Intro: (MIMS is the accepted acronym for Mineral Insulated Metal Sheathed as used to describe finished cable, formerly known as simply MI Cable with the sheath material assumed)

USES: Crushable insulators are used to produce Mineral Insulated Metal Sheathed (MIMS) cable (in long lengths) or probes (typically shorter complete units). The conductor wires may be thermocouple pairs, resistance wire for heating, RTD conductors, or simply extension wiring for extreme environments. Crushable insulators are not suited for stand-alone applications and must be enclosed in a protective sheath.

These insulators are designed to be inserted into the metal sheath with conductor elements already threaded in them. The assembly is then swaged or drawn to crush the insulator into a fine, flowable powder which will allow elongation of the cable and tightly compact to maintain separation and electrical insulation of the metallic components.

Description: Crushable insulators are rigid tubes, rods, and multi-hole configurations available at lengths up to about 200 mm

They may be designed to crush very easily to protect soft platinum components, or with more integrity to improve handling and performance properties. Please refer to the strength specifications.

Properly engineered insulators permit the fabrication of very fine cables in very long lengths resulting in tremendous value added for the producer. Very precise conductor placement is assured in comparison to powder fill methods.

Caution – It is important to protect MgO materials from humidity if the ceramic is exposed to the air. A pre-bakeout ritual is recommended even though the parts are shipped in sealed polyethylene packaging.

For some purposes, very soft and weak insulators should be specified, which of course makes fabrication and assembly more difficult.

TATEHO OZARK offers a variety of MgO choices as well as sister offerings in Al2O3 and HfO2. New materials under consideration are fused SiO2 and MgO Al2O3 Spinel.

The following are typical analyses or limits for powders used to produce extruded insulators. Most raw materials are high quality fused grain powders which are processed in-house to obtain optimum particle size distributions for end use as probes, cables, and heaters.

MgO (%)99.7099.4098.0.0003ND
Al2O3 (%)
SiO2 (%)<.02
Fe2O3 (%)<.02
CaO (%)
B (PPM)<10<10<10NDND
Cd (PPM)<10<10<30NDND
S (PPM)<10<10<10NDND
C (PPM)<50<50<50NDND

Insulators are fired to uniform mechanical strengths which have been established for optimum handling and swaging characteristics for different applications. A true mechanical test does not exist to characterize “crushability”. A Modulus of Rupture measurement is normally provided for historical tracking purposes. This test is a modified three point bending test performed according to ASTM standard E1652 Appendix 4 and has many limitations.

MOR range (psi)Apparent StrengthGradesRecommended Usesn
2000-4000softCSM, CCAPlatinum thermocouple, >.75" OD thermocouple
3000-6000medium soft to mediumCSM, CKMThermocouple
4000-8000mediumCKMThermocouple, coil heater on ID, Cold Core
5000-10000medium hardCKMCartridge Wind Core, Cold Core
7000-12000hardBKMSmall OD Cartridge Core, tubular core, spacer stock