At the most basic level, any RS232 interface is one line in, another line out of serialized bits. No matter what is hooked up to the PC's serial port, if you put a scope there you are going to see 1,0, high, low, +12, -12. These bits are accumulated in the serial buffer and fetched by labview who interprets them as ascii characters. What you need to do is build an intermediate vi that gets the ascii characters from the labview read vi and determine the corrosponding tempature value.
Jared