Got a TDS/EC tester. Have some questions

Advanced Techniques
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I am currently on my third grow ever and have finally invested in a TDS/EC meter. Tested my water today after putting nutes in and got 505 ppm & 1002 EC.

I’m using Fox Farms Hydro nutes & Cal Mag. My nutrient dose chart says to keep EC between 2.1 & 2.3 during this stage of growth and to have PPM at 1470 to 1610. Are these the same measurements just stated in different terms?

With my PPM at 1002, does this mean I am starving my plants? I’ve been using about 50% of the recommended dose of nutes and the plants seem to be doing very good. In my research this is what I’ve seen over and over, do not overfeed. The older I get the harder it is to understand this technical stuff.

Any help is greatly appreciated.

SkyDog

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Good question. They are the measuring the same thing but there is a difference that I’ll try to describe below.

PPM (part per million) is a quantity of a substance. For a liquid, we are talking about 1 milligram of substance per liter of solution. For a solid we are talking about 1 milligram per kilogram.

EC (electrical conductivity) measures the amount of salt in a solution by determining how much current flows through the solution (between the two electrodes on the probe). Recall, pure water is non-conductive until you add ions (e.g. a salt).

Different substances along with the quantity of that substance creates different amounts of conductivity when placed into a solution.

When using a TDS/EC probe, you are measuring the EC (electrical conductivity). The manufacturer will then apply a conversion to derive the PPM based on some standard substance.

For instance, a manufacturer may use sodium chloride as a reference. They will add 1000mg/liter of sodium chloride to a solution and then measure the electrical conductivity. They will then know the EC of a solution based on a 1000 PPM solution. Another manufacturer may use some other substance, such as potassium chloride, as the reference for the conversion between EC and PPM. Because the two manufacturers are using different standards, the measurement of PPM between the two different probes in the same solution will be slightly different. The conversion to PPM differs but the EC between the two is the same (or at least should be)!

When you measure some unknown solution, the meter will use whatever reference the manufacturer has chosen to convert the EC into a PPM value. It will tell you the PPM but it will be only relative to the calibration solution and will not be the true PPM. It is only an estimate.

Because of this, the use of PPM can cause some confusion.

Based on your 505PPM and 1002EC, this indicates that this probe has a (500/1000) ~= 0.5 conversion factor. However, based on the fox farms conversion of (1500/2200) ~= 0.7 conversion factor. Confusion.

If you can, the suggestion is to use EC only. It’ll be the most reliable way to track and communicate the concentration for a pre-defined nutrient mixture.

If the manufacturer does not provide EC (only PPM), then you will need to know the conversion factor of your EC probe and also what the manufacturer is using for their conversion factor. Then do some math.

So after all of that, using EC you are (correctly) measuring about 50% of what the manufacturer is suggesting (more precisely 1002/2100 = 48%). Which is not necessarily a bad thing if the plants are doing fine.

ps. If someone is creating their own nutrient formulation using a bunch of individual chemicals. The most accurate method in that case is the use of PPM. e.g. IF you are weighing and measuring the individual substances before they are added to a liquid. Not the case for pre-mixed solutions.

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Wow, @Northern_Loki, that is one of the best, and most informative answers to one of my not so well stated questions i think I’ve ever seen! I really have a much better understanding of TDS & EC. I’ve searched it on the internet but have always come away confused. Thanks a million!!! I believe I can experiment from here to come up with the best conditions in which to grow.

There are so many variables to take into consideration when growing and I’m trying to be as well informed as I can. I am currently monitoring temperatures and humidity and logging my findings as well as I can.

My next adventure will be to get a temperature and humidity monitor so I can log at more frequent intervals so I can fine tune my operation. I’m currently looking at some devices that will give me the information using wireless communications and a phone app so I can see and document them in real time and by looking at trends and patterns.

Thanks again for taking the time to help me get a better understanding of this subject!

Regards,
SkyDog

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PPM-EC-CF.jpg465×665 76.3 KB

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I rarely use the manufacturers suggested dose of nutes. At times I’ve almost reached it by the end of flower though. Just stick with what the plant tells you. If it starts to get lime green then slowly up her feed.

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I use the ppm/EC meter as a exact measure for how much nutrients I’m giving them. I NEVER use the nutrient manufacturers levels. I raise the levels each week by 100 ppm/0.2 EC until I see a negative reaction from the plants ie nutrient burn then I back it off to the last ppm/EC level I had before I saw the burn/problems and just keep it there for the remainder of the grow. At the moment they sit happy at 1000 ppm/2.0 EC. The meters allow me to push them to the absolute limit. More food = bigger fatter buds :+1:

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Thanks for the info @LabRat, @Meesh, and @Esrgood4u! Information is knowledge. All of this info will go into my memory banks. The chart is very helpful and will save me lots of calculations.

