I’m trying to pull everything Thad and Micah said together into one post, and I did a little math on my own…let me know if I got it right or wrong.
[B]Pressure Washing Cleaning Solution Recipe[/B]
2 gallons of fresh 12.5% sodium hypochlorite
3 gallons of water
16 ounces of Simple Cherry powder
The sodium hypochlorite must be new or “fresh” because its strength breaks down quickly in storage.
Pre-mix the Simple Cherry in one of the gallons of water to aid dissolving. Add the sodium hypochlorite and remaining water. For really bad mold/mildew, increase the strength of the cleaning solution by using three gallons of sodium hypochlorite and two gallons of water.
[B]Determining the Amount of Cleaning Solution Applied[/B]
If you want know the exact amount of solution you are applying, fill up a 5-gallon bucket with water, and time how long it takes you to empty that bucket with your downstream injector. Then, compare that elapsed time with the gallons-per-minute (GPM) rating of the pressure washer. Once you know exactly what percentage you’re pulling through the injector, you can calculate your ratio of cleaners to mix into the 5-gallon bucket for proper cleaning.
Let’s say you have a 5 GPM pressure washer, and it takes 15 minutes to empty a 5-gallon bucket of cleaning solution. The total amount of liquid applied to the house is determined as follows:
[U]5 gallons[/U] x 15 minutes = 75 gallons
Since five of those gallons were solution, 70 gallons must have been water. Next, determine the ratio of water to cleaning solution.
[U]70 gallons[/U] = 14 or a 14:1 ratio
[U] 5 gallons[/U] = .06666… or approximately 7%
Thus, the ratio of water to cleaning solution was 14:1. Looking at this another way, approximately 7% of the liquid applied was cleaning solution.
If it takes less than 15 minutes to empty your bucket, then the ratio is higher than 14:1; if it takes longer, then the ratio is less than 14:1. The formula above will help you determine the ratio for your pressure washer/injector/tip combination.
The keys to getting a decent downstreaming ratio are a good injector, a high GPM gun, and a short downstreaming intake hose.
While these calculations are helpful, they will only provide an approximate calculation of your cleaning solution strength because you don’t actually know the real strength of your “12.5%” sodium hypochlorite due to its relatively short shelf-life.
As a general rule of thumb, you want the mold and algae to disappear after about 5-7 minutes of dwell time. Any slower than this indicates that your mix is probably too weak (add more sodium hypochlorite); any faster means it’s too strong (add more water to your downstreaming bucket).