FAQ's about metal detectors

Q: How deep will a metal detector/locator go? [top]

A: This is the question most frequently asked. Unfortunately, it has no absolute answers. The following variables, in addition to your own detector's capabilities, all have an effect on the answer.

1. Conductive properties of the soil: Heavily mineralized soil will tend to reduce the penetration power of your detector. Soil mineralization may vary greatly, and you may have to re-ground balance your detector to adjust for soil mineralization; unless, of course, your detector has automatic ground balancing. In this case, you may need to decrease sensitivity and/or increase discrimination on models like the Fisher 1266-X, which has extraordinary sensitivity.

2. The length of time an object is buried: Various chemicals in the soil have a corrosive action on metal. Some metals corrode faster than others. A modern zinc penny is attacked by these soil chemicals quite easily, whereas the action on copper and silver is much less, and corrosive action on gold is hardly noticeable, if at all. As these chemicals eat away at the metal, oxidation (rust) takes place, which is absorbed into the surrounding soil. This causes the soil to become more conductive, which in turn makes the metallic object appear larger than it actually is and easier to detect. This is known as the "halo effect."

3. The size of an object: The larger the metallic object, the easier and deeper it can be detected. For example, a bucket can be detected much easier than a single coin. The more surface area seen from above, the deeper the metallic object will be detected.

4. The shape of an object: Every metallic object reradiates at least part of the signal transmitted by your metal detector. In this way, objects function like additional antennas, and consequently their shape becomes important. Ring or loop-shaped objects lying flat, on or under the ground, produce the best results; flat or dish-shaped objects are similarly easy to detect. Rod-shaped items, especially when scanned end-on, are very difficult to detect unless they're made of iron and you're using a ferromagnetic detector, such as the Fisher FX-3.

5. The degree of magnetization: With ferromagnetic locators, such as the FX-3, the degree of magnetization has a strong influence on depth. A magnet, for example, can be detected at much greater depth than an equivalent mass of iron. The more magnetization an object has, the deeper it can be sensed by a ferromagnetic metal detector.

Q: Regarding the size of search coils, is bigger better? [top]

A: Several coil sizes are usually offered for each detector. Each size offers specific strengths and weaknesses. A small coil is better than a larger coil for picking out good targets among many trash targets. The standard, 8-inch coil is an all-purpose coil. It is the most popular coil size, and it performs well under most conditions. Larger coils are best suited for low-trash and low-mineralization areas. They cover more ground and increase depth penetration 5-15 percent. However, larger coils are heavier, and some detector users feel more comfortable during long searches if they use a smaller coil. Having more than one coil size is often helpful for varying conditions you will encounter. Elliptical coils, which get into nooks and crannies easier, are also available. All Gold Bug and Gold Bug-2 coils are elliptical in shape and measure 6 1/2, 10 and 14 inches in length. A 10-inch elliptical coil is also available for the 1200-X series Fisher detectors. Although it offers slightly less depth penetration, its lighter weight makes it the favorite of Treasure Hunters who attend competition hunts.

Q: How does discrimination work? [top]

A: A better word for discriminator is perhaps "differentiator." At minimum or no discrimination, all metal within the detectable range is detected. As you slowly increase discrimination, small pieces of metallic trash and ground mineralization are ignored (rejected). As you increase the discrimination, pull tabs, small nails, foil, and even some good targets (such as gold rings and nickels) will be rejected. The best way to learn the discrimination points (the lowest discrimination setting at which an object is rejected) of your detector is to scatter some sample targets, such as coins, pull tabs, and foil on the ground from 1-2 feet apart. Starting at 0, or your detector's lowest discrimination point, scan each target. Gradually increase discrimination and record the results. With practice, you should be able to determine whether or not to dig by listening closely to the target signals.

The Fisher Gemini-3 and TW-6 "two-box" metal detectors can locate large metallic objects at great depthsobjects like pipes, cables and treasure caches. These "two-box" detectors consist of a signal-receiver box and a signal-transmitter box connected by a handle.

Q: What is the sensitivity control and how is it used? [top]

A: The sensitivity control on metal detectors is probably the most misunderstood control on the instrument. Sensitivity is usually set to its maximum level and ignored. This does not always allow for maximum operation of the detector. To use an analogy, think of the sensitivity control as the throttle of a car. You don't drive everywhere at full speed. In fact, posted speed limits are for normal conditions. But what about rain, snow, or even high wind? Of course, you decrease your speed. Likewise, you should adjust your sensitivity control for varying conditions. Heavy ground mineralization, nearby power transformers, and nearby radio stations are all reasons to lower your sensitivity. Although you might experience a slight loss of depth, you may be losing more good targets than you think by listening to the false signals and chatter of high sensitivity.

