What device are you using to measure the voltage from the thermocouple? Are you using a National Instruments DAQ board, or something else to input the voltage to the PC? Is the voltage from the thermocouple amplified by anything before you measure it? That would help a great deal in determining the answer to your question.
But this might be useful info:
According to this website: http://www.picotech.com/applications/thermocouple.html
Type K (Chromel / Alumel)
Type K is the 'general
purpose' thermocouple. It is low cost and, owing to its popularity, it
is available in a wide variety of probes. Thermocouples are available
in the -200 °C to +1200 °C range. Sensitivity is approx 41 µV/°C. Use
type K unless you have a good reason not to.
So, at 100°C, without any amplification, the thermocouple would output a voltage of
.000041V/°C * 300°C = 0.0123V (the TC range starts at -200, so add 200 to the desired temp)
And at 25°C, the TC output would be
.000041V/°C * 225°C = 0.009225V
So, I would say that you have some kind of device that is amplifying your TC to a higher voltage. Therefore, you need to know what that amplification factor is before converting that voltage to a temperature. If, just as an example, you have a circuit or box that inputs a TC voltage and outputs 10mV/°C, then your 25-100°C range would be .250V - 1.00V. In that case, the .010 is your scale factor, and your 0.4V output would translate to 40°C. Again, that depends on the hardware you are using.
Does that help at all, or just confuse you more?
B-)
.
