01 July 2003
Harsh turbine engine test
An accelerated comparison stress test of sensors made with the new mineral insulation material and those made with MgO took place at 1200°C.
The test was conducted using a Lindberg box furnace with an upper temperature limit of 1371°C. A 22-inch long ceramic tube was inserted through a hole in the furnace door, with the tip of the tube located 5 inches from the back interior wall of the furnace.
Lindberg box furnace
Sensors were bundled together and wrapped with platinum wire to keep them in thermal equilibrium. The sensor bundle inserted into the ceramic tube to a point where the tip of the bundle was one-half inch from the end of the ceramic tube.
The ceramic tube was held in place with two ring stands and appropriate clamps to keep the ceramic tube at a fixed location during the test.
Five type-K sensors were bundled together with a type-S working reference standard and held in a constant temperature zone of the furnace at 1200°C until all had failed.
Analysts registered sensor outputs daily and compared them to the type-S working reference standard. The working standard had previously been calibrated versus a NIST-traceable primary reference type-S sensor.
The test samples were type-K sensors in Inconel 600 sheaths. Three well-known companies manufactured the sensors using MgO mineral insulation and sold them as within special limits of error tolerances.
Two were manufactured using MI-Dry as the mineral insulation and were as close in design dimensions to the three purchased sensors as possible. Special limit wires were on board the MI-Dry sensors.
The 1200°C test temperature is near the upper limits of use for the materials in a type-K sensor, but below the high-end temperature of Inconel 600 so that this did not become a sheath test.
The test protocol defined that any sensor with two consecutive daily readings outside standard limits of error would be unacceptable for industrial use and therefore disqualified. These sensors were manufactured to perform within special limits of error.
The mean-time-to-failure of the two dry sensors was four times as long as the three MgO sensors. In addition they were still indicating within special limits of error at the time of their failure. The new technology insulating material demonstrated superior performance to MgO.
Separately, a major turbine engine manufacturer has also performed cycling tests of the dry sensors. The tests simulated accelerated turbine engine operating condition temperatures and cycles.
The test covered 18,181 cycles between 300°F and 2150°F over 2,597 actual hours that simulated about 150% of the normal engine cycles and 500% of the time at maximum temperature.
There were at least 10 other type-K MIMS sensors in the test. At 18,181 cycles the test ended. All the sensors had failed except the dry sensor.