Q: Why do serious metal detectorists use headphones? [top]

A: Headphones can greatly reduce outside noise (wind, waves, traffic, etc.). They also enhance the audio target signal, which will help you determine which targets to dig and which targets to ignore. Overall, a good set of headphones will improve the number of your good finds and greatly reduce the time you spend digging trash. Headphones also reduce distractions, extend battery life and keep you from attracting unwanted attention from curious onlookers. Fisher offers two different headphone models: one standard set and specially designed Fisher Phones that give Fisher detectors superior performance. Acoustically designed for faint-target response, Fisher Phones make faint, deep target signals easier to hear. No additional batteries are required for these headphones.

Q: What are false signals? [top]

A: False signals are sometimes called "phantom" signals. They occur any time your detector responds to metal when there is actually no metal there. False signals are most often caused by naturally occurring iron oxides, such as magnetite and hematite. Magnetite is the primary constituent of the "black sand" that commonly occurs in placer gold deposits. Most Fisher models have automatic ground balancing to accommodate without a false signal but the most severe deposits of mineralization. If your detector has a manual ground-balance adjustment, you can usually tune out false signals. Sometimes you may need to ground tune frequently if the mineralization of the soil changes abruptly as you move from one place to another. If ground mineralization is excessive, turning down your detector's sensitivity may be the only solution.

Q: Which detector is best for finding large, deep targets? [top]

A: A "two-box" detector, such as the Fisher Gemini-3 or TW-6, is best for finding large, deep targets, such as caches (hidden treasure troves) or buried pipes and cables. But using a "two-box" detector on a connecting handle is different from using a standard, hand-held metal detector. Although it goes deeper, the smallest metallic object you can hope to find with a "two-box" detector is about the size of a cantaloupe melon, even if the melon-sized object is resting on top of the ground. However, that same melon-sized object may be detected at a depth of up to 6 feet with a Fisher Gemini-3 or TW-6. Maximum depth with these "two-box" detectors is achieved by carrying the unit low to the ground, suspended by a strap. An extra-long handle is available for the Gemini-3 and TW-6 that also increases depth penetration by widening the separation of the boxes on the handle.

Q: Are metal detectors safe for people with pacemakers?

The question of whether pacemakers present a safety issue in relation to metal detectors is one which we have paid close attention to over the years. We are not aware of any report of a hobby type metal detector ever interfering with a pacemaker or other electronic medical device, or having any other adverse health effect.

The magnetic field to which a hobby metal detector exposes the user in normal use is much weaker than the geomagnetic field which already surrounds us, many times weaker than the electromagnetic radio waves near a broadcast transmitter or CB radio or cellphone, and weaker than the magnetic field of walkthrough security metal detectors such as are used in airports and courtrooms.

Because hundreds of millions of people each year are exposed to the field of walkthrough metal detectors, government agencies involved in security have conducted a lot of research into the potential for interference with pacemakers. Over the years there have been perhaps one or two reports of a brief malfunction which resulted in no serious harm to the “searchee”. Because there can be no absolute guarantee that there will be no such interaction, it is customary for security to post a notice where walkthroughs are being used, advising “searchees” who are wearing or who have implanted electronic medical devices that they may be hand searched without going through the walkthrough.

The manufacturers of pacemakers design them to be resistant to interference from electromagnetic fields. An obvious concern to manufacturers of pacemakers and other implantable and wearable electronic medical devices is fields from walkthough security metal detectors. Pacemakers and most other electronic medical therapy devices require FDA approval, and reliability and safety are criteria which must be met in order to receive that approval. However the bottom line is that we have no control over how pacemakers are designed and manufactured and therefore can make no absolute guarantee what they will or will not do.

The magnetic field of a hobby type metal detector is concentrated within a few inches of the searchcoil. It is probable that most medical devices would continue to function properly with the searchcoil brought up next to them. If there were a malfunction it is unlikely that there would be any damage: normal operation would likely resume once the searchcoil was moved away from the device. Despite all that, common sense would dictate that a person with an implantable or wearable electronic medical therapy device should not bring the searchcoil up next to the device. This of course is not something that a person would normally do anyhow.

--Dave Johnson
Chief Designer, First Texas Products and Fisher Research Labs