[nhellman]

Aren't there symbols missing for the mid central vowels which are common in English? I.e. https://en.wikipedia.org/wiki/Mid_central_vowel.

How do you write e.g. "learn" or "bird"?

It seems to be needed in the first "This is a sentence..." example as well. Right now it uses U for the lone "a" and I for the second syllable in "sentEnce". But U represents /ʌ/ (as in but) and I represents /ɪ/ (as in bit), but a symbol for /ə/ would be a better fit for these two, right?

[dragonwriter]

The given symbols match the way the vowels represented are pronounced in at least some dialects/accents of English; one problem with trying to do a new phonetically-accurate writing system for English is that English has lots of dialects and accents, and pronunciation (even relatively—like which parts of words match which parts of other words) isn't consistent between them.

[vkethana]

Thanks for pointing out these mistakes - I added the schwa back into the vowel inventory (forgot to include in the original post) - added clarification for how to write the “ir” in “bird” or “firm”