Measuring fertilizer with nutrient pens that read PPM (parts per million) is inaccurate.
All nutrient pens first detect EC (electrical conductivity) and then convert it to PPM. But different manufacturers use different conversion ratios!
This makes accurate nutrient solution measurement impossible.
The rest of the horticulture world measures EC when determining the overall fertilizer strength in their nutrient solution. PPM only makes sense when speaking about the presence of individual nutrients such as nitrogen, potassium, and phosphorus.
But for measuring overall nutrient solution strength (or leachate), skip the guesswork and just measure EC.