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Myola

Inconsistent TDS readings

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Myola

Hi all,

I currently have two different brands of TDS metres - one that I use all the time (Xiao Mi) and a cheap no name one that is actually the same one that Mark Peggie (Mark's Shrimp Tanks) uses. Today, when I was doing my weekly water parameter check and water change, I decided to see if there was a difference in the TDS readings between the two metres. Well I was surprised to discover that there was a huge difference in the reading - the Xiao said 252ppm and the no-name said 203ppm. Which one should I believe? My GH is 6, KH is 1 and the readings were taken at the same time in the same sample of water so no temp difference. Neither of them mention anything about calibrating in the instructions so consequently I have not calibrate them. Has anyone else had the same situation? As I'm only keeping neocaridinas I guess the TDS is a little less critical but I would like to start keeping caridinas in the future, and if I am setting my water up to the wrong parameters it is going to cause some big problems. Any suggestions or info would be great!

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jayc
2 hours ago, Myola said:

the Xiao said 252ppm and the no-name said 203ppm. Which one should I believe? My GH is 6

If GH is indeed 6, I would be more inclined to believe 252 TDS.

Try testing pure RO or rain water to see what the difference is at the low end.

And test at a high TDS to see what the difference is. Currently you have a 50ppm difference. If it remains 50ppm measuring Pure RO, then you know the cheaper unit is out of calibration. And if it is still 50ppm difference at high then you can compensate for it.

 

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sdlTBfanUK

I know your problem well, not with TDS personally but different brands of PH and GH test kits though. You could take a small sample of water to an aquarium place and ask them to test the water to find out which one is most accurate and discard the other. Obviously do JayC check first (I bow anyway to his greater knowledge/experience) as that may make things very clear, but if not:

In theory TDS 150 should be around Gh6 if you start with GH and TDS of zero and remineralise (with dennerles product I use anyway) it with specialist products, but in theory the titanic was unsinkable and look how that turned out??? My tank (Taiwan bee) runs at TDS 160 and GH4. The difference was my fault I think as I used zerowater Gh0, TDS000 but didn't remineralise it from day 1, ie, even when I was cycling the un-occupied tank. Again, IN THEORY, my GH should very slightly increase with each water change even though the TDS should remain 160 - time will tell on that but everything is doing well in the water as it is and I only do 7% water change per week so it will be a very slow change anyway. It will depend on your source water.

I don't think you need worry though with neocaridina  as they are pretty tough/adaptable, mine get completely ignored and I just use (dechlorinated) tap water and they reproduce like crazy (can't give them away quick enough).

Caridina however are a whole different kettle of fish in my experience and it took me YEARS to get that to work, if you are interested and setting up a completely new tank you may want to see the thread I added recently in the forum, under 'shrimp keepers forum','shrimp tank setup', 'TAP WATER...'

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Myola
On 10/1/2018 at 4:48 PM, jayc said:

If GH is indeed 6, I would be more inclined to believe 252 TDS.

Try testing pure RO or rain water to see what the difference is at the low end.

And test at a high TDS to see what the difference is. Currently you have a 50ppm difference. If it remains 50ppm measuring Pure RO, then you know the cheaper unit is out of calibration. And if it is still 50ppm difference at high then you can compensate for it.

 

Im using filtered (but not RO) rain water from our rain water tanks. It usually measures 45ppm TDS on the Xiao (GH3). I have to remineralise to get the GH up and use Salty Shrimp GH/KH+, which also increases the TDS. I've just tested some RO water that I got from the LFS. They claim it's 0TDS and the Xiao measured 3ppm, so close enough. The el-cheapo measured 0ppm. Then I made up some really minerally water at home and the Xiao said 455, the cheapie said 407, so again, close to the 49 ppm difference I was getting before. So obviously there is a 48/49ppm difference between the two that I will have to take into account if I ever have to start using the cheapie. I'm keeping neocaridinas for now but down the track would like to start up a CRS colony. What brand of metre do people use when keeping caridinas, as I know TDS is more critical for them? 

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Myola
On 10/1/2018 at 11:09 PM, sdlTBfan said:

I know your problem well, not with TDS personally but different brands of PH and GH test kits though. You could take a small sample of water to an aquarium place and ask them to test the water to find out which one is most accurate and discard the other.

Hi sdlTBfan,

Thanks for the reply. I have taken water samples to my LFS in the past but they know NOTHING about shrimp and they generally use API test kits. They are generally unskilled in interpreting the results and quite often they tell my something that I can clearly see is the wrong result. So unfortunately I don't live in an area that I could take my water in and get tested because I wouldn't be confident of the answers. It was a good idea though and I will read your thread about tank water. 

Cheers

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jayc
6 hours ago, Myola said:

I've just tested some RO water that I got from the LFS. They claim it's 0TDS and the Xiao measured 3ppm, so close enough. The el-cheapo measured 0ppm. Then I made up some really minerally water at home and the Xiao said 455, the cheapie said 407, so again, close to the 49 ppm difference I was getting before. 

Based on this new result, i would be inclined to believe the XiaoMi. LFS RO water is rarely 0ppm TDS. Only if and when they change their RO filters will it be 0. And filters are not cheap, so they rarely change it. But 3ppm TDS is actually not too bad. If it really was 0ppm TDS, then both of your meters would have read 0.

And now we know that there is a consistent 49ppm TDS difference in readings no matter if it's low or high ... it means that it is a calibration issue with your cheapie unit. So use it as a back up and remember to always subtract 49 from the reading.

 

6 hours ago, Myola said:

What brand of metre do people use when keeping caridinas

I use a HM Digital TDS meter. It's a lot more accurate than cheap ebay units, but also a lot more expensive.

 

 

6 hours ago, Myola said:

They are generally unskilled in interpreting the results and quite often they tell my something that I can clearly see is the wrong result.

LOL! unfortunately quite a few LFS are like that. They can be staffed by students on work experience, or for a summer holiday job, and have been only given a 5 minute tutorial on the subject. I am often the one educating THEM about their stock or products. Just this last weekend I was at my LFS and asked if they had Blue Ram fish. The guy said yes, and proceeded to show them to me in the tank. "Here they are" ... I looked at it and told him "Those are Gold rams, not Blue Rams". He said, but the label says Blue Ram. Sigh... just because the label is wrong does mean JayC is wrong. "Look, these are gold in colour. Blue Rams are ... blue at the very least". He said "Oh". :embarrass: I guess getting them to pick male from female Blue Rams is out of the question, when their staff can't even tell Gold from Blue. Exit stage I left <-

 

One last thing...

If you are willing, you can try "fixing" the cheapie.

There should be two metal prongs on the TDS meter. Try squeezing them apart from each other with some pliers. Not too much force. We are just trying to move them ever so slightly further apart to compensate for that extra 50ppm reading.

See if that works, or at least reduces the 50 ppm discrepancy to something less.

Oh, if prying them apart causes the reading to increase, then you need to squeeze them closer. I forget which way is which, as I haven't had to tune a TDS meter for the longest of time. I take no responsibility if it doesn't work :crazy:.

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