Looking at the

**specific gravity standard**, your recipe says to make a 3.714 weight % NaCl solution, dissolve 1 tsp (6.2 grams) of iodized Salt in 161 mL (makes 163 mL).

I am assuming by weight percentage you mean weight in volume or grams per 100mL. I will cross multiply 6.2 / 163 = x / 100 gives 3.8 (3.85 when using 161).

**So it looks to me like you are making a 3.8 w/v% solution which is 35.8 PSU according to the table and not a 3.714 w/v% which is 35 PSU.**

Looking at the

**refractive index standard**I also get a slightly different number. The instructions say a 3.65 weight % NaCl solution can be made by dissolving 3.65 grams NaCl in 96.35 grams (mL) of purified freshwater. 3.65 / 96.35 = x / 100 gives 3.79 and if I account for the volume added which I calculated to be 1.17 then it's 3.65 / 97.53 = x / 100 which is 3.74.

**So again it looks like I am making a 3.74 % solution and not a 3.65 % solution.**

I am off by basically the same amount on both recipes, my math shows me making solutions just over 2% stronger than advertised. I'm sure there is something you accounted for that I've misunderstood, what is it?