Saturday, 8 December 2012

Thermocouple woes

Looking at the Wikipedia page on thermocouples I found this table:
TypeTemperature range °C (continuous)Temperature range °C (short term)Tolerance class one (°C)Tolerance class two (°C)IEC Color codeBS Color codeANSI Color code
K0 to +1100−180 to +1300±1.5 between −40 °C and 375 °C
±0.004×T between 375 °C and 1000 °C
±2.5 between −40 °C and 333 °C
±0.0075×T between 333 °C and 1200 °C
IEC Type K Thermocouple.svg
BS Type K Thermocouple.svg
MC 96.1 K Thermocouple Grade Color Code.svg
J0 to +750−180 to +800±1.5 between −40 °C and 375 °C
±0.004×T between 375 °C and 750 °C
±2.5 between −40 °C and 333 °C
±0.0075×T between 333 °C and 750 °C
IEC Type J Thermocouple.svg
BS Type J Thermocouple.svg
MC 96.1 J Thermocouple Grade Color Code.svg
N0 to +1100−270 to +1300±1.5 between −40 °C and 375 °C
±0.004×T between 375 °C and 1000 °C
±2.5 between −40 °C and 333 °C
±0.0075×T between 333 °C and 1200 °C
IEC Type N Thermocouple.svg
BS Type N Thermocouple.svg
MC 96.1 N Thermocouple Grade Color Code.svg
R0 to +1600−50 to +1700±1.0 between 0 °C and 1100 °C
±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C
±1.5 between 0 °C and 600 °C
±0.0025×T between 600 °C and 1600 °C
BS Type N Thermocouple.svg
BS Type R Thermocouple.svg
Not defined.
S0 to 1600−50 to +1750±1.0 between 0 °C and 1100 °C
±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C
±1.5 between 0 °C and 600 °C
±0.0025×T between 600 °C and 1600 °C
BS Type R Thermocouple.svg
Not defined.
B+200 to +17000 to +1820Not Available±0.0025×T between 600 °C and 1700 °CNo standard use copper wireNo standard use copper wireNot defined.
T−185 to +300−250 to +400±0.5 between −40 °C and 125 °C
±0.004×T between 125 °C and 350 °C
±1.0 between −40 °C and 133 °C
±0.0075×T between 133 °C and 350 °C
IEC Type T Thermocouple.svg
BS Type T Thermocouple.svg
MC 96.1 T Thermocouple Grade Color Code.svg
E0 to +800−40 to +900±1.5 between −40 °C and 375 °C
±0.004×T between 375 °C and 800 °C
±2.5 between −40 °C and 333 °C
±0.0075×T between 333 °C and 900 °C
IEC Type E Thermocouple.svg
BS Type E Thermocouple.svg
MC 96.1 E Thermocouple Grade Color Code.svg
Chromel/AuFe−272 to +300n/aReproducibility 0.2% of the voltage; each sensor needs individual calibration.
I also had a closer look at the sheaths that the wires for the thermocouple are insulated with. The one going to the +ve port had red lines and the one to the -ve port had what appeared to be black lines in it. The background colour is white.

Based on that table then and if we assume the black is actually a very dark blue then the thermocouple used is most likely going to have been a K type which appear to be fairly popular so it should be easy to find a new one.

With 41 µV/°C I can calculate the maximum voltage I'll need to be able to spoof the thermocouple up to 600°C. I picked this as it shouldn't even go close but in case it does I can still test it.

With 3.3V avaliable from an old ATX PSU and (with some help from here) a voltage of 25.703mV(measuring 600°C reference 20°C) accross the thermocouple:

Voltage drop across fog machine=0.025703V
Voltage drop across resistors=3.274297V
Current flow from fog machine=V/R
                                                 =0.025703/1000 000
                                                 =0.000000026A
Resistance required=V/I
                               =3.274297/0.000000026
                               =125934500Ω
                               =125934.500kΩ
                               =125.934500MΩ
Which is pretty close to 126MΩ

However when I run this through KTechLab (Better than a SPICE simulator for a quick calculation) 10MΩ seems to be enough... perhaps I neglected to calculate current flow through the resistance array? I'll re-post once I have more info...