Description

The FOXBORO FBM42 (part number 3A99242G01) is a 16-channel thermocouple input module designed for the Foxboro I/A Series® distributed control system (DCS). It provides high-precision temperature measurement by directly interfacing with industry-standard thermocouples (Types J, K, T, E, R, S, and B), making it ideal for applications such as boiler tube monitoring, turbine exhaust, reactor vessels, and heat exchanger control.

Each channel features built-in cold junction compensation (CJC) using a high-stability RTD sensor at the terminal block, ensuring accurate conversion of millivolt signals to engineering units (°C/°F). The module supports galvanic isolation, open-circuit detection, and software-configurable filtering—delivering reliability in electrically noisy industrial environments.

Application Scenarios

At a 600 MW coal-fired power plant in the Midwest, repeated false high-exhaust-temperature alarms on a steam turbine led to unnecessary load reductions. Investigation revealed that ground loops between thermocouples and the legacy DCS input cards were injecting offset voltages into the mV signals. The plant replaced all exhaust TC modules with FOXBORO FBM42 units installed in grounded FBM chassis with shielded twisted-pair cabling. The FBM42’s channel-to-channel isolation and differential input design eliminated ground-loop errors. Additionally, its open-circuit diagnostic flagged a failing Type K probe during routine operation—preventing a potential blade overheating event. “The FBM42 didn’t just measure temperature—it measured truth,” said the controls engineer. This upgrade reduced nuisance trips by 90% and extended probe life through early fault detection.

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Technical Principles and Innovative Values

Innovation Point 1: True Differential Inputs with High CMRR

Each channel uses a precision instrumentation amplifier with >100 dB common-mode rejection—critical for rejecting noise in grounded thermocouple systems.

Innovation Point 2: Per-Channel Thermocouple Type Selection

Unlike fixed-type modules, the FBM42 allows mixing of TC types in one module—ideal for multi-process skids.

Innovation Point 3: Built-In Diagnostics for Predictive Maintenance

Open-circuit detection triggers alarms before complete signal loss, enabling planned replacement during outages.

Innovation Point 4: Seamless I/A Series Integration

Configured via Foxboro Workbench, the FBM42 appears in the plant hierarchy with live values, alarms, and historical trending—no third-party drivers needed.

Application Cases and Industry Value

In a petrochemical refinery in Texas, catalytic reformer inlet temperatures were drifting due to undetected thermocouple degradation. The old system lacked open-circuit alerts, so operators only noticed issues during product quality excursions. After installing FOXBORO FBM42 modules, the DCS began logging gradual increases in loop resistance—a precursor to failure. Over six months, three incipient TC failures were caught early, avoiding off-spec product batches estimated at $ 2.1M in losses. “The FBM42 turned our temperature sensors into predictive assets,” noted the process control lead.

Related Product Combination Solutions

FOXBORO I/A Series CP60 or CP80: Controller—hosts FBM42 data for logic and HMI

FOXBORO FBM-TB42: Terminal base—provides screw terminals and CJC sensor for FBM42

FOXBORO FBM44: RTD input module—complements FBM42 for mixed-sensor applications

FOXBORO Workbench: Engineering software—configures TC types, ranges, and diagnostics

Rosemount 6888: Smart TC transmitter—alternative for 4–20 mA systems, but adds cost and latency

ABB AC 800M with TB850: Can integrate via Modbus—but lacks native millivolt support

Honeywell Experion: Competitor DCS—but requires different I/O hardware

FOXBORO FBM47: High-density analog output—can drive heaters based on FBM42 readings

Installation, Maintenance, and Full-Cycle Support

Installing the FOXBORO FBM42 requires:

Mounting in an I/A Series I/O chassis with proper grounding.

Using shielded, twisted-pair extension wire (e.g., Belden 9915) for thermocouples.

Connecting shields to the terminal base ground lug (not at both ends).

Ensuring thermocouple alloys match the configured type (e.g., no copper extension for Type K).

Best practices:

Perform a loop calibration using a dry-well calibrator during commissioning.

Enable open-circuit detection in Workbench to catch broken wires.

Avoid running TC wires parallel to power cables (>30 cm separation recommended).

Maintenance includes annual verification of CJC accuracy and visual inspection of terminal tightness. If a channel fails, the entire module can be replaced hot—configuration resides in the controller.

Our technical team provides end-to-end support—from I/O list validation and cabinet design to FAT testing and migration from legacy systems (e.g., FBM01/02). Every FBM42 undergoes functional test with live thermocouples before shipment. We offer a 24-month warranty and access to Foxboro-certified engineers.