A simple data logger example program for Sauter SU 130 (and maybe others). The 2.5mm jack is RS232 (I measured around 3.7V from the device) and the cable I got already contains a PL2303. The software which comes with the cable works but only runs in Windows. I wasn't able to run it in wine.
The software and cable are the same for Sauter TC, TE, TF, TG, SU, HD, TD.
The device runs at 2400baud and sends 0x10 around twice a second (which indicates that a new measurement is available). Each time you answer with 0x20, the device responds with a message containing the mode (Lp, Lq,.. fast/slow,..) and the value.
On the SU 130, the message is 0x08 0x04 MODE 0x0A 0x0A V1 V2 V3 0x01 CHECKSUM
Look into the code for further details.