EC and TDS Information

Heres just a little information I came upon about EC and TDS and conversion rates and such, thought it might help some ppl who are starting out . . . Sometimes all the conversions get a little confusing HTH

"Q: What is the difference between EC and TDS?
A:There's a lot of disagreement over the relative merits of measuring Electrical Conductivity (EC) as opposed to Total Dissolved Solids (TDS), so I'll do my best here to explain the debate in the simplest terms possible. Both measurements are used by indoor gardeners to determine the strength of a hydroponic nutrient solution, and either one can deliver the information a gardener needs to make adjustments and corrections in the reservoir.

The source of all the trouble is that while the information gardeners need to optimize plant growth is the TDS of the nutrient solution in Parts Per Million (PPM), no method exists to measure TDS directly. Every meter out there only measures the EC of a liquid, usually in milliSiemens (mS) or microSiemens (µS). “Hey, wait a minute,” you say, “I've got a TDS meter, and it works great!” Well, that may be, but really that instrument is measuring EC, and then converting it into TDS using a standard algorithm. “So why wouldn't I want that?” you ask. This is where it starts to get a little tricky.
The problem is that different elements change the EC of a solution to different extents. A 1200 PPM solution of one element may have an entirely different EC than a 1200 PPM solution of another element. This means that an instrument that delivers readings in PPM has to make assumptions about what it is it's measuring. There are two different conversion scales that are common in the hydroponics industry. One is the 442 scale, which assumes that the solids in solution are 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride – some folks say this scale is the closest approximation of what is actually in a hydroponic nutrient solution. Then there's the NaCl scale, which assumes that the solids in solution are 100% sodium chloride – other folks say this is the closest thing to a hydroponic solution. Neither conversion scale is inherently better than the other; which one will give you a more accurate estimate of your TDS probably depends on the specific composition and brand of nutrient you're using more than anything else. But while the NaCl conversion is approximately 510 x EC (in mS), the 442 conversion is approximately 750 x EC (in mS). So a TDS meter dipped into a solution with an EC of 2.5 mS could give a reading of 1275 PPM or 1875 PPM depending on the algorithm chosen by the manufacturer.

So how should we deal with all of this as gardeners? Well the only truly accurate and scientific method I can think of is to get an EC meter, call the manufacturer of every nutrient and supplement you're using to get information from them on how their products affect EC, and then develop your own conversion algorithm. . . good luck! But most of us just use the nutrient manufacturer's suggested usage rate as a starting point, find out what works well for us, and then use an EC or TDS meter to maintain a consistent level of concentration"
GJ17