Above image showing nail lying close to a coin, a metal detector with good 'Recovery Speed' will still give clear signal indicating a good target.
Let’s start with the technology bit. Apart from a few specialist exceptions, most metal detectors are VLF (very low frequency) MOTION machines. The “motion” in the title refers to the fact that these machines need to be in motion in order to generate a signal prompted by a metal object below the ground; hence the easily recognisable sweeping action associated with the hobby. You will note that all detectors have a frequency rating. This will fall into one of three categories: Low frequency (1-6kHz), medium frequency (7-15kHz) and high frequency (16-19kHz). In simple terms the low frequency models are regarded as being the deeper seeking machines that handle iron (see below) and mineralisation (see below) well but are not the best in terms of recovery speed (see below). In contrast, the higher frequency machines have faster recovery speeds and are particularly good with smaller and thinner targets. Most introductory metal detectors fall into the middle frequency bracket.
We offer advice to prospective owners of new machines who are confused by the plethora of available models and technology. Hopefully, the information provided below will clarify these matters and enable your to make an informed decision.
Arguably the most important function that a metal detector offers. It allows the machine to “discriminate” between different metals. This metal discrimination technology allows the user to instruct the machine to ignore (”discriminate” against) metals of the detectorists choice.
The soil of the British Isles is littered with iron. It seems that no corner, however remote, is entirely free of it. Agriculture and industry over the millenia are responsible and, naturally, this is bad news for the artefact hunter who does not want to spend his time digging up rusty lumps of ferrous material. This is what makes discrimination so important (and why any detector worthy of the name should offer this function) and allows juicy targets to be retrieved from even the most iron saturated area.
Recovery speed refers to the time it takes a detector to recover from identifying a ferrous object and signal the presence of a non ferrous target. For instance, imagine a gold ring lying close to an old nail. A machine with poor recovery speed will not be able to respond quickly enough, after ignoring the iron, to flag up the gold. No signal will be received. A machine with fast recovery will identify the nail, ignore it and recover swiftly enough to indicate the non ferrous element.
In addition to all the iron, the ground contains (to varying degrees) iron oxides and other minerals which modify and moderate the magnetic field that the detector uses to identify targets (referred to as MINERALISATION). The ground balance function can be implemented to significantly reduce these effects.
Inextricably linked with depth, the higher this function is set the deeper the machine will detect. However, where mineralisation occurs the sensitivity setting will need to be moderated (reduced) accordingly. Failure to do so will compromise signal integrity and lead to “broken” and misleading signals.
This is a common question and of course an important one, it is also a very difficult question to answer as many factor influence a machines performance, which is why most manufacturers shy away from depth specifications. The main influence is the size of the object, it's simply a case of the larger the object the deeper it can be detected, for instance an average machine will detect a Victorian Penny at around 6" - 8" below the surface. The same machine would struggle to pick up a small earring at 2". Having said this most finds are retrieved within the first 8" and so most machines stand a good chance of detecting medium sized objects.