We have a Chinese made ultrasonic transducer, 120kHz approximately, 25mm x 25mm dia. It has three wires - red, green and white. The body is PVDF We have determined that the red and white are used to drive the crystal, but cannot understand what the green is for. The sparse data sheet had no explanation. The green wire seems to have no connection to anything that we can determine. The only thing I can think of is some kind of internal EMI shield
Does anybody know why it exists?