sensitivity settings

[jtalley007]

I'm not that sophisticated but for relic hunting and beach hunting I run manual as high as I can and back down as needed. In 41 years and every detector known to man the CTX is the deepest in good ground. I think the noise cancel helps get the extra edge as well. Dug a measured civil war bullet in wood 16" deep in a pounded place that has bad EMI. Some days I can run the 3030 on max 30 with 75% gain. Manual, you get to decide depth-sensitivity, In auto if you have EMI it will dial you down below what's necessary. JMHO

 

["Dr.Tones"]

Auto +3 for me. Very seldom do I change it. If I do it's down a notch or two. I think digger describes it best with the "headlight" analogy. Too much sensitivity is like driving in the fog with high beams. (picture "fog" as mineralization)

 

[laplander]

Auto is great here in the States but I have personally found manual the best for tiny coins on UK trips. These guys are as small as 9mm in (farthings, 1/4 staters) diameter and it doesn't matter if their gold are silver they low tone. My point is if you think you know it all your eyes are half open, the 3030 is an awesome detector only limited by the detectorist

 

[Johnnyanglo]

My understanding (which is probably flawed):

The CTX (and E-Trac) automatically measures the ground signal as you sweep. The detector receives (and xmits) on two channels (freqs), low and high. It uses the ground signal to normalize the receive waveform. For the algorithm to work properly there needs to be a minimum of ground response (typically the reactive signal (+X) that enhances the receive signal due to the ferromagnetic nature of the ground. The ground signal (+X) increases the target inductance (due to the higher permeability of ferro minerals which bend the field lines into themselves - somewhat akin to metal attracting lightning as it is easier to travel through). The FBS detectors measure the time constant of the target, and the Tc increases in the presence of ground minerals because they affect target inductance. Think of Tc as a ratio of the target Inductance / Resistance (L / r), so if the inductance (L) (eddy spin time) rises the Tc value increases.

The low freq (LF) and high freq (HF) used by FBS detectors (3.125 KHz is LF, 25 KHz is HF) measure the received waveform at several time intervals (I think three) and measured each separately after a series of long and then short period pulses end, to determine the nature of the target. An iron target has large resistance (to electron flow) but the inductance is also high (because of its high permeability). The Tc would be L / r = High / High, producing a long Tc. In iron targets (magnetic) that are somewhat conductive the resistance is high and the inductance (eddy flow ability) is low. The Tc would be L / r = Low / High, producing a short Tc. Comparing the signals received during coil on and at several points during coil off can determines if the target is ferrous or non-ferrous.

But to do this correctly, the signals must be ground balanced. There is no reactive signal when the xmitter is off (can't bend in or bend away a non-existent magnetic field, so no +X or -X reactive signal present). Any signal measure when the xmitter is off and target eddies (and magnetic domains) are dying do not have much reactive components (but do have a resistive component). But the measurements while the xmitter voltage is on and current is rising would be influenced by the ground's +X signal. The detector can get the ground values wrong (for instance, when the ground signal is clean, no +X mineralization) or having a conductive salt mineral component instead (like saltwater beaches with a -X reactive signal with opposite polarity). The Auto balancing of the FBS detector could under or over compensate the sensitivity erroneously in these circumstances (and may be why the detector does poorly in air test while in Auto sensitivity mode).

It is my understanding, for the FBS detectors to properly work the ground measurement needs a mineralized background. In neutral soil (like some coral beaches or dark sand beaches composed of tiny glass beads where lava met water and cooled and then fractured) the soil is either without +X (magnetite, maghematite) or neutral. On salt beaches the ground is diamagnetic (-X because field lines are bent away due to eddies having opposite field polarity to the primary field and oppose it, i.e., negate it). When the expected background minerals are not present the detector should be set to Manual rather than Auto, or else the sensitivity is likely to be set in error by the algorithm applied by the detector.

So, on neutral soil or salt beaches without appreciable magnetite (use a magnet of dry beach sand - did you pick up any black sand?) just set the sensitivity manually to a level that the detector can still operate without falsing radically.

With the CTX the green displayed number (on the right) is the average sensitivity determined by the ground measurement at the three measured points. It can be wrong as explained above. Remember that the green number is always an average of all three ground measurements.

