TDS Meter Question

[skipper]

I recieved a new Hanna TDS meter of Christmas and have a question. It reads EC and also in KCL, 442, and NaCL in ppm. Does it really mater which of these scales I use, as long as it is always the same? The readings for KCL and NaCL are almost the same, with the 442 reading being higher. My old meter had only one scale and I do not know what it is. It has not ever been calabrated so a comparison with the new meter is a problem. Thanks

Dale

[Ray]

You have hit EXACTLY on the issue with TDS meters.

An EC meter measures the electrical conductivity of a solution. Your fertilizer provider should be able to tell you the added conductivity of its formulation. Add that to that of your water, and you can control it well.

TDS meters are just cheap EC meters with a built-in conversion - and they vary, as you have noted. Your meter apparently offers various options in that regard.

If you know what you like applying to your plants, any one of them will allow you top duplicate prior conditions.

[jpyerry]

This is the second posting asking about TDS meters recently and since we are on the subject, I actually am curious about something. I have noted that meters that test for TDS can range in price from $30 to thousands of dollars. I guess I am curious as to whether or not a, as some literature indicates, research grade TDS meter is still simply a cheap EC meter with a build-in conversion. As I understand it, measurement of TDS in order to ascertain a high degree of accuracy is performed gravimetrically with an analytical balance by allowing the solvent to evaporate off and measure the difference in mass before the evaporation commenced to afterwards and then calculate a ppm measurement based on the solutes known to be present in solution. I probably misunderstand the nature of TDS measurement since I have only had General and Organic Chemistry coursework and never an instrumental analysis course. I guess I am just curious about this from an academic standpoint. If the accuracy of a TDS measurement is so difficult to obtain using a meter, then why even bother making them, why not just sell EC meters and let the end user derive TDS measurements. I guess I am just confused on the issue. I would definitely like to know more about all of this.

Justin

[skipper]

Justin, I have always been chemistry challenged, so I will not be able to help with that end of your question. As far as the testing meters are concerned, they are very easy to use and read. A meter like the one that I have lets the user pick the chemical compound that you would like to test for. I have been using the scale for NaCL to test for that salt. You can calibrate your meter to a purchased solution, so you know you are getting the correct reading. I find this information to be of great value when I am mixing fertilizer or flushing my plants, as well as fun to do. After all this is a hobby, and I am retired, so why not. If it will help you to grow better plants, than that is a good thing. The meters range in price from about $25.00 to $100.00. In the upper end of that range, there are many nice units to look at. The real high end units are of little value to what we do as hobby growers. You sound like the type of person that would enjoy one of these. It just adds to the enjoyment of the hobby, and they are very helpful, as well as easy to use. Just a thought.

Dale

[Ray]

Justin, Dale is right. Any TDS meter can be used for "confirmation of sameness", but unless they are well calibrated, cannot give you an absolute value equivalent your your concept of gravimetric analysis.

Very simply, electrical conductivity of a solution is a measure of the mobility of charged particles (ions). If I have two solutions of equal concentration, each containing equally sized ions - giving them equal mobility - but one carries a +1 charge and the other a +2, the EC of the second will be double that of the first, so our TDS meter will also show a difference, which is incorrect.

If you look at the complexity of the chemistry of a fertilizer, and recognize that both the anions and cations may play a role in conductivity, you can see impossibility of TDS meter accuracy across several formulas, or even as the actual concentrations change.

That's why a professional grower will work with the EC instead. Professional fertilizer manufacturers provide EC-versus-concentration data with each formula. By adding that value to the EC of your water supply, you know exactl;y what you're shooting for, and can measure it.

For example, at 100 ppm N (which yields roughly the same TDS), the two Greencare "MSU" formulas will add 0.8 mS (RO) and 0.68 mS (Well Water) to the incoming water supply. I use the first with RO water (EC= essentially zero), so would shoot for the 0.80 mS conductivity. I'd have to measure my tap (well) water's EC, then add that value to the 0.68 for the target with that one.

And beating the dead horse - again - if dissolved in the same, pure water, a TDS meter would indicate the TDS of the RO formula was 0.80/0.68=18% higher, even though they are equal.