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Autologic Controls is a dedicated Mineral Insulated (MI) Thermocouple manufacturer based in Ahmedabad, India, supplying a range of 10 MI thermocouple variants built for demanding industrial temperature measurement applications. Our MI thermocouples are available in all standard types — K, J, T, N — with sheath diameters from 1mm to 8mm, sheath materials including SS316 and Inconel 600, and customizable lengths to suit your exact process requirement.
Whether you need a standard head-type MI thermocouple for a process vessel, a 1mm hot runner thermocouple for an injection moulding machine, an L-type assembly with a process connection for a pipe installation, or a fully customised design to your drawing, we manufacture it at our Ahmedabad facility and supply across India with fast lead times.
What Is a Mineral Insulated (MI) Thermocouple?
A Mineral Insulated Thermocouple, also called an MI thermocouple or MgO thermocouple, is a temperature sensor in which one or more thermocouple wires are tightly packed inside a metal sheath using magnesium oxide (MgO) powder as the insulating material. The entire assembly of wires, insulation, and sheath is drawn down together through a die-drawing process, producing a compact, flexible, and highly durable sensor cable.
The MgO insulation serves two critical functions: it electrically isolates the thermocouple wires from each other and from the sheath, and it provides excellent thermal conductivity, allowing the sensing junction to respond quickly and accurately to temperature changes in the surrounding process. The outer metal sheath protects the entire assembly from moisture, pressure, vibration, and chemical contamination.
The result is a thermocouple that is smaller in diameter, more flexible, faster in response, and far more rugged than conventional bare-wire or ceramic-sheathed assemblies, making MI thermocouples the preferred choice wherever the process environment is harsh, space is limited, or fast response and long sensor life are required.
Key Advantages of Mineral Insulated Thermocouples Over Conventional Types
| Feature | MI Thermocouple | Conventional (Ceramic/Head Type) |
|---|---|---|
| Flexibility | Fully bendable — can be routed through tight spaces | Rigid — fixed straight configuration |
| Response Time | Very fast (small diameter, direct thermal contact) | Slower (air gaps in ceramic tube) |
| Vibration Resistance | Excellent — MgO is tightly compacted, no movement | Poor — ceramic tubes can crack under vibration |
| Minimum Sheath OD | From 0.5mm (MI cables) to 1mm (assembled) | Typically 6mm and above |
| Moisture Resistance | Fully sealed metal sheath — IP65 or better | Prone to moisture ingress without extra protection |
| Length Availability | Long continuous lengths (metres) | Limited by the sheath rigidity |
| Pressure Rating | Can be rated for high-pressure applications | Not pressure-rated without additional fittings |
| Temperature Range | Up to 1260°C (K type) — type dependent | Up to 1750°C (ceramic, for R/S/B types) |
Mineral Insulated Thermocouple – Technical Specifications
| Parameter | Available Options |
|---|---|
| Thermocouple Type | K (Chromel/Alumel), J (Iron/Constantan), T (Copper/Constantan), N (Nicrosil/Nisil) |
| Temperature Range | -200°C to 1260°C (type and sheath material dependent) |
| Sheath OD | 0.5mm, 1mm, 1.5mm, 2mm, 3mm, 4.5mm, 6mm, 8mm (custom on request) |
| Sheath Material | SS304, SS316, SS316L, Inconel 600 (application-specific) |
| Insulation Material | Magnesium Oxide (MgO) — high-purity, compacted |
| Junction Type | Grounded (fast response), Ungrounded/Isolated (EMI protection), Exposed (fastest response, unprotected) |
| Element Configuration | Simplex (single element), Duplex (dual element / redundant) |
| Length | Standard lengths or custom — from 50mm to 50+ metres |
| Process Connection | NPT/BSP threads, compression fittings, flanged, bayonet, L-type, spring-loaded |
| Termination Style | Bare leads, PVC/Teflon extension cable, head-type terminal block, connector |
| Standards | IEC 60584, IS 2486 / ANSI MC96.1 |
| Customization | Full — sheath dia, length, junction, connection, element count, sheath material |
Specifications shown are indicative. Final design is confirmed against your application datasheet.
