Skip to main content
Shops & Outbuildings Free Pro Features Available

4-20 mA Signal Helper - Scale, Troubleshoot & Verify Analog Loop Signals

Convert between engineering units and 4-20 mA current, check percent-of-span, and diagnose out-of-range signals

Free 4-20 mA signal calculator for instrument technicians and controls engineers. Enter your transmitter range (LRV and URV) and a process value to see the expected milliamp output, or enter a milliamp reading to calculate the equivalent process value. Supports linear and square-root extraction scaling for flow transmitters. Flags out-of-range conditions, burned-out upscale/downscale signals per NAMUR NE 43, and helps diagnose loop faults by comparing expected versus actual mA readings.

Pro Tip: When a 4-20 mA signal reads exactly 4.00 or 20.00, you're probably looking at a saturated transmitter, not a coincidental process value. NAMUR NE 43 defines the valid signal range as 3.8 to 20.5 mA, with anything below 3.6 mA indicating downscale burnout and above 21.0 mA indicating upscale burnout. If your reading is pegged at exactly 4.0 or 20.0, check whether the transmitter is in saturation or if the process truly is at the range endpoint.

Check I/P converter output for valve control loops

I/P Converter Sanity Checker →

Visualize PID loop response to setpoint changes

PID Loop Quick Visualizer →

Diagnose pneumatic signal issues downstream of I/P

Pneumatic Troubleshooter →

Read the full guide on 4-20 mA signal fundamentals

4-20 mA Signal Explained →
Start Using This Tool See How It Works

PREVIEW All Pro features are currently free for a limited time. No license key required.

4-20 mA Signal Helper
Pop Out

How It Works

  1. Enter Transmitter Range

    Input the lower range value (LRV) and upper range value (URV) in engineering units. For example, a pressure transmitter ranged 0-100 PSI would have LRV=0 and URV=100. For suppressed-zero or elevated-zero ranges, enter the actual calibrated endpoints.

  2. Select Scaling Type

    Choose linear scaling for most transmitters (pressure, temperature, level) or square-root extraction for differential-pressure flow transmitters. Square-root scaling is needed because flow is proportional to the square root of differential pressure.

  3. Enter Known Value

    Enter either a process value in engineering units to calculate the expected mA output, or enter a measured mA reading to calculate the equivalent process value. The calculator shows percent-of-span for both.

  4. Compare Expected vs Actual

    Enter both the expected process value and the actual mA reading from your multimeter. The calculator shows the deviation and flags whether the error exceeds typical transmitter accuracy (0.1% of span for smart transmitters, 0.5% for analog).

  5. Check Signal Health

    Review the signal status indicator. The calculator flags NAMUR NE 43 fault conditions: below 3.6 mA (downscale burnout), 3.6-3.8 mA (warning low), 20.5-21.0 mA (warning high), and above 21.0 mA (upscale burnout).

Built For

  • Instrument technicians calibrating 4-20 mA pressure, temperature, and level transmitters
  • Controls engineers verifying analog input scaling in PLC and DCS configurations
  • Maintenance techs troubleshooting loop errors between transmitter output and control room indication
  • Operators converting control room mA readings to process values during manual operations
  • Commissioning engineers validating analog signal ranges during loop checks on new installations
  • Reliability engineers diagnosing intermittent signal faults using expected-vs-actual comparison
  • Apprentice instrument techs learning 4-20 mA signal math and NAMUR NE 43 fault conventions

Features & Capabilities

Bidirectional Conversion

Convert from process value to mA or from mA to process value. Enter either known quantity and the calculator derives the other, along with percent-of-span and percent-of-signal.

Square-Root Extraction

Supports square-root scaling for DP flow transmitters where flow is proportional to the square root of differential pressure. Shows both raw DP signal and extracted flow value.

NAMUR NE 43 Fault Detection

Automatically flags out-of-range signals per NAMUR NE 43 conventions: downscale burnout below 3.6 mA, warning range 3.6-3.8 mA, normal range 3.8-20.5 mA, warning high 20.5-21.0 mA, and upscale burnout above 21.0 mA.

Error Percentage Calculation

Compares expected versus actual mA readings and calculates the error as a percentage of span. Flags readings that exceed typical transmitter accuracy specifications.

