TDS is the measuring of the amount of salts in a solution. For alot of applications the amount of salt is indicitive of the levels of other stuff in a solution. TDS/PPM meters sold for gardening and aquariums figure the amount of salt in Parts Per Million by measuring the Electrical Conductivity of the solution under test. So a PPM/TDS meter is a EC meter that converts the EC value into PPM values. There are alot of descriptions online about why and how to measure PPM/TDS or EC so google for them if you want alot more details.

EC is a measure of Electrical Conductivity from two probes 1cm apart. 1 EC is = 1 microsiemens, to convert from EC to siemens multiply by 1E-6. EC can be converted to PPM by multiplying by 500. PPM can be converted to EC by dividing by 500. To convert from siemens to Ohms is s=1/ohms, you can also go the other way and do ohm=1/s for siemens to ohms. Siemens is also known as Mhos, which comes from ohm written backward.


The number 500 used to convert between PPM and EC is called the Conversion Factor. Different salts will have different conversion factors because some conduct better or worse than others. NaCl's is 500, this seems to be the most common standard used, and is what was used for the calibration solutions.
If EC/PPM is just measuring the Conductance (or resistance) then why not use a volt/ohm meter directly?? Because they pass DC current thru the probes and you cannot measure conductivity of salts with DC current because it will rip the molecules apart, and since the molecules are what conducts the electricity you get a constantly changing reading that is useless. Overcome this by using an AC signal. If the frequency is high enough (>1khz it seems) the molecules dont have time to move apart before they are pulled in the opposite direction. Sort of like how high DC current will throw a person off of it, where as AC forces them to hold on and get shocked to death or something.


Something very important, temperature will effect the conductivity! The chart shows the same solution measured at 3 different temperatures. Measured Conductivity
at different Temperatures
EC Temp C
3.43 13
4.16 22
4.73 30

I´ll try to gather some more info on EC (or PPM´s) common values during a Cannabis Plant life cycle.Meantime if u have a record of ur EC or PPM values during a grow plz post them...Peace and Pot

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Trichome - http://www.rollitup.org/marijuana-pics/index.php?n=445
Hydro and Soil Water ph Range - http://www.rollitup.org/marijuana-pics/index.php?n=444
Light Colour Temperature Chart - http://www.mediacollege.com/lighting...mperature.html
Plant Abuse Chart and Photos - http://www.drugs-forum.com/growfaq/97.htm

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Moderator ...

Can you make the above post a "sticky?"

Excellent post, Omega.

Vi