In order to use a PPM scale which at best is a guess, You will need to know what conversion factor the manufacture of your meter used. Some use 500 PPM per EC while others use 700 PPM per EC.

http://www.gthydro.com/plantguide.html will give you
PH, PPM, and cF for most plants. cF is EC x 10.

Norm

Like Norm said, the meter itself has two different ways of interpreting EC. You can set it to divide the PPM by 500 or 700 to arrive at the correct EC.

EC is measured in Micro Seconds. So your meter may not say EC when it is in the correct mode. I was dumb enough to stick with the PPM function on my meter for a long time because I could not find the EC function.

I have a black Hanna 3 combo meter that does PH, PPM, and EC. I can use either 500 or 700 to convert the PPM. The correct one to use for the US is 700 as our PPM numbers are usually given based upon that conversion factor.

One last bit of advice is that the EC and PPM charts come with a low and a high number or a rang that you should use. Baby plants should get half as much as the lowest EC or PPM and have it slowly increased over time. The max number is what you would use for a monster plant. What I mean by that is that if your tomato plant is only 3 ft tall, do not use the max PPM or EC which is actually for a 10 ft tall tomato plant. It is much better to stick with the lowest number if you want nice looking plants.

Thank you for the explanation.

Hi tntpointer,
Perhaps you may find this list useful: pH/PPM/CF for Various Vegetables

Have fun

I recommend the blue lab truncheon. It has all 4 on it. What makes no sense to me is why anyone uses anything but EC, especially nutrient manufacturers. All those numbers are based on the EC of the water as Norm kind of pointed out. You use a probe that measures the electrical conductivity. The ppm estimate is the conversion of EC into a ratio. My understanding is that the ppm aspect is actually from water quality testing to determine specific ppm's of specific particles. Ever read your local water quality reports? All in ppm, but per chemical/particle. That's because ppm will indicate dosage and dosage is what makes a substance a poison or not. "PPM" meters don't give ppm readings. They give estimates based on a totally different concept. No ppm reading is ever taken with a "PPM" tester that uses EC to come up with a number. That is why it can't tell you how much of each individual nutrient is in the water like a true ppm test does. This is just my understanding from dealing with water treatment people (I'm in the pump/motor market as well as a thermographer). I'm no expert by any means.

That truncheon is hands-down the best meter on the market.

Everyone who's "someone" in hydroponics uses those.

They aren't the cheapest, but they're tough, they last forever, and they only need calibrated like every 5 years or so.