Monday, August 12, 2013

Common problems of thermocouples used in data acquision systems

Common problems of thermocouples used in data acquision systems:

The following are some points to consider in thermocouple connections in data acquision systems. These points may help in troubleshooting.

□ Open channel: always generates random large unreasonable readings. The hardware solution is to place a thermocouple free to ambient or use a jumper (when using a jumper, the reading will be the same as ambient assuming the data acquision card has a built-in Cold Junction Compensation CJC). The software solution is to detect open channels and eliminate their readings 
(software filter)

§  □ Inverted/exchanged poles of thermocouple wire in the male socket plug

§  □ Inverted/exchanged  poles of thermocouple wire in the female socket

§  □ Inverted/exchanged poles of thermocouple at the connection with the data acquision card 

§  □ Thermocouple junction is not established, wires are not welded together, or wires are not twisted on each other.

§ □ Thermocouple tip is touching a large conductive part:  a continuity tester (buzzer) is used to detect such a problem. One probe is connected to the large conductive surface and the other probe is connected to one thermocouple plug pin. If buzzer works then this means that thermocouple junction is not electrically insulated.

§ □ Thermocouple wire is in touch with a conductive part with leaking current

§  □ Thermocouple socket plug has oxidized pins, so they are not conducting

§ □ Non-smooth/unstable thermocouple readings: occur when thermocouple is placed in low thermal capacity stagnant mediums like air, and when placed in a flowing streams of fluids like water or air

§  □ Noise in readings: occur when thermocouples are placed near power sources and conductors with high and/or alternating magnetic fields. This problem appears in the single-ended thermocouple connections.

§    □ Un-calibrated system: the system should be calibrated as overall in a process called on-site calibration. Calibrating thermocouple alone is not useful because it does not take in consideration thermocouple wires, wires connections, and ambient noise.