This is actually not such a simple question as it seems! The reason being that even those thermocouples designed for extremely high temperatures have a maximum range that can be significantly affected by factors such as the;
- type and state of the product (liquid, solid, gas)
- environment (reducing/oxidizing/vacuum)
- type of thermocouple alloy and possible sheath material used
- diameter of the thermocouple wire and whether it is exposed or sheathed
- accuracy required
- variations in working temperature
However, generally speaking, refractory metal tungsten-rhenium wire Type C and Type D are considered the highest temperature wires, capable of being used for temperature measurement up to 2300ºC, provided it is not an oxidizing environment.
In an oxidizing environment the noble metal platinum-rhodium thermocouple wire Type R (Pt13%Rh-Pt), Type S (Pt10%Rh-Pt) and Type B (Pt30%Rh-Pt6%Rh) are considered the highest temperature TC wires with a maximum range varying from 1400ºC up to 1700ºC.
You can read about complete high temperature thermocouples on our product page and in our brochure which you can download on the product page.
What is the difference between these wire types?
Kamet provides various high temperature thermocouple wire types: S, R, B, C and D. Each thermocouple type has its advantage and can be completely customized to your requirements.
| Thermocouple wire | Type | Symbol | Standard limits of error* | Recommended temperature range | |
|---|---|---|---|---|---|
| + | – | Per ASTM E 230 | |||
| Pt10%Rh | Pt | S | S | ± 1.5°C or 0.25% per ASTM E 230 | 0-1450°C |
| Pt13%Rh | Pt | R | R | ± 1.5°C or 0.25% per ASTM E 230 | 0-1450°C |
| Pt30%Rh | Pt6%Rh | B | B | ± 0.5% per ASTM E 230 | 870-1700°C |
| W5%Re | W26%Re | C | AE | ± 4.4°C (0-426°C) ± 1% (426-2315°C per ASTM E 988) | 0-2200°C |
| W3%Re | W25%Re | D | AO | ± 4.4°C (0-426°C) ± 1% (426-2315°C per ASTM E 988) | 0-2200°C |
What is difference between Type S and type R TC wires?
Type S and Type R thermocouples wires are technically very similar. The only difference is that the positive leg of a Type R consists of 87%Pt, 13%Rh and a Type S of 90%Pt, 10%Rh.
The main difference between type S and R is the higher percentage of Rhodium in the type R. This makes this thermocouple cable or wire more expensive yet gives a (small) higher output and is overall a bit more stable than the type S. Type S sensitivity is between 6 and 12 µV/°C. Type R has a sensitivity between 6 and 14µV/°C.
Both type R and type S thermocouples wires can cover a temperature range of 0°C to 1450°C with uncertainties as good as 0.15°C over most of the range.
What is the difference between Type C and Type D TC wires?
The positive leg of a Type C thermocouple consists of 95% Tungsten, 5% Rhenium while the positive leg of a Type D wire consists of 97% Tungsten, 3% Rhenium. The slight difference in output is that the average EMF of Type D is marginally higher than that of Type C.
It is better to not use Type D thermocouples in temperatures below 500° C. More about Type C and D thermocouple wire.
Mechanical properties and physical properties
| Tungsten | W3Re | W5Re | W25/26 Re | |
|---|---|---|---|---|
| Tensile Strength, ann. (x 103 psi) 8°F 1832°F 3632°F | 80 35 15 | 172 60 10 | 220 65 26 | 200 95 24 |
| Elongation (% in 10”) 68°F 1832°F 3632°F | 0 10 23 | 15 18 23 | 20 24 24 | 11 19 27 |
| Resistivity (W · circ. mil / ft.) 68°F 1832°F 3632°F | 33 199 398 | 57 228 420 | 70 235 434 | 170 331 524 |
| Therm. Exp. Coef. (in./in./°F) 68°F 1832°F 3632°F | 1.7×10-6 2.3×10-6 3.1×10-6 | 2.9 x 10-6 3.1 x 10-6 3.9 x 10-6 | ||
| Density g./cm3 lb./in3 | 19.3 0.697 | 19.4 0.700 | 19.4 0.701 | 19.7 0.714 |
| Melting Point °C °F | 3410 6170 | 3325 6017 | 3350 6062 | 3120 5648 |
Go to the product page about high temperature thermocouples for a complete thermocouple sensor configuration.