Metal Detectors

There are two main types of metal detector on the market at the moment. They are:

Transmit Receive (T.R). Also known as non motion machines.

These were first developed in the 1960’s. They need to be manually tuned and re-tuned every few minutes due to the signal drifting. This type of machine is normally less expensive but has the disadvantage of suffering from ground effect.

Motion machines.
A motion machine is a fully automatic ‘ground cancelling’ detector. In most cases you turn it on; it tunes itself and retains its optimum tuned state automatically because it is constantly re-tuning itself. You cannot stop moving the search coil over a target as it will tune out. The coil therefore has to be moving to be able to detect anything, hence the term motion machine. This movement is the swing from left to right and back which is your search swing.

All detectors are affected by the conductive minerals in the ground. TR machines suffer more than motion machines in this respect. The effect these minerals have varies from one piece of ground to the next, with no way of determining what is ‘clean ground’ (with little or no ground effect) or what is bad ground (highly mineralised).

To test this, on inland or above the high tide mark on a beach you would find negative ground and typically a TR machine would lose its sensitivity (depth) on this type of ground but a motion machine would perform at a greater depth. If you now walk down onto the wet sand below the high tide mark on the beach you would now have a positive ground effect. This is where the detector is giving a signal when it detects the salt water conductive sand the same as it would as if picking up metal.

If the TR machine has a discrimination control you can get rid of this effect by turning this up and rejecting the positive effect. If you are using a motion machine, you would turn the sensitivity control down to reduce any false signals.

Discrimination
Most metal detectors will allow you to discriminate between good targets and bad unwanted items. There is a safe level you can set all discriminating detectors to where you will reject most of the unwanted items but still find the good stuff.

Depth
Metal detectors are ideal for finding buried objects but the question most commonly asked is how deep they will detect? Unfortunately this is a question we cannot answer with any accuracy. The depth of a detector will depend on many aspects:

The type of metal detector
The technology used for detection is a major factor in the capability of the detector.

Size of the object
The larger the object you are detecting, the greater the depth it can be detected at. For example if we take a £1 coin certain detectors will pick it up at 6-7 inches in air. In the ground the distance will be less due to ground effect, which is arbitrary. If we now take a handful of £1 coins it could be up to 12 inches, a box full of coins around 18-24 inches, PC sized container full of coins 2-3 feet and a car at 4-5 feet.

The type of metal
Different metals have different conductivity and are easier or more difficult to pick up. For example gold & silver are good and stainless steel is poor.

How long has it been in the ground
If you bury a 2p piece in your back garden at 6 inches you probably wouldn't’t be able to pick it up. If it was left for 6 months for the soil to stabilise, the coin will now be in range. If left undisturbed the detection range will increase slightly as the metal leaches slightly into the surrounding soil.