NEPP - Thermistors

For use in extended space flights, NASA Goddard Space Flight Center qualified epoxy-coated, ceramic bead thermistors to GSFC S-311-P-18 back in 1974. The manufacturer retains samples from each production run and requalifies to the specification every year. This specification also includes screening to ensure these thermistors will perform in the rigorous requirements of space flight applications without premature failures. Parts manufactured to this specification are NTC (negative temperature coefficient) thermistors, meaning that the resistance decreases with an increase in temperature, either has a result of self-heating or an external source. Each thermistor procured to this specification comes with a calibration curve showing the expected resistance values at various temperature points across the specified temperature range. These thermistors are primarily used for temperature measurements.

For PTC (positive temperature coefficient) thermistors, or other NTC types, for use in temperature compensation or control applications, as well as for temperature measurements, Goddard recommends thermistors procured to MIL-PRF-23648. There is considerable heritage showing that parts manufactured to this specification are quite suitable for space flight.

Major Issues

Thermistors have decided advantages over commercial resistance temperature detectors, such as a PRTs (platinum resistance thermometers). Thermistors have faster response times and more sensitivity, making them better suited for precision temperature measurements. Thermistors are smaller and more rugged. Thermistors are available with high resistance values, which allows them to be used with long extension leads with little loss of accuracy since lead resistance effects are minimized. However, due to the nonlinear nature of thermistors, the usable temperature range is limited compared to PRTs. Interchangeability is also limited unless specially manufactured. Thermistors manufactured to GSFC S-311-P-18 are available with either a ± 0.2°C or ± 0.1°C interchangeability tolerance.

In addition to bead type thermistors, metallized surface contact types are also available. These may be in the form of chips, disks, wafers, washer or rods. These types, with the exception of the emerging SMT thermistors, are generally not suitable for high reliability applications requiring precision temperature measurements. The GSFC Parts, Packaging, and Assembly Technologies Office (Code 562) has prepared test plans for the qualification and screening of SMT thermistors for space flight applications. In time, as more lots are processed and the test methodologies refined, a companion specification to GSFC S-311-P-18 will be issued for surface mounted thermistors.