Using Thermocouple Probes in Extreme Conditions

In environments that experience extreme high temperatures, such as in solid waste incinerators, sintering powdered metals, or gas or oil fired-furnaces, your thermocouple probe needs to be able to stand the heat. Mineral insulated cable (MI cable) is the perfect choice due to its low flammability even when exposed to high temperatures. It is also resistant to oxidation, to ensure constant precise performance when measuring.

Selecting the right MI cable sensor that will maintain their accuracy and durability over time is vital when they play such a key role in high-temperature processes.  For example, it would be disastrous if the temperature controllers used in nuclear plants to monitor core temperatures were not receiving accurate readings, and accessing the cable for repair or replacement would be very difficult.

Solution

Testing MI Cables for Performance

In order for Omega to compare their own probes manufactured with MI cable against others on the market, they conducted a cycle test to determine their accuracy and durability. Twelve probes were tested in all, including four sample probes from two competing manufacturers and four probes from each of the three Omega sheath combinations. All probes had a diameter of 3.0 mm.

There were several parameters to the test, including one based on a real customer application of a thermocouple probe placed inside the exhaust system of a diesel engine with a turbocharger. This involved the rapid heating of each probe to 1100 °C within 15 seconds and immediately cooling the probe down to ambient temperature within 45 seconds.

Cycle Test Results

The results of the cycle test indicated that the Omega sheath combinations outperformed the competitor probes by a wide margin, based on the average of each combination. The highest performing of all the probes was the Omega™ 310/XL sheath; it lasted on average 8 times longer than either of the competitor’s probes and even around twice as long as other Omega combinations.

However, in most cases, the 15 second heat cycle had to be extended up to 18 seconds as the probes aged in order to reach the 1100 °C temperature limit. A possible explanation for this is the oxidation of the sheathing and conductor materials.

Figure 1

Figure 2 is an evaluation of the five poorest performing probes. It revealed that every one of the five had broken the negative thermos-element at approximately 2½” from the hot end tip. This is the same depth each probe saw when cycled into the induction coil.

Figure 2

An electro motive force test (EMF) of all probes after each 1000 cycles indicated that most probes had slight changes in their EMF outputs at the various temperature points tested over time. The only exception to this was one Omega™ 310 XL probe that lasted 15,900 cycles and experienced an extremely small amount of EMF shift for the duration of the test.

Figure 3

The final examination was long term static testing, as drift is a high concern in thermal monitoring. Figure 3 shows the probes tested at 2200 °F, and it can be seen that three of the Omega probes show no drift until 60-100 days. The probes produced by competitors begin to drift almost immediately.

Result

Conclusion

Although the lifespan of an individual probe cannot be guaranteed, the data from Omega’s own controlled testing environment suggests that the Omega 310/XL probes demonstrated superior performance over all other MI cable probes tested. As well as beating the competitor probes it also showed better performance than other probes in the Omegas range.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.