Although there is no formal recommendation on distance, sensors can be placed several hundred feet from the Sensaphone without a problem as long as the resistance on the line does not exceed 50 ohms. Temperature sensors may supply readings that are a few degrees off when they are wired several hundred feet from the Sensaphone, but you can re-calibrate the Sensaphone to make the appropriate adjustment to offset the resistance and report an accurate temperature reading. You can also use a larger gauge wire to minimize resistance.
