Metal Detecting Q&A

[Steve-B]

[u]A few basics</u>

Most modern metal detectors are what are known as Motion machines (some still work on the Non Motion technology) basically this refers to the fact that on Non Motion machines no movement of the search coil is require for elicit a response to a metallic target.

Conversely, some movement is required on a Motion detector for it to respond to a target.

The main differnce are that a Non Motion machine machine has to be tuned in and normally operates with a faint hum ( the Threshold Tone) in the back ground, they can tend to wander out of tune and have to periodically be retuned by means of a reset button.

Motion machines on the other hand are silent search (no tone until a target is located), they are automatic tuning and generally a little more usr friendly. The amount of movement required is minimal and is supplied by the normal sweep pattern.

It would be wrong to state that one system is better than the other they are just different.

Concentrating on Motion machines, the two main controls are the Sensitivity and Discrimination controls.

[u]Sensitivity</u>

Loosly speaking this is the machines depth control. There are some machines available where this is a fixed setting, however they can sometimes be rendered unusable.

The setting at which the sensitivity level can be set on any machine is in the main determined by then ground mineralisation conditions, namely the ferrous oxides in the ground. The lower the level of mineralisation, the higher the setting of sensitivity that can be used and the greater the depth capabilites to a degree.

The amount of mineralisation present in the ground can vary greatly and over very short distances, so if the detector suddenly becomes erratic, this may indicate an increase in mineralisation and the need to lower the sensitvity level.

There is another system present on the detector called ground balance that also deals with this effect, but I will cover that at a later date.

[u]Discrimination</u>

The discrimination circuitry allows the detector to recognise different metals, based mainly on their conductivity level (this is a simplification as there are also other factors).

The Discrimination control allows the user to reject certain targets. The noraml discrimination control will reject everything below the setting that the user has established. In the main, detectorists use this to eliminate only small ferrous targets..from the hobby perspective, everything else is potentially a 'good' target.

It would be impossible to eliminate larger ferous targets such as horse shoes oe spear heads for example as to do so would meant settng the level so high as to also elimnate other more desirable targets.

The second discrimination system is called Notch Discrimination. In this system the user is able to select a certain 'block' within the conductivity spectrum and still find all targets below this level and all targets above this level.

Mainly this system is used by beach detectorists to help eliminate pull tabs, however I am not a fan of this system as other targets will also be lost.

other types of discrimination can be found on some of the moreexpensive machines, where they have a learn accpet/reject edit system and certain types of finds can be presented to the machine which can then be programmed to respond to those targets or ignore them.

Discrimination and target information can be presented to the user either audially via the speaker or headphones and/or visually via and LCD meter.

I am not a great fan of using too much discrimination, notch or accpet/reject programmes.

However the sophisticated the technology may appear, it is however limited. By this I mean that modern metal detectors are normally very good at informing the user of the targets conductivity level, however, they cannot effectively differentiate between dissimilar targets exhibiting similar conductivity levels.

This is a basic explaination of these functions, please feel free to ask any further questions.