A duplex sensor houses two independent sensing elements — thermocouple junctions or Pt100 elements — within a single sheath, providing a primary measurement element and a backup in the same probe. When the primary fails, operations switches to the backup without a process shutdown or maintenance intervention to change the probe.
| Configuration | Element 1 | Element 2 | Primary Use | Typical OD |
|---|---|---|---|---|
| TC/TC Duplex | Thermocouple (measuring) | Thermocouple (redundant) | High-criticality process; auto-switch on deviation | 3/16" to 1/2" |
| RTD/RTD Duplex | Pt100 Class A (measuring) | Pt100 Class B (backup) | Control loop + independent safety trip | 1/4" to 1/2" |
| TC/RTD Combo | Pt100 RTD (accurate control) | Thermocouple (OT protection) | Control RTD + high-temp overtemp TC in one probe | 3/8" to 1/2" |
| Noble Metal Duplex | Type S (primary calibration) | Type S (backup) | Furnace control; one element fails without losing control | Ceramic tube |
| Parameter | TC/TC Duplex | RTD/RTD Duplex |
|---|---|---|
| Element 1 Type | K, J, N, R, S (specify) | Pt100 IEC 60751 Class A |
| Element 2 Type | Same as Element 1 (std) | Pt100 IEC 60751 Class B |
| Element Isolation | Electrically isolated (separate conductors) | Electrically isolated — separate MgO columns |
| Sheath OD | 3/16" min; 1/4" typical | 1/4" min; 3/8" typical |
| Sheath Material | 316SS or Inconel 600/625 | 316SS or Inconel 600 |
| Process Connection | NPT, flanged, compression | NPT, flanged, compression |
| Head Type | Dual-compartment or large-bore Form B | Dual-compartment or DIN-rail head |
| Wiring | 4 conductors (2 per element) | 6 wires (3 per element) standard |
| Calibration | Both elements calibrated; matched pairs available | Both elements calibrated and serialised |
In a refinery, power plant, or critical manufacturing process, a failed temperature sensor on a key control loop can force an unplanned shutdown that costs tens of thousands of dollars per hour in lost production. A duplex assembly gives the control system an immediate backup without requiring any physical intervention — the backup element is already installed, calibrated, and wired to the system. Switching is either automatic (on deviation alarm) or manual at the DCS operator station.
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