Unless your machine (internal) thermocouples are each calibrated and compensated no two machines will deliver eactly the same results.
Here are your challenges:
1. Data acquisition equipment A-D converter errors 2. PCB measurement thermocouple (t/c) error 3. Machine temperature A-D converter errors 4. Machine tempeature t/c errors
And thats just the measurement side of the equation. All of these have a tolerance. Wires typically are +/- 1.25% or 2.25 degrees (which ever is greater). Items 1-4 can stack-up for a total + or - tolerance which may explain why one machine is different than the other. Also, you did not mention if it was 7 degrees Celsius or Fahrenhiet.
Affects of anealing, insulation contamination, cold junction compensation, wire routing techniques, bond integredity, atachment techniques....these add up to an integrated error that typically is not accounted for. Remember profiling in Electronics is not absolute. There will always be a tolerance.
On the machine side of the equation you have issues like insulation (isolation), cross zonal heating uniformity, flow rates....these too can affect change in your result.
I had 4 furnaces once. two were build togeter (a and b) the others were built a year apart (c and d). All the furnaces were the same "model number"
a and b were within 2 degrees celsius of each other. c was 7 degrees hotter that a and b. d was 13 degrees hoter thatn a and b
I inquied with the manufacturer as to the differences. Different insulaiton material, different amounts of insulation, different blower manufacturer. Everything else was identical.
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