I’ll start this with an editorial comment: “Who gives a damn about TDS!”
People buy “TDS meters”, then end up using them to control their fertilizer and additive use, without really understanding what they’re doing. I’ll actually limit that a bit more - orchid growers are guilty of that more than anyone.
There are several facets to the misunderstanding. First is an understanding of what “total dissolved solids” really means.
Don’t get me wrong, I think folks grasp that the units, ppm, or parts-per-million, or milligrams per kilogram just fine and understand that it’s a mass of solute per mass of solution, but the question is, “do they grasp the meaning of the magnitude of it?”. For example, folks talk about keeping the TDS below 200 ppm to make phrags happy. Where did that number come from and what does it really mean?
I feed my plants weekly at 100 ppm N (a concentration of nitrogen, not TDS), and many folks do something relatively close to that. They tend to “freak out”, however, when confronted with the fact that the true TDS of a 100 ppm N solution is many multiples of that, varying by the fertilizer formula. 100 ppm N solutions made using MSU WW, MSU RO, & K-Lite, for example, have true TDS levels of 530, 740, & 770 ppm, respectively. My stance is “That’s interesting, but so what?”.
Now let’s throw in the meters themselves, as they are almost worthless.
A TDS meter does not measure the concentration of a solution. It is actually a cheap electrical conductivity (EC) meter to which a built-in “conversion factor” is applied. Some meters even offer a couple of different scales, further lulling people into thinking their measurements are meaningful. The trouble is that the EC/TDS conversion requires a different “factor” for each and every solution to be tested, and the built-in one is often done with a sodium chloride solution, which is nothing like a fertilizer solution.
The EC of a solution is a measure of the flow rate of electrical charges. That flow rate is determined by the charge on each ion (higher charge equals greater EC), the physical size of the ions (bigger ions move more slowly, so contribute less to the EC), and their concentrations, so how can a single “conversion factor” be correct for all solutions? It can’t, and it can be way off - I tested a known, 100 ppm N solution of MSU RO in distilled water using two TDS meters; one gave me a reading of 380, and the other 475, while the true reading was 770 ppm.
Pure water has zero TDS and EC. Let it stand around and absorb carbon dioxide from the air, and the EC increases due to the formation of carbonic acid, so a TDS meter will show an increase in the dissolved solids, too. Let that water evaporate, and the carbon goes with it, leaving you with nothing. Technically, there was a level of dissolved solids (C), but as true TDS is determined by evaporating a know mass of solvent and weighing the residue, we can safely say it was zero.
A KelpMax discussion let to this thread, so let’s look at other additives. All that I can truly say is that anything you add to your solution may affect the conductivity, giving you a “high” TDS reading, but what does it matter? Yes, KelpMax (Kelpak) does increase the EC of a solution, but within the range of recommended dosage, it makes no difference. Epsom Salt is often used to add magnesium to solutions, and it affects the EC, as well, but again, “so what?”, unless you overdo it.
As far as I’m concerned, TDS meters are only useful in monitoring changes, not in measurement of absolutes. If my known 100 ppm N solution gives me TDS readings of 100, 500, or 2000 ppm, as long as my testing tells me it’s reasonably close to that, I’m happy.
I have yet to see a decent, controlled study tell me what TDS is correct or even acceptable. Actual concentrations, sure, but not nebulously TDS readings.
People buy “TDS meters”, then end up using them to control their fertilizer and additive use, without really understanding what they’re doing. I’ll actually limit that a bit more - orchid growers are guilty of that more than anyone.
There are several facets to the misunderstanding. First is an understanding of what “total dissolved solids” really means.
Don’t get me wrong, I think folks grasp that the units, ppm, or parts-per-million, or milligrams per kilogram just fine and understand that it’s a mass of solute per mass of solution, but the question is, “do they grasp the meaning of the magnitude of it?”. For example, folks talk about keeping the TDS below 200 ppm to make phrags happy. Where did that number come from and what does it really mean?
I feed my plants weekly at 100 ppm N (a concentration of nitrogen, not TDS), and many folks do something relatively close to that. They tend to “freak out”, however, when confronted with the fact that the true TDS of a 100 ppm N solution is many multiples of that, varying by the fertilizer formula. 100 ppm N solutions made using MSU WW, MSU RO, & K-Lite, for example, have true TDS levels of 530, 740, & 770 ppm, respectively. My stance is “That’s interesting, but so what?”.
Now let’s throw in the meters themselves, as they are almost worthless.
A TDS meter does not measure the concentration of a solution. It is actually a cheap electrical conductivity (EC) meter to which a built-in “conversion factor” is applied. Some meters even offer a couple of different scales, further lulling people into thinking their measurements are meaningful. The trouble is that the EC/TDS conversion requires a different “factor” for each and every solution to be tested, and the built-in one is often done with a sodium chloride solution, which is nothing like a fertilizer solution.
The EC of a solution is a measure of the flow rate of electrical charges. That flow rate is determined by the charge on each ion (higher charge equals greater EC), the physical size of the ions (bigger ions move more slowly, so contribute less to the EC), and their concentrations, so how can a single “conversion factor” be correct for all solutions? It can’t, and it can be way off - I tested a known, 100 ppm N solution of MSU RO in distilled water using two TDS meters; one gave me a reading of 380, and the other 475, while the true reading was 770 ppm.
Pure water has zero TDS and EC. Let it stand around and absorb carbon dioxide from the air, and the EC increases due to the formation of carbonic acid, so a TDS meter will show an increase in the dissolved solids, too. Let that water evaporate, and the carbon goes with it, leaving you with nothing. Technically, there was a level of dissolved solids (C), but as true TDS is determined by evaporating a know mass of solvent and weighing the residue, we can safely say it was zero.
A KelpMax discussion let to this thread, so let’s look at other additives. All that I can truly say is that anything you add to your solution may affect the conductivity, giving you a “high” TDS reading, but what does it matter? Yes, KelpMax (Kelpak) does increase the EC of a solution, but within the range of recommended dosage, it makes no difference. Epsom Salt is often used to add magnesium to solutions, and it affects the EC, as well, but again, “so what?”, unless you overdo it.
As far as I’m concerned, TDS meters are only useful in monitoring changes, not in measurement of absolutes. If my known 100 ppm N solution gives me TDS readings of 100, 500, or 2000 ppm, as long as my testing tells me it’s reasonably close to that, I’m happy.
I have yet to see a decent, controlled study tell me what TDS is correct or even acceptable. Actual concentrations, sure, but not nebulously TDS readings.
