Text version of the page

7. Immersion Depth— Heat can beconducted away from sensors that are not sufficiently immersed into theprocess being measured. The resultwill be a reading lower than actual temperature of the system. As a ruleof thumb thermocouples should beimmersed to a minimum depth of 10 times the sensor diameter. RTDsand thermistors should be immersed a minimum of 10 times the diameterplus

Sensor catalog - 7347 GeneralInformation

Application Hints

8. Changing of ThermocoupleImmersion Depths— Thermocouples can develop inhomogeneities due to oxidization, corrosion, contamination andmetallurgical changes under someprocess conditions. If the sensordepth is changed to shift the inhomogeneities into steeptemperature gradient zone, the output can be greatly altered. It is suggested that thermocouples not be repositioned once they areplaced in a process. 9. Sensor Life— Every systemexposes sensors to a wide varying range of operational environments. Selection of sheath materials, protection tubes, temperature cycles and sensor type influence overall usable life. Experience provides the method of determining the need to examine, test and replace sensors. Watlow recommends that each customer establish a preventative maintenance program for periodic inspection and replacement of all sensors.

The sensor appears to bereading incorrectly. What might be wrong?

(Continued)requires thermocouple to be atuniform temperature and best results are obtained at room temperature. This prevents the voltage generated by thermocouples at temperature from conflicting with multimeter resistance measurement function. The resistance value will vary by calibration and can be approximated by values given in the chart on page 24. For thermistors, resistance measurement at room temperature should equal nominal resistance value.
1 �
2 inch to provide proper heattransfer in most applications. 28