adjusting ph with phosphoric acid

I decided to start using RO water and was following Gordon Strongs process in which he adds 1/4 tsp. phosphoric acid per 5 gallons to his brew water.  This, according to him, should bring the ph down to about 5.2-5.3.  I've now added 1tsp. and my ph is still around 6.  I put a test strip in the phosphoric acid and it turned dark red so I know it's acidic!

Should I just keep adding more?  Why would I need so much more?

I stirred it more and let it sit and now seems to be about 5.  I need to get a ph meter!  I'm still curious why it took 4 times as much as Strong needs?

It wasn't clear if you are measuring the pH of the water or the mash. The pH of the water is not important; it is the pH of the mash that matters. The grains have acid and will lower the mash pH. Darker grains will lower the pH more than light grains, so you can't have a universal formula that works for all grain bills.

--GF

I was talking about the water pre-mash.

This is from Gordon Strongs Pale Ale recipe on BYO:

"This recipe uses RO water. Add 1⁄4 tsp. 10% phosphoric acid per 5 gallons (19 L) of water. Use 1⁄2 tsp. calcium chloride and 1⁄2 tsp. gypsum in the mash."

In his book Modern Homebrew Recipes he says the 1/4 tsp of phosphoric acid will bring 5 gallon RO water to 5.2-5.3 ph.

If memory serves me, Gordon says that he acidifies his brewing water much like Sierra Nevada does. Sierra Nevada brings all of their their water to a pH of about 5.6 with acid. Without darker grains in the mash this will usually get your mash into the 5.2 to 5.3 pH range. Gordon also states that he steeps his dark grains and adds them post mash.

You were trying to bring your strike water pH to 5.2 or 5.3, which would drop your mash pH below the optimum range. Also, if you're not using a properly calibrated pH meter, you aren't getting a good pH reading. If you're intent on following this recipe, follow it as it is written. I have done the 1/4 tsp in 5 gallons of RO water and it works at bringing your mash pH into range as long as you have little or no dark grain in it.

Thanks Bob.  I went back and looked at the book again and the strike water was 5.5 to 5.6.  I'll just go with the recipe as you suggest.

Any suggestions on a good ph meter?

Thanks