How to Air Test (Bench Test) Various Non-Ferrous and Ferrous Targets
Learn Your Metal Detector's Depth Potential, Audio and Visual Responses to Targets
Generally, Air Test (Bench Test) is a test to determine the maximum sensitivity of your metal detector to various targets that are manually passed across a search coil under IDEAL conditions, i.e. there is nothing but air between the search coil and the targets.
Setting up your Air Test and starting learning your metal detector's language through simple testing procedures is the first thing that you MUST do after reading and re-reading the User Instruction Manual as many times as necessary to understand your detector's capabilities.
Do not even think about taking the metal detector outside and starting metal detecting right away unless you need a good doze of frustration and self-pity. There are certain times for certain things, and as many of us - weathered treasure hunters, learned it back in the day, the smaller steps you take "hiking up" a steep learning curve, the more successful you will be after you get on top of it.
Through Air Testing with the current search coil to be regularly used, you foremostly FAMILIARIZE YOURSELF WITH YOUR DETECTOR'S AUDIO RESPONSES TO VARIOUS TARGETS, and then will be able to obtain the following important information:
1) Detector's DEPTH POTENTIAL - DISTANCE LIMITS of audio responses to desirable targets.
The Depth Potential for targets can be observed only during the Air Test. Under real metal detecting conditions, the detector's actual OPERATIONAL DEPTH RANGE for the same targets will never surpass the air test results, and may only reach close to them if you detect on neutral ground that is free of iron junk, using the same search coil and utilizing OPTIMAL program settings.
Your detector's DEPTH PENETRATION ability will most likely be challenged as well under real conditions of metal detecting (see details on factors that negatively affect the metal detector's depth penetration in the same above-mentioned article). Nevertheless, using the air-test results for fine-tuning your search program settings will enable you to optimize your detector's performance.
Your goal is to fine-tune your metal detector to the point of establishing the Operational Depth Range as close to the detector's Depth Penetration as possible. Just keep in mind that depthwise the DEPTH POTENTIAL is superior to the DEPTH PENETRATION which is superior to the OPERATIONAL DEPTH RANGE (DETECTING RANGE).
2) VDI (Visual Display Indicator) numbers (also called Conductivity numbers, VID (Visual ID) values, or Conductive properties) of desirable and/or undesirable targets, which can be used for adjusting Accept and Reject ranges of Discrimination, or NOTCH Discrimination, on the 1-dimensional Discriminate scale.
On the 2-dimensional Discriminate scale of FBS (Full Band Spectrum) metal detectors, FE-CO numbers (FERROUS and CONDUCTIVE properties of targets) are obtained through the air test and used for creating Discrimination patterns, or just "dots" of Discrimination on a Smartfind screen.
If you use a FBS metal detector (Minelab Explorer, E-Trac or CTX 3030) which incorporates the 2-dimensional Discrimination scale and utilizes the 'LEARN' feature in AUTO EDIT mode, the air testing is a good way to "teach" your metal detector which targets to accept or reject. Just make sure that the Sensitivity is set on a moderate level to avoid erratic visual Target ID responses bouncing all over the 2-dimensional Discrimination scale ('Smartfind' window on Minelab FBS detectors).
3) WIDE RANGE of VDI numbers (Conductivity values) or FE-CO numbers of nonferrous targets for creating a "Target VDI Chart", also called "Targets Conductive Relationships Chart" or "Targets FE-CO Relationships Chart", to be used for your references when learning your metal detector's responses to various targets in a field.
4) DISTANCE LIMITS of Visual ID (VID) responses to various targets. It is important to know at what maximum distances from the coil various size targets produce signals that are strong enough to trigger the visual Target IDs.
Just like the depth range values for the targets' Audio IDs, their VID depth ranges will be smaller under real detecting conditions and vary depending on the ground mineral content level.
Nevertheless, using the referential test data will be helpful in learning your machine and making decisions whether or not to dig up "difficult" targets.
Always follow this rule: when you receive a good signal, and it stays good at any direction of your search coil sweep, and a depth gauge indicates a deep target, and there is no VDI reading on a display, dig the target up! In many cases, it is a valuable!
Please keep in mind that there are also "Factors That Affect VDI Readings of Non-Ferrous Targets" during metal detecting under real conditions.
How To Set Up and Conduct Air Test To Determine Your Metal Detector's Depth Potential and Learn Audio and Visual Responses to Targets
If you do not have a "Coins" search program customized for specific metal detecting conditions in your area, it is best to use a factory preset program "Coins" (or "Coins and Jewelry") as a basic program to be modified during the air test.
And, of course, you need coins, including silver ones, of different denominations and a variety of other items consisting of both the valuable targets (gold and silver rings, earrings, metal buttons, etc.) and junk targets (foil, screw caps, beer bottle caps, pull tabs, brass casings, nails, etc.). The more, the better! Just make sure you know the metal composition of each item.
To conduct Air Test, you should follow these steps:
1. Place your metal detector horizontally on a non-metallic surface. If it is a table or bench, make sure a search coil is not near any screws, nails, or bolts. I suggest to conduct air testing outdoors - away from any house wiring, electric devices or sources of electromagnetic interference (EMI), like in the field. But first locate a spot on the ground free of metal and highly non-conductive objects such as hot rocks and black sand deposits as well as "hot mineral pockets" of high intensity.
