• 4mA: Corresponds to the measurement zero point (lower limit)
• 20mA: Corresponds to the full-scale measurement (upper limit)
• Intermediate values: Linearly correspond to the measured parameter

Calculation Formula:
Output Current = 4 + (Measured Value / Range) × 16 mA

Reverse Calculation:
Measured Value = (Output Current – 4) / 16 × Range

• Broken Wire Detection: When the signal drops below 4mA (e.g., 0mA or 2mA), the receiving device can immediately detect a broken wire or fault condition
• Two-Wire Power Supply: The 4mA minimum current can power two-wire transmitters (the “live zero” concept)
• Fault vs True Zero Distinction: 0mA could be a true zero value or a fault; 4mA clearly indicates the zero point

Ohm’s Law: U = I × R

When I (current) is constant: Resistance R changes → Voltage U changes proportionally, but current I remains unchanged.

매개변수:

• Cable resistance: ~40Ω/km × 2km = 80Ω (round trip)
• DCS input resistance: 250Ω
• Total load: 80 + 250 = 330Ω

Calculations:

• At 20mA voltage drop: U = 0.02A × 330Ω = 6.6V
• At 4mA voltage drop: U = 0.004A × 330Ω = 1.32V
• Required supply voltage: ≥6.6V + transmitter operating drop (~3V) = 9.6V

Conclusion: With a standard 24VDC supply, 2000-meter transmission is completely feasible! With 0-10V voltage signals, cable voltage division would severely degrade the signal, potentially reducing the actual voltage reaching the DCS to only 7-8V with enormous error.

• Constant Current Source Characteristics: Externally induced interference voltage cannot change the current magnitude
• Low Loop Impedance: Current loop impedance is much lower than voltage signal circuits, making it difficult for interference voltage to couple
• Differential Reception: Receiving devices only care about current magnitude, not common-mode interference

Analogy: Voltage signals are like “water flow”—easily affected by external influences. Current signals are like “fixed water volume”—regardless of how complex the path, the volume remains constant.

Instrument Technician’s Diagnostic Logic:

• Seeing 0mA → Immediately judge broken wire, check wiring
• Seeing 3.5mA → Transmitter internal fault, replace
• Seeing 21mA → Overrange, check range settings or process conditions

• Transmitting the 4-20mA signal
• Powering the transmitter

No additional power wiring required!

Factory Project Example: Assuming 1000 instruments
Two-wire savings: 1000 sets of cable terminals, wiring labor
Total cost savings: Can reach hundreds of thousands of dollars

Power Supply (24VDC)
|
├─ [Resistor 250Ω] ──┬─ Ammeter (display mA)
│                    |
└────────────────────┴─ [Two-Wire Transmitter]
|
Field Measurement

• Function: Converts physical quantities to 4-20mA current
• 유형: Pressure, temperature, flow, level transmitters, etc.
• Output Characteristics: Constant current source output

Function: Converts current to voltage for A/D conversion
Calculation: 20mA × 250Ω = 5V (standard input voltage)

• Standard: 24VDC ±10%
• 유형: Switching power supply, linear power supply
• Requirements: Stable output, low ripple, short-circuit protection

• Specification: Twisted pair shielded cable (e.g., RVVP 2×1.5)
• Shield Layer: Single-end grounded (prevents ground loops)

1. Disconnect transmitter from DCS connection
2. Connect calibrator in series with the loop
3. Have calibrator output 4mA, 12mA, 20mA respectively
4. Check if DCS/PLC readings correspond correctly
5. Record errors, adjust or replace as needed

• Signal Generator: Simulates 4-20mA signals
• Multimeter: Measures actual current
• Standard Meter: Compares accuracy

문제 해결 절차:

1. Check local indicator
   • Local indicator has reading → Problem in signal transmission or DCS
   • Local indicator also shows zero → Problem with field instrument
2. Assuming local indicator is normal, measure mA signal
   • Use multimeter to measure mA in series → Actual signal is 12mA → Problem in DCS card
   • Actual signal is only 4mA → Problem with transmitter or wiring
3. Further inspection
   • Check terminal connections → Found loose connection
   • After tightening, returns to normal → Fault cleared

• Signal value stability check
• Terminal connection tightness
• Power supply voltage measurement
• Environmental conditions (temperature, humidity)

• Use high-quality cables and connectors
• Ensure proper shielding and grounding
• Regular calibration (1-2 times per year)
• Maintain maintenance records

• Digital signal superimposed on 4-20mA analog signal
• Frequency: 1200Hz FSK modulation
• Advantage: Compatible with traditional 4-20mA while supporting digital communication

• Remote diagnostics
• Online calibration
• Multi-variable transmission
• Asset management

FOUNDATION Fieldbus:

• Fully digital, bidirectional communication
• Control functions distributed to field devices
• Higher accuracy and reliability

Profibus PA/DP:

• Open fieldbus standard
• Widely used in process industries

WirelessHART:

• Wireless transmission
• Suitable for locations difficult to wire

But! 4-20mA remains the mainstream:
✓ Excellent compatibility
✓ Mature technology
✓ Low cost
✓ Simple maintenance

Background: A new unit at a refinery with pressure transmitters 1500 meters from the DCS control room, using 4-20mA signal transmission.

Design Calculations:

• Cable resistance: 1.5km × 40Ω/km × 2 = 120Ω
• DCS input resistance: 250Ω
• Total load resistance: 370Ω

Voltage drop at 20mA: 0.02A × 370Ω = 7.4V
Required supply: 7.4V + 3V (transmitter) = 10.4V
Selected 24VDC supply: Sufficient margin ✓

Implementation:

• Selected RVVP 2×1.5 shielded cable
• Shield grounded at DCS side only
• Field terminals properly waterproofed
• Commissioned successfully on first attempt

Background: A flow meter in a pump room showed severe reading fluctuations, making it unusable for control.

Fault Diagnosis:

Symptom: Flow readings fluctuating rapidly within ±10% range

Investigation:

1. Measured mA signal with multimeter → Confirmed fluctuation
2. Disconnected transmitter, connected standard signal generator → DCS reading stable
   → Conclusion: Problem with transmitter or field conditions
3. Inspected field: Large VFD motor adjacent to transmitter
   → Conclusion: Electromagnetic interference

Solution:

• Rerouted signal cable through metal conduit (magnetic shielding)
• Grounded shield at both ends (eliminates interference)
• Added filter to transmitter power supply
• Increased spacing between signal and power cables (>30cm)

결과: Readings stabilized within ±0.5% range

• Understand the principles: Knowing why we use it enables us to use it well
• Master fault diagnosis: 4-20mA faults are the most common in daily work
• Perform proper maintenance: Regular calibration, wiring checks, ensuring reliability
• Embrace new technology: HART and smart transmitters are the evolution of 4-20mA

The 4-20mA signal may seem simple, but it embodies profound engineering wisdom. Mastering it is fundamental knowledge for every instrument technician.

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