I found some more info about this here:
(Fatman's) DIY nutrient mixing guide
Contacting mega crop now to ask what ppm scLe they use
If you go to that link and then follow the first link in the post called “Nutrient Calculator” and then go to the bottom of the page, you will see this:
“If you've tried to reconcile the elemental parts per million (ppm) shown in the nutrient profiles with metered TDS ppm figures, you're probably pulling your hair out by now. In order to avoid confusing the two, it's important to distinguish between their contexts when the term
ppm is being used. For more details about TDS/EC metering and how it relates to GH Flora nutrient profiles, see this Cal-O-Rama link (Cal is for calibration).”
Then follow the link for the cal-o-Rama article (sometimes it goes to a weird site) but once you get to the article, scroll all the way down to the bottom and you will see this:
“An interesting blurb from Hydroponic Food Production by Gordon Resh on salts and conductivity:
"Ammonium sulfate conducts twice as much electricity as calcium nitrate and more than three time that of magnesium sulfate, whereas, urea does not conduct electricity at all. Nitrate ions do not produce as close a relationship with electrical conductivity as do potassium ions (Alt, D. 1980). The higher the nitrogen to potassium the lower will be the electrical conductivity values for the nutrient solution."”
So I guess to feed by ppm is not very straight forward if you are trying to use an E.C. meter to measure your nutrient solution strength
And blue lab info helps alot
What are the different conductivity scales? What do they mean?
EC, CF, ppm500, ppm700, TDS... What's the difference?
EC and CF are measures of electrically charged nutrient ions in a solution. Pure water will not conduct electricity. Water usually conducts electricity because it is full of impurities, in our case, electrically charged nutrient ions. The two black dots on the end of Bluelab conductivity probes are called electrodes. When these are placed in a solution, an electrical current passes from one electrode, through the water to the other electrode and counts the number of electrically charged ions present. The reading is then presented as an Electrical Conductivity (EC) reading or Conductivity Factor (CF) reading.
PPM measures parts per million. Known as dimensionless quantities, they are pure numbers with no associated units of measurement. A mass concentration of 2mg/kg - 2 parts per million - 2ppm - 2 x 10-6.
There are many different scales used for different industries around the world and for many different reasons! Did you even know there are more than two scales? Widely used ppm scales in hydroponics are the ppm500 scale, ppm650 scale and the ppm700 scale.
What's the difference between ppm500 and ppm700 scale?
- The ppm 700 scale is based on measuring the KCl or potassium chloride content of a solution.
- The ppm 500 is based on measuring the NaCl or sodium chloride content of a solution.
- The ppm 500 scale is also referred to as TDS - total dissolved solids.
It is worth noting that while this are what each scale is
based on, in reality these scales do not measure
only the KCl content or NaCl content of the solution, but rather the overall conductivity of all electrically charged ions in the solution, this reading is then converted to give you a ppm500 or ppm700 representation.
The true ppm of a solution can only be determined by a chemical analysis, ppm cannot be measured by an EC meter.
They are present on Bluelab products as a conversion guide only.
Bluelab meters will measure the EC of the solution, and then convert this to a ppm reading if the desired ppm scale is selected on the Bluelab device. The conversion from EC to ppm is as follows:
- ppm500 = EC x 500
- ppm700 = EC x 700
Some examples of how this works, using a conductivity reading of 2.4EC:
- ppm500 scale: 2.4EC x 500 = 1200ppm[500 scale]
- (or 1200ppm / 500 = 2.4EC)
- ppm700 scale: 2.4EC x 700 = 1680ppm [700 scale]
- (or 1680ppm / 700 = 2.4EC)
If you are reading from a book that says you should grow your crop at 1100ppm - how do you know which scale the writer is referring to?
It is very important to match the scale on your meter to the scale being referred to in the book. Is the scale on your ppm meter right for the job? If the book was written in the USA, it could be the 650 or 500 scale. If the book is written in the UK, it could be the 700 scale. If it was written in Australia, well it could be any of the 3!
Because of the complications of the ppm scale(s), we highly recommend the use of EC, the international scale for conductivity.
If you must grow using ppm, you will need to know the following;
- What ppm scale is the book or feeding schedule referring to? Match your meter to this!
- What ppm scale is your meter using? Ensure it matches the feeding schedule or book being followed.
- What ppm scale is the nutrient formula referring to? Match your meter to this!
Which ppm scale should you use? Which ever matches the rest of your system
