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InvisibleDr. Penguin
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Registered: 05/19/08
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EC, PPM, TDS: A Guide Understanding Nutrient Concentration
    #541311 - 03/29/11 03:37 PM (12 years, 8 months ago)

EC, PPM, TDS: A Guide Understanding Nutrient Concentration

  Many people new to hydroponic growing get confused by the variety of terms and acronyms used to discuss nutrient strength and the various means used to calculate them. I'm going to attempt to dispel some of that confusion with a little mini guide. Lets start from the top!

EC: Electrical conductivity
TDS: Total Dissolved Solids
PPM or Parts Per Million: a measure of TDS
TDS or PPM Meter: An electronic device used to test EC in a nutrient solution

How the meter functions:
  Most nutrients in a hydroponic fertilizer dissolve into ions which your plants can absorb and use to grow. Consequently, the ions in solution change the electrical conductivity of your nutrient. When you use a digital meter to check your nutrient concentration, you are testing the EC of the water.

Why we use TDS at all:
  We as growers are generally not all that interested in EC by itself because the EC doesn't really matter to the plant, all that matters is how much of the various nutrients are in the solution. Now I say EC isn't important to the plant because many different ions will affect EC in different ways IE, one gram of Sodium Chloride (table salt) in one liter of water will have a different EC than one gram of Potassium Chloride in a liter of water. Also, some organic nutrients like green sand, blood/bone meal, and fish emulsion produce very little change in EC, but when broken down by soil bacteria can yield usable nutrients.

  To try to mitigate this problem to some degree, most American TDS meters try to change EC to PPM. Converting EC to PPM is generally a very simple calculation involving a constant (either .5 or .7) multiplied by the EC in microSiemens. A quick aside, EC as read on most meters, is in milliSiemens, very important to remember.

Let Constant = .7 or .5
Siemens: 1,000 microSiemens(┬ÁS/cm) = 1.0 milliSiemens(mS/cm) = 1.0 EC(as read by the meter) = .001 Siemens(S/cm)
PPM = Constant * (EC * 1000)

Which constant should I use:
  The reason there is so much confusion about which of the two constants to use is because the manufacturers of these devices cannot agree on a reference solution. Some manufacturers use NaCl as a reference solution (.5 constant) and others use a "442" solution (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) (.7 constant). The disagreement stems from which of the calibration solutions most resembles a hydroponic nutrient solution in its makeup of ions.

When talking about nutrient concentrations, which should I use? EC or PPM:
  When two people give you the same EC readings, of the same nutrient formula, the concentrations will be exactly the same. On the other hand, if two people with two different ppm meters (one NaCl and one 442) give you the same ppm reading they will not be the same concentration. For that reason it is almost always a better idea to use EC when talking to people on these forums.

  That should clear up most of your questions I think, if not feel free to ask questions. Good luck!

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Lord of the Flies

Registered: 05/05/08
Posts: 9,372
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Re: EC, PPM, TDS: A Guide Understanding Nutrient Concentration [Re: Dr. Penguin]
    #544997 - 04/10/11 01:45 PM (12 years, 7 months ago)


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OfflineMr Ploppy
Registered: 04/02/12
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Re: EC, PPM, TDS: A Guide Understanding Nutrient Concentration [Re: Dr. Penguin]
    #615809 - 04/02/12 12:17 PM (11 years, 8 months ago)

Thank you for the post, it was helpful. I am a long time grower, but just dipping my toe in the world of hydro and aero. It has been fun. I am trying to determine what type of meter to purchase. Can anyone explain the difference in functionality between an EC meter and a wand. I read a posting on another site recommending a TRUNCHEON meter, and that they can be purchased on Ebay for about $40. For $40 on Ebay I see a TRUNCHEON wand, however the meters are more than $100. Any insight or information would be greatly appreciated.

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Magnar of Thenn

Registered: 04/08/13
Posts: 5
Last seen: 9 years, 8 months
Re: EC, PPM, TDS: A Guide Understanding Nutrient Concentration [Re: Dr. Penguin]
    #723748 - 03/29/14 02:40 PM (9 years, 8 months ago)

Good info Dr. I think another point that's good to make along with this post is the benefit of pH fluctuation in hydro.
Many new hydro growers will see a fluctuation from their set 5.8 to 6.5 and will immediately think of it as a problem. This is natural! This is ideal. If your root zone's pH is being taken from 5.8-6.5 your plants will be able to make full use of all the nutrients available within your system. Take a look at this chart:
You may notice that peak uptake for nearly everything starts just below 6.0. Everything else (with the exception of iron) will continue to become more (or just as) available as you make your way to the 6.5 mark.
So, don't freak out when your see your pH drift upward to 6.5 or so over a day or 2. My best advice is to not let it go above 6.7, and to use a pH down additive every time it gets in the 6.5 range, so you can knock it back down to 5.8
This is especially important in flower, as a lot of new growers will keep their pH constantly at 5.8. This isn't a good idea... Phosphorous is absolutely one of the most important nutrients throughout the bloom, and it will not be taken up as readily at a set pH of 5.8.
This brings me to another point... beneficial bacteria. As I have no experience with Flood&Drain I won't tell you to use it or not for F&D, however with DWC. I would recommend that you find a bennie nutrient that is right for you. These will help regulate your plants' root zones and will help keep this gradual pH slide regulated. They will also help to fend off bad bacteria (slime). Anyways, I'm just gonna stop it here before I continue ranting.
Once again, wonderful post Doc, hope this helps some folks out as well.
Peace :mariopeace: , Love :forbiddenlove: , & Apathy :jah:

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