Metal Detector's Discriminate Mode: Less Discrimination Lets You Find More

Negative Effects of Ample Discrimination, page 1


There are many variables in the equation for successful metal detecting, and Discrimination is as important a factor as the detector's Depth Penetration (see details below) or the hunt site's potential when it comes to getting the most out of any metal detecting site.

To avoid passing up many valuable targets at the hunt site, you need to understand the negative effects of Discrimination and prevent them from interfering with your search process.

Just to refresh your memory, Discrimination is the metal detector's mode of operation allowing a user to intentionally mute detector's responses to different metal types starting with iron.

The Discriminate circuitry uses the 'Phase Shift' related information to function. Phase Shift is the duration of time required for the detector's transmitted electromagnetic field to return to the searchcoil's receive loop in a form of electromagnetic field generated off a detected target.

The high-conductive targets (i.e. silver and copper coins) are INDUCTIVE and have large phase shifts, and the low-conductive targets (i.e. iron nails) are RESISTIVE and have small phase shifts. When a target is detected, the Discriminate circuit measures the phase shift and references it against your set-points either on the 1-Dimensional Discrimination scale - Phase Shift scale exhibiting only conductive properties of detected targets, or your rejected/accepted areas in the Discrimination pattern - a 2-dimensional Phase Shift scale exhibiting both the conductive (CO) and ferrous (FE) properties of targets.

If the acquired phase shift value "lands" into any accepted segments/areas of the Discrimination scale, the Discrimination circuitry does not filter it out and allows the detector to respond with both visual indication (numeric value - VDI number, or FE-CO numbers, and a graphic indicator such as a notch on a Conductivity scale or Target Crosshair in the Smartfind window - a CO/FE scale) and an audio signal.

If the phase shift value "falls" into any rejected segments/areas of the Discrimination scale, the detector is not allowed to respond with an audible tone, but may still display the visual indication. The latter is provided by the Visual Target ID circuitry that measures the phase shift but do NOT filters it out like the Discriminate circuit. The visual Target ID (TID) circuits operate INDEPENDENTLY of the Discriminate circuits to provide the visual TIDs (VDIs) even when minimum or no Discrimination is used.

Many enthusiasts believe that "Less Discrimination = More Detection Depth". However, this is incorrect. Although using less Discrimination indeed leads to receiving more audio signals that indicate deep targets, this effect has nothing to do with DETECTION DEPTH. You may want to read my short article "Quieter Operation of A Metal Detector vs. Detecting More Deep Coins" (it is included in the "Search Programs for Minelab FBS Metal Detectors" section) for details on myths about Sensitivity and Discrimination.

DETECTION DEPTH (more often called DEPTH POTENTIAL) CAN NOT be adjusted by an operator unless the detector has a specific feature (like a 'TX Power' setting of an XP Deus metal detector) especially designed for adjusting the detector's TRANSMIT POWER. Otherwise, the Detection Depth can be only reduced by adverse effects of ground mineralization; thus, becoming a different characteristic - DEPTH PENETRATION. However, the detector's Depth Potential can be increased by using a larger search coil.

DEPTH PENETRATION reflects the depth to which the metal detector's electromagnetic field can penetrate the mineralized ground matrix so that the detector can respond to targets lying at that depth. To learn what negatively affects the Depth Penetration, read my article "Metal Detector's Depth Penetration". The search coil's size and type can also affect the Depth Penetration.

DETECTING RANGE (also called OPERATING DEPTH RANGE), on the contrary, can be easily REDUCED (impaired) by either the operator's own wrong-doing in adjusting the detector settings and modes or using search programs that are unsuitable for particular metal detecting conditions (read details in my article "How To Prevent Reduction Of Operating Depth Range"). This is why the Detecting Range never exceeds the Depth Penetration which, in its turn, can never exceed the Detection Depth and can be equal to it only during an Air Test. Under real search conditions, only a highly experienced detectorist can make the Detecting Range's outer limit reach very close to the outer limits of Depth Penetration and Detection Depth of the metal detector.

If you decrease the Discrimination level to minimum or zero (All Metal mode), yes, you will notice an improvement in sound characteristics of some audio responses to non-ferrous targets lying close to nails, and hear more responses to desirable targets, both deep and small. But you will also have to put up with a lot of additional noise indicating the now-accepted iron targets, some of them exhibiting both ferrous and conductive properties - Iron Falsing, and mentally decipher all that "audio rainbow". At least, it will not be as crazy as when you crank up the Sensitivity level (see details here).

If right away your metal detecting results noticeably improve - the ratio of junk targets to good targets brakes in favor of the good targets, you have successfully overcome one of the negative effects of Discrimination of iron. When it is employed, the following factors greatly affect recognition of desirable targets lying within the detector's Operating Depth Range:

1) Using A LOT OF FILTERING (ample Discrimination) may lead to MASKING of desirable targets lying in close proximity to rejected targets if your detector does not have a high RECOVERY SPEED (also called Response Speed, Recovery' and Reactivity) - time duration required for the detector's circuitry to recover from the last detected target before responding to the next. The Recovery Speed is a vital setting that determines the detector's performance in terms of speed of analysis and selectivity. When even a minimum Discrimination is used, detectors with long recovery time are prone to "ignoring" targets lying close to rejected targets.

For example, when a detected target is rejected, the audio response to it is turned off by the Discriminate circuit. And this response remains inaudible during the recovery time of the detector. If an accepted target is detected at this moment, the detector will not respond to it either.

On the "1 to 5" Reactivity scale, the Recovery Speed of most metal detectors on the today's market is 1. The Minelab E-Trac's Recovery Speed may reach 1.5 if a setting 'FAST-ON' is enabled, and the ground is neutral or low-mineralized. The Minelab CTX 3030's Recovery Speed may reach close to 2 under similar conditions. And the XP Deus has the fastest discriminate circuit as its Reactivity can be set up at 5!

By lowering Discrimination or not having any at all, you free the detector's electronic circuit of excessive filtering so that the circuit can process the detected target information faster, and you can hear better sounding responses to targets that have been within PENETRATION DEPTH all the time. That is why the Detecting Range will always be greater in the All-Metal mode of the detector's operation than in the Discriminate mode under any search conditions.

In terms of target separation, the modern Multi-frequency detectors have an advantage over the single- and dual-frequency metal detectors because, for example, the 28-frequency FBS metal detector receives 28 times more target information than a single-frequency machine, in addition to new advanced features such as FERROUS-COIN Target Separation incorporated into the latest model of Minelab - CTX 3030. However, if the Recovery Speed is low, and some Discrimination is employed, even with the advanced Multi-Frequency transmitter, you will still miss quite a few targets.

Please Do Not Use Any Discrimination At This Metal Detecting Site


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