2. Set a search coil perpendicular to the ground surface, and onto a 16-inch (40cm) long plastic or wooden ruler in the way that the search coil's edge would rest at a "0" mark (this is a set up for steps ).
NOTE: Do not place your detector's control box too close to the search coil or the control housing may disrupt your air testing.
3. Remove your wrist watch and all jewelry from your hands. Also take off your jacket or windbreaker if it has metal buttons on sleeves.
4. Turn the Sensitivity control to maximum.
5. Set Discrimination at zero (All Metal mode).
6. Set the Ground Rejection Level at 75%-85% by adjusting the Manual Ground Balance control (if it is available).
7. Set your metal detector's Audio in MULTI TONES (if this feature is available and not to be confused with just a 3-tone (low, medium and high) audio mode). In the Multi Tones (usually around 250 tones) audio mode, for example, audio responses to silver and copper coins will be of a high-pitched tone while responses to gold items and foil will sound in lower tones, and iron targets will produce the lowest buzz.
It is important to memorize tonal IDs of certain targets and train your ears to discern sound characteristics of various types of targets, for instance, classified by shape, such as coins and tokens (disc shaped targets), brass casings (targets of complex but regular shape), foil and can slaw (targets of irregular, sometimes complex shape), etc.
8. Prepare a few wooden or plastic slats with coins attached to them. It is much easier to observe an accurate distance reading on the ruler when using a coin slat than when just holding a coin in your hand. I use the slats made of transparent Plexiglas, and scotch the coins to them to be able to see without confusion what coin I am actually testing. Initially I made these sharp-pointed slats for the "Depth Penetration Test" (a link will be posted soon) to determine effects of current mineralization on the detector's Depth Penetration ability, which would require insertion of the slats into a side wall of a test hole.
Get Familiar with Targets' VDI Values and Record Them into a Target VDI Chart
9. Pass the coin slat in front of the search coil and observe which coin's position in relation to the coil makes the detector respond with the strongest and clearest signal. Obviously, when the coin is parallel to the search coil's plane and positioned on a height level of the coil's center, the audio response sounds the best, and the coin's VDI readout is not changing with every sweep of the slat.
This is when you should record the coin's VDI value into your Target VDI Chart before you proceed further with steps described below.
After you complete your chart, your goal will be to memorize which conductivity zones are occupied by which metals or metal alloys, and at which points of the Conductivity scale VDI readings (conductivity values) of certain targets in focus (specific coins or jewelry) "landed".
Of course, you will not be able to memorize all numbers right away, that is why, later, in the beginning of your metal detecting activity at a real hunt site, you should carry your VDI chart with you so that it would help you navigate among various VDI readouts of real targets. The VDIs of some real targets may be slightly different but you should not worry about it at that point.
Learn Your Detector's Audio Responses to Various Targets
You will soon learn that superficial coins emit "clean", "solid" and "unmistakable" signals because most metal detectors are calibrated on coins (targets of a perfect disc shape). Audio responses to the rest of the non-ferrous targets will vary in their sound characters according to the targets' types categorized by shapes, metal compositions and other properties.
If your hearing capacity is average or better, you, for instance, will be able to hear difference between sound characters of a .75 cal musket ball and a 1982 Memorial cent, even though both targets have the same VDI values and, therefore, audio responses to them would have the same high-pitched tones. The more you practice, the more sensitive to slight changes in sound character your ears become, provided your metal detector is designed to give you such information.
Obtain Depth Potential for Assorted Targets
10. Gradually move the coin slat outward until you can just barely hear the response. The "stop point" should be determined based on your judgment: whether you would dig up a target emitting this signal or not in real detecting situation. Then the test results would be fair. I suggest to designate stop points at the spots where the responses to targets become faint but still sound "two-way".
11. Record the obtained distance ("depth") for the coin in a "Target VDI Log" (if you are testing just this particular coin with different settings of Sensitivity, Ground Balance, Discrimination, etc.), or in a "Target VDI Chart" if you are air testing various targets, both desirable and undesirable.
12. If you have a search program that you will be using for your upcoming metal detecting project, adjust the Sensitivity, Ground Balance, Discrimination, and other key-settings according to this program, and repeat the test with targets in focus. Record all test data and compare with previous results. This info will be essential for your understanding how different program settings interact with each other and affect the detector's responses to desirable targets. You may want to air test targets in focus with a larger search coil to see how different the detector's Depth Potential for these targets can be with a different coil. But this will be just "half job" done...
Optimize Your Search Program Through "Mineralization/Depth Penetration Test" or at Test Garden
Once again, under real conditions, there are many variables that present obstacles to the detector's transmit power and, therefore, to its Depth Penetration. The actual VDI readouts and operational depth ranges for the same targets may greatly deviate downward from both the Air Test and Mineralization/Depth Penetration Test results.
If you construct an outdoor Test Garden, you will have almost real metal detecting conditions for testing various targets and optimizing your search program settings. You will certainly be able to factor the combined effects of local mineralization ( if it is high, the High Mineralization Effect will take place) and surrounding iron content (Mixed VDI Value Effect) in test conditions. The "job" will be done after you complete your search program optimization. And then you will be ready for real action!
