Good question. They are the measuring the same thing but there is a difference that I’ll try to describe below.
PPM (part per million) is a quantity of a substance. For a liquid, we are talking about 1 milligram of substance per liter of solution. For a solid we are talking about 1 milligram per kilogram.
EC (electrical conductivity) measures the amount of salt in a solution by determining how much current flows through the solution (between the two electrodes on the probe). Recall, pure water is non-conductive until you add ions (e.g. a salt).
Different substances along with the quantity of that substance creates different amounts of conductivity when placed into a solution.
When using a TDS/EC probe, you are measuring the EC (electrical conductivity). The manufacturer will then apply a conversion to derive the PPM based on some standard substance.
For instance, a manufacturer may use sodium chloride as a reference. They will add 1000mg/liter of sodium chloride to a solution and then measure the electrical conductivity. They will then know the EC of a solution based on a 1000 PPM solution. Another manufacturer may use some other substance, such as potassium chloride, as the reference for the conversion between EC and PPM. Because the two manufacturers are using different standards, the measurement of PPM between the two different probes in the same solution will be slightly different. The conversion to PPM differs but the EC between the two is the same (or at least should be)!
When you measure some unknown solution, the meter will use whatever reference the manufacturer has chosen to convert the EC into a PPM value. It will tell you the PPM but it will be only relative to the calibration solution and will not be the true PPM. It is only an estimate.
Because of this, the use of PPM can cause some confusion.
Based on your 505PPM and 1002EC, this indicates that this probe has a (500/1000) ~= 0.5 conversion factor. However, based on the fox farms conversion of (1500/2200) ~= 0.7 conversion factor. Confusion.
If you can, the suggestion is to use EC only. It’ll be the most reliable way to track and communicate the concentration for a pre-defined nutrient mixture.
If the manufacturer does not provide EC (only PPM), then you will need to know the conversion factor of your EC probe and also what the manufacturer is using for their conversion factor. Then do some math.
So after all of that, using EC you are (correctly) measuring about 50% of what the manufacturer is suggesting (more precisely 1002/2100 = 48%). Which is not necessarily a bad thing if the plants are doing fine.
ps. If someone is creating their own nutrient formulation using a bunch of individual chemicals. The most accurate method in that case is the use of PPM. e.g. IF you are weighing and measuring the individual substances before they are added to a liquid. Not the case for pre-mixed solutions.
