Detector Discriminate Mode: Use No Discrimination for Best Detecting Results
Negative Effects of Discrimination of Iron, page 2
With high levels of Discrimination, even the most advanced metal detectors may still miss some targets if
2) targets are DEEPLY BURIED or SMALL-SIZED. TARGET DEPTH and TARGET SIZE play big roles in forming adequate phase shifts of detected targets. If a target is detected right at the outer limit of the detector's transmitted electromagnetic field, only a small portion of the target's generated electromagnetic field reaches the detector's receiver coil. The same occurs when a small target is detected. If the target is a low conductor, its induced electromagnetic field is even smaller.
In all cases, the target's phase shift may be too small for the Discriminate circuit to allow the detector to respond to it with a full signal. As a result, the user hears only a broken response or, in a worst-case scenario, no audible signal (in a Silent Search Discriminate mode of operation) or just nulling of Threshold (Audio Threshold Discriminate mode of operation) - a real deep target can cause the Threshold nulling in Discriminate mode, and you might not get a response in Pinpoint mode. The incorrect low visual Target ID (TID) most likely ends up in the FERROUS part of the Discrimination scale. If this area is fully rejected, the detector's audio response to this target will be muted by the discrimination circuit, and the target is left out.
In this case, decreasing Discrimination will let you hear the deep or small targets not only because less filtering will be implemented, but also because the reduction of Discrimination inversely works like increasing Sensitivity without causing false signals. So now the weak electromagnetic fields of deep or small targets will be enhanced to manifest themselves with adequate phase shifts in the visual/audio Target ID circuits.
NOTE: Deep low-conductive targets in magnetic soil may not be detected at all if hot rocks are rejected, or Discrimination is set too high.
And even if you use least Discrimination, you may still miss some deep and small non-ferrous targets because
3) MAGNETIC IRON MINERALS are present in the ground. Discriminate circuits of today's advanced metal detectors can simultaneously reject unwanted metals and reduce the ground mineral effect. However, muting responses to IRON is the main objective of Discrimination. And when millions of magnetic iron particles generate the electromagnetic field of high intensity, the weak responses to the deep small- and medium-sized non-ferrous targets become lost in the overall noise when Mineralization is high, and/or Manual Sensitivity is set above the detector's stability level (read details on Sensitivity here and here).
Even if the Sensitivity level is properly adjusted (high sensitivity circuit noise is eliminated), but the Discrimination level is still high, the TIDs of medium-sized targets get distorted and are indicated with very low Conductivity readings on the 1-Dimensional Conductivity scale, or with low FE values in the FE-CO read-outs on the 2-Dimensional Discrimination scale.
The more Discrimination, the higher the probability of these distorted Target IDs to end up in rejected area on the Discrimination scale because their phase shifts become similar to those of rejected iron targets. If these readings "land" in the rejected area, the corresponding low-tone audio responses are turned off by the Audio Discriminator, so you are not aware of the valuable targets being present under your search coil. The visual read-outs of these non-ferrous targets also get distorted. As you receive neither audio nor visual indication of the detected good targets, you simply pass them up.
Factors That Can Cause Accidental Rejection of Coins by Discriminate Circuit:
1) Coin's Depth.
The deeper the coin within the detecting range, the less accurate the coin's target ID. This causes the visual target ID to shift lower on a Discrimination scale. If Discrimination is used, and the coin's VDI number ends up in the rejected area, the detector's response to the coin is muted. In a worst case, the Discrimination and Sensitivity levels may be too HIGH to allow a detector to penetrate ground minerals and generate an audio response to the coin's weak electromagnetic field.
2) Coin's Size.
The smaller the coin, the weaker its generated electromagnetic field, the less accurate the coin's target ID. Like in the previous case, this causes the visual target ID to shift lower on a Discrimination scale. If Discrimination is used, and the coin's VDI number ends up in the rejected area, the detector's response to the coin is completely muted. In a worst case, the Discrimination and Sensitivity levels may be too HIGH to allow a detector to penetrate ground minerals and generate an audio response to the coin's weak electromagnetic field.
3) Coin's Position in the Ground
A metal detector may respond to coins "standing" on edge in the ground or perpendicular to the search coil with signals similar to nail responses. In particular, when the search coil is moved along the axis of the coin's edge, false readings of lower CO values may be displayed on the detector's screen. If iron is REJECTED, the coin's visual ID is likely to end up in rejected area of the Discrimination scale, and there will be no audio response (you may want to read my article "Search Coil Detecting Techniques: Pinpointing Coins On Edge").
4) Coin's Metal Content
Coins with low conductive properties are more likely to be rejected when ample Discrimination is used. This category of coins includes nickel-alloyed coins, small and thin gold coins, and small ancient Roman coins, just to name a few. As you can see, not just the coin's metal content is responsible for the coin's conductive properties, but also the coin's size (surface area) and thinness. But, of course, the low conductive coins are more susceptible to the effects of Discrimination and other negative factors than the high conductive coins. When the Discrimination level is set too high and/or hot rocks are rejected, the deep low-conductive coins in magnetic soil may not be detected at all.
5) Coin's Oxidation
Only gold and gold-alloyed coins have the greatest resistance to their surface oxidation. Silver coins oxidize when subjected to salt environment (sea water) and highly acidic soils. Coins of all other metal contents, especially the low conductive nickel-alloyed coins like the US 5-cent pieces, if buried in corrosive ground over a long time, acquire heavily corroded surfaces and spread them into the surrounding soil developing halo effects.
These halo effects may work to the advantage of the low-conductive coins when the ground is moist. However, a problem arises when the low-conductive non-ferrous targets, such as pull tabs, are rejected by Discrimination, and you face a case of a good response suddenly vanishing during target recovery (see details in my article "Disappearance of A Good Signal"). Many valuable coins are regularly left undetected because of the non-ferrous Discrimination.
6) Coin's Defects or Deformed Shape (broken and bent)
So called Eddy currents play a big role in generating an electromagnetic field around a target. When a metal detector’s transmit electromagnetic field is present, the tiny electrical Eddy currents are induced off the target, and then the target's generated electromagnetic field can be detected by the metal detector's receiver coil. For example, if the detector picks up a coin that has been cut, chipped, or even bent by a plow or lawn mowers, the coin's Eddy currents will be too degraded. As a result, the coin's electromagnetic field will be too weak to be registered with the VDI numbers (FE-CO numbers on FBS detector's display) that are typical for a whole and round coin. And the coin's audio signal will have a low tone pitch accordingly.
In my experience, I recovered a few coin-halves with the XP Deus metal detector (it incorporates a 1-dimensional Discrimination scale), and their VDI values were half of the VDI values of corresponding "whole" coins. Similarly, the gold rings with breaks in their bands may read very low or not at all on the screen.
7) Worn Coins
The coin's surface texture plays a big role in Eddy current generation. If no design relief is present on a coin surface, the coin's generated electromagnetic field is too weak to be correctly classified by the Discriminate circuit and trigger an adequate audio response. As a result, the coin signal sounds lower than it is supposed to, and the coin's visual ID is registered at the low end of the non-ferrous area on the Conductivity/Discrimination scale, if not in the ferrous area. The higher the Discrimination, the more chances the coin is rejected.
To sum everything up, decreasing Discrimination has been proved as the most practical and effective approach to increasing your detector's ability to recognize more deep and small targets.