Hi Henrik!
The sensor is the infamous MS5541-CM and I believe the protocol is SPI. Basically the sensor to a 3v to 5v converter to the PIC then from the PIC to a MAX232-cpe to the computer. At short distances from the 3-5v converter the sensor readings are "normal", that is you will accurately read room temperature for example. As the cable from the 3-5v converter becomes longer the room temperature reading becomes abnormal such as -1434C. First cable was a shielded 6-wire, 22 gauge cable. Then I tried a ribbon cable with signals on every other wire with ground at one end only for the wires inbetween the signal wires to avoid cross talk and any ground loops.

Best, Ed