Common Range Presets

Quick-select common transmitter ranges: 0-100 PSI, 0-250 PSI, 0-150°F, 0-500°F, 0-100% level, and custom. Saves time during repetitive calibration work.

Frequently Asked Questions

The 4 mA live zero serves two critical purposes. First, it allows the system to distinguish between a transmitter reading zero process value (4 mA) and a broken wire or dead transmitter (0 mA). Second, the 4 mA minimum current powers loop-powered (two-wire) transmitters, eliminating the need for separate power wiring. If 0 mA represented zero, there would be no way to differentiate a failed loop from a legitimate zero reading.
Square-root extraction converts a differential pressure signal into a flow signal. Because flow through an orifice plate or other DP element is proportional to the square root of the differential pressure, a linear 4-20 mA signal from a DP transmitter does not represent flow linearly. Square-root extraction can be performed in the transmitter, in the DCS/PLC, or in a separate signal conditioner. It should only be applied once in the loop. Applying it twice (e.g., in both the transmitter and the DCS) produces incorrect flow readings.
NAMUR NE 43 is an international standard that defines how 4-20 mA transmitters should indicate fault conditions. The valid measurement range is 3.8 to 20.5 mA. Signals below 3.6 mA indicate a downscale failure (the transmitter has detected a fault and driven its output low). Signals above 21.0 mA indicate an upscale failure. The ranges between 3.6-3.8 mA and 20.5-21.0 mA are uncertainty zones. Most smart transmitters can be configured for either upscale or downscale burnout direction depending on process safety requirements.
Calibration accuracy depends on the transmitter technology and the process requirements. Modern smart transmitters (HART, Foundation Fieldbus) typically specify accuracy of 0.04-0.1% of calibrated span. Conventional analog transmitters are typically 0.25-0.5% of span. During calibration, the applied standard (pressure source, temperature bath, etc.) should be at least four times more accurate than the device under test. A calibration error exceeding the manufacturer's published accuracy specification indicates the transmitter needs adjustment or repair.
Yes, most PLC analog input modules provide loop power for two-wire (loop-powered) transmitters. The module supplies a DC voltage (typically 24 VDC) through a sensing resistor, and the transmitter regulates the loop current between 4 and 20 mA. Check the module specifications for the maximum number of loop-powered channels and the available voltage. If the loop includes long cable runs, barriers, or additional devices in series, verify that the total voltage drop does not exceed the available loop voltage minus the transmitter's minimum operating voltage.
Disclaimer: This calculator provides 4-20 mA signal conversion estimates for reference purposes. Actual transmitter performance depends on calibration accuracy, environmental conditions, and installation quality. Always verify signal readings with calibrated test equipment. NAMUR NE 43 fault thresholds may vary by manufacturer. ToolGrit is not responsible for signal interpretation, calibration accuracy, or process control outcomes.

Learn More

Shops & Outbuildings

4-20 mA Current Loops: The Complete Instrument Tech Guide

How 4-20 mA current loops work, why 4 mA is live zero, how to convert between signal and process values, and how to detect loop faults.

Read Guide →
Shops & Outbuildings

I/P Converter Calibration & Troubleshooting Guide

How I/P converters work, 5-point calibration procedure, common failure modes, and when drift means the converter needs replacement.

Read Guide →
Shops & Outbuildings

PID Tuning Basics: What Every Instrument Tech Should Know

Practical PID tuning guide for instrument technicians. Understand P, I, and D actions, interpret step responses, and apply Ziegler-Nichols and Lambda tuning methods.

Read Guide →

Related Tools

Shops & Outbuildings Live

Shop Heater BTU Sizing Calculator

Calculate the exact BTU output your shop or garage heater needs. Factors in wall R-values, ceiling insulation, slab edge loss, overhead door infiltration, and air changes per hour to size propane, natural gas, and electric heaters correctly.

Launch Tool →
Shops & Outbuildings Live

Overhead Door Infiltration Loss Calculator

Calculate heat loss through overhead doors in shops, garages, and warehouses. Compares open-door vs closed-door losses, seal condition impact, and annual cost of infiltration with payback on door seals and high-speed doors.

Launch Tool →
Shops & Outbuildings Live

Long-Run Voltage Drop Calculator

Calculate voltage drop for long wire runs to detached shops, barns, garages, and outbuildings. Compares copper vs aluminum, shows motor starting voltage impact, and recommends the right wire size for your distance and load.

Launch Tool →