Moore Industries has been continuously running its NET Concentrator System (NCS) data acquisition system at temperatures hovering around the boiling point of water for more than three years in an environmental chamber.
“The only problem we’ve found so far is the labels are turning brown from the continuous, intense heat,” reports Fred Adt, director of quality assurance at Moore Industries. “We’ve taken care of that problem by using more heat-resistant labels in new products.”
The NCS is equipped with several modules, including an EIM (Ethernet Interface Module), CPM (Concentrator Power Module), AIM (Analog Input Module), TIM (Temperature Input Module), DIM (Discrete Input Module), and ROM (Relay Output Module), all of which have also been in the environmental chamber for three years. Except for their labels turning brown, no problems have occurred.
Adt explains that conventional testing and life-test procedures are not enough when products are intended for use in severe operating conditions, such as in the Middle East. The NCS is intended for use as a remote control and data acquisition system in unheated and non-air conditioned enclosures, so it has to endure wide temperature extremes.
“The MTBF (Mean Time Between Failures) for the NCS, determined in accordance to the Bell Core reliability model, exceeds 300,000 hours @25°C,” explains Adt. But this model isn’t good enough for Moore Industries. Adt wants to find out how long the NCS will last under severe ambient conditions.
So Adt is using the Highly Accelerated Life Test (HALT) concept and uses the accumulated on-time to demonstrate MTBF at increased environmental stress from elevated temperatures.
Running at 90°C provides a factor of 6.5; that is, every hour running at 194°F (90°C) is equal to running 6.5 hours at normal ambient temperatures. Fortunately, meeting the test specs was not difficult. Moore Industries’ products are designed for operation at ambient temperatures of -40oC to +85oC, so the test is just barely above its maximum design temperature.
“HALT lets us demonstrate performance of the NCS outside its specified environmental operational envelope,” explains Adt.
“The idea is that any failures induced that way might be consequently eliminated by design, thus creating a product with more margin to failure. Therefore, we have been running a continuous HALT on the NCS since February 2005, at temperatures from 90 to 105°C (194 to 221°F), with open air applications in the Middle East in mind.” So far, it hasn’t failed.
The NCS reached a HALT-demonstrated MTBF of 171,000 hours in February 2008, and remains in continuous test. The HALT accumulates about 57,000 hours per year, so the test has just over two years to go before reaching 300,000 hours. “We intend to reach 300,000 hours, and maybe beyond,” notes Adt.
“We knew the NCS could take it,” says Adt, “but we were starting to worry about the environmental chamber. This test proves that both the NCS and the environmental chamber are very tough products.”
The specifications for the NCS HALT are:
- Ambient temperature: 90oC (194°F)
- Test commenced: February 8, 2005
- Models installed: EIM, CPM, AIM, TIM, DIM, ROM
- Operation mode: Temperature inputs are monitored via the EIM’s Ethernet communications port, and data is logged on a daily basis
- MTBF status: 1100 days times 24 hours/day = 26,400 hours
According to the Arrhenius equation, the acceleration factor for 90oC is 6.5. This means a demonstrated MTBF of 171,600 hours has accumulated as of February 13, 2008.