VLF Discrimination, Multi Frequency, Speed

[Decanfrost]

Hi,

Regarding the VLF detectors.Can someone answer for me what constitutes a VLF with good discrimination?.I feel to discriminate that speed is a factor as well.Otherwise no matter how good,if a target is next to iron it will read off a good hit.

The Deus has fast reactivity,so is this now a good discriminator.Between ferrous and non ferrous?. How does this compare to say the Nokta Impact for speed discrimination.

Thanks in advance

Ash

[Steve Herschbach]

Speed is a factor but so is depth. To get the best depth possible requires gathering as much information about the target as possible, and then relaying it to the operator in a useable fashion.

As a general rule target separation (recovery speed or reactivity) and maximum depth fight each other. For maximum depth it is best to have a target in the clear (no nearby targets) and a fairly slow sweep speed to give the detector plenty of time to gather information from the target. Using more than one frequency to analyze the target helps, and due to processor speeds this has also made for slower detecting requirements to get the most accurate target id at maximum depth. Finally, delivering a nice, clear audio signal to the operator also takes time.

However, as you note if things are too slow, adjacent targets can "run over" each over and in overlapping cause target id inaccuracies or targets to be missed completely while the detector is still thinking about the last target. By speeding up the detector reaction time you can help separate targets that are close together.

The problem when you speed things up you give the detector less time to analyze the target resulting in less information to work with, which hurts target id accuracy and depth. Also, the audio response must be speeded up also, which means shorter or "clipped" audio responses that can be harder for some people to hear. Older machines like most White's have a large "boombox" sound on targets and older Garrett's have the "bell-tone". Newer fast models like the Deus have very short beeps or even "bips" that indicate the target but that also require faster mental processing.

What we are discussing is usually called "Recovery Speed" by most manufacturers. From the White's XLT User Manual:

"Recovery Speed - Speeds target responses, so several targets that are close together can each respond.

When a metal is detected, it takes a fraction of a second for the detector to process the signal before it can respond to another metal target nearby. The time it takes to process the first metal target signal so that the second metal target signal can respond is called RECOVERY SPEED.

There are advantages and disadvantages to fast (high numbers) and slow (low numbers) RECOVERY SPEEDS. Faster RECOVERY SPEEDs work well in high trash areas. However, they will have some difficulties with very deep targets as well as double responses on shallow targets. Slower RECOVERY SPEEDs do not work very well in high trash areas. However, they will have better responses on very deep targets. Slower speeds also have more definitive discrimination sounds. A custom setting needs to be found that suits the preferences of the individual and the conditions in the area. As a general rule, the closer together the metal targets are in an area, the faster the recovery speed should be. The more spacing between targets, the slower the speed should be. Don't use the fast speed if you don't need to.

In very trashy areas it is recommended to switch to a loop smaller in size than the standard 9.5 inch black loop. Smaller loops offer better separation between targets. However, larger loops detect deeper and cover more area with each pass. RECOVERY SPEED combined with a smaller loop can be used to search severely trashy areas."

Just to confuse people White's decided to call it "Recovery Delay" on the V3i. A low recovery delay equates to a fast recovery speed.

From the White's V3i User Manual:

"Recovery Delay - 1 – 200 200 = slowest. Additional and separate (beyond filtration) selection for the signal response time. Short response time benefits performance in high trash by providing better target separation. A longer response time allows a larger window to detect deeper targets. Ideal Recovery Delay is dependent on Ground Filter selection, ground mineralization, trash density, and your average sweep speed (how quickly you move the search coil)."

It would seem detector manufacturers abhor standard terminology, even the same manufacturer! XP has decided to call Recovery Speed by an even newer term - Reactivity. From the Deus User Manual:

Note this quote from the page above "In terms of pure performance, the greatest detection ranges are obtained with low reactivity levels. However, you will find more targets and will be deeper on mineralised ground with medium or high reactivity levels."

It is important to note that it is not just adjacent trash that can mask targets; adjacent hot rocks can also mask targets. Fast recovery times can aid depth in highly mineralized ground by allowing more ability to "see between" the hot rocks which can make up a large portion of the ground matrix.

The Nokta Impact has a fast recovery speed or reactivity setting analogous to one of the medium Deus settings. Personally I could discern little difference between the Nokta / Makro detectors and the Deus when it comes to depth and recovery speed during in field testing as long as coil sizes are about the same. You can find opinions on both sides of the aisle I am sure as to "which is best" but as far as I am concerned they are close enough so as to not really matter to me.

As a rule detectors with fast recovery times have been a specialty of the European manufacturers whereas U.S. manufacturers and Minelab have focused on target id accuracy at maximum depth. What I refer to as "Euro machines" excel at target separation and ferrous/non-ferrous separation but are not noted for being very good at target id accuracy. The Minelab and "U.S. style detectors" tend to have more accurate target id. This makes Euro machines better for field and relic type detecting, and U.S. type detectors better at cherry picking certain target id numbers in park and groomed yard scenarios. I have a detailed article about this at European Detectors Versus U.S. Style Detectors On Target Masking.

The bottom line is there is a trade off between fast recovery speed (reactivity) and the most accurate target id at maximum depth. Fast recovery times provide better separation of adjacent targets, while slower recovery speeds aid in maximum depth. The ideal detector has a control like the Deus reactivity setting that allows a person to customize the detector for different ground and target mixes.

My final tip reiterating what White's said above: using small coil sizes on any detector can provide benefits that match or exceed faster recovery speed in dense trash.