Appendix A. Measurement Considerations176 Model PM880 Portable Hygrometer User’s ManualA.1 Choosing a Measurement Site (cont.)• Protect the probe cables from excessive physical strain (bending,pulling, twisting, etc.). In addition, do not subject the cables totemperatures above 65°C (149°F) or below –50°C (–58°F).• Observe the proper cable restrictions for the probes. You can locate theMoisture Image Series probe up to 915 meters (3,000 feet) away fromthe electronics unit with an unshielded twisted-pair cable. The M Seriesand TF Series probes require specially shielded cable. You can locate theM and TF probes up to 600 meters (2,000 feet) from the unit. If you aremeasuring pressure with a TF probe, the cable length should not exceed152 meters (500 feet).A.2 Moisture/Temperature Probe ConsiderationsThe M Series, TF Series, and Moisture Image Series probes consist of analuminum oxide sensor mounted on a connector head. Standard probemounts include a protective stainless steel shield.The probe sensor materials and housing maximize durability and insure aminimum of water absorbing surfaces in the vicinity of the aluminum oxidesurface. A sintered stainless steel shield is used to protect the sensor fromhigh flow rates and particulate matter. The end cap should not be removedexcept upon advice from GE.The sensor has been designed to withstand normal shock and vibration. Youshould make sure that the active sensor surface is never touched or allowedto come into direct contact with foreign objects, since this may adverselyaffect performance.Observing these few simple precautions will result in a long and usefulprobe life. GE recommends that probe calibration be checked routinely, at12-month intervals, or as recommended by our applications engineers foryour particular application.
