In an extreme heat environment, such as during the sintering of powdered metals, in solid waste incinerators, or in oil or gas fired-furnaces, thermocouple probes need to be able to withstand extreme heat. Because of its low flammability, even when exposed to high temperatures, mineral insulated cable (MI cable) is the ideal choice. It is also oxidation resistant, enabling precise measurement.
As MI cable is typically used in high-temperature environments during critical processes the selection of MI cable sensors that will maintain their durability and accuracy over time is an important consideration. As an example, it would be extremely dangerous if the temperature controllers used to monitor core temperatures in nuclear plants were not able to take accurate readings, and it would be difficult to replace or repair cables in that environment.
Solution
Testing MI Cables for Performance
In order to determine the performance of different thermocouple probes manufactured using MI cable, a cycle test was conducted, comparing the durability and accuracy of a range of competing brands. The test measured the performance of twelve probes, including four probes from three Omega sheath combinations, as well as four sample probes from two competing manufacturers. All probes tested had a diameter of 3.0 mm.
During the test, the temperature of each probe was ramped up to 1100 °C over 15 seconds, followed by immediately cooling the probe to ambient temperature within 45 seconds. The parameters used during this test were based on a proposed real application of a thermocouple probe, in which it would be placed within the exhaust system of a diesel engine which included a turbocharger.
Cycle Test Results
Reviewing the test data, it was shown that the Omega sheath combinations significantly outperformed the competitor probes, based on the average of each combination. The highest performing probe was the Omega™ 310/XL sheath which lasted on average 8 times longer than the probes from either of the other manufacturers. It was also seen to last approximately twice as long as the other Omega combinations tested.
In most cases, however, the initial 15-second heat cycle had to be increased to 18 seconds as the probes aged in order for the 1100 °C temperature limit to be reached. This could have been caused by the oxidation of the sheathing and conductor materials.
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Figure 1
Electro motive force tests (EMF) of all probes were carried out after every 1000 cycles. The results of these tests showed that slight changes occurred in the EMF output of most probes over the range of temperature points tested over time. The one exception to this was one Omega™ 310 XL probe, which lasted 15,900 cycles while displaying very little EMF shift over the entire test.
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Figure 2
Drift is a high concern in thermal monitoring, and in order to address this, a long term static test of the probes at temperatures of 2200 °F was carried out. The results of this are shown in Figure 3, in which it can be seen that three of the Omega probes show no drift until 60-100 days. Probes from other manufacturers were seen to begin to drift almost immediately.
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Figure 3
Conclusion
While the lifespan of any individual thermocouple probe cannot be guaranteed data from the controlled testing environment used in this study indicates that the Omega 310/XL probes demonstrate superior performance when compared to other MI cable probes tested, including other Omega probes as well as probes from other manufacturers.
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.