(02-06-2017, 18:33)danner Wrote: can I use it with just the single 82PB715?
Yes, I'm fairly sure that you can.
Standard I2C devices are designed to work with a "stray" load capacitance of up ro 400 pF (to Earth). The 82B715 effectively spliits the bus into two "halves" so that each half can have a capacitance up to 400 pF. CAT5 cable has a capacitance of only just over 50 pF/metre* so if only that and the SHT31 are attached to the "second" side of the 82B715, then the cable length might be as much as 7 metres (20+ feet). Personally I wouldn't go as far as that because there are other factors such as crosstalk and "radio frequency" noise, etc., but 3 feet should be absolutely fine.
The 82B715 chips themselves are able to drive a much higher capacitance, so if you had a cable with very high capacitance (say 3000 pF) then you would put a 82B715 at each end, so that all the standard I2C chips are "proitected" (buffered) from that capacitance.
* The only cable I can think of with a capacitance that might approach 300 pF/metre is small-diameter "individually screened" coaxial cable as might be used for audio connections (e.g 4 cores for in/out and left/right in a sheath of < 5mm diameter).
Cheers, Alan.