4–20 mA Calibration Procedure

Calibration ensures that the transmitter output accurately represents the process value across the entire 4–20 mA range.

Typical Calibration Loop Setup

Loop must always be measured in series.

Required Tools

• Calibrated multimeter or loop calibrator
• Process calibrator (pressure, temperature, etc.)
• 24 VDC loop power supply (if required)
• Manufacturer range data

Step 1 – Isolate the Instrument

• Inform control room and take necessary permits
• Isolate process (valves, manifolds)
• Put loop in manual or bypass mode

Step 2 – Connect Calibration Setup

• Break the loop at test terminals
• Insert multimeter or calibrator in series
• Confirm loop power is available

Step 3 – Zero Adjustment (4 mA)

Apply the lower range value (LRV).

• Expected output: 4.00 mA
• Adjust zero if required

Step 4 – Span Adjustment (20 mA)

Apply the upper range value (URV).

• Expected output: 20.00 mA
• Adjust span if required

Step 5 – Intermediate Verification

• Apply 25%, 50%, 75% input
• Confirm linear response
• Document as-found / as-left values

Calibration Tolerance Calculation

If transmitter accuracy = ±0.5% of span:

Span = 16 mA (20 – 4)

0.5% of 16 mA = 0.08 mA

Acceptable 4 mA range = 3.92 to 4.08 mA
Acceptable 20 mA range = 19.92 to 20.08 mA

Common Field Mistakes

• Calibrating without isolating impulse lines
• Measuring mA in parallel (wrong method)
• Ignoring power supply instability
• Not documenting as-found condition
• Forgetting damping configuration reset

Field Rule

Never calibrate a transmitter before confirming correct loop wiring and stable power supply. Calibration does not fix wiring errors.

Related Blog Articles