Non-intrusive pig signallers detect the passage of a pig using sensors mounted on the outside of the pipeline.  Unlike intrusive models, they do not penetrate the pipe wall or rely on the mechanical triggers.  This article explains the operating principles behind the main non-intrusive detection technologies – magnetic, ultrasonic and acoustic – and outlines how each method identifies a pig as it passes key points along a pipeline.

What Is A Pig Signaller?

A pig signaller, sometimes called a pig detector, is a device used to confirm that a pig has passed a specific point on a pipeline.  While “signallers” and “detector” are often used interchangeably, both terms describe equipment designed to identify a pig’s movement and indicates its passage.  Signallers are commonly installed at launchers, receivers and critical points along the pipeline to support safe and efficient pigging operations.  When a pig is known to have passed one signaller but not the next, the search area can be narrowed quickly before tracking equipment is deployed.

There are two types of pig signallers:

• Intrusive – use a mechanical trigger inside the pipeline
• Non-intrusive – use external sensing technologies to detect the pig without intruding into the pipe.

This article focuses exclusively on how non-intrusive signallers work.

Technology Overview

Before diving into the many smart features and benefits of non-intrusive signallers, we will outline the different technologies available and their application.

Magnetic

Magnetic pig signallers are not a new technology but they have evolved to become more reliable, configurable and easier to use.  The work across any pipeline medium – the only requirement is that the pig carries magnetics.

Magnetic Sensor: A magnetic sensor – either integrated into the unit or supplied on a cale for buried lines or limited-access locations – monitors the change in magnetic field strength as the pig approaches.  When the field reaches a pre-set threshold, the signaller registers “Approach.”

Pig Detection: As the pig passes the signaller and continues along the pipeline, the magnetic field drops.  This change is used to confirm “Pig Passage.”

Detection Alert: Once passage is detected, the signaller activates an alert.  This may be a local visual indication (e.g LEDs), and/or a remote signal to a control room or handheld device.  The event is also logged with date and time for later review.

Reset and Readiness:  After signalling, the unit resets automatically after a pre-determined interval so it is ready for the next pig.  If preferred, it can also be reset manually via the push button or from the control system.

Ultrasonic

Ultrasonic pig signallers offer a sophisticated approach to pig detection and have the added advantage of being able to detect the position of a stationary pig, not just the passage of a moving one.  They also do not require magnets or transmitters on the pig to operate.

Ultrasonic Pulse Emission: The signaller is equipped with an ultrasonic transducer, supplied on a cable, and uses the pulse-echo method to monitor the time of flight of ultrasonic pulses through the pipe.  When the unit is first switched on, it establishes a baseline internal diameter for the pipeline from the returned signal.  Provided the line is liquid-filled, the device can “see” a stable reaction from the opposite side of the pipe.  

Interruption and Detection: When a pig passes the sensor location, it disturbs the flow path and breaks the beam.  This interruption changes the apparent diameter reading relative to the baseline the unit established at start-up.  The device processes this change to confirm pig passage.

Filtering: The detection algorithm can be calibrated to filter out smaller disturbances – such as debris – ensuring that only meaningful changes in the ultrasonic path trigger a pig-passage event.

Detection Alert: As with the magnetic pig signallers, ultrasonic signallers have the capability to locally or remotely signal pig passage and log pig passage data

Ultrasonic Locating: As pigs approach receivers, they can lose momentum and stall after the main line tee but before the isolation valve.  To determine the location of the pig, the operator moves the ultrasonic sensor incrementally along the pipe and monitors the live diameter reading: where the pig occupies the bore, the measured diameter deviates from the baseline; where the bore is clear, the reading returns to baseline.  In this way, the operator can map sections of the pig (e.g cups/discs vs gaps) and pinpoint the exact stall position, avoiding unnecessary delays.

Acoustic

Acoustic pig signallers operate on the principle of sound detection and can be used in any pipeline medium without the need for magnets or transmitters.  Here is a breakdown of how they function:

Sound Detection: Acoustic Pig Signallers monitor the vibrational and acoustic noise generated as a pig moves through the pipeline. Each pig produces a characteristic acoustic signature and the device listens for these changes at the sensor location.

Filtering and Processing: To avoid false alarms, the detection algorithm filters out background noise and focuses on the sound associated with pig movement.  Correct placement is important – these units should not be installed close to valves, pumps or other features that may create similar noise profiles to a passing pig.  A short test run is required during installation to calibrate the unit to the specific pig type and to confirm the chosen location provides clear, reliable detection.  

Alert Activation: Once the device identifies the sound of a pig approaching, it will signal “Pig Approach.”  Once the pig passes the detector and the sound level drops below the threshold, it will signal “Pig Passage.” This indication can be viewed locally or remotely.

System Reset: After a successful detection and alert, the signaller resets to its default state so it is ready for the next detection.     

Magnetic, ultrasonic and acoustic signallers work by detecting distinct physical effects at the pipe wall.  Knowing how each method generate and validates an indication helps operators install, calibrate and interpret results correctly. 

For wider context and selection guidance, see our Complete Guide to Non-Intrusive Pig Signallers (2026) and Pig Signaller Buyer Guide – Choosing the Right Technology for Your Pipeline