The white number (on the left) is the number currently in use (either you set it manually or it was chosen by the detector in Auto). In mineralized ground you cannot match the performance of the Auto sensitivity - it can micro adjust and get much better depth (sensitivity) than some manual number you dial in. The white number in Auto is the highest value of the three ground measurements - not an average, it's the highest one to push operations toward the peak.

In Auto, the green value is the average, so it can be the same as the white value (on left) but never higher. Recall that the ground is measured at three time periods, prior to xmit coil shut off and again after shut off. The ground will react differently at these three points (and because the xmit pulses are different - long and short). So, the three ground channel values will rarely respond the same, the left value is likely to be higher than the green average sensitivity value on the right.

When you use Auto+3 you are simply telling the machine to take the three ground measurements and increase them all by 3. The green value will be the average of the three ground channels + 3. That is, if in Auto+0 you had channel 1, 2, and 3 as (15, 17, 19) then in +3 they'd be (18, 20, 22) . In the first case the white value would be 19 (the highest) and the second case (+3) the value used would be 22 (3 points higher). The first averaged is 17 and the second is 20, so you'd see White/Green in Auto+0 as 19 / 17 and in Auto+3 as 22 / 20.

When you are in Manual the value you choose overrides the ground settings. The green value (on right) still shows the average ground values for reference. Your manual setting entered will be the white value (on left).

Since the green average could be in error, just set the manual value to whatever keeps the detector from becoming sparky. Since the ground values aren't adjusting automatically, the detector stability will be off as the ground changes and can produce intermittent chirps from the incorrect ground affecting the audio/display. If it bothers you, return to Auto sensitivity - it is doubtful the error would be so great as to cause a drastic loss of sensitivity and a peaceful audio is sometimes worth it (at least for me).

Johnnyanglo

 

[Drjohn71a]

[pre]
Thanks for the explanation, Johnny! In my creek area, the auto runs 25-27 on the left number, about 3-5 over the right number. Also, I couldn't keep it in Auto +3 or + anything today - way too much noise. So Auto worked best.
[/pre]

 

[INSAYN]

Johnnyanglo...That's one very detailed explanation!

I never had the Manual vs Auto explained in that context before and curious about something.

Not doubting you or where this great info came from, I am merely observing my results from the field.

Out here in the PNWet, our ground can be really bad and changes a lot within the same hunting area.
For example, just this morning I was hitting a new location that was pretty low on EMI, but Green was all the way down to 8.
In Auto +3, I believe I saw the White go as high as 12. Feeling that I was losing depth, I switched to Manual.
Running in Manual in the same spot I could run it up quietly to about 27 before I got some sparky chatter. Green was still showing 8.
VDI's were certainly not stable, but I expected that. I kept moving around the park and decided to move about 200 feet south.
The Green number eventually climbed up as far as 12, and if I switched back to Auto+3, I saw the White hit as high as 17-18 at best.
At this location, I could run Manual all the way up to 30 without noticeable chatter.

I then moved to another park that I have been hitting for the past 16 years. Targets are getting slim pickings, shy of nails, bolts, nuts, beaver tails, and the occasional old coin.
Running in Manual, I chose to run at 28 just for the heck of it, even though it was stable at 30 (tones, not the VDI's). As I hunt, I try to double check each of my target hits with both Manual and Auto+3.
Most of my coin targets are 7" or less. With the very occasional 8" coin.


Are you saying I should be able to run Auto+3 in any of my conditions listed above, and still have equal or better depth than anything a higher Manual setting could produce?
Or, is there some give where Manual settings that are audibly smooth, could produce more depth than Auto+3?

I would really love to trust Auto+3 all the time as the VDI's are pretty solid, were the Manual VDI's are pretty jumpy.

EMI is a whole nuther beast. I can be in one park and it would be super quiet. Pick another park, or even a location in another park and my screen looks like the 4th of July with cursors and VDI's going nutz on the screen. Switch to Auto+3 and it takes the Green number down to a single digit yet again to calm the machine down. Even after auto and manual re noise cancelling attempts.

Thanks,

 

[jvc55349]

I have started using mine on manual and have opened it right up a few times. Certainly seems that I am getting more depth/stronger signals but its been only recently that I have been trying it.

 

[Johnnyanglo]

Insayn, I know the soil in the Pacific NW, hunted there for years (my average values typically ran from 12-17). Unbeknownst to the user, the soil conditions under the coil can change radically from spot to spot, even just yards apart. That's why in the mineralized ground the Auto keeps the detector better adjusted - not deeper, just less sparky. Decreases audio confusion.

For instance, if the Green sensitivity value was 8, that is the average of the three channels - some higher and some lower. If the Auto +3 was using say 12, then the highest of the three at that point was 9 (12 - 3 = 9). We can assume if the Green value is 8 then the three values were 7, 8, 9, with auto sensitivity chosen as the highest of the three. While the soil mineralization is high, the variation is low. Note: the actual manner the CTX/E-Trac uses to set the auto sensitivity setting is a trade secret and is no doubt a more involved algorithm than just taking the maximum value of three ground channels. But we'll play along.



On the other hand, if you see in Auto+0 White-Green values of, say, 21-14 that would mean the average is 14 but the highest value was 21. That's a large spread (rms), which can be interpreted as a more complex but reactive soil condition, with more outliers reacting to the short-long pulses at the two frequencies. I would expect switching to Manual-30 would open the floodgates to these sparky outliers and the machine would become very unstable, probably unusable.

I think the greater the variation between Green and White sensitivity values the less able Manual sensitivity will be able to run smoothly. When the variation is large, even a slight error in ground setting causes a flurry of wild hits in audio and TID that otherwise would have been eliminated in Auto mode. While depth is suffering somewhat with the greater volume of iron minerals (being permeable they shunt the field energy laterally along their horizontal strata rather than allowing the field to enter into the ground, decreasing depth), the low Green value (high mineral content) does not necessarily mean you can't overdrive sensitivity in Manual.

But, even on low-mineral beaches (salt only), the even slight instability due to conductive salts in Manual causes too many false chirps, some from the waves, that I'm almost always hunting in Auto+3. I'd rather switch to Manual+30 when interrogating a weak target than for general hunting. In the iron zones in the NW, I could boost the sensitivity to high levels without too much problem if the White-Green spread was close.

If you TID is going wild with random hits bouncing everywhere - even with the coil stationary, that's EMI. Could be underground cables, power fence, cell tower or cell phones, another detectorist, transformers, radio transmissions, etc. If it only happens when the coil is held vertically that is sferics, from distant lightning discharges. I've been hit with EMI in certain areas and there was no way to mitigate it.

 

[McClod]

I much prefer the stability of auto +3 (I'm also in the PNW) to manual sensitivity when hunting, but over time, in learning my ctx I've found while hunting in manual that I can pick up coins that auto +3 wouldn't reach. So far the experience I've had has shown me that most of the coins I've found while using manual sensitivity could also be detected using auto +3, to some lesser degree depending on depth, but there are those deeper coins that I'll find in manual that auto just wouldn't let me hear. So over time I've come to use manual sensitivity most often while hunting, but I always switch to auto +3 once I hear a signal in manual to better isolate the target from all the jibberish signals that manual produces, this is why I've found over time that usually, but not always I can detect the targets in both settings.

Still, it's pretty frustrating hunting in manual because of all the noise and jumpy signals. I'd rather hunt in auto but I'm afraid I'll miss a good target. I've kept my gain up pretty high all the time, probably no lower than 27, I don't know how this might affect things, my logic for that is that I don't care if a target "sounds deeper", because gain is lower, targer depth indicator tells me that, and I don't want to miss a target because it's much quieter because it's deeper, but I wonder if having my gain set at a higher value will cause more instability or have other negative effects, or if that's even a concern?

 

[Johnnyanglo]

Well, I think high gain values (26-30) just make the target sound bigger, and can overcome the audio threshold on weak (deep) targets so they can break through and be heard. So it is good to go with high gain on deep targets. But if set too high, then it is possible that noise from internal components or EMI can create too much audio chirping. Needs to be reasonable for the hunting situation.

I was not keen in having all targets (deep or shallow) sound the same. I like to know what is deep from shallow, the deep stuff is usually more note worthy than the shallow surface junk. I especially prefer lower gain when there are multiple targets under the coil or lots of iron hits. I think a lower gain (like perhaps 15, with partial amplification of weak targets) reduces the audio impact of iron and foil, making their report less impressive on the ears.

If the gain is set low you can turn Deep ON (E-Trac only) but not on the CTX.

It has been reported that high gain causes the TID numbers to hang ... changing more slowly, which is unwanted. Not sure if this happens with CTX, maybe someone has experience with it. I keep my gain at a compromise value of 20, but I mostly beach hunt.