Pulse Induction Detector Testing (Target Size)
A useful pulse-induction target-size test changes one factor at a time and records repeatable responses. Test measured targets at set depths, while holding coil height, sweep speed, settings, and orientation steady. Compare target sizes separately from materials, repeat each pass at least five times, and report the detector and test conditions rather than claiming a universal minimum detectable size.
A controlled test can make an opaque detector response easier to understand. The process is also easier to care for over time: use the same marked path, measured targets, and log sheet for each session. In my research on screening and detection systems, I treat a test result as evidence about one setup, not a promise about every detector or search condition.
Pulse-induction (PI) detectors send electrical pulses through a coil and sample the changing signal after each pulse. A metal target can develop induced currents, which then decay; the detector measures part of that transient response. Target size matters, but so do material, shape, orientation, depth, detector settings, soil, and interference.
Planning the Target-Size Test
This section defines the test goal and the controls needed to make results useful. The dimensions and settings below form a practical, controlled protocol; they are not code requirements, an industry mandate, or a universal PI-detector rating. The purpose is to compare repeatable responses under stated conditions.
Start by writing down the question. A size comparison asks how the same detector responds to different-sized targets when other variables stay fixed. A material comparison asks how different materials respond at the same size and shape. Keep those questions separate: changing size and material together makes the result hard to interpret.
Choose and measure targets
A target is the object placed in the detector’s field during a test. For the size series, use copper disks measuring 10, 20, and 40 mm in diameter, each 1.0 mm thick. Hold each dimension to ±0.1 mm and record actual measurements, not only nominal sizes.
For a separate material comparison, use a 20 mm diameter × 1.0 mm aluminum target and a 20 mm diameter × 1.0 mm low-carbon steel target. Keep the 20 mm copper disk as the reference. Record target material and dimensions; do not treat these three targets as a pure size series.
Record the starting conditions
A controlled comparison needs a written baseline. Record coil diameter, detector model if known, operating mode, gain or sensitivity, pulse-delay setting if adjustable, and any other setting shown by the unit. Also note target orientation, test medium, and ground condition.
Survey the test area for nearby or buried metal and possible electromagnetic interference. Document what you find and keep conditions as stable as possible. A change in ground response or interference can affect readings, so it belongs in the test record, not in an unmentioned assumption.
Access, Coil Clearance, and Target Placement
This section covers how to keep the coil path and target position consistent. Mechanical control matters because a small change in coil height or path can alter the signal. Use a nonmetallic fixture and marked scan path, and measure distances from clear reference points.
Set clearance, height, and speed
Keep other metal objects at least three coil diameters from both the coil and target. For example, with a 200 mm coil, this protocol calls for at least 600 mm of clearance. This is a test-control distance, not a safety rule or a guarantee that interference is absent.
Fix the coil’s test height within ±2 mm of the chosen height. Mark the scan path and use a measured sweep speed of 0.5 m/s ±0.1 m/s. A guide, metronome-assisted practice, or video review may help you keep speed steady, but record the method and check the actual movement.
Define depth and orientation
For this protocol, target depth means the distance from the coil’s lower face to the target’s nearest face. Test at 0, 10, 20, 30, and 40 mm. If the detector still responds at 40 mm, extend the range in recorded steps rather than treating 40 mm as a maximum.
Use a nonmetallic holder to set depth and orientation. For a flat disk, a simple orientation record is “flat face parallel to coil” or “edge toward coil.” Choose the orientations before testing and keep them unchanged within each comparison. Do not assume a detector responds equally to every angle.
Executing the Size and Material Matrix
A test matrix is a planned set of target, depth, and orientation combinations. It helps ensure that each result has a clear comparison. Test one target at a time, change one variable per comparison, and record every pass, including weak or inconsistent responses.
Run size and material tests separately
First test the copper size series: 10, 20, then 40 mm disks, all 1.0 mm thick. At each selected depth and orientation, make at least five sweeps per target. Keep the detector settings, ground condition, coil path, height, and speed unchanged.