I’ve always read that when feeding nutes you should start at half the recommended dose but never thought to up the dose when the leaves start to get lime. At on time I read that the lighter color was the result of low Nitrogen levels and that Cal Mag would help. Never thought to just up the nutes.

Now that I have the meter I can start looking at increasing doses in a predictable way. I like the idea of increasing levels by 100 ppm at a time. On my first grow I went from 1/2 the recommended dose to 3/4. Now on this, my third grow, I have seen good growth and color just leaving it at half dose. I have a couple weeks more of feeding and will try increasing ppm a little. I like the idea of bigger, fatter bud!

SkyDog

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To add confusion, the truncheon has both scales, 500ppm=1 EC and also 700PPM=1EC…

Generally, you will want to measure the EC of your tap water then add no more than 1EC to that. Much much less for cuttings and seedlings. If you have added 1EC to your tap water and your plants are showing nutrient deficiencies, it is more likely you have a PH problem.

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More good info @MicroDoser! I just started researching a good PH meter yesterday. Until now, I have been using solution to test after adjusting and I just don’t feel like I can get very reliable results. My tap water starts out around 7.0 and I have been shooting for 6.0 or just a little lower but am not sure if I am really getting that. Looking through a little tube and comparing to a color chart on my cell phone is hard for an old man whose sight is not what it used to be.

SkyDog

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When you do decide to get a PH meter, the better ones have a two-point calibration and a glass bulb as a rough rule (they also start at twice the price of the cheapest ones). PH calibration needs to know where 7.0 is (which is a one-point calibration), and also where 4.0 is as well (the second point) so it can figure out the slope that the device currently has so it can tell everywhere in between 4.0 and 7.0 far more accurately.

Some of the cheaper ones are no better than looking at the liquid, some are worse because the liquid will always tell you the same, correct, result instead of bouncing around 5 different incorrect values. If you can, get the testing solution that comes with a little card instead of looking at your phone, the screen is backlit unlike the tube and this can make it hard to check one colour against the other.

Just under 6 is a good place to aim for. It gets you most of the things you need except zinc, which is more free at 5.5 (but not needed as much as the chemicals you get at 5.9)

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Thanks! I got a card with my test solution but the liquid in the ph solutions leaked on the card in the mail and I haven’t got new solution yet. Probably something I should do.

Glad you gave me the info on the two-point calibration and glass bulb. I was looking at a cheapy, but will follow your suggestion. They are definitely more expensive so I will have to save a little longer, knowing that my current method is acceptable. Again, thanks for the info. Like all my other hobbies, this is turning into a passion very quickly especially considering that I can’t imbibe anything right now due to a new policy of random testing at work and at the Dr’s office since I am on opioids for pain relief. Hopefully, all of those conditions will disappear soon.

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I have seen too many people buy the cheapest one, have problems that cost them far more than the difference in price, then go out and get the expensive one anyway…

Good luck!

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Check out this thread for some additional ideas on PH meters:

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Useful info, how come there are two equivalences for EC in ppm? How do we know which one are we using? Thanks …

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Measuring PPM by electrical conductivity is an estimate. The manufacturer will base their PPM estimation on a standard salt solution. And, there are different standard solutions that have different conductivities. Depending on which standard they choose it will determine the conversion factor.

You’ll need to check with the manufacturer to verify the conversion ratio to PPM if the meter itself does not perform the conversion for you.

More info about that in post #2 above…

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Also wanted to chime in with a reminder that EC/PPM meters for our purposes should be used mainly for repeatable results, and not necessarily to dose our plants at exactly the manufacturers recommendations.

Find what works and use your meter to repeat it.

Cheers!

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Thank you both,I have learnt with nutrient burns in the past and flushing too soon these days that you never have to give credit to product charts or breeders estimation for flowering stage, only my further experience and your help will count from now on …

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The one most people use is the 500 scale, where 500PPM = 1EC

Some devices like the truncheon show other scales but mostly it will be 500PPM=1EC unless stated otherwise.

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Thanks @MicroDoser, mine’s a Milwakee EC59 and haven’t seen anywhere what scale it uses, so guess it will be 500 as you say …

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Programmable apparently:

to change the TDS factor value: after setting the temperature unit, press ON/OFF once to show the current value (e.g. 0.50 CONV). Select the desired value by using the SET/HOLD button, then press ON/OFF twice to return to the normal measurement mode.

via http://www.milwaukeeinst.com/site/db/doc/manEC59_EC60_ENG.pdf

It appears to provide both TDS and EC by switching modes. You can also determine the conversion factor by first measuring a solution with the EC mode and then measuring the same solution in TDS mode (PPM). Divide the TDS by the EC readings = conversion factor.

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