I had a look at the driver and spec sheet. The device is line powered and I don't see anything in the driver to to bring the RTS line high. You can do this with a VISA property node. Right click on it and select Properties>Modem Line Settings>RTS State. Right click again and select Change to Read. You can then right click on the node and select Create Constant or Create Control. This will be an enum and you can select Unknown, Asserted, or Unasserted. According to the data sheet, you want the RTS line to provide a positive voltage only when you want to take a reading. Double check the wiring to the sensor and make sure you've connected everything correctly. Pin 1 should be connected to the TD line of the pc's serial port (pin 3), pin 2 to the RTS line (pin 7), and pin 3 to the RD line (pin 2). The data sheet also says that it comes with a utility program. Have you run that to see if you can get data?