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Military specification (mil spec) resistors have been used in nearly every well logging tool ever built, and in a good deal of surface electronics as well.  This is true because these high grade resistors are capable of performing dependably at the high temperatures encountered in deep well logging applications, and because they look cool.

There are dozens of mil spec resistor styles, and dozens of applicable mil spec or mil performance spec documents (if you cannot find a spec in the format MIL-R-xxxx, try the newer MIL-PRF-xxxx).  Check out the Defense Supply Center, Columbus (DSCC) for more information on military specifications and standards than you ever dreamed existed.  The better mil spec grades have "Established Reliability", with the best having a known failure rate not exceeding .001% per 1000 hours.  Many are also characterized for operation to 175ºC, with the power derrated by half at 125ºC from the 70ºC values and further derating to 175ºC (but see the power rating comments below).

Mil spec resistors are typically physically larger than their industrial or commercial counterparts for a given power rating.  This invariably confuses users not familiar with the mil spec power rating convention.  In RN, RNC, and other mil spec resistors sharing the sizing number code, a 55 size resistor is rated at 1/8 watt at 70ºC, but an industrial 55 size resistor would be rated at 1/4 watt up to 125ºC.  Likewise, a mil spec 60 size resistor would be rated at 1/4 watt at 70ºC, but an industrial 60 size resistor would be rated at 1/2 watt up to 125ºC.  The mil spec 55 size resistors are further derated to 1/10 watt at 125ºC, and the mil spec 60 size resistors derated to 1/8 watt at 125ºC (no such derating is specified for the industrial version of these resistors such as the Vishay Dale CMF industrial line discussed below, but of course both the mil spec and industrial versions are subject to (additional) derating above 125ºC).  This very conservative power rating of mil spec resistors is presumably to insure reliability and long life.  For the sake of convenience, and our sanity, we stock resistors here at AnaLog by their industrial equivalent size; hence, RN55 1/8 watt and industrial 1/4 watt resistors are both stocked in our "1/4 Watt Cabinet".

The following tables summarize information about mil spec resistors commonly encountered in well logging electronics:

While the RN mil spec is no longer active for new military designs, these resistors are still manufactured and readily available.  They continue to be used in many downhole applications.  Vishay Dale markets these resistors as military qualified (see their CMF Military RN / RL data sheet), but they market the exact same resistors as their CMF Industrial line (see their CMF Industrial data sheet).  If one shops around, it is possible to purchase even small quantities of these excellent industrial equivalents in their 50 ppm / 1 % version at about the same price as premium color coded 1% commercial resistors (color banded resistors make downhole tools look like cheap consumer grade electronics to some critics, and their published tempco specs may be questionable).

The "Characteristic" column in the above tables gives the temperature coefficient (tempco) specifications available for each of the mil spec resistor styles covered.  In general, the temperature coefficient is the relative change of a physical property when the temperature is changed by 1º C or K.  Tempco values are simply an expression of how stable a resistor ohmic value will be with changing temperature.  Temperature coefficient is especially important in downhole oil well logging tools where a tool may have to operate over a very wide temperature range (cold outdoor temperatures up to say 200º C / ~400º F).  As a simple example, a one (1) kilohm (1,000W) resistor with a 100 ppm tempco can be expected to change ten ohms (10W) in value over a 100º C / 180º F temperature excursion or one percent (1%).  Mil spec resistors are often marked with a "T" code to indicate the tempco rating; here is a list of common codes encountered and the ppm values associated with them: