Only EC exists. PPM can't be measured by the kit we are sold. You can pick up two of the best PPM meters, both properly calibrated, and get readings 40% apart. Then boil off the solution and weigh the dry matter, to find both were wrong anyway.
The problem is, that PPM isn't being measured by the PPM meter. No. The PPM meter is measuring EC and then using a conversion factor to guess PPM. Much like using a Lux meter and correcting it for PAR.
As in my previous post, the Lux to PAR conversion is different for different meters. With the EC to PPM conversion, it's different for every salt. Opening up the issue further, you can use a PPM meter to make up a solution in two reservoirs, with a different feed in each. set both for 1000ppm, and there is no real hope of both being 1000ppm in reality.
PPM's exist when you weigh the stuff in yourself. Then you measure the EC, and monitor it. That is the truth. Converting to PPM is just kidding ourselves. It serves no purpose. It just introduces errors that are accepted as 40% between meters. So when you read someone chatting away in PPM, it's quite useless information in regards of neutrient strength. Just as with people measuring lights with smart phones.
To give ourselves a chance of sharing numbers, I used this meter beside a known meter. I can only tell you this meter says 20,000 at 600 and that's not transferable to any other meter
It's a good job I posted that image.. I imagined a button. It has just 3 not 4.
I plotted it out (did I say?) and it was linear to about 800ppfd, where I ran out of light to test further (I was being lazy. I could of moved it, but had what I needed)
When people talk Lux or PPM without sharing their equipment, it's not usable information. The internet has years of records regarding peoples grows, that are near useless because they used unknown measurements. Spoons. Eggs. Gallons. PPM's. Lux. Frankfurters. It has to stop. People have their boots on their own necks, holding themselves down.
I'm ranting again aren't I. Totally gone off on one. Left the building, made it to the airport and boarded for lala land. Sorry :)