Rock solid and compatible with *most* commercial gym equipment (cycles, ellipticals, treadmills, stair steppers, etc.). I use it almost every day so the machine gets my heartbeat without my having to place my hands on the machine's sensors. (Machines use your heartbeat to compute various parameters for the workout, display, calories burned, etc.) The coded transmitter works with both coded and uncoded machines. The uncoded transmitter works with undoded machines - but, I don't know if the uncoded transmitter works with the coded machines. Probably does. Get the coded just to be sure.
These are sealed units, so there is no battery replacement. You just throw it out and buy a new one when it dies. I have both a coded and uncoded unit that I have been using interchangeably and about equally for 5 years, and both are still functioning.
Super high recommend.
At the heart of your training is the Polar T31 Coded Transmitter. Its purpose is simple: to transmit your heart rate to your wrist unit. Slim and light, the ECG (electrocardiogram) accurate transmitter is fully water resistant and designed for all weather conditions. Compatible heart rate monitors include, but are not limited to: FT1, FT2, FT4, FT7, FT40, FT60, FT80, RS100, RS300X, RS400, RCX5.