If you have ever faced a situation where your transmitter is healthy, PLC is working fine, wiring is OK, but the reading is still wrong, then most likely you are facing a problem related to Instrument Loop Impedance and It affects 4–20 mA Accuracy.
This is one of the most ignored but critical topics in instrumentation.
Many engineers focus only on the transmitter and PLC but forget that the entire 4–20 mA loop is an electrical circuit, and every circuit has impedance (resistance/load).
If this loop impedance exceeds the capability of the transmitter, your signal accuracy silently starts degrading.
In this article, I will explain in very simple words the 7 hidden effects of Instrument Loop Impedance on 4–20 mA Accuracy, using practical field logic, formulas with examples. So lets start !
What Is Instrument Loop Impedance?
In simple terms:
Instrument loop impedance is the total electrical resistance (load) seen by a 4–20 mA transmitter in the complete current loop.
This total loop-impedance includes:
1) Resistance of signal cables
2) Resistance of PLC or DCS analog input
3) Resistance of barriers and isolators
4) Resistance of intrinsic safety devices
5) Any additional series resistors in the loop
This total impedance directly decides whether the transmitter can push full 20 mA accurately or not.
That is why Instrument Loop-Impedance and Its Effect on 4–20 mA Accuracy is such an important design and troubleshooting topic.
The Basic Rule You Must Always Remember
Every 2-wire transmitter has a defined compliance voltage.
For example:
12 V, 18 V, 24 V
The maximum allowable loop resistance is calculated by:
Maximum Loop Resistance (Ω) = (Supply Voltage − Minimum Transmitter Voltage) ÷ 0.02
This simple formula controls everything related to Instrument Loop Impedance.
The 7 Hidden Effects of Instrument Loop Impedance on 4–20 mA Accuracy
Now let us go step-by-step through the 7 practical effects observed in real plants.
1. Output Signal Saturates Before Reaching 20 mA
This is the most common hidden effect of Instrument Loop Impedance on 4–20 mA Accuracy.
What You See in the Plant:
- Transmitter never reaches full-scale reading
- PLC or DCS shows 85–90% even when process is at maximum
- Calibration fails at upper range
What Is Really Happening:
Due to high loop-impedance, the transmitter runs out of voltage and cannot push full 20 mA.
Simple Example:
- Supply = 24 V
- Transmitter minimum operating voltage = 12 V
- Available voltage for loop = 12 V
- Maximum loop resistance = 12 ÷ 0.02 = 600 Ω
If your actual loop impedance becomes 750 Ω, the transmitter will saturate before 20 mA.
This is a direct field example of Instrument Loop-Impedance and Its Effect on 4–20 mA Accuracy.
2. Non-Linear Signal Behavior Across the Range
This is a very dangerous hidden effect because the signal appears OK at some points and wrong at others.
What You See:
- Lower range looks accurate
- Mid range shows small error
- Upper range shows large error
Why This Happens:
As loop impedance increases, the transmitter output starts compressing the upper part of the signal, causing non-linear distortion.
This means the signal is not proportional anymore, even though scaling looks correct in PLC.
This is a serious manifestation of Instrument Loop Impedance especially in flow and level measurement.
3. Increased Measurement Error at Long Cable Distances
Long cable runs are the silent creators of loop-impedance problems.
What You See:
- Accurate in local testing
- Inaccurate after long cable installation
- Error increases with distance
The Hidden Reason:
Signal cable resistance adds directly to loop impedance. For example:
- Typical copper cable ≈ 18 Ω per km (per core)
- For a 1 km round trip, total resistance ≈ 36 Ω
- Add PLC input + barrier + isolator → easily crosses 300–500 Ω
Even though 36 Ω sounds small, in marginal designs it pushes the loop beyond safe limits.
This is a textbook case of Instrument Loop-Impedance.
4. Transmitter Works on Bench but Fails in the Plant
This is one of the most confusing field issues.
What You See:
- Transmitter calibrated perfectly in workshop
- Once installed in the plant, readings become unstable or incorrect
- Engineers wrongly blame the transmitter
The Real Cause:
On the bench:
- Only power supply + meter
- Very low loop impedance
In the plant:
- Long cables
- Barriers
- Isolators
- PLC input resistance
The total loop impedance increases drastically, exposing the true effect of Instrument Loop-Impedance and it will affect your 4-20mA accuracy.
5. Reduced Accuracy in Intrinsically Safe (IS) Loops
This effect is very common in hazardous areas.
What You See:
- Normal loop works fine
- Same transmitter in IS loop becomes inaccurate
- Upper range becomes unstable
Why It Happens:
Intrinsic safety barriers add 200–350 Ω resistance directly into the loop.
This additional load drastically increases loop-impedance.
This is why Loop Impedance must be checked separately for IS and non-IS loops.
6. Slow Dynamic Response of the Signal
This effect is often misunderstood and blamed on the sensor.
What You See:
- Process changes fast
- PLC value responds slowly
- Control loop becomes sluggish
The Hidden Electrical Reason:
Higher loop resistance increases the RC time constant of the circuit, slowing down the current change. This causes the signal to respond electrically slower, even though the sensor itself is fast.
This is a critical dynamic effect of Instrument Loop Impedance, especially in pressure and flow control loops.
7. Transmitter Overheating and Premature Failure
This is the most dangerous hidden effect, often ignored.
What You See:
- Transmitter heats up abnormally
- Repeated transmitter failures
- Burnt electronics in hot environments
Why It Happens:
When loop impedance is too high, the transmitter operates at the edge of its voltage capability, causing:
- Internal power dissipation
- Excess heat
- Component stress
- Reduced lifespan
This long-term reliability issue is a severe outcome of Instrument Loop Impedance.
How to Calculate Instrument Loop Impedance for a 4–20 mA System
Let us do a simple calculation.
Assume:
- Supply voltage = 24 V
- Transmitter minimum voltage = 12 V
- PLC input resistance = 250 Ω
- Barrier resistance = 300 Ω
- Cable resistance = 50 Ω
Total Loop Impedance:
250 + 300 + 50 = 600 Ω
Maximum Allowed Impedance:
(24 − 12) ÷ 0.02 = 600 Ω
This loop is running at the absolute limit, meaning any small increase will cause signal distortion.
This is a clear calculation of Loop Impedance.
What we learn today?
If you truly want accurate, stable, and reliable 4–20 mA measurements, then you must stop thinking only about:
- The transmitter
- The PLC
And start thinking about: The complete electrical loop
Most “mysterious” measurement errors in the plant are not mysteries at all but they are simply the hidden consequences of poor loop impedance design.
Now that you fully understand Instrument Loop Impedance, you are already ahead of many practicing engineers.