In my opinion all growers should be using conductivity as a measurement of nutrient strength. The reason for this is very simple TDS and PPM is nothing short of a worthless reading. The fact is that two different TDS sensors can have readings hundreds of PPM off from each other and both be 100% accurate! In other words one TDS meter can read 1400 ppm and the other 1000ppm and both be spot on as far as the manufactures spec sheets are concerned. The reason for this is simple. ALL TDS/PPM sensors measure the conductivity of the nutrients and use a conversion value to guess at PPM. And different manufacturers use different values.

Conductivity is the ability for the water to conduct electricity. Fun fact, if you sat in a bath tub of distilled water and dropped a toaster in most likely nothing would happen at least until the salts in your skin disolved into the water. Pure water is not conductive. It's the salts mixed in with water that allows it to conduct electricity (act like an electrical wire). The more salt that is in the water the more conductive it is. Put simply plant nutrients are nothing more than different kinds of salts. So us growers can get an idea of the strength of a nutrient solution by measuring how conductive it is. More nutrients in the water the more conductive it is.

All TDS Meters first take the conductivity of the solution (the reservoir water) and then convert it into PPM (Parts Per Million). The value used to convert from conductivity to PPM varies between meters. Some use 500ppm per unit of conductivity many others use 700ppm per unit of conductivity. The reason for two different types of meters boils down to this. Different salts change the conductivity of a solution to varying degrees.

If you put 5 grams of table salt into 1 liter of distilled water. You would have a 500 PPM solution of table salt (sodium cloride). This solution on a conductivity meter would measure 1.0 mS/cm2 (many times referred to as EC which stands for electrical conductivity). So 500ppm of table salt in water equals 1.0 electrical conductivity.

However if you used the same PPM meter that read 500ppm in the above solution. And took 1 liter of typical nutrient solution made up with a hydroponic nutrient and it also measured 500ppm. Then boiled off the water so only the salt was left. It would typically have about 7 grams of nutrient salts (as a reminder 500ppm of table salt is made of 5g of table salt per liter of water).

Companies have made TDS meters that will read accurately based on the specific conductivity of common nutrient salts (700ppm per EC) and others use the most common (500ppm per EC) that is based on table salt. Say you have a reading from a conductivity meter that reads 1.0. While one TDS/PPM sensor will read 500ppm in the same solution the other will read 700ppm. The problem now becomes apparent. If you speak to a master grower and he tells you to keep your PPM's at 1000ppm. If his meter is one of the 500ppm versions and you have a 700ppm your nutrient stength will be much lower than he is recommending. You'd need your PPM reading at 1400 to have the same strength nutrients.

Many growers have failed because of this. Some over fertilize because the people advising them were giving them ppm values they got from their 700ppm meters. While they were using a 500ppm meter. Others under fertilized because the people that were advising them had a 500ppm meter and they have a 700ppm meter.

This can all be avoided if you use conductivity readings directly. The units of conductivity can be a bit scary if you haven't been exposed to them. Generally it's mS/cm2 that is Micro Seimens (pronounced see-muh nz) per centimeter squared. Many have found it's easiest to just use the term EC or electrical conductivity. To us this just means it's one unit of conductivity. We don't care about the physics of what mS/cm2 means (see footnote if your interested). Most will be inclined to wonder what a particular EC value is in PPM. The EC scale makes this easy. 1 EC equals either 500ppm or 700ppm depending on the meter your converting too. So 1.5 * 500ppm = 750ppm or 1.5 X 700ppm = 1050ppm. If you continue to make the conversion in your head it is only a short period of time until you will instinctively think in EC units. If someone ever recommends a PPM value be sure to ask them what conversion scale their meter is using. The information is difficult to use without it.