The equipment does not read in error. It just not read in specifics. For an example. A nutrient sample reads an EC of 2. It could be 2000 ppm of Calcium nitrate or it could be 1600 ppm of Magnesium sulfate. It could be many different combinations of concentrations.
TDS, PPM, and EC
If you have any desire to mix your own fertilizers, it is important that you have at least basic understanding of these terms
Electricity is conducted due to the presence of ions (electrically charged) in a solution. The ions get there by introduction of salts via our ferts!
EC (electric conductivity) is a representation of how much potential a solution has to conduct electricity...SO, by testing the ability of a solution to conduct electricity, we can indirectly determine the amount of salts present....thereby knowing if we have the right concentration of ferts.
E.C. is a measure of salinity by measuring its conductivity. You want an E.C. under 2.0
Despite many peoples natural aversion for ppm measurement, it is a good idea when mixing ferts as a beginner.
TDS is a measurement, by weight, of the Total Dissolved Solids in a solution and is measured in PPMs (Parts Per Million). Basically, when you hear someone say they introduce nutrients at a rate of 500 PPMs, it means that they have 500 milligrams of solid dissolved in a Liter of water.
You can figure the PPM of your ferts one of two ways.....you can precisely measure the ferts and water you mix together...or you can make a solution and measure the PPM of IT. The shortcut lies in hand-held meters which measure the EC of a solution and then apply mathematical conversion to the EC figure to arrive at PPMs. Keep in mind, even this is an approximate measurement...but plenty accurate for growers!
Some meters actually display the EC AND the PPM readings. Some only express the EC value and this can be tricky because European and American made meters measure at different rates.
An American device that displays an E.C. of 1.0 has 500PPMs.
A European device that displays an E.C. of 1.0 has 640PPMs.
So if your device only displays E.C., use a bit of simple algebra and set the two given equations equal to each other. For example, let's say your E.C. reading is 1.2.
Using the known ratio given just above, just cross multiply to solve for x to get the unknown ppms.
1.0/500ppm=1.2/X, so (1.2 *500)/ 1.0) = X
X=600
Therefore 600PPMs.
If you want to figure out the PPMs yourself, its pretty easy. Each 1 gram of fertilizer per liter of water gives 10 PPMs of each given nutrient per percentage point. Sounds clumsy, wish I could state that better....here's an example.
Use 1 gram of SuperPhosphate 0-20-0 in a liter of RODI water. The solution would have 0 ppm of N, 200 ppm of P, and 0 ppm of K. Also keep in mind that tap water already has dissolved solids...most likely anywhere between 200-400 PPMs. Use RODI water when possible...0 PPMs!!
Short of testing for each fertilizer individually, an EC or TDS meter is the cheapest, easiest method of providing an approximate ppm. For an example the cheapest ISE (ion specific electrode) I have seen retails for about $250. It is nearly as adequate as a laboratory grade. The calcium ISE is the only ISE I have ever seen that selss chepaly and y that is because the volume sold to reef aquarium keepers is large enough for volume sells at low mark upos. It is a Pin Point Calcium Monitor Meter at $149.95. I use laboratory grade ISE's such as the Thermo Orion ISE that sells for over $700 for just the electrode (probe). The meter costs equally as much or more.
I try to buy from Ebay as much as possible. I also use ISE testing for Nitrate, ammonium and phosphate. Phosphorus I check with a Hach spectrophotometer as well as iron and chlorine. Magnesium I test by titration as it is part of a hardness test that confirms the calcium test and also tells me the magnesium and the bicarbonate hardness.
In general I mainly just check, ammonium, nitrate, pottasium, chlorine and iron. Except when changing strain, nutrient formulations, EC or any major parameter I usually just monitor EC and pH.
I would likely not own the test equipment I have if it was not tax deductible due to my profession and if I did not acquire most of it ion eBay. I also have slat water reef aquariums so it sll gets used fairly often. It is nice buying a new, never used ion specific probe that usually sells for $800 or more for $20 to $100. I recently bought an ammonium and a nitrate probe sold together for $28.77. They retail for over $800 each generally. The reagents for testing with the spectrophotometer usually run about 20 to 25 cents per test.
If you ever get that interested in that aspect of growing send me a PM. I have lots of testing equipment, meters, controllers and analyzers, peristaltic pumps etc.
1.2
not necessarily MJ) 
