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4-20mA Signal Calculator

Calculate the mA output signal for any process value within a transmitter’s configured range, or determine the process value from a measured mA reading. Essential for commissioning and troubleshooting 4-20mA loop instruments.

mA → Process Value

Process Value → mA

mA → Percentage

Lower Range Value (LRV)

Upper Range Value (URV)

Current Signal (mA)

Engineering Unit (optional)

4-20mA Calculation Formula

The 4-20mA current loop is the most common analog signal standard in industrial process control. The relationship between the current signal and the process variable is linear:

mA = 4 + (PV – LRV) / (URV – LRV) × 16

PV = LRV + (mA – 4) / 16 × (URV – LRV)

Where: PV = Process Value, LRV = Lower Range Value, URV = Upper Range Value

The 4 mA baseline (called the “live zero”) is a key design feature: it allows the control system to distinguish between a transmitter reading zero and a broken wire (which reads 0 mA). A healthy 4-20mA loop should never read below ~3.8 mA or above ~20.5 mA during normal operation.

Some transmitters support extended ranges: NAMUR NE 43 defines 3.8-20.5 mA as the normal operating band, with values outside this range indicating fault conditions. Values below 3.6 mA indicate a downscale failure, while values above 21 mA indicate an upscale failure.

Worked Examples

Example 1: Pressure Transmitter Reading
A pressure transmitter is configured for 0-10 bar range (LRV=0, URV=10). The measured signal is 12 mA. What is the pressure?
PV = 0 + (12 – 4) / 16 × (10 – 0) = 0 + 0.5 × 10 = 5.0 bar

Example 2: Temperature Transmitter Signal
A temperature transmitter is configured for 0-200°C (LRV=0, URV=200). The process temperature is 75°C. What mA signal should it output?
mA = 4 + (75 – 0) / (200 – 0) × 16 = 4 + 0.375 × 16 = 10.0 mA

Example 3: Level Transmitter with Offset
A level transmitter measures 200-800 mmH₂O (LRV=200, URV=800). The signal reads 8 mA. What is the level?
PV = 200 + (8 – 4) / 16 × (800 – 200) = 200 + 0.25 × 600 = 350 mmH₂O

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4-20mA Transmitters in Stock

These transmitters use the 4-20mA standard you just calculated. Paired here: one for pressure, one for temperature — both ship with factory calibration certificates.

HM20 General Purpose Pressure Transmitter

HM20 Pressure Transmitter (4-20mA)

Standard 2-wire 4-20mA pressure transmitter

Signal: 4-20mA 2-wire, 10-36 VDC loop
Accuracy: ±0.25% FS including linearity & hysteresis
Ranges: Gauge, absolute, compound — 20+ options
Process connection: G1/4, 1/2 NPT, G1/2 and more

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HM100 Integrated Temperature Transmitter

4-20mA temperature transmitter — universal RTD / TC input

Signal: 4-20mA 2-wire, 10-36 VDC loop
Accuracy: ±0.1% of span (better than 0.2% Class A)
Input: Pt100 / Pt1000 / Cu50 / K / J / T / E / N / S / R / B
Options: Explosion-proof Ex d IIC T6, HART

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Frequently Asked Questions

4-20mA is an analog signal standard used in industrial process control. A transmitter outputs a current between 4 and 20 milliamps, where 4 mA represents the minimum measurement value (LRV) and 20 mA represents the maximum (URV). The current is proportional to the measured process variable.

The 4 mA “live zero” is a deliberate design choice that allows the control system to distinguish between a transmitter reading its minimum value (4 mA) and a broken wire or failed transmitter (0 mA). If the loop current drops below approximately 3.6 mA, the system knows there is a wiring fault.

The maximum loop resistance depends on the transmitter power supply voltage. A typical formula is: R_max = (V_supply – V_min) / 0.02, where V_min is the minimum voltage the transmitter needs to operate (typically 10-12V). For a 24V supply with 12V minimum, R_max = (24-12)/0.02 = 600Ω.

Yes. HART (Highway Addressable Remote Transducer) protocol is superimposed on the standard 4-20mA signal. The analog 4-20mA relationship between process value and current remains the same. HART adds digital communication on top of the analog signal without affecting the current reading.

A 2-wire (loop-powered) transmitter gets its power from the same two wires that carry the 4-20mA signal. It is limited to 20 mA maximum power consumption. A 4-wire transmitter has separate power and signal wires, allowing it to draw more power for features like displays or additional outputs.

Connect a milliamp meter in series with the loop. Apply known process values (e.g., zero pressure = 4 mA, full scale = 20 mA). The reading should match the calculated value within the transmitter accuracy specification. If the signal is stuck at 0 mA, check wiring. If readings are offset, the transmitter may need recalibration.

Need a 4-20mA Pressure or Temperature Transmitter?

Our transmitters output standard 4-20mA signals with configurable ranges. Available with HART, RS485, and display options. Engineers typically respond within 12 business hours.

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4-20mA Signal Calculator

4-20mA Calculation Formula