T31 model. Ergonomic design that is 25% smaller. Smaller means increased comfort. 100% waterproof, 2500 hours of battery life with coded transmission to prevent cross talk. Backed by two year maintenance free warranty
Although it has a replaceable battery, I have found that after the initial replacement, the next time I have had to replace the unit. New batteries no longer work.
The T-31 coded model works well with my old "S series" X-Trainer plus. I was on my second T-61 transmitter when I discovered that it was almost out of gas. I called Polar USA, and was told to use this model as a replacement. They were correct.
Regarding the last comment, I called Polar yesterday regarding the t61 coded chest transmitter. Polar no longer sells it though the t31 will work. I still use my S710 myself. However, they no longer sell the interface transmitter between a pc and the S710 itself. Polar wants you to spend $480+ for their now HR wrist monitor w/ the polar protrainer 5. money, money, money...
This strap doesnt work with the S-series monitors, which includes many of the ones with cycling functions that Excel sells, including the S-710, S-710i, S-720, etc. Those require the T-61 coded transmitter, which isnt sold here (bummer).