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Limited Equivalent Conductivities and the use of EC meters

Posted by greystoke South Africa(11) (My Page) on
Thu, Nov 26, 09 at 0:31

In the first place I would like to plead with our members to NEVER-EVER use a dedicated TDS (Total Dissolved Solids) meter, or the ppm scale on your EC meter. Always use the EC (or S) scale.
The reason ?
Well, there are no such things as tds-meters. Those devices that you stick in a watery solution, which then display a tds in ppm are in fact converted EC-meters, which measure the reverse electrical resistance of the solution, and the problem those meters have is that they cannot add ppm values together without serious errors.
Example:
Lets say that you have a solution of 100 ppm Potassium Nitrate, and a solution of 100 ppm Calcium Nitrate. Suppose you mix the two solutions and stick in your tds meter to check, expecting to read 200 ppm, but . . . alas, because, depending on the meter you bought, it will either read more or it will read less, but never: 200 ppm (100+100).
However, if you were to use the EC-scale you would read: 143 S for the KNO3 solution, and 123 S for the Ca(NO3)2 solution. And surprise . . surprise the mixed solution will read: 266 S.

Many members have come short on this confusing aspect of EC-meters, and many have adopted fallacious ways of suiting the ppm-scale. That wasnt necessary. All that was needed is to change-over to the EC-scale.

But how do you relate grams per liter (ppm) to conductivities ?
The answer is that every type of anion and kation contributes its own conductivity to the solution. However, these contributions are known to science, and they are called: Limited Equivalent Conductivities (LEC), and are generally quoted in S/ppm/mol.

LEC values

Example:
KNO3 -100 ppm has a total LEC of 73.5 + 71.4 = 145 S/ppm/mol, = 145*100/101 = 143 S
Ca(NO3)2 100ppm has a total LEC of 59.5 +2x71.4 = 202 S/ppm/mol, = 202*100/164 = 123 S

Note:
A) Strictly speaking, LECs are not constants, and valid only in low concentrations. Particularly low dissociated salts (salts of weak acids such as acetates and bicarbonates) may give problems. However, in our hobby that hardly ever applies, and the errors caused by it are not significant for our purpose.
Even so, I personally try to stick to the area around 1000 S, and if necessary I dilute a sample of a strong nutrient solution 1:1 with RO water, just to do a reading.

B) Tap water also has a conductivity, and you need to measure it before using, so that you can subtract its value from your reading to arrive at your nutrients concentration. You can also try to incorporate the calcium content into your formulas, but then, you need to know what you are doing.
Otherwise, just remember that you are getting some extra calcium for free out of your tap.


Follow-Up Postings:

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Correction

Greystoke wrote:Example:
Lets say that you have a solution of 100 ppm Potassium Nitrate, and a solution of 100 ppm Calcium Nitrate. Suppose you mix the two solutions and stick in your tds meter to check, expecting to read 200 ppm, but . . . alas, because, depending on the meter you bought, it will either read more or it will read less, but never: 200 ppm (100+100).
However, if you were to use the EC-scale you would read: 143 S for the KNO3 solution, and 123 S for the Ca(NO3)2 solution. And surprise . . surprise the mixed solution will read: 266 S.

This is not what I meant to say: The salts should be mixed together. Not the solutions. Doing it as descibed you would - of course - get the average of the two, ie: 133 S, although the relative error would still be the same.

Sorry about that.


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Corrected version

Just for those who don't want to get confused: The corrected version: devil

In the first place I would like to plead with our members to NEVER-EVER use a dedicated TDS (Total Dissolved Solids) meter, or the ppm scale on your EC meter. Always use the EC (or S) scale.
The reason ?
Well, there are no such things as tds-meters. Those devices that you stick in a watery solution, which then display a tds in ppm are in fact converted EC-meters, which measure the reverse electrical resistance of the solution, and the problem those meters have is that they cannot add ppm values together without serious errors.
Example:
Lets say that you have a solution of 100 ppm Potassium Nitrate, and a solution of 100 ppm Calcium Nitrate. Suppose you make a mixed solution of the two salts and stick in your tds meter to check, expecting to read 200 ppm, but . . . alas, because, depending on the meter you bought, it will either read more or it will read less, but never: 200 ppm (100+100).
However, if you were to use the EC-scale you would read: 143 S for the KNO3 solution, and 123 S for the Ca(NO3)2 solution. And surprise . . surprise the mixed solution will read: 266 S.
Many members have come short on this confusing aspect of EC-meters, and many have adopted fallacious ways of suiting the ppm-scale. That wasnt necessary. All that was needed is to change-over to the EC-scale.
But how do you relate grams per liter (ppm) to conductivities ?
The answer is that every type of anion and kation contributes its own conductivity to the solution. However, these contributions are known to science, and they are called: Limited Equivalent Conductivities (LEC), and are generally quoted in S/ppm/mol.
LEC values
Example:
KNO3 -100 ppm has a total LEC of 73.5 + 71.4 = 145 S/ppm/mol, = 145*100/101 = 143 S
Ca(NO3)2 100ppm has a total LEC of 59.5 +2x71.4 = 202 S/ppm/mol, = 202*100/164 = 123 S
Note:
A) Strictly speaking, LECs are not constants, and valid only in low concentrations. Particularly low dissociated salts (salts of weak acids such as acetates and bicarbonates) may give problems. However, in our hobby that hardly ever applies, and the errors caused by it are not significant for our purpose.
Even so, I personally try to stick to the area around 1000 S, and if necessary I dilute a sample of a strong nutrient solution 1:1 with RO water, just to do a reading.
B) Tap water also has a conductivity, and you need to measure it before using, so that you can subtract its value from your reading to arrive at your nutrients concentration. You can also try to incorporate the calcium content into your formulas, but then, you need to know what you are doing.
Otherwise, just remember that you are getting some extra calcium for free out of your tap.


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