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Transmitter Signal Fault – Initial Troubleshooting (4–20 mA)

This guide describes a systematic, field-proven approach for troubleshooting transmitter faults on LiqTech filtration systems. It applies to pressure, flow, level, temperature and other transmitters.

When to use this guide

Use this guide when:

  • A transmitter alarm is active

  • The HMI shows out-of-range values

  • The PLC receives no, unstable, or unrealistic signal

  • Measured values do not match expected process conditions

Preconditions and safety

Before starting troubleshooting:

  • The system must be in STOP state

  • Troubleshooting may be performed with the process live, as this is often the fastest way to isolate the fault

  • Live troubleshooting shall only be carried out by personnel with sufficient electrical and process know-how

  • Where possible, troubleshooting should be performed under 24 VDC only (LAUS – Low Voltage DC)

⚠️ Always follow site-specific safety procedures.

Typical symptoms

  • Fixed minimum or maximum reading

  • Sudden signal spikes or dropouts

  • No signal detected in PLC

  • Alarm reappears shortly after reset

Step-by-step troubleshooting procedure

1. Eliminate or assess process influence

  • If possible, isolate or depressurize the transmitter

  • Alternatively, observe whether the measured value behaves plausibly under known stable process conditions

2. Check the transmitter locally

  • Inspect the transmitter for physical damage or contamination

  • Check local display or status indicators (if available)

  • Verify correct supply voltage (typically 24 VDC)

  • If feasible, dismount the transmitter and verify that it reads 0 at atmospheric pressure (or equivalent reference condition)

3. Swap transmitter or components to move the fault (fastest isolation step)

This is often the most efficient troubleshooting method.

  • Replace the suspected transmitter with a known-to-be-good transmitter from another position in the system

  • Observe on the HMI whether:

    • The fault moves to the new position

    • Or remains at the original position

If applicable:

  • Swap transmitter hats / electronics between two nearby transmitters (if cable length allows)

  • Observe whether the fault follows the swapped component

👉 If the fault moves, the swapped component is defective.

4. Inspect wiring and signal path (end-to-end)

If the fault does not move, inspect the entire signal path, not just a single section.

The signal path typically includes:

  • Transmitter terminals

  • Transmitter hat / connector

  • Cable gland

  • Field cable

  • Terminal block (input side)

  • Terminal block (output side)

  • PLC I/O card

Checks to perform:

  • Inspect all terminals for loose or intermittent connections

  • Measure continuity from transmitter terminals all the way to PLC input

  • Be aware that faults may exist:

    • Between transmitter and hat

    • Between hat and cable

    • Inside the cable

    • Between cable and terminal block

    • Between terminal block and PLC I/O card

5. Measure and simulate signal using test equipment

Multimeter checks

Using a multimeter:

  • Continuity test (“buzz”)

    • Verify uninterrupted signal path conductor-by-conductor

  • Voltage check

    • Confirm correct 24 VDC supply at transmitter terminals

  • Current measurement (if applicable)

    • Verify actual loop current corresponds to expected signal value

⚠️ Ensure the multimeter is correctly configured before measuring current in-loop.

I/O tester (recommended)

An I/O tester can be used to:

  • Simulate a 4–20 mA signal

  • Inject a known signal at:

    • Transmitter terminals

    • Terminal block

    • PLC input

👉 This allows verification of wiring and PLC input independent of the transmitter.

6. Verify PLC I/O card and channel

  • Confirm the PLC I/O channel responds correctly to a simulated signal

  • Swap to a spare I/O channel if permitted

  • Inspect I/O card diagnostics and status indicators

7. Reset and verify on HMI

Whenever a change is made:

  • Press “Reset” on the HMI

  • Observe whether:

    • The alarm clears

    • The signal updates correctly

👉 Many alarms will not clear until a reset is performed after the fault condition is removed.

Common root causes

  • Defective transmitter electronics or hat

  • Broken or intermittent wiring

  • Loose terminals or connectors

  • Faulty PLC I/O channel

  • Missing reset after signal restoration

When to stop and contact LiqTech Service

  • If the fault persists after transmitter/component swapping and signal simulation

  • If multiple transmitters show similar behavior

  • If signal instability affects control logic or system safety

💡 Note

After replacing or swapping transmitters, always verify correct type, range, and scaling in PLC and HMI.

⚠️ Field insight – multiple transmitters showing abnormal signals

If multiple transmitters suddenly display abnormal or implausible signals at the same time, this often indicates a shared root cause rather than individual transmitter failures.

In such cases:

  • Review the electrical documentation to determine whether the affected transmitters are connected to the same PLC I/O card

  • Check the I/O card status LEDs and diagnostics

  • Verify whether the I/O card has lost indication (e.g. no diode / status light)

👉 If several coinciding faults originate from the same I/O card, troubleshooting should start at the I/O card, not at individual transmitters.

This approach can significantly reduce troubleshooting time and prevent unnecessary component replacement.

Transmitter Signal Fault – Alarm ID references (examples)

The following alarm IDs are non-exhaustive examples of transmitter signal–related alarms observed across different LiqTech Crossflow systems.
They are provided to help support agents recognize the alarm type when a customer references a specific alarm number.

Other alarm IDs with similar alarm texts may exist depending on system generation and project configuration.

FTxx – Flow Transmitters

Examples include:

  • MK6: 30, 31, 32, 76, 77, 78, 127, 128, 129, 225

  • MK8 (D-series): D0007, D0501

  • MK8 (F-series): F0109, F0110, F0111, F0112, F0140

These alarms typically indicate missing, unstable, or implausible flow signals.

PTxx – Pressure Transmitters

Examples include:

  • MK6: 24, 25, 26, 27, 28, 29, 70, 71, 72, 73, 74, 75, 123, 124, 125, 126

  • MK8 (F-series): F0107, F0138, F0139

LTxx – Level Transmitters

Examples include:

  • MK6:

    • 34 (Permeate tank)

    • 131 (Permeate tank)

    • 197 (Concentrate tank)

    • 204 (Reject tank)

    • 223 (Precipitation tank)

  • MK8 (D-series): D0006, D0502

TTxx – Temperature Transmitters

Examples include:

  • MK6: 33, 130

  • MK8 (F-series): F0108

These alarm IDs are references only.
The troubleshooting steps in this guide may also apply to other alarms with texts indicating transmitter signal faults, such as missing signal, out-of-range values, or unstable measurements.