Chemistry help? pH issues driving me crazy
so i'm in SF, with water avg pH 8.5, TDS avg 108, alkalinity/hardness avg 61 and 62, respectively. i've got a ton of container plants, some in gritty mix, others in a 3-1-1 of bark/turface/lava rocks, even a few in potting soil. very sunny patio that traps heat, starting to need to water/fertigate up to 3-4 times a week.
i use dyna gro nutrients and acidify with white vinegar shooting for about a 6 pH. i bought some hoses, clamps and valves from local hydro shop and make large batches of fertigation solution for gravity drip in 5 gal buckets. i fertilize with foliage pro and a little bit of protekt, and adjust pH with vinegar. that mix is fairly reliable in its ratio.
but when i mix up my solutions for irrigation i can get seriously different pH readings from the same exact amount of nutrients in each 5 gal bucket. for instance- this morning i went to irrigate with water, protekt and vinegar. 5 gal water, 1.25 tsp protekt, and 1.75 tsp vinegar put me right around 6-6.5pH.
but- when i went to fill the second bucket up i had to use 2.25 tsp of vinegar and that got me maybe to 6.5-7pH. thats nearly 30% more vinegar and still wound up with a slightly higher pH than the first batch.
i'm trying to get my ratios correct to cut down on the amount of time i have to spend watering, (mix, check pH, adjust, rinse, repeat, etc) and this is driving me crazy. i know our water pH fluctuates throughout the year based on our water supply mix, but it shouldnt change in the same day, or even week. and i'd think with so little TDS/hardness/alkinity/etc that the water isnt really the issue. the chemicals maybe?
any body have a similar experience? and ideas what i might be unwittingly doing to cause this- or things i need to do to prevent it?