Without the SilverTron, the only way to calculate the ppm (reasonably) accurately is using the standard formula which calculates the amount of silver released according to current and time:-
Mins run time = Volume of DW (Ltrs) x ppm AgNP required x 15
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milliAmps on constant-current power supply
for example:
0.25 ltrs DW x 20 ppm x 15 / 5 mA = 15 mins run time required
(credit: cgcsforum.org)