Radiant heating of the thermocouple? Can the thermocouple "see" the heater? >>> rashid.chaoua@lmco.com 21 Feb 08 09:50:47 >>> A few of you offered assistance a couple of weeks back regarding some issues I was having with my thermocouples. Unfortunately, I tried most of the suggestions and I am still in the same place I was then. Problem: I have two type of thermocouples from Omega Engineering: TJC36-CPSS-020E-12 and HGTMQSS-125E-6. They are connected with 15' to 50' of TT-T-20S-TWSH-200 (Omega). The wires are hooked into a National Instruments data acquisition board (cFP-1804 with Isothermal Connector Block for the thermocouples). The operating temperature is 0C to -30C. We have taken independent readings using the NI board and compared them to the HH-25TC (Omega handheld meter) and have gotten similar readings. Additionally, we have tested all thermocouples, including the wires that they are attached to, in an ice bath and all readings are around 0C when taken with the HH-25TC. The thermocouples and NI board are inside of a low-temperature refrigerated container (insulated steel shipping container) capable of reaching -30C. The container is divided into 3 rooms and each room will go through a different temperature cycle. Four 500watt heaters are in each room and can be turned on to achieve a desired ramp rate. Note the tips of the probes are not in contact with any surfaces. Room 1 contains TC1-1 and TC1-2 as well as "next to 1-1" which is an independent self-contained temperature sensor/data logger hanging from the ceiling with the sensor close to TC1-1 Room 2 contains TC2-1 and TC2-2 as well as "next to 2-2" which is an independent self-contained temperature sensor/data logger hanging from the ceiling with the sensor close to TC2-2 Room 3 contains TC3-1 (which showed as an error "1770") and TC3-2 as well as "next to 3-2" which is an independent self-contained temperature sensor/data logger hanging from the ceiling with the sensor close to TC3-2 Please note that the independent data loggers are sheathed and they will therefore have ~20 minute lag and currently they are used to provide an independent temperature reading. We ran a trial in which the chamber temperature started at ~0C and decreased to -30C. During the temperature decrease, the thermocouples were somewhat accurate and the duplicate measurements in an individual room are at times acceptable. After 5 hours of decreasing temperature, we turned the heaters on while leaving the cooler running. During the temperature increase, the readings appear to fall apart and the discrepancy between the actual room temperature and the temperature read at the end of the extension wire (near the NI device) is often more than 10C. This discrepancy is also measured when the HH-25TC is used. . We've had a few suspicions for the cause of these problems which we have disproved: 1. Electrical noise or vibration when the refrigerator condenser unit is on (we disproved this by verifying that readings with the unit on vs off were the same) 2. Electrical noise or interference when the breaker box is on which feeds the heaters and NI unit (we disproved this as well) (additionally we tested having both the condenser and the breaker off and there was no difference) 3. Wire problems (Disproved: thermocouples and extension wires give accurate readings in ice bath) 4. Improper connections (for the type T thermocouple the Copper (Blue) is Positive & the Copper/Nickel is Negative). I have checked that this is consistent throughout each run of wire. We are at a loss for ideas here and would like help to get this problem resolved as soon as possible. Thanks for the help. Rachid Chaoua -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist