Hi Bruce,
Think of EC as a universal measurement, all tds meters measure EC and then convert it to a ppm by using a conversion factor. the conversion factor depends on the meter.
If you wanted to copy someone`s nute ppm you`d have to know which conversion factor the meter used or you could be way out. If the value is expressed as EC it wouldnt matter.

342 calibration is a 0.5 conversion so 500ppm = 1 EC, 442 is 0.7

try this ;)
Here we go, my base water test out to 238ppm / 0.5 = EC: 0.476. When I first mix my nutrients they test out to 894ppm / 0.5 = EC: 1.788, after the first week they drop to 603ppm /0.5 = EC: 1.206 , now at 13 days they are at 523ppm /0.5 = EC: 1.046

This post was edited by hex2006 on Fri, Apr 4, 14 at 14:40

Basically, what hex says, and just to add that a 342 calibration is a weak solution and more suited for testing most tap water. But such hair-splitting accuracy won't play a roll in your hydro need as long as you have a way of telling stronger from weaker on any calibration at all.

The actual calculations the stick is giving are really a black box if you start splitting hairs and ... conversions get hairy when you deal with summer vs. winter on top of who's calibrating with what and at how strong and at what temperature.

Sticking with EC frees you from ever worrying about ppm conversion factors again, except when helping others wade through the mess ppm has caused. So, you can complicate life with ppms or just use the actual signal of the stick gives, which is EC, and never look back, because the ppm is not what is being measured, it's the EC.

The only benefit I see to "ppm meters" is that they are cheaper, but not by much. If you wonder how something that has programmed into it an extra level of conversions can be cheaper that the one that simply gives the raw electrical number, it's because mass produced meters are TDS since tap water is usually measured in ppm, and the guy that started making EC meters makes them in smaller runs.

In hydro, there's a case to be made for those using a single brand of powdered fertilizer that ppm is easier to relate to gram readings, but to be honest I have never once seen a single post of someone actually using it for that. If you add up the ppms we are actually interested in, like N, P, K, S, Mg, Ca, Fe, etc., the total won't add up to the ppm on your meter, so why bother?

Since you already bought TDS, consider it like a speedometer. Mix the ferts new, and check the reading. The reading is your rough speed limit. No one will give you a ticket for going a little bit faster or slower, but major differences and your plants will need to pull over into the shoulder. If you use EC, though as already noted, you will be able to share your results without everyone degenerating into a ppm vs. EC food-fight!

EDIT: Let me add, for your purposes, it is not the NaCl calibration with 342 ppm NaCl that will make a squirrel's ass difference. What you want to make sure is that it is compensated for temperature. A 342 ppm calibration would be extremely inconvenient for me if it were only done at 25 C, and not automatically compensating for temperature ("ATC"). Many of the cheap meters do not have ATC, so watch out since even one degree make a difference in solution measurement. for a typical 10 C temp dif, the uncompensated readings of the same brew will be 20% different - and that becomes a problem!

This post was edited by PupillaCharites on Fri, Apr 4, 14 at 15:56

Thank you so much for your help with this. And thank you so much for going that extra step to edit and add the equations for me. Now I understand why people are talking about conversion factors and whatnot... I'm still not too sure how I can use that to my advantage but at least now I understand what everyone's talking about.

As far as my nutrients go I'm just using general hydroponics flora series.. I'm still a long way away from custom mix's or anything like that
I am still trying to understand how things work, why they work the way they do and what not...

Once again thank you for your time, and happy growing
Bruce

I use a bluelab truncheon for the nutes and an el cheapo dual probe tds meter to constantly monitor the inlet and outlet ppm of the aero setup which is run to waste. I wouldnt trust a tds meter for anything critical but they do have their uses.

Nice nutrient strength meters, and an investment I'll make one of these days. Between the cheapo ones' flimsy battery clips, really little metal strips, that deform and the destruction of one when it falls in the reservoir, I think anyone who does this a lot will come out ahead with one as well built and *waterproof* as that!

Its worth spending a bit extra for quality ec and ph meters. Like most people i made the mistake of buying cheap when i started out. I bought a constant monitor ph meter with a replaceable probe, its still going strong and hasnt been switched off in 8 years.

This post was edited by hex2006 on Fri, Apr 4, 14 at 22:59