KeithRB:
No you didn't. thermocouples output a voltage and have an almost zero resistance. Each type of thermocouple has a large polynomial you use to convert the voltage to temperature. You used a thermistor - which might be an option, but not what the OP asked for.
Right-on, Keith,
Having done this for a Rotax engine dual exhaust gas temperature, I can add a little.
First, the thermocouples MUST have a 'real' or 'simulated' Cold Junction ... that is, a reference point that is 0C... as one may expect, this is usually done in hardware/software on a customized chip: called a thermocouple interface (sometimes known as a transconductance amplifier.) Inside this chip is a custom uC and a lookup table for the specific type of thermocouple utilized (the metal combinations.)
A common thermocouple amplifier is: Thermocouple Amplifier MAX31855 breakout board (MAX6675 upgrade) : ID 269 : $14.95 : Adafruit Industries, Unique & fun DIY electronics and kits
NIST has the official ITS-90 documents: ITS-90 Thermocouple Database, Download Files
My experience with thermistors indicates around +/- 2C can be obtained rather reliably... with precision 1% components, +/- 1C can be managed. My most recent wireless thermometer: ESP8266 Wireless Temperature
When measuring the outside temperature of a pipe, you have a number of negative issues you need to address:
- a rather large delta-t associated with fluid temperature in the pipe before a corresponding change in the outside temperature
- pipe outside temperature may be several degrees (or more) lower than the internal fluid temperature
The pipe material affects the above concerns ... while metal pipe conducts temperatures better, they also dissipate temperatures quicker and act as a large, long heatsink. Plastic materials are inferior to metal for heat conductance. The best method of determine if the outside pipe monitoring is appropriate is Empirical Testing - that is, experiment.
Ray