Type-T T/C
The copper positive leg and constantan negative leg produce a thermoelectric voltage related to the temperature difference between the sensing junction and reference connections. Type T has its own nonlinear reference curve. A single microvolts-per-degree figure is only a local sensitivity, not a conversion constant for the whole range.
The copper and constantan leads join at the sensing point. A temperature difference between that point and the reference connections produces a small voltage. Measure the reference temperature, convert it to the equivalent thermocouple voltage, add the measured voltage, then use the correct type's conversion curve. Do not simply add temperatures as if the response were perfectly linear.
In plain terms
Think of it as a thermometer that compares two places. To find the sensing-point temperature, it also needs to know the temperature where its wires meet the measuring instrument.
Why designers use it
- For measurements designed around Type T materials and its documented conversion curve.
- To measure temperature with a small sensing junction and a suitable low-noise interface.
- To match an existing instrument's configured thermocouple type, provided the replacement probe meets the application requirements.
Best for
- Thermocouple measurement circuits
- Compatible temperature instrumentation
- Probe designs with documented operating limits
Key specifications
- Positive leg: copper
- Negative leg: constantan
- Local sensitivity at 0 °C: 38.74 µV/°C (TI Table 1-1; sensitivity changes with temperature.)
- Conversion: Type T reference curve (Cold-junction compensation is required.)
- Usable probe range: Check the exact probe specification (Construction, atmosphere and wire size matter.)
When not to use it
- Outside the exact probe's temperature, atmosphere or mechanical ratings.
- When the instrument is configured for a different thermocouple type.
- As evidence of food-contact, medical, aviation or other safety compliance without approval of the complete assembly.
Common mistakes
- Using the wrong type's reference curve or extension wiring.
- Ignoring cold-junction compensation.
- Treating ADC resolution as the total measurement accuracy.
- Using one constant sensitivity across the whole temperature range.
- Confusing a reference-curve range with the safe range of a particular probe.
Where you will find it
- In an illustrative bench circuit, a Type T probe connects to a suitable differential ADC and a reference-temperature sensor. The calculation uses the Type T curve and the measured reference temperature.
- The TI guide provides examples of biasing, open-sensor detection and cold-junction compensation. Choose the circuit according to the ADC and probe requirements.
A short history
This standard thermocouple type uses copper for the positive leg and constantan for the negative leg. Standard reference curves relate its voltage to temperature, but the sensor measures a temperature difference: cold-junction compensation is still needed. Probe construction, atmosphere and wire size determine the usable range of a real assembly.
Good to know
- Cold-junction compensation is a voltage-domain correction using the appropriate thermocouple curve, not simply adding two temperatures.
- Extra dissimilar-metal junctions can introduce errors if their temperatures do not cancel in the measurement path.