PPM is an abbreviation for "parts per million.”
This measurement is the mass of a chemical or contaminate per unit volume of water. One ppm is equivalent to the absolute fractional amount multiplied by one million. A better way to think of ppm is to visualize putting four drops of ink in a 55-gallon barrel of water and mixing it thoroughly. This procedure would produce an ink concentration of 1 ppm. Here is a video that helps give other examples of ppm concentrations.
Our sensitivity for Nima’s gluten sensor is 20 ppm (the FDA threshold for gluten free). Our sensitivity for Nima’s peanut sensor (in development & available to reserve) is 10 ppm.