Mineral Insulated Cable 101


Mineral Insulated (M.I.) Cable or AerOpak® is a variety of thermocouple or conductor wire inside a metallic sheath, insulated by compacted ceramic insulation.  ARi’s mineral insulated cables starts from three (3) components:

Thermocouple Mineral Insulated Cable Wire Conductor-2

  1. Metal Tubing / Pipe – turns into the outer sheath.

    • Various alloys can be used
      • Different grades of stainless steels (304, 310, 316…)
      • High oxidation resistant alloys such as Inconel 600
      • Copper, Nickel
      • Refractory metals such as Tantalum, Platinum.
  2. Crushable Ceramic Insulators that becomes the compacted insulation and our heat conductor out to the outer metal sheath.  Cable designs can be simplex, duplex or complex depending upon the customer’s requirements.
  3. Electrical Conductors (Wires) – AerOcoax cable has one (1) wire going through the insulation.  AerObiax has two (2) wires evenly spaced and isolated from each other and the metal sheath.  ARi currently stock insulators that can carry up to 8-wires though the cable.

Mineral Insulated Cable Tubes-2These three (3) components become mineral insulated cable.  Sounds simple but it is a very long drawn out process as you will see.  The starting point is actually our finishing point.  Designing and Engineering of the finished product is the first step of production.  What is the final product we are making?  In electric heater cable we are looking for a certain Ohms/Ft on certain wire/wires.  In thermocouple cable production we need the right alloy wires to give us the right sensor signals.  Conductor cables are made for carrying signals out from sensors, and hazardous areas.  We also need a certain outer metal sheath material that has the properties that meets ARi’s or customer’s custom requirements.  On heaters with hot/cold lengths we start with wires that will give us the final Ohms/Ft on the heated section, and we also need to look at heated length, and the proper number of wires to make the design.  Some customer’s want extra heavy sheaths to protect the process critical signals carried inside of the MI cable.  The combinations are almost limitless.


When a Work Order is created;
o All specified materials to be use are pulled out from inventory.
o For later traceability each material (Tube, Insulator, Wire) certification number are recorded against a job number which is assigned on every work order created.
o Hand clean tube from any remaining lubricant.
o Inside of the tubes are scrub with alcohol to remove traces of dirt or oil.
o Insulation pellets are carefully strung on the conductors.
o The whole assembly is push into the tubes.
o In the work environment all workers wear gloves.
o Stainless steel table tops are used to make sure all materials stay very clean.
o This will ensure the finished cable performance is not adversely affected by any contamination or foreign object damage (FOD).
o Wow that was easy, right? Not really. We still have air gaps and things sliding-floating around inside of our tubing/pipe. What happens next is where the magic comes in.
o The whole assembly must have a point put on it so the end will fit through the drawing die and can be grip by the draw bench carriage. This step is called “Swaging”.
o After swaging it is then pulled thru the Drawing Die, the diameter gets smaller at the same time the whole assembly getting longer.
o Drawing Die is like a metal donut with the hole bigger on the front side than the back side. It is made of very hard metal, Tungsten Carbide, but do wear out. All dies are cleaned and check after each use to ensure that it remains within the precise tolerance band.
o For each draw process everything gets smaller and the looseness of the internal structure goes away as we get compacted tighter and tighter. The drawing process has another effect on the assembly that’s not exactly the properties we want on our final cable.
o After each draw the metal turns hard and brittle, it can break or split if not heat treated.
o Heat treating the metal or Annealing is the process of conditioning metal to make it soft enough to cold work.
o Annealing the cable involves heating it up to a very high temperature near its melting point for a certain time period and cooling it quickly.Mineral Insulated Cable Insulators
o Time and temperature are the key factors in proper annealing and both must be carefully controlled or the metal will stay brittle and causing cracks / splits / breaks during the next draw.
o Inert gas such as Hydrogen is used as a cover gas to prevent oxidation of the outer sheath and inside conductors.
o The cable comes out of the annealing process and is again able to be processed further.
o Mineral insulated cable production is a repeating sequence of steps (draws & anneal operations) which brings the cable to the final size.


It sounds really easy only in words. The process has been kept to the bare minimum here to offer a glimpse into Mineral Insulated cable production. The demands on our MI cable products have been developed since 1952 by ARI to give our customers some of the highest quality MI cable produced in the world today. The simple to the complex is shipped to our and our customers demanding specifications. MI cable is used around the world in some of the harshest industrial/military/scientific processes you could ever imagine. Let ARI’s experience help fast track your needs for tomorrow’s shared challenges.