TDS Meter Calibration Issues

I'm hoping that there are some really smart folks out there that can help me understand something of the world of meter calibration.

After making my purchase of an Oakton TDSTestr11, I asked the young genius behind the counter for an appropriate calibration solution, for which he hands me General Hydroponics Standard Calibration Solution. Here's where the confusion starts, as the package indicates that it calibrates to 1500ppm (NaCl), 2190ppm (442), and 2930S (microSiemens). So everything I've been led to believe tells me to calibrate the meter to 1500ppm when using this solution. That's certainly what Einstein told me to do.

However, when I got it home and started using it, it certainly appeared as though it was terribly out of scale, compared to two other reference meters that I use. I eventually became so concerned at the apparent discrepancy between devices (both Oakton) that I called the company's support department to ask for some hand-holding through the calibration process.

Rude awakening #2. The 1500 calibration scale that I and all my cronies so blithely use and assume to be correct is for the NaCl scale, and should only be calibrated to that point if the solution you are measuring is the same. NaCl, for those of us who found Chemistry slightly beyond Wood or Auto Shop, is Sodium Chloride. Salt. Brine, to be more specific. When pressed for my application, hydroponic nutrient measurement, his immediate response was that I should be using the 442 scale to calibrate with, as it is more of a brackish water scale, in line with what I am measuring.

This has two immediate ramifications for me, if it is factual! and that's still a big IF, because, this would seem to mean that I have been far out of calibration with my nutrients for a long time. Specifically, if I have been calibrating my meters to 1500ppm using a calibration fluid that rightfully requires me to calibrate to 2190ppm instead, just what have the values actually been that I have been watering with for so long?

And does any of this really matter? Are we talking about values that are so small to begin with, 600 or 700 parts per MILLION, fer crying out loud, that it's immaterial. Is this just another example of how amazing this plant is in being able to take a tremendous amount of abuse and keep on keeping on?

Or does anybody out there really know if there's something to this different scale business, and I (we) should be looking at how we calibrate our TDS meters differently somehow?

If so, I'm going to be returning a bottle of Calibration Solution that our erstwhile young entrepreneur managed to foist off on me as suitable for my TDS meter. As the meter maxes out at 1999ppm, it's not even able to be calibrated to the 2190 that, supposedly, it should be for all of the purposes that it is sold for at the store I purchased it at.

Anyway, I'd like to hear what any others have to think about this.

existential insanity, eh?

hmm Oakton TDSTestr11...

I've had one that takes a very specific ppm solution, 1687ppm or something along those lines.

If you're off by a fraction, lets say you have ro water thats actually 20ppm, but it reads 25, then 40 will equal 50, and 80 will equal 100, 100 will equal 150, etc.. (probably not that linear hah)


Let me try to find the calibration solution for the Oakton TDSTestr11

Ok....I have no clue what you just said.....but I have an oakton tds basic and I asked my hydro guy (whom I would trust with my firstborn) what to calibrate it with and he told me 1500/tds solution (which is what he then sold me). I unscrew the little top and press the buttons to calibrate. I've never had a problem with it and I've been using it for a couple months now.

That's my story and I'm sticking to it. :D

seems like it has both auto, and manual calibration

since its max is 1999ppm, you could try finding a calibration solution like 1500ppm, dip it in- set the pen to say 1500 (since you know the fluid is 1500)


Still wish i could find you the auto calibration fluid number...

Ps...He also sold me the oakton meter in the first place.....and did I mention I'd trust the guy with my life savings? :D

If you ever hear people talk about TDS measurements, you'll notice that they often indicate (or should indicate) whether the measurement was taken at 0.5 or 0.7 conversions. Those numbers are important because it ensures that you're comparing apples to apples when discussing TDS.

There's nothing wrong with the calibration solution they hydro store sold you. It's a standard TDS solution that a lot of us use. The problem is you need to figure out how to set your TDS meter so it accurately reads the calibration solution. To do that, you need to be sure that your meter is set to either 0.5 or 0.7 (your meter can do either), and then *manually* calibrate it to match the number on the bottle - 1500 for 0.5, or 2190 for 0.7.

Doc, it looks like you are in very good hands here. I'm only posting to say that I got a HUGE laugh out of your post. A big welcome to canncom! :hippy:

Well his meter only reads to 1990 so he's going to have a hard time calibrating it to 2190.

If it's the basic meter i'm thinking of it reads at a 0.7 conversion scale and is calibrated by sticking it into "something with a known tds number" or what not and calibrated to that. I don't see anything on mine that switches to a 0.5 conversion and nothing in the instructions either so I'd be interesed to know if that actually works although I doubt I would change at this point because I'm really used to the 0.7 now.

Meters that read e.c should be calibrated with a conductivity solution instead of a tds solution (at 0.7 conversion I use 141.3 solution)

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Originally Posted by Weedhound

Well his meter only reads to 1990 so he's going to have a hard time calibrating it to 2190.

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LOL...ooops. I was tired and a bit distracted.

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If it's the basic meter i'm thinking of it reads at a 0.7 conversion scale and is calibrated by sticking it into "something with a known tds number" or what not and calibrated to that.

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I've also got the basic one, so I looked up the TDSTer11 before I posted. It's got a lot more bells and whistles than the basic one does, including the ability to adjust the conversion ratio.

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I don't see anything on mine that switches to a 0.5 conversion and nothing in the instructions either so I'd be interesed to know if that actually works although I doubt I would change at this point because I'm really used to the 0.7 now.

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You sure it's 0.7? Damn, I've been running under the belief that mine was 0.5 all this time. That might explain why my plants get nute hungry about half way through flowering :rolleyes:

Yeah I've heard good things about the oaktons but maybe yours is one that can swing both ways. Mine is ULTRA basic(what a great name :D) ....not a bell or whistle to be found.

Mine is actually a back up tds meter....I use a BlueLab e/c meter and it not only reads e/c but a 0.7 AND 0.5 conversion all at the same time. Otherwise I would constantly forget which one I use......so yes, I'm sure of the conversion there.

Do you see defs in flower? What kind of issues are you seeing?

Mine is their TDSTestr Low, which is still pretty basic. All I know is that I've been using the same GH calibration and calibrating it to 1500ppm. If it's really a 0.7 conversion, that explains a lot.

I've mainly been seeing Mg deficiencies in mid-late flowering. I'm using the Lucas formula, and I know Lucas has stated that the Mg deficit is usually the first to show when your concentrations aren't high enough, since the formula has *just* enough Mg for the needs of the plant. The LF recommends a nute concentration of ~950ppm @0.5, and ~1330ppm @0.7. I had my white widows on my most recent grow running between 1050 and 1100 (on my meter, whatever it actually is lol) throughout almost the entire grow, and WW is supposed to be fairly nute sensitive. If my meter is really a 0.7 conversion, that explains why my widows liked the concentration so "high".

Ok my meter is .5 conversion...What the hell does that mean? I never new there was a diff. Shucks hope someone can tell me if my ppm's are too low because it's a .5? Thanks.