waters-of-the-world/united-kingdom/highland-spring

JPC


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## Adam E (22 Nov 2019)

Thanks everyone for all the replies and suggestions. I will get onto them over the weekend.

Tam - the pen is only a few weeks old? Maybe it's a faulty pen?

Dw1305 - next time I'm at the super6ill get some di water or I might just pop into the fish store and ask them to check against there pen/tds levels

Sparky weasel - the pen is only a few weeks old so I hope so? I'll pick up some new ones and try it again.

Akwarium - according to the manual it is temperature corrected. It also can tell the temp although the temp seems to be about 2°c lower than what the thermometer say.

Jaypeecee - I'll grab a bottle on way to work tomorrow and take pen with me will test it on my break.


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## dw1305 (22 Nov 2019)

Hi all, 





Adam E said:


> Dw1305 - next time I'm at the super6ill get some di water or I might just pop into the fish store and ask them to check against there pen/tds levels


I'd probably buy some DI water. TDS meters are pretty robust, but you don't know exactly what the LFS may have done with theirs.

It is really easy to contaminate your standard (and sample), conductivity standards aren't like pH buffers, any addition of salts <"raises conductivity">.

cheers Darrel


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## Adam E (26 Nov 2019)

Hello everyone. Sorry it's taken a while I've had a hectic week! Finally bought some di water and some new batteries just incase. Tested the water with the pen and it read 2ppm. I changed the batteries anyway and still got a reading of 2ppm at 22°c. That potentially rules out the pen being at fault. I dunno maybe I'm doing something wrong? I did another test with a bucket of water. It came out the tap this time at 272ppm left for 12hrs with a heater set to 24°c and an air stone to agitate the water to help remove chlorine. ( I done this as I want to lower the tds in my tank and obviously adding de-chlorinater will raise the tds) after 12hrs the bucket had a reading of 330ppm? Is this normal?( I thought I would of gone down due to the chlorine coming out) I really dont understand the pen and dont want to do something terribly wrong and kill my fish off.


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## Adam E (26 Nov 2019)

I should add that if I add de-chlorinater straight to the bucket with water out of the tap it gives a reading of 300-330ppm depending on the tds of the tap water as that also varies. Although I've never added treatment and waited 12hrs to see if the tds changes again.I think, if I work it out right, about 1ml of de-chlorinater per 9ltr adds about 30ppm to the bucket


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## jaypeecee (27 Nov 2019)

Adam E said:


> Hello everyone. Sorry it's taken a while I've had a hectic week! Finally bought some di water and some new batteries just incase. Tested the water with the pen and it read 2ppm. I changed the batteries anyway and still got a reading of 2ppm at 22°c. That potentially rules out the pen being at fault. I dunno maybe I'm doing something wrong? I did another test with a bucket of water. It came out the tap this time at 272ppm left for 12hrs with a heater set to 24°c and an air stone to agitate the water to help remove chlorine. ( I done this as I want to lower the tds in my tank and obviously adding de-chlorinater will raise the tds) after 12hrs the bucket had a reading of 330ppm? Is this normal?( I thought I would of gone down due to the chlorine coming out) I really dont understand the pen and dont want to do something terribly wrong and kill my fish off.



Hi @Adam E 

At least the TDS pen is now looking more promising. You should be able to get a TDS reading of your tap water from your water company. You can then compare their figure with what you have measured.

JPC


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## Adam E (27 Nov 2019)

this is what my local water company states. If I'm right that's 286.5ppm?? Which is what it is out the tap if I let it run for a minute before i fill up the bucket. 

So the pen works!..My only question now is why the pen gave me a different reading 20 mins after testing it the 1st time.


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## Adam E (27 Nov 2019)

And why the tds rose up after 12hrs of being aerated and not having anything added?


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## jaypeecee (27 Nov 2019)

Adam E said:


> View attachment 129376 this is what my local water company states. If I'm right that's 286.5ppm?? Which is what it is out the tap if I let it run for a minute before i fill up the bucket.
> 
> So the pen works!..My only question now is why the pen gave me a different reading 20 mins after testing it the 1st time.



Hi @Adam E 

Yes, it looks as though you're now in business. As for the rise to 330ppm, my only thought is that the conductivity of the water may have risen as a result of dissolved CO2 introduced during aeration. In this respect, the term 'TDS' is slightly misleading. Because TDS is total dissolved _solids_, it's confusing, isn't it? We have to remember that a TDS meter is actually measuring electrical conductivity (in microSiemens/cm) and then applies a conversion factor to arrive at the TDS figure. So, if the conductivity rises as a result of CO2 being dissolved, this will form carbonic acid that I would expect to raise the conductivity. @dw1305 will correct me if I'm wrong.

JPC


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## tam (28 Nov 2019)

A container of water will also gradually increase in TDS due to evaporation - the water reduces leaving the TDS more concentrated. Doubt that would account for the full increase but it would contribute to it.


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## dw1305 (28 Nov 2019)

Hi all,
It might just be the warming, as the water warms the <"conductivity will rise">, even if you don't have any more ions present.

Standard conductivity readings (in microS.) are measured at 25oC. Usually I don't worry too much about temperature, because aquarium water is usually near enough 25oC for it not to matter.






 "_Specific conductance at 25 degrees C is used as a standard of comparison for different water sources as conductivity ratios change with temperature._"

cheers Darrel


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## Adam E (28 Nov 2019)

Thanks everyone for all your help and replies. Its Certainly made things easier and a lot clearer for me. I have tendencies to over think certain things haha and this is one of many! Hopefully that's the last of my confusions for now, but, I know where I'll be heading the next time I am!


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## MJQMJQ (30 Nov 2019)

Chlorine will evaporate from water too!Thats why u aerate it.Not sure about yr chlorine concentration and other things that might possibly evaporate.


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