Bristol FB300 Analog Input Scaling and Calibration

Quick Answer

Analog input scaling on the Bristol FB300 converts raw 4–20 mA or 1–5 V signals from field transmitters into engineering units (PSI, °F, feet, GPM) using a linear zero/span mapping configured in OpenBSI or ACCOL.

How Analog Scaling Works

Every analog transmitter in the field sends a standardized signal — most commonly 4–20 mA — that represents a process variable. The 4 mA signal corresponds to the zero (low) end of the measurement range, and 20 mA corresponds to the span (high) end. The FB300 analog input card converts this current signal to a digital value (raw counts), and the scaling configuration translates those counts into meaningful engineering units.

The scaling formula is a simple linear interpolation:

Engineering Value = Zero + (Raw - Raw_Min) × (Span - Zero) / (Raw_Max - Raw_Min)

Where:

• Zero = engineering value at 4 mA (e.g., 0 PSI)
• Span = engineering value at 20 mA (e.g., 500 PSI)
• Raw_Min = raw counts at 4 mA
• Raw_Max = raw counts at 20 mA

Step 1 — Identify Transmitter Ranges

Before configuring scaling, document each transmitter's range from its datasheet or nameplate:

Step 2 — Configure Scaling in OpenBSI

1. Open the point configuration for the target analog input.
2. Set the Input Type to match the physical signal (4–20 mA, 1–5 V, or thermocouple type).
3. Enter the Zero value — the engineering-unit value when the transmitter outputs 4 mA (e.g., 0 for a 0–500 PSI transmitter).
4. Enter the Span value — the engineering-unit value when the transmitter outputs 20 mA (e.g., 500 for a 0–500 PSI transmitter).
5. Set the Engineering Units text (PSI, °F, FT, GPM) for display in OpenBSI and at the SCADA master.
6. Configure clamping if desired — this limits the displayed value to the zero/span range even if the raw signal goes slightly above 20 mA or below 4 mA due to transmitter overrange.

Step 3 — Configure Alarm Limits

1. Set alarm setpoints based on process requirements:
   • High-High (HH): Emergency shutdown threshold (e.g., 28 ft tank level triggers high-level shutdown).
   • High (H): Warning level (e.g., 25 ft).
   • Low (L): Warning level (e.g., 2 ft).
   • Low-Low (LL): Pump protection threshold (e.g., 1 ft triggers pump shutoff to prevent dry running).
2. Set alarm deadbands to prevent alarm chattering when the process value oscillates near a setpoint. A typical deadband is 1–2% of the measurement span.

Step 4 — Calibration and Verification

1. Zero check: Apply exactly 4.000 mA to the analog input using a precision milliamp source. Verify the engineering-unit reading in OpenBSI shows the configured zero value (±0.1%).
2. Span check: Apply exactly 20.000 mA. Verify the reading matches the configured span value.
3. Mid-range check: Apply 12.000 mA. Verify the reading is exactly half of the span. This confirms linearity.
4. Overrange check: Apply 20.5 mA and 3.5 mA. Verify clamping behavior if configured, or confirm the correct over/under range reading.
5. Record all calibration results in the commissioning package with the milliamp source serial number and calibration date for traceability.

Common Scaling Mistakes

• Swapped zero/span: Entering the span value in the zero field and vice versa results in an inverted reading (high pressure reads low). Always double-check by applying 4 mA and confirming the zero value.
• Wrong input type: Configuring a channel for 1–5 V when the transmitter outputs 4–20 mA produces erratic or pinned-high readings.
• Missing square root extraction: Differential pressure flow transmitters require square root extraction to convert ΔP to flow rate. If the transmitter doesn't have built-in square root, enable it in the FB300 scaling configuration.
• Ground loops: Multiple ground connections on instrument shield wires cause offset errors that shift the entire reading. Use single-point grounding at the RTU end.