Then run the material comparison using the 20 mm copper, aluminum, and low-carbon steel targets. Use the same depth and orientation sequence. This comparison isolates material only as far as the targets’ stated geometry permits; differences in alloy or exact composition may still affect response.
| Test question | Targets | Keep constant | Change |
|---|---|---|---|
| How does size affect response? | Copper, 10/20/40 mm diameter × 1.0 mm | Material, thickness, settings, orientation, depth | Diameter |
| How does material affect response? | Copper, aluminum, low-carbon steel, each 20 × 1.0 mm | Nominal size, settings, orientation, depth | Material |
| How does depth affect response? | One selected target | Target, settings, orientation, medium | Coil-to-target distance |
Log the detector’s own indication
Record the detector’s native response, such as its displayed signal, tone, or other built-in indication. Do not invent a common signal scale across different models. For each condition, note the number of detections out of five or more passes, along with the response indication for each pass.
| Field log item | What to record |
|---|---|
| Setup | Coil diameter, detector settings, test date |
| Target | Material, actual diameter and thickness |
| Position | Depth from coil lower face to nearest target face; orientation |
| Movement | Coil height, sweep speed, marked path |
| Conditions | Ground or test medium, nearby metal, observed interference |
| Result | Every response, detection count, unusual or inconsistent pass |
Field-style test examples
These are test-planning examples, not claims of measured detector performance. For a size comparison, a researcher could place each copper disk in the same holder, test at 10 mm depth, and complete five passes at the same orientation. If the 10 mm disk gives fewer responses, that finding applies only to the recorded detector and setup.
For a material comparison, the same researcher could test the three 20 mm targets at 20 mm depth with settings unchanged. If the responses differ, the log supports a material-related difference in that setup; it does not prove that all aluminum or steel targets behave the same way. For either example, do not put multiple targets in the ground together.
Verifying Repeatability and Reporting Results
Repeatability means the same setup gives a similar result when the test is run again. It is a check on the quality of the comparison, not proof of universal detector performance. Repeat a baseline target at the end, and investigate drift, inconsistent passes, or interference before drawing conclusions.
After each target series, repeat the baseline, such as the 20 mm copper disk at a chosen depth and orientation. Use the same settings, path, height, and speed. If its response has changed, pause and check for altered settings, target movement, ground changes, battery or equipment issues, and interference.
A “miss” does not establish a universal minimum detectable size. PI response depends on target conductivity, geometry, orientation, coil design, pulse delay, gain, ground response, and interference. A small or lower-conductivity target may produce a transient that falls outside the detector’s sampling window. Report the observed result as “not detected in this setup,” with conditions and pass count.
A clear report states the test method before interpreting the result. Include the target dimensions and material, coil diameter, depth definition, settings, orientation, test medium, speed, height, and all response records. If you extend the depth range, show the added distances. Do not turn a single test into a product-wide detection claim.
Conclusion and FAQ
A useful PI target test is controlled, measured, and modest in its claims. Separate size from material, use one target at a time, keep the coil movement steady, and repeat each condition. The result is a record of how a particular detector behaved under stated conditions, not a universal size threshold.
What does a PI detector measure?
It measures a changing signal after sending pulses through its coil. Conductive targets can develop induced currents that decay, and the detector samples part of that response.
Which targets should I use for a size test?
Use copper disks of 10, 20, and 40 mm diameter, each 1.0 mm thick, measured to ±0.1 mm. Record the actual dimensions.
How should I compare materials?
Use same-size targets, such as 20 mm diameter × 1.0 mm copper, aluminum, and low-carbon steel. Run this as a separate test from the size series.
How is target depth measured here?
Measure from the coil’s lower face to the target’s nearest face. This definition keeps the distance reference consistent.
How many passes should I make?
Make at least five sweeps for each target, depth, and orientation. Record every response, not just the strongest one.
What sweep speed and coil height should I use?
This protocol uses 0.5 m/s ±0.1 m/s and holds coil height within ±2 mm of the chosen test height. These are controlled test conditions, not universal detector requirements.
Does a missed target show the detector’s minimum size limit?
No. A miss applies only to the recorded setup. Material, shape, orientation, depth, settings, ground response, and interference can all affect detection.
Why repeat a baseline target at the end?
It helps reveal drift or changed test conditions. If the baseline response differs, investigate before comparing target results.
(This article was written by one of our staff writers, Steven Langford. Visit our Meet the Team page.)
