Why people prefer method 2. Is there better accuracy?
Or method 1 is wrong correc method is second?
I have on many small processor people usually apply method 1?
Method 2 is physically correct and therefore is in principle more accurate. Method 1 is not strictly correct and is therefore less accurate.
However, the achievable accuracy in practice depends on many variables, and one always looks to the largest source of error in the system. If you have a 2 degree source of error in one place, there is no point fussing over a 0.05 degree error somewhere else.
- How accurately can you measure the reference junction temperature?
- How accurately can you measure the small thermocouple voltages?
- What is the tolerance and accuracy of the thermocouple itself?
- How repeatable are your measurements?
- How much noise is present in the measurement system?
- How much will the properties of the measurement system drift over time?
- How much accuracy do you actually need for your application?
In short, method 1 may be perfectly fine for your needs.
Note also that over small enough temperature ranges (e.g. 100 degrees) you may not even need the lookup table. Over small ranges the relationship between thermocouple voltage and temperature is linear to well within other sources of measurement error.