Take a know amount of a dye ...
Published by Terry Scheurman, President at Applied Specialties, Inc.
Take a know amount of a dye whose concentration you can accurately measure. add this to the tower deck. Take an estimate of the system volume and using the recirculation rate determine what 3 turnover rates would be. Wait that long and take a dye reading. That is your zero point.
Take readings every 2 hours for 8hours. You can now plot those concentration drops and determine the rate of loss from the system. That will equal your water loss or bleed. Remember Bleed is water loss and blowdown is what you do intentionally.
Evaporation/[cycles -1] = Bleed Bleed = drift + leaks + blowdown