The SEM203W is a transmitter that accepts variable resistance sensors such as potentiometers and converts the sensor range selected to a 4 to 20mA analog signal.  A simple push button allows the user to select the transmitter range manually by setting the potentiometer position and pressing the button to output 4 and 20 mA at offset and span ends of the range required.  Any value potentiometer can be used between 1k and 100K