I’m sure a newbie question… Wherever I see the table of environmental conversion factors, I also see something like “It is not valid to apply to these conversions to Mean Time Between Critical Failures (MTBCF)…”
So, it is acceptable to apply to MTBF, but not MTBCF. Why the difference? Because of the outside influences in determining critical failures?
The MTBF environment conversion factors presented in MIL-HDBK-338B (and various versions of the System Reliability Toolkit) are derived from reliability predictions that were performed on a variety of electronic military systems. These reliability predictions were performed for each military system at various environments using the failure rate models described in MIL-HDBK-217. Ratios for the MTBF conversion from one environment to another were then developed from the prediction results and follow directly from the MIL-HDBK-217 PI E factors for the different component types that comprise the systems. The total quantity of components included in these predictions was approximately 18,000, and the relative percentages of each part type in the database were as follows: transistors-2%, capacitors-28%, resistors-27%, integrated circuits-18%, inductors-17%, diodes-5%, miscellaneous-3% (reference the pie-chart in Figure 10.3-1 of MIL-HDBK-338B). These percentages are important because each component type has different PI E ratios between environments.
On the other hand, failure rates presented in NPRD are not based upon MIL-HDBK-217 reliability predictions. Instead, they are based upon field data and are mostly for mechanical equipment. Since the MIL-HDBK-338B conversion table is not based upon field data and Figure 10.3-1 does not contain mechanical equipment, it would be inappropriate to use this table to adjust the MTBF data from any version of NPRD. To my knowledge, there are no such conversion factors available.
Finally, the environmental conversion factors are not applicable to MTBCF since the component percentages pertaining to critical failures may be quite different from the percentages for the system as a whole, and the conversion factors were based upon system level MTBF data.