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="What the heck is going on here? Shouldn't I be hitting stuff deeper than this? It really makes me question everything I thought I knew about this detector in the field...."
Your results are the reason why having a test garden is so important to finding the truth. You can listen to second-hand stories or get the answer for yourself.
In Washington State dirt a buried dime is gone at 7" and no repeatable audio beyond 5.0" in AUTO+3 with the CTX and E-Trac.
In the same dirt a buried dime is fringe at 7" with occasional hits every 2-3 coil passes in MAN-30 with the CTX and E-Trac. At 6.5" the dime is almost diggable.
So, to reliably get a dime in Auto+3 keep the dime buried less than 5.5". To reliably get a dime in MAN-30, keep the dime buried less than 6.5".
If you watch the cursor, a dime signal will begin to jump about deeper than 4.5" ... such as, 11-45 to 01-33 to 11-37 to 01-44. But the general area of jumpiness is within the non-ferrous zone; this is your clue to dig even when at the same time the audio is blanking on every third pass - the TID is hinting the target is non-ferrous. So dig it.
It is also worth mentioning that the CTX-3030 and E-Trac behave very similarly - their sensitivity is nearly identical (as is there related depth capability).
It is interesting to note that while MAN-30 gives perhaps a 15-20% increase in depth in the dirt, you can go even deeper All-Metal Pinpoint. That is, where AUTO+3 has checked out at 6.5" on a buried dime (random audio hits) the AM PinPoint easily centers the audio over the target. Granted in All-Metal there is no ferrous/non-ferrous discrimination, but remember the TID cursor is probably jumping around in the non-ferrous zone if it's a coin (that's your clue). If you have a fringe signal switching temporarily to Pinpoint can help center the target as it is another 15-20% deeper than even MAN-30.
Now that is in slightly mineralized Washington soil. The FBS detectors will show greater detectable distance in the air than the ground, even getting more than 12" in MAN-25 and 7.5" in AUTO+3 on the same dime. Yes, AUTO+3 cuts detectable distance by nearly 40% as compared to MAN-25+. That shouldn't be shocking. AUTO sensitivity is a happy medium of averaged signal processing to ensure stability. You get easy detecting without the sharp blips and crackles of MANUAL, but at a price.
But when you compare AUTO to MANUAL in the soil, the difference drops to half that of the in-air distances, in other words the difference in capability is less dramatic thanks to the overpowering effects of soil minerals. If you want to get an extra bit of depth, switch to MANUAL, but if ear fatigue is bothersome go back to AUTO. You'll be losing 15-20% of the total depth capability but you'll be keeping your sanity.
The chart below shows the affect each change in MANUAL and AUTO sensitivity has on target detectability in air. If you use AUTO+3 regularly it is if you were hunting in MAN-14 (the max is MAN-30). Again, in the soil the difference isn't that dramatic, but still has impact on depth.
You'll notice that the reduction/increase in sensitivity with each MANUAL adjustment is linear. The actual forumula would be:
Max Depth = 0.3945*(MAN Value) + 2.187
Which means the slope is about 40/100 or 40%, every 1 change in MANUAL value yields 0.25" depth increase (in the air). That is, if you go from MAN-15 to MAN-19 (a change of 4 values) you get 1" more depth. If you go from MAN-20 to MAN-30, a change of 10, you get 10 x 0.25" = 2.5" more depth (in the air). Assuming bad soil it is more likely that in the ground any change would be about 15-20% less variable. So, from MAN-20 to MAN-30 you might get you 2.0" more depth (in the ground) instead of 2.5".
Recall that a buried dime in MAN-30 is maxed out at about 7.0", so dropping to MAN-20 might put you around the 5.0" depth mark, depending on your soil conditions.
You'll be happy to know that the rate of change in AUTO is about the same, 0.25" per AUTO value. Over the range of 6 values (-3 to +3) the change of 6 x 0.25 = 1.5" change. In other words, AUTO+3 changed to AUTO-3 will drop your depth in the air by 1.5". Knowing that AUTO+3 is like using MAN-14, then dropping to AUTO-3 is like using MAN-10 or so, where the dime in air is maxed out at 6.0" instead of 7.5" at AUTO+3.
What have we learned? If you want to exceed the capabilities of AUTO+3 use MAN-15 or higher, then you'll get more depth. From MAN-15 to MAN-30 (the max) you'll gain almost 4" in depth in the air and perhaps 3" in the ground.
Anything else? Well, if you're looking for that Rosie dime in moderately mineralized soil, and you know it has sunk deep, say at least 6.5" down, then AUTO+3 will miss the target about 80% of the time and will miss it 95% of the time at 7.0" down (that's if you use audio alone ... the TID may hint its location down to 7.0", even when the audio is silent as described above).
If you want to go deeper, get a bigger coil or change to a single freq VLF detector. Can you get a dime deeper than 7.0 inches? Yes, if your soil is more mild, but probably the increase won't be identical to air (12"), perhaps in coral sand you can get near 12" but for most of us clean soil is probably maxed out at 10" or so on a dime. So, forget about getting any dime-sized target in moderately mineralized soils deeper than 6.0" without being extra slow, extra careful, watching the display, listening for the occasional tone every third sweep (non-repeatable), and keeping the DD coil centered for max depth while using MAN-30 (and no Discrimination - nope, none at all or all bets are off).
Hope this helps you find those deep coins,
Johnnyanglo