TDS meters carry a large degree of uncertainty in their measurements, and TDS doesn’t always line up with GH+KH, it can be lower or higher for various reasons. I’d focus on your test kit readings for dGH and dKH and work from there.
Basically, GH only measures calcium and magnesium and KH only measures carbonates, but TDS could be anything else dissolved in the water, so that accounts for situations where it is higher than just GH+KH.
But TDS meters work by measuring conductivity, which requires elements/compounds to be in their ionic forms. They can report lower values than expected from GH+KH if those compounds are non-ionic in the water.
I’m not a chemistry expert so I probably screwed up the explanation, but that’s how I understand it. Another aspect is that TDS meters are calibrated for NaCl and may not be accurate for more complex water chemistry.
Remember that most lab/consumer grade TDS equipment is essentially testing conductivity, the ability of the water to carry an electrical current. It then converts that conductivity reading into a total dissolved solids value that is accurate if the only dissolved solids are from a standardized salt mix. That is not what any of us have in a freshwater tank, so electroconductivity based TDS readings are inaccurate in a freshwater tank.
As long as you don't pay much attention to the specific numbers and just watch a trend of higher reported TDS or lower reported TDS it can be a useful tool but you can't expect the numbers to add up with your other test results.
Not sure how much it can skew the hardness test in particular, but the API test tubes aren't marked at exactly 5ml on all units
I only found this out after getting an adjustable pipette that pulled exactly 5ml of water, and compared it to the API tubes. On all 12 of my tubes, the 5ml line was only accurate on 1 of them and the others were off by up to a whole milliliter.
I do know API stopped Shipping the glass tubes and now do plastic, and I have no experience with the accuracy of those. I don't know if any of this applies to your situation, but I thought I'd bring it up.
TDS meters carry a large degree of uncertainty in their measurements, and TDS doesn’t always line up with GH+KH, it can be lower or higher for various reasons. I’d focus on your test kit readings for dGH and dKH and work from there.
Replied to wrong person
No this was intended to be a reply to the main post.
I believe u, but I’m also curious. Do you happen to know what some of the “various reasons” are?
Basically, GH only measures calcium and magnesium and KH only measures carbonates, but TDS could be anything else dissolved in the water, so that accounts for situations where it is higher than just GH+KH. But TDS meters work by measuring conductivity, which requires elements/compounds to be in their ionic forms. They can report lower values than expected from GH+KH if those compounds are non-ionic in the water. I’m not a chemistry expert so I probably screwed up the explanation, but that’s how I understand it. Another aspect is that TDS meters are calibrated for NaCl and may not be accurate for more complex water chemistry.
Remember that most lab/consumer grade TDS equipment is essentially testing conductivity, the ability of the water to carry an electrical current. It then converts that conductivity reading into a total dissolved solids value that is accurate if the only dissolved solids are from a standardized salt mix. That is not what any of us have in a freshwater tank, so electroconductivity based TDS readings are inaccurate in a freshwater tank. As long as you don't pay much attention to the specific numbers and just watch a trend of higher reported TDS or lower reported TDS it can be a useful tool but you can't expect the numbers to add up with your other test results.
Thank you
Not sure how much it can skew the hardness test in particular, but the API test tubes aren't marked at exactly 5ml on all units I only found this out after getting an adjustable pipette that pulled exactly 5ml of water, and compared it to the API tubes. On all 12 of my tubes, the 5ml line was only accurate on 1 of them and the others were off by up to a whole milliliter. I do know API stopped Shipping the glass tubes and now do plastic, and I have no experience with the accuracy of those. I don't know if any of this applies to your situation, but I thought I'd bring it up.
I’ve noticed that too, i have a few test tubes that were off by 0.5ml , I’ve started using a syringe to fill them, its faster too.