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I/P Converter Sanity Checker - Verify 4-20 mA to 3-15 PSI Signal Conversion

Check I/P converter output pressure against expected mA input and diagnose common converter faults

Free I/P converter diagnostic tool for instrument technicians. Enter the 4-20 mA input signal and see the expected 3-15 PSI (or 0.2-1.0 bar) pneumatic output. Compare against your actual gauge reading to identify zero shift, span errors, and nonlinearity in current-to-pressure converters. Includes built-in diagnostic checks for common I/P failure modes including diaphragm leaks, nozzle blockages, and supply pressure problems.

Pro Tip: Before blaming the I/P converter, always check the supply air pressure first. An I/P converter requires clean, dry instrument air at 20-25 PSI to produce a full 3-15 PSI output. If your supply drops below 18 PSI, the converter physically cannot reach 15 PSI output and will appear to have a span error. A five-dollar pressure gauge on the supply regulator saves hours of unnecessary I/P calibration troubleshooting.

Verify the 4-20 mA signal feeding the I/P converter

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I/P Converter Sanity Checker
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How It Works

  1. Enter Input Signal

    Input the 4-20 mA signal value being sent to the I/P converter. This can be the actual measured mA from a multimeter in series with the loop, or the expected mA based on the controller output percentage.

  2. Select Output Range

    Choose the I/P converter output range. Standard ranges are 3-15 PSI (North America), 0.2-1.0 bar (metric), or 20-100 kPa. Some converters use split ranges or non-standard ranges for specific valve applications.

  3. Enter Actual Output Reading

    Input the actual pneumatic output pressure measured at the I/P converter output port with a calibrated test gauge. Measure at the converter, not at the valve actuator, to eliminate tubing losses from the comparison.

  4. Review Diagnostic Results

    The checker compares expected versus actual output and identifies the error type: zero shift (offset at 4 mA), span error (correct at 4 mA but wrong at 20 mA), nonlinearity (error varies across range), or erratic output (possible mechanical fault).

  5. Check Supply Pressure

    Enter the supply air pressure feeding the I/P converter. The checker flags inadequate supply pressure that could limit output range, and warns about excessive supply pressure that may indicate a missing or failed supply regulator.

Built For

  • Instrument technicians calibrating I/P converters during scheduled preventive maintenance
  • Controls techs diagnosing valve positioner problems by isolating the I/P converter signal
  • Maintenance planners verifying I/P converter performance before planned shutdowns
  • Commissioning engineers checking I/P converter output during initial loop checkout
  • Reliability engineers tracking I/P converter drift trends to predict failures
  • Apprentice instrument techs learning the relationship between mA input and PSI output

Features & Capabilities

Linear Conversion

Calculates the expected output pressure for any mA input using the linear relationship: Output = ((mA - 4) / 16) * span + LRV. Handles standard 3-15 PSI, 0.2-1.0 bar, and custom ranges.

Error Classification

Automatically classifies the error type based on the deviation pattern: zero shift, span error, combined zero-and-span, nonlinearity, or erratic behavior. Each classification points to specific mechanical causes.

Supply Pressure Check

Validates that the instrument air supply pressure is adequate for the converter to produce full-range output. Flags low supply conditions that mimic converter calibration problems.

Multi-Point Check Mode

Enter readings at 4, 8, 12, 16, and 20 mA to perform a five-point calibration check. The tool plots expected versus actual and calculates linearity, hysteresis, and repeatability errors.

Common Fault Diagnosis

Links error patterns to likely mechanical causes: zero shift suggests a nozzle or flapper adjustment, span error points to feedback bellows or supply issues, and erratic output indicates a leaking diaphragm or contaminated air supply.

Frequently Asked Questions

An I/P (current-to-pressure) converter transforms a 4-20 mA electrical signal into a proportional 3-15 PSI pneumatic signal. Inside the converter, the mA signal energizes a coil that positions a flapper relative to a nozzle. The nozzle backpressure acts on a pneumatic relay that amplifies the signal to produce the output pressure. The feedback bellows closes the loop to ensure the output pressure tracks the input current linearly. I/P converters are essential in hybrid control systems where electronic controllers drive pneumatic control valves.
Most I/P converters require an instrument air supply of 20-25 PSI (1.4-1.7 bar) to produce a full 3-15 PSI (0.2-1.0 bar) output range. The supply must be at least 2-5 PSI above the maximum output pressure to maintain control authority at the top of the range. The air must be clean, dry, and oil-free per ISA-7.0.01 (Class 3 or better). Dirty or wet supply air causes nozzle blockages, corrosion, and erratic output that is often misdiagnosed as an electronic calibration problem.
Calibration intervals depend on the application criticality and the converter's drift history. A common starting point is annual calibration for critical loops and biennial for non-critical loops. Smart positioners with built-in I/P converters often have longer intervals because they continuously auto-calibrate. Track calibration as-found data to establish drift trends. If a converter consistently drifts less than 0.5% of span between calibrations, the interval can be extended. If drift exceeds 1% of span, shorten the interval or investigate the root cause.
The most common cause is insufficient supply air pressure. If the supply is only 17 PSI, the converter cannot produce 15 PSI output because it needs a pressure differential to operate. Other causes include a leaking output diaphragm (pressure bleeds off faster than the relay can supply it), a partially blocked nozzle (limits the pilot signal), or a span calibration error. Check supply pressure first, then check for leaks by plugging the output and monitoring pressure decay, then recalibrate if the mechanical components are sound.
An I/P converter only converts the mA signal to a pneumatic pressure. A valve positioner does the same conversion but also includes a position feedback mechanism that compares actual valve stem position against the demanded position and adjusts the output accordingly. Positioners compensate for friction, packing drag, process forces, and other disturbances that would cause a simple I/P-to-actuator setup to position inaccurately. Modern smart positioners (HART, Foundation Fieldbus) incorporate the I/P function internally and add diagnostics, auto-calibration, and communication capabilities.
Disclaimer: This tool provides I/P converter signal conversion and diagnostic estimates for reference purposes. Actual converter performance depends on supply air quality, installation conditions, and maintenance history. Always use calibrated test equipment for official calibration. ToolGrit is not responsible for I/P converter calibration, valve control accuracy, or process control outcomes.

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