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  Metal Detectors  
Metal Detectors :
Mention the words metal detector and you'll get completely different reactions from different people. For instance, some people think of combing a beach in search of coins or buried treasure. Other people think of airport security, or the handheld scanners at a concert or sporting event. .
Metal detectors can provide hours of enjoyment when you use them to search for buried treasure.
The fact is that all of these scenarios are valid. Metal-detector technology is a huge part of our lives, with a range of uses that spans from leisure to work to safety. The metal detectors in airports, office buildings, schools, government agencies and prisons help ensure that no one is bringing a weapon onto the premises. Consumer-oriented metal detectors provide millions of people around the world with an opportunity to discover hidden treasures (along with lots of junk). 
In this article, you'll learn about metal detectors and the various technologies they use. Our focus will be on consumer metal detectors, but most of the information also applies to mounted detection systems, like the ones used in airports, as well as handheld security scanners. 
Anatomy of a Metal Detector
A typical metal detector is light-weight and consists of just a few parts: 

1. Stabilizer (optional) - used to keep the unit steady as you sweep it back and forth 
2. Control box - contains the circuitry, controls, speaker, batteries and the microprocessor 
3. Shaft - connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height 
4. Search coil - the part that actually senses the metal; also known as the "search head," "loop" or "antenna" 
Garrett GTI 1500 metal detector

Most systems also have a jack for connecting headphones, and some have the control box below the shaft and a small display unit above. 
Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. In most cases, you sweep the coil (search head) back and forth over the ground in front of you. When you pass it over a target object, an audible signal occurs. More advanced metal detectors provide displays that pinpoint the type of metal it has detected and how deep in the ground the target object is located. 
Metal detectors use one of three technologies: 
• Very low frequency (VLF) 
• Pulse induction (PI) 
• Beat-frequency oscillation (BFO) 

In the following sections, we will look at each of these technologies in detail to see how they work. 
VLF Technology
Very low frequency (VLF), also known as induction balance, is probably the most popular detector technology in use today. In a VLF metal detector, there are two distinct coils: 
• Transmitter coil - This is the outer coil loop. Within it is a coil of wire. Electricity is sent along this wire, first in one direction and then in the other, thousands of times each second. The number of times that the current's direction switches each second establishes the frequency of the unit. 
• Receiver coil - This inner coil loop contains another coil of wire. This wire acts as an antenna to pick up and amplify frequencies coming from target objects in the ground. 
A Garrett walk-through metal detector
In addition to recreational use, metal detectors serve a wide range of utilitarian functions. Mounted detectors usually use some variation of PI technology, while many of the basic handheld scanners are BFO-based. 
Some nonrecreational applications for metal detectors are: 
• Airport security - screen people before allowing access to the boarding area and the plane (see How Airport Security Works) 
• Building security - screen people entering a particular building, such as a school, office or prison 
• Event security - screen people entering a sporting event, concert or other large gathering of people 
• Item recovery - help someone search for a lost item, such as a piece of jewelry 
• Archaeological exploration - find metallic items of historical significance 
• Geological research - detect the metallic composition of soil or rock formations 
A Garrett Super Scanner handheld metal detector
Manufacturers of metal detectors are constantly tuning the process to make their products more accurate, more sensitive and more versatile. On the next page, you will find links to the manufacturers, as well as clubs and more information on metal detecting as a hobby. 
eXp 4000
The eXp 4000 is the successor of the eXp 3000. Advanced technology allows measurements in brillant resolutions. The Handling is absolutely easy and possible without the need of a laptop. The measurement unit of the eXp 4000 includes a pc-module which what the data will be displayed immediately on the color screen. All measured data will be stored in the internal 128 MB memory and can be transfered to a normal computer to evaluate and determine depth, size and position of targets with our professional software Visualizer 3D.
The eXp 4000 has many different functions especially for treasure hunters. So it has:
Distinction of metals ( gold, silver, iron ) 
Detection of cavities ( bunkers, tunnels, burial chambers, graves ) 
Intuitive menu navigation with voice output 
Easy handling and compact equipment 
No laptop is needed for working 
To ease the work with eXp 4000 and the subsequently image evaluation the following functionality was integrated:
  • Automatic display of object differentiation 
  • Automatic signal stabilization during measurement 
  • Automatic antenna identification 
  • Pre-filtration of potential mineral structures 
The comprehensive equipment of the eXp 4000 is only one point that makes it a unique treasure hunting unit. Every single probe was optimized for its own purpose. So you can work with the following antennas:
  • GPR-Antenna ( 25cm, 50cm, 75cm, 100cm ) 
  • Antenna for metal discrimination ( DDV-System ) 
  • Antenna for tunnel detection 
  • Super Sensor for high resolution ground scans 
With a weight of only 1500g the eXp 4000 is ideal for fast searching an area. 
The integrated color monitor is also usable for night searching. 
But also in the sunlight the display is very clear in contrast to a laptop.

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