- Posts: 9529
- Joined: Sun Nov 27, 2011 8:21 pm
- Location: North Lincolnshire
- Has thanked: 1550 times
- Been thanked: 2304 times
What causes a metal detector to produce a sound ?
A detector’s search coil is actually two coils in a single unit, comprising a ‘transmit’ and a ‘receive’ coil. When switched on, set-up correctly and with no metal target present, the two coils transmit a balanced magnetic field. When a target comes within range of the search coil, it's magnetic field causes a small electrical eddy current to be produced on the surface of the target/find. This in turn produces a secondary magnetic field which affects the balance of the search-coils to produce an audible response (beep).
On wet sand my detector repeatedly indicates a target but nothing is found when I dig a hole
Detecting on dry sand with most metal detector holds few problems for detectorists. However, on salt wet sand, spurious signals can occur. There are several causes of such signals but they are usually due to the electrolytic reaction of salt water on the sand rendering negatively charged minerals to become ‘positive'. This has the effect of overwhelming the search coil’s balance causing the detector circuitry to produce a signal response.
Why do some stones, coke and other non-metallic items produce a signal?
This phenomenon can be experienced on both beach and inland sites and is due to the electrical properties of a wide range of items. Coke, some pottery and earthen-ware are examples.
These items have ‘capacitive’ rather than ‘conductive’ properties. When such targets come within range of the magnetic field, they absorb and momentarily retain, a small amount of the electrical charge produced by the magnetic field. This in turn, generates a secondary magnetic field creating a change in the coil’s state of balance and resulting in a positive target response.
Tip:- To differentiate between the response from a stone and that from a metal target, the target should be checked from several different angles. The audible signal produced by a stone or similar capacitive object, will be audibly identical each time. This will rarely be the case when locating metal targets unless they are uniformly circular in shape - e.g. coins/washers etc.
Will a larger search-coil locate deeper targets?
There is much conjecture on this subject but it’s commonly felt that any depth increase over and above the ‘standard sized’ coil will be relatively modest. Large coils are said to find ‘large’ targets at greater depth than a standard coil, which is why they are generally preferred for searching for hoards and larger artefacts. Large coils are less sensitive to the smaller desirable objects such as coin fractions (cut halves and quarters). Large coils come into their own when searching a site of large acreage as their size allows greater ground coverage in less time than a standard coil.
Can I use any search-coil on my detector which has the same coil connection?
Whilst it is possible, it would be wise to check with the manufacturer first as there can be several incompatibilities such as coil fittings, wiring and operating frequency variations.
Why does my detector sometimes produce a back-ground humming noise on some fields?
Assuming the anomaly is not due to operator settings, the culprit may be one or more of the following:
1. The presence of high voltage overhead power lines nearby. In rural areas, power supply lines are sometimes laid underground. In both cases the interference level may vary becoming more pronounced nearest the source or during peak supply times.
2. Electric Fences - These produce an audible pulsing signal which varies in strength depending upon proximity. Moving away from the source usually improves the situation.
3. Some detectorists report interference from their mobile phone, switching the phone to ‘Airplane' mode is said to resolve the issue.
Why does a detector sometimes produce a ‘signal’ when it’s knocked or during searching?
This is quite a common and there are several possible causes.
1. If wrapped very loosely around the base of the lower stem, the coil cable may be within detection range of the coils magnetic field. To prevent this, the first 30cm or so should be fed straight up the lower stem, secured at the 30 cm point and then coiled (not too tightly) around the upper stem and secured immediately at the point where the coil cable plug enters the control box. This will prevent the majority of ‘false signals’ occurring.
2. Although these items are usually constructed in plastic, some detectors still have a coiled metal cable protector fitted over the coil cable at the control box plug. To prevent this device from sliding down the coil cable, it is held in place by being partially inserted into the cable's plug. Over time they can ‘work’ their way further into the cable plug resulting in an intermittent short circuit of the cable terminals. Remedy – occasionally give the coiled cable protector a light ‘tug’ but be careful not to pull it out altogether!
What is a Pulse Induction detector and what are the advantages/disadvantages in using them?
Pulse Induction (PI) detectors are generally regarded as a ‘specialist’ machine for use in specific conditions. They differ from VLF machines in several ways. There are both advantages and disadvantages over the usual VLF detector.
1. They are amongst the deepest seeking commercially available metal detectors.
2. They are very sensitive to metal targets (especially small ferrous targets – tiny nails etc.).
3. They’re not affected by salt wet sand.
1. They currently lack effective discriminating ability (some experienced operators claim an ability to differentiate between ferrous/non ferrous signals).
2. They require higher capacity batteries or an increased number of single cells.
3. They are much heavier machines.
4. ‘Pinpointing’ a target accurately by using the search-coil, is difficult to master effectively.
5. Although ignoring most levels of ground effect, they are still capable of producing spurious signals when encountering particularly highly mineralised targets e.g. hot rocks.
How does a Pulse Induction machine differ in operation?
In operation a Pulse Machine produces a strong magnetic field by issuing very powerful and rapidly occurring ‘pulses’ of electric current (up to a thousand per second).
Between each generated pulses the search coil effectively ‘switches off’ and becomes a receiver ‘listening in’ for a response which would indicate that a target has been located.
Once a target is 'sensed' during the coil's 'switching-off period', a positive target indication is produced.