Almico wrote:With the new RTD set up this should get even better.
I'd be careful with that. I can not argue with the better results Marko is obtaining. And the internet searches seem to generally say RTDs are more immune to noise. However, the fact is, there are many ways a circuit can measure the RTD resistance. In the case of the Phidget 1200, specs say max RTD current is 62uA. That equates to only 23uV per degree C at 200C. Compare this with a K thermocouple, which is 41uV per degree C. And the thermocouple has much lower resistance. So in theory, the noise immunity of the thermocouple should be better. But that depends on other factors such as common mode rejection, which is not specified.
The analog filtering, which is very desired, is accomplished in the tip of the RTD. The noise likely couples to the wires after the RTD and so this anolog filtering does no good IMO. The best place for filtering is at the A/D converter within the Phidget. Putting a capacitor at the terminals would do exactly this.
That said, a better RTD circuit would put more current through. I've read 1mA is pretty common. But higher current results in some self heating. So there is a trade with accuracy.
Lastly, I would be a little worried of ruggedness of cheap RTDs. Often you see that they cannot be bent. Buy commercial probes would likely mitigate this issue.
But RTDs, IMO can be used to achieve very robust, highly accurate and noise immunity better than thermocouples.
I'd be inclined to construct a circuit using the Phidget voltage sensors.