In general, titratable acid (TA) relates pretty well to the 'acid taste' of a juice or cider. If the TA doubles, people will tend to perceive it as twice as acidic. The pH relates more to things like microbial stability and susceptibility to mould and bacterial spoilage. In particular, the antimicrobial effectiveness of sulphur dioxide is very pH dependent (see the sulphite page), and this is the reason that commercial cidermakers measure it. TA is measured by titration, and pH by a pH meter or by narrow-range pH test strips*. A pH meter is tricky to set up and calibrate, and only really worthwhile if it's being used daily in a laboratory environment. The cheap pH 'dipsticks' do not have replaceable electrodes and may only have an effective life of a year or so. For most non-commercial cidermakers, I used to recommend pH test strips as a better bet. But now after doing a formal comparison in 2011 I'm not so sure. Read what I found here at pH measurement comparisons. If you want to measure titratable acidity