Our MI Thermocouple Product Variants – What We Manufacture
Our mineral-insulated thermocouple range covers 10 specific designs, each engineered for a distinct application scenario:
1. Bare Lead MI Thermocouple
A mineral-insulated thermocouple cable terminated with bare wire leads — no process connection or termination head. This is the base-build format used as raw cable in OEM assemblies, custom panel wiring, or wherever the installer fits their own termination. Available in all standard types, sheath diameters, and lengths.
2. Head Type MI Thermocouple
The standard process-ready MI thermocouple with a terminal head (connection head) at the cold end. The head houses terminal blocks for wiring and protects the connection from ambient conditions. Available in aluminium, cast iron, or stainless steel heads — suitable for general process temperature measurement in industrial plants, heat exchangers, and vessels.
3. MI Thermocouple in Bayonet Fitting
A bayonet-style process connection allows quick plug-in installation and removal without threading — widely used in the plastics industry for hot plate, die, and heated runner applications. Our customised MI thermocouple in bayonet fitting is available in 1mm, 1.5mm, 3mm, and 6mm sheath OD to suit different machine models and nozzle sizes.
4. Hot Runner MI Thermocouple (1mm and 1.5mm)
Specifically designed for injection moulding hot runner systems — the most demanding thermocouple application in the plastics industry. The 1mm and 1.5mm sheath diameters are required to fit the tight thermocouple pockets in hot runner manifolds and nozzles. Our hot runner MI thermocouples use a K-type element, grounded junction for fast response, and are available with standard bayonet, J-type, or custom terminations matching OEM hot runner system specifications.
5. Insert MI Thermocouple for Thermowell
A thermowell-insert MI thermocouple is designed to be installed inside a thermowell, protecting both the thermocouple and allowing it to be replaced without opening the process. The MI thermocouple insert fits snugly inside a flanged or threaded thermowell available in lengths to match standard thermowell bore dimensions. The spring-loaded design ensures constant contact with the thermowell tip for optimal heat transfer and accurate temperature reading.
6. L-Type MI Thermocouple with Process Connection
The L-type (right-angle) MI thermocouple is used where the sensing element must be inserted perpendicular or at an angle to the process — common in pipe installations, duct mounting, and tank side-entry configurations. The flexible MI cable is bent to a 90° angle with a rigid process connection fitting. Available with NPT or BSP threaded fittings, compression fittings, or flange-mounted designs.
All 10 variants are listed in the product grid above. Contact us with your application details for a customised configuration not listed.
Sheath Material Selection Guide – Which One Is Right for Your Application?
| Sheath Material | Suitable Up To | Recommended Application |
|---|---|---|
| SS304 | 900°C | General industrial use, mild environments, non-corrosive fluids and gases |
| SS316 / SS316L | 900°C | Chemical, pharmaceutical, food processing — superior corrosion resistance, including chloride environments |
| Inconel 600 | 1260°C (K type) | High-temperature, oxidising atmosphere furnaces, heat treatment, petrochemical reactors |
| Copper | 400°C | Cryogenic and low-temperature laboratory applications (T-type) |
Which Junction Type Should You Choose?
| Junction Type | Response Speed | When to Use |
|---|---|---|
| Grounded | Fast | Standard industrial use — junction welded to sheath tip for good thermal transfer. Best for most applications. |
| Ungrounded (Isolated) | Moderate | Use when measuring across multiple points on a single instrument, or in environments with EMI/ground loops (motors, drives, inverters nearby). |
| Exposed | Fastest | Use in clean gas streams with no pressure and no corrosion risk. The bare junction directly contacts the medium — very fast response, zero protection. |
Industries and Applications for MI Thermocouples
- Plastics and Rubber Processing: Hot runners, extruder barrels, injection moulding machines, blow moulding, pipe extrusion — where 1mm and 1.5mm bayonet-type MI thermocouples are the industry standard.
