||09-28-09 08:17 PM
Other posters may be on the right track, but it sounds like you caught a corrosion problem. T31 transmitters have an average life of 3 years and no replacement batteries. Sounds like you have a Wearlink transmitter with replaceable batteries though, and the straps are only around $10 if you aren't getting good contact any more. Sometimes, problems appear when the contact of the transmitter with the skin isn't the best. Usually, that goes away when people are sweaty or they use electrode gel. For women, an underwire brazzier can interfere as well.
Another possibility is radio frequency interference. Certain tail lights and flashing headlights have been reported to interfere with bike computer components, and the transmitter (chest strap) sends a radio frequency signal to your Polar 720i. The receivers (watch) seem to pick up radio frequency noise creating junk data from time to time in our cardio room and around our wifi ports. So if you added a new device recently, it could cause radio interference.
As far as changing batteries goes, no reason not to if you plan to keep the receiver dry, are careful not to strip the screws, and you use the correct battery type. Mostly, what you are paying Polar for is their technician's assurance that they place a fresh "O" ring and buttons lined up in the right positions so it remains water proof, and that they shouldn't strip your screws. That's especially important if you are a triathlete. At my school where we use downloadable Polar HR monitors for Physical Education classes, we change our batteries every year or so. It does void the warranty which I think is typically two years, so it may be worth paying for the first battery to be changed by Polar if you do it during that period. We never use them in the pool because chlorine kills technology even if it is designed for that kind of abuse.