During the run I noticed that the two exhaust temp gauges behaved very different to each other, so last night I did some experimentation with different thermocouple arrangements, I made a test chamber from a steel tube that I heated with a propane torch to get the same environment for both temp probes.
As a reference I used a hand held unit with a Fluke thermocouple, I tested it against the analogue gauge with its original thermocouple, the analogue gauge with a Fluke thermocouple, and a digital gauge with both types of thermocouples.
* The reference/Fluke and the analogue/Fluke measured the same, but the analogue gauge was much slower in response.
* Compared to the reference the analogue gauge/original thermocouple was both slower and read aprox.100°C lower in temp.
* The reference/Fluke and digital/Fluke read the same up to 800°C give or take couple of degrees.
The last test was the heat image, I managed to take a pic of the thermocouples when the temp had settled close to 750°C. (had to let go of the cables to take the pic so the right one tilted upwards and got a little bit hotter than the left one, the upper thermocouple was probably a tad hotter than 750°C when I took the picture.)
Here is a temp chart blacksmiths use to judge the temp of the metal, the tip color seems to be somewhere between the 704°C and the 760°C mark in the chart so I think I can assume that the reading is pretty correct.
So, what have I learned from this? I will remove the analogue gauge and its probe and couple a Fluke thermocouple to the digital display I used for measuring T2 earlier. Much faster response and an, as far as I can tell, accurate reading.
Cheers!
/Anders