I couldn't find anything in the datasheet for the 'older' HMP45C sensors, which have the orange power control wire.
There is a wiring table at the end of the actual datasheet, but it doesn't tell if these control inputs are ok with 12V DC.
Anyone?
You can find Campbell's manual for the HMP45C at:
http://www.campbellsci.com/documents/manuals/hmp45c.pdf
The older probes with the orange wire contain their own power (12V red lead) switch in their cable. The orange wire goes to a control port which is set high to turn the sensor on prior to measurement, and then set low following the measurement to turn off power to the sensor.