Figured I'd throw this little air depth chart I made up in here of all the coils I've owned/been loaned.
Only wish I would have done this with the 15x12 I owned for a while as well, but based on it's use in my mineralized soils it did not seem to get any deeper or maybe not even as deep than the stock 10" Tornado in my mineralized soils on coin/ring sized targets, while it did in my mineralized sands for some odd reason. I suspect it was just beyond the point of no return in terms of coil size for my soil on coin/ring sized targets, sucking up too much ground stew I would guess and washing away the target signal, because for one thing it will seem to give me better depth or at least ID/tone at depth if I lowered the sensitivity much more than what was max stable.
The 15x12 is a great coil though and very stable, with fantastic ground coverage, but I feel having one in my line up would be too much like replicating my 12x10 I use 99% of the time on land, which *in my soil* seems to be the deepest coil I've ever used or owned and with fantastic left/right separation like I've never experienced before. It's stability in minerals or EMI is also something that is very impressive to me, and the audio report it gives is louder, crisper, and more treble like to my ears than any coil I've compared it to, with perhaps the Ultimate coming very close or perhaps just as good in that respect.
The 13" Ultimate loaner I had is different enough in traits from the SEFs and even compared to round DD coils that I plan to add one to my line up for big wide open fields and such when I want to grid even faster than the 12x10's nice improvement in that respect over the stock 10" Tornado. Always nice to have a different feeling/functioning coil in my line up, even if I'm still not sure yet if the Ultimate is as deep or deeper than my 12x10 in my soil.
Just finished this video testing the depth of my new eight inch Tornado coil. Actual size is 7.25". Very small...
http://youtu.be/cNPPYn4moOM
But if you don't want to watch that then check out this chart below I put together of the depths for all these coils I tested thus far- 8" Tornado, Sun Ray S-5, Sun Ray S-12, 10" Tornado, 13" Ultimate, and SEF 12x10. The depths of this little Tornado were pretty impressive, and we all know these BBS units can and do get better depth in some soils or sands than they do in air tests due to the unique way they handle (and thus need) a ground load, where as it's usually the reverse of that for other machines, where they get far less depth in the ground than in the air.
All these tests were done using max sensitivity, unless otherwise noted where I turned sensitivity down to half on the 8" Tornado to see how it fared that way too, at this same low-emi site so all coils were able to run stable at full blast manual sensitivity. I wanted to do that to rule out any differences in sensitivity settings as well as have all the other settings exactly the same- Iron Mask ON Disc Mode, Noise Band 2, Threshold Mode & just loud enough to hear, Full Volume, No disc or notch dial used. Test target used for all of them- clad dime.
These air depths for various coils, listed in order of shallowest to deepest, are demonstrated in other videos on my channel, and all were used on the Sovereign GT with the exception of the noted 2nd test of 10" Tornado on a new Excalibur II.
NOTE: Air testing the Sovereign or Excalibur is pretty controversial. These machines often will out perform their air tests in certain soils or sands. I only use this as a starting point, not the final answer on how a coil might perform. It could very well be that certain coils need a ground load according to their design to perform at optimal levels, and that's excluding the obvious nature of these BBS units in needing a ground load based on the unique way they handle a ground load, unlike other detectors.
8.5" - Sun Ray S-5 (5.5") Intruder
8.5" - Minelab 8" (7.25" actual size- tiny thing) Tornado at half (12PM position) sensitivity
9.0" - Sun Ray S-12 (12.5") Intruder (This is first audible response from it)
9.5 to 10" - Minelab 8" Tornado at full sensitivity
9.5" - Sun Ray S-12 (12.5") Intruder (This is the first response from it, a null)
10" - Minelab 10" Tornado (On brand new Excalibur II)
10.5" - Minelab 10" Tornado (This time tested on Sovereign GT)
11" - Detech 13" Ultimate
11.5" - Detech SEF 12x10
Pretty impressive depths for the little Tornado, and I've read/heard of others getting more than that in their soils. Just recently I read of it being tested in an old coin test garden and banging a coin at 10" too.
The odd thing though is that your 18x15 is a tad heavier than the FBS version Kered weighed in the chart (with coil cover). Everything I've seen thus far is that the FBS versions of the SEF coils are a good bit heavier than the BBS versions, so it's odd that the BBS version of the 18x15 is heavier than the FBS one? Not sure why that is. Are you sure about the weight, and I assume that was with the coil cover on the scale too along with the cable then?
BBS 8" with cover and cable:shocked, also on a food scale: weight was 21.8 oz.