The Foxboro 43AP series is a pneumatic process controller. Its primary function is to automatically maintain a process variable (like Pressure, Flow, or Level) at a desired setpoint.
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Input: It receives a pneumatic signal (typically 3-15 psi, or 0.2-1.0 bar) representing the measured process variable. In this specific model, the internal mechanism is calibrated for a differential pressure equivalent to 0 – 1500 mmH₂O, which is a common range for flow measurement using orifice plates.
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Output: It computes a control action and sends out a corresponding pneumatic output signal (also 3-15 psi) to a final control element, such as a control valve.
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Power: The “engine” of this controller is a clean, dry supply of compressed air, typically at 20 psi (1.4 bar).
2. Detailed Model Number Breakdown (43AP-PA42C/PC)
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43AP: This is the base model number for this series of pneumatic indicating controllers from Foxboro.
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PA42C: This suffix defines the specific configuration and features of the unit.
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P: Likely denotes a specific housing or mounting style.
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A42: Specifies the type of control action and the internal mechanism. “A” often refers to a proportional-plus-reset (PI) control action. The “42” further defines the internal bellows and flapper-nozzle assembly.
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C: Often indicates a local setpoint adjustment.
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/PC: This is a critical suffix that defines the controller’s functionality.
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PC: Stands for Proportional Controller. This is a simple, “bare-bones” controller mode.
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Proportional Band (Gain): The operator can adjust the proportional band (e.g., 5% to 500%). A narrow band means high gain (aggressive response), and a wide band means low gain (sluggish response).
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No Reset (Integral) or Rate (Derivative): As a pure “PC”, this unit does not have automatic reset (to eliminate offset) or rate action (for anticipating error). This makes it suitable for simple processes where a small steady-state error is acceptable.
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3. Key Features and Components
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Local Indicating Scale: The unit has a large, easy-to-read scale and pointer that shows the current process value. The operator can also see the setpoint pointer.
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Manual/Automatic Transfer: A key feature allowing the operator to switch between:
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Automatic: The controller automatically adjusts the output based on the process variable.
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Manual: The operator manually adjusts the output pressure using a separate knob. This is essential for startup, shutdown, and troubleshooting.
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Setpoint Adjustment: A knob allows the operator to change the desired setpoint locally.
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Proportional Band Adjustment: A knob or screw (often behind a cover) to tune the controller’s response.
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Pneumatic Construction: Built with robust components like bourdon tubes, bellows, a flapper-nozzle amplifier, and a relay. These are metal parts designed for long life in harsh industrial environments.
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“Foxboro Stabilflo” Relay: A patented component that provides a high-capacity, stable air flow for the output signal, ensuring fast and accurate response.