- Chemical and Petrochemical Plants: Reactor vessels, distillation columns, heat exchangers — SS316 or Inconel sheath provides corrosion resistance and pressure compatibility.
- Oil and Gas: Subsurface and surface process piping temperature measurement — Inconel or SS316 MI cables with appropriate explosion-proof termination heads.
- Power Plants and Boilers: Steam line temperature, economizer, superheater, and bearing temperature measurement — vibration-resistant MI construction handles these demanding environments.
- Pharmaceutical Manufacturing: Vessel jacket temperature, autoclave, sterilizer, and clean-room HVAC measurement — SS316L for compliance with hygiene standards.
- Food and Beverage: CIP (Clean-in-Place) lines, pasteurizers, storage vessels — hygienic surface finish SS316 MI probes with sanitary fittings.
- HVAC and Building Automation: Duct air temperature, chiller measurement, fan coil units — compact 6mm OD K-type MI assemblies.
- Automotive and Motorsport Testing: Exhaust gas temperature (EGT), turbocharger outlet, engine bay — fast-response 1mm grounded junction MI thermocouples.
- Metal and Heat Treatment Furnaces: Annealing, case hardening, carburizing, nitriding — Inconel 600 sheath K or N type for high-temperature oxidizing environments.
- Research and Laboratories: Calibration baths, environmental chambers, test rigs — precision duplex MI thermocouples for redundant measurement.
Frequently Asked Questions
What is a Mineral Insulated (MI) Thermocouple?
A Mineral Insulated thermocouple is a temperature sensor where thermocouple wires are packed inside a metal sheath using compacted magnesium oxide (MgO) powder as insulation. The MgO electrically isolates the wires while conducting heat efficiently to the sensing junction. The sealed metal sheath protects the assembly from moisture, pressure, vibration, and chemical contamination — making MI thermocouples smaller, faster, and far more rugged than conventional ceramic-tube thermocouple assemblies.
What is the temperature range of a Mineral Insulated Thermocouple?
The temperature range depends on the thermocouple type and sheath material. A K-type MI thermocouple with an Inconel 600 sheath measures from -200°C to 1260°C. A J-type with SS316 sheath covers -40°C to 750°C. A T-type covers -200°C to 350°C for cryogenic use. For applications above 1260°C, ceramic-sheath R, S, or B type thermocouples are recommended instead.
What is the difference between a grounded and ungrounded MI thermocouple junction?
In a grounded junction, the sensing wires are welded directly to the sheath tip giving the fastest thermal response but electrically connecting the measuring circuit to the sheath. In an ungrounded (isolated) junction, the wires are insulated from the sheath by MgO response is slightly slower but electrical interference and ground loops are prevented. Grounded junctions suit most industrial applications; ungrounded junctions are specified in electrically noisy environments near VFDs or large motors, or when multiple sensors connect to a single controller.
What is a hot runner MI thermocouple and why must it be 1mm or 1.5mm?
A hot runner thermocouple is a mineral insulated thermocouple designed for injection moulding hot runner systems. Hot runner manifold and nozzle manufacturers precision-machine thermocouple pockets to accept either 1mm or 1.5mm OD cartridges exactly a larger diameter will not fit, and a smaller one gives poor thermal contact and inaccurate readings. These thermocouples use a grounded junction for the fast response required by hot runner temperature controllers, which cycle heating rapidly to maintain tight temperature bands.
Can MI thermocouples be bent or shaped for non-standard installations?
Yes this is one of the key advantages of MI thermocouple construction. Because the MgO insulation is a compacted powder inside a flexible metal sheath, the cable can be bent to a minimum radius of approximately 3 times the sheath OD without damaging the sensor. A 6mm OD MI thermocouple, for example, can be bent to an 18mm radius. MI thermocouples can be pre-formed into L-shapes, U-bends, or custom configurations before installation complex shapes should be specified at the time of order.