temperaturethermocouple
Notice that the temperature in the graph is in °C, not even in °K.
0°C is the border between temperatures of water and ice under 1 atm pressure. Why do thermocouples give 0V voltage at a temperature of 0°C? Is there any relationship between water and thermocouples, are the thermocouples designed to behave like this, or is this just a coincidence?
I think you need to use an instrumentation amplifier like the AD8221. It does sound like common mode noise. My company fit plenty of thermocouples on aircraft engines directly to blades and other hot and large pieces of metal and we still get noise but it's livable with when using an IA.
If you read the datasheet, the conversion time of that chip is 100ms maximum. It effectively samples the input voltage then goes off and does a conversion and cold-junction compensation.
Offhand, I don't see any number indicating how long the valid voltage must be present at the inputs before doing a conversion. You may be able to overlap acquisition and conversion to some degree (perhaps start a conversion, wait 50ms then switch to the next channel, then you could immediately start a new conversion after 100ms for 10 channels/second), but that's not entirely clear from my skimming of the datasheet. In any case you can't do any better than 100msec/channel (reliably) though some samples of the chip may be a fair bit faster.
Best Answer
The thermocouple voltage is from the difference in temperature between the sense junction and a reference junction; the reference junction always exists and may be the connection of the thermocouple wires to your voltmeter; note that brass, copper, plating materials all form additional intermediate junctions that cancel if they are at a uniform temperature. Thermocouple tables are based on the reference junction being at 0C (ice-point reference, a convention) so that is why the sense junction temperatures in these tables are all 0V at 0C. Omega Engineering website has great reference documentation on thermocouples that explains all about reference and intermediate junctions. http://www.omega.com/techref/
There are many temperature compensation methods. One is to put the reference junction in ice slush, a stable and known reference point. Another is to produce a voltage to compensate for the actual junction temperature so that the measured voltage will be the "table temperature" with no offsets. Another method is to use an above-ambient and stabilized junction temperature in which case a fixed offset can be used to compensate. While this may all sound complicated, thermocouples are frequently used for extreme temperatures, and the accurate measuring or stabilization of a reference junction can be done with lower temperature electronic methods.