Puzzling Measurement Swings Using Thermocouples
Puzzling Measurement Swings Using Thermocouples
(OP)
I am not that experienced using thermocouples (E and K type) and may be overlooking a glaringly obvious fact.
When attaching thermocouples to a National Instruments signal conditioner, I am having difficulty achieving stable temperature readings using a LabView virtual instrument written by a former employee. The leads appear to be securely attached, but when I gently touch the thermocouple lead wire, the temperature readings fluctuate wildly and even go to a different nominal value 20 - 30 F apart. The hot junction is not in contact with anything but the same air.
In my acquaintance with thermocouples, this is the first time I have experienced this kind of problem.
What am I not considering?
When attaching thermocouples to a National Instruments signal conditioner, I am having difficulty achieving stable temperature readings using a LabView virtual instrument written by a former employee. The leads appear to be securely attached, but when I gently touch the thermocouple lead wire, the temperature readings fluctuate wildly and even go to a different nominal value 20 - 30 F apart. The hot junction is not in contact with anything but the same air.
In my acquaintance with thermocouples, this is the first time I have experienced this kind of problem.
What am I not considering?





RE: Puzzling Measurement Swings Using Thermocouples
Might also be that you are acting as a shield/ground when you touch the TC wires. How is the board isolated relative to ground? Are you using shielded TC wire or running it thru EMT, is the shield/EMT grounded to the same reference as the A/D board sees, and what environment does the TC wire travel through in terms of electrical/magnetic fields?
Finally, some longer-term drift errors occur due to local heating/cooling of the board's temperature sensor it uses for cold junction compensation. Are there any rapid temperature swings that the board might be experiencing, i.e. a sudden cold draft when the door to its room is opened, or the AC kicking on/off?
RE: Puzzling Measurement Swings Using Thermocouples
rp
RE: Puzzling Measurement Swings Using Thermocouples
RE: Puzzling Measurement Swings Using Thermocouples
RE: Puzzling Measurement Swings Using Thermocouples
RE: Puzzling Measurement Swings Using Thermocouples
ISZ
RE: Puzzling Measurement Swings Using Thermocouples
Say in my testing I am measuring temperatures at 3 TC points of an electrical equipment. I generally run the thermo couple from a point where I want to measure to the temperature reader.
Which grounding are you guys taling about? Do you mean that the equipment must gave a proper grounding wire connection?
RE: Puzzling Measurement Swings Using Thermocouples
If you have a thermocouple which has its junction connected electrically to its metallic sheath, that thermocouple junction is considered to be "grounded". If the junction is embedded in MgO insulation such that it is in thermal contact but not in electrical contact with the sheath, or the junction is exposed beyond the end of the sheath, that junction is considered "ungrounded". Whenever we have used grounded junction thermocouples in the interest of having the fastest responding and most accurate junction possible, we have very often experienced problems with getting good data out of them due to ground loops or other electrical interference.
If you use a "grounded" junction T/C, what you have to worry about is a difference in potential between the ground that the sheath is connected to electrically (i.e. the equipment) and the ground that the thermocouple amplifier/reading device is connected to (i.e. the instrumentation ground). If there's an isolating transmitter in there, the problem is reduced of course, but ONLY if it has electrical isolation as part of its design. MANY thermocouple input cards for I/O systems do not have proper isolation as part of their design, and hence they are subject to ground loop problems. Of course when the tech comes around and plugs the T/C into his battery-powered (i.e. isolated) thermocouple reader/calibrator, everything looks fine.
EMI/RFI is a different matter. Unshielded wires, particularly unshielded wires connected to a very high input impedance amplifier (such as, say, unshielded thermocouple extension wires connected to a thermocouple reading device) are particularly good antennas to pick up this kind of noise, which can be generated in great quantities by devices such as motor speed controllers etc. Twisting the wires helps to reduce the noise, but a shield helps more still. That shield has to be connected to ground somewhere, but not such that it becomes a ground loop path itself. But it is still necessary to keep the T/C junction isolated electrically from the sheath ground.
Does this make things clearer?
RE: Puzzling Measurement Swings Using Thermocouples
Now if we look at a ground loop it isn't hard to see that any voltage drop in a ground path, with a grounded TC, will result in an erroneous output. (And realize that this voltage drop will change with temp, humidity, stray voltage from nearby electrical equipment, dust, etc.) Next comes a person with some amount of static charge, and viola, output changes again.
The easy answer is to use ungrounded whenever possible. Generall the response time penanty isn't an issue, but if you have to go ungrounded you must take special precautions to prevent this.
ISZ