I grow plants in a hydroponic system. It holds 78 gallons of nutrient solution at a 450ppm(parts per million) concentration. Currently each day 7 gallons of 0ppm water are added from a top-off reservoir to replace water removed through transpiration. At the end of each day my solution is reduced to 400ppm and I have to add nutrients directly to my system to return it to 450.
How do I calculate the concentration required in the top-off reservoir to keep my system at a steady 450ppm?
My calculation
71 gal * 400 ppm/10^6 = 0.0284 gal nutrient left at end of day
7 gal make up water at x ppm nutrient = 7x/10^6 gal nutrient
78 gal * 450ppm/10^6 = 0.0351 gal nutrient required by system
7x/10^6 = 0.0351 -0.0284=0.0067
x= 957 ppm conc of nutrient in make up water
I guess it all comes down to how one interprets the problem ...
... if the concentration is 400 ppm before adding the 7 gal of pure water, then bjhopper's calculation is correct.
... if the concentration is 400 ppm after adding the 7 gal of pure water, then my calculation is correct.