so far as production is concerned.
As a rule (although not all manufacturers follow the 'rule'), metal detectors have been calibrated with the particular coil they are shipped with. I have been involved in that phase of the industry and have seen different approaches to achieving this.
Most of the time, during the final part of production the control housing is mated with a coil and final calibrations are made, using that specific search coil. Usually, a piece of ferrite of some type is used to set a GB reference for the turn-on-and-go models, while the manual GB models have their own external adjustment for that.
A manufacturer can direct set-up procedures based upon readings off of different test-points, and often this is going to be fine, as long as the engineer writing the set-up instructions has a proper, functional setting developed for the procedure.
Search coils have to be made to function within a certain tolerance range, and this can and will vary. Additionally, while some makes and models are designed to operate on a specific, crystal controlled frequency, such a 6.59 kHz, other manufacturers use a different principle, such as an LC Oscillator or Phase-Lock Loop principle making the coil itself an integral part of what result.
This is why, for example, Tesoro has made models from 9.6 kHz (Shadow X2) to 15 kHz (Golden Sabre Plus), with most being in the 10 kHz to 12 kHz range, but ALL OF THEM still able to share the same search coils. Not easily achievable if you're using a tightly controlled operating transmit frequency.
With one coil size and type, such as the 8" brown coil, it is possible for the coil's inner workings to get 'out of tune' or 'out of spec.' That calls for replacement, repair, or on occasion a slight tweaking of the GB that might make things work better.
<STRONG>"Seems like that would be the thing to do since they offer so many coil options."</STRONG>.... True, but the coils can vary one to another, and then there can be more significant differences when changing sizes and/or types.
<STRONG>"Are units "optimized" for the coil they are shipped with and therefore you dont get peak performance when you get a unit calibrated to handle "all coils"."</STRONG>.... Yes, that is basically correct. Not everyone uses a coil other than the stock coil, and that's why it's important to make sure the detector works right out of the factiry as supplied.
I have seen 'holiday specials' offered by one large manufacaturer (not from Arizona or Oregon) who supplied a 4" coil is all the 'special' packages. A dealer friend and I opened all of the inventory he got in of a partiular mid-priced model. Of the 8 similar units, only 1 of them worked properly with both the supplied 8" & 4". We had to do some mixing and matching to try and mate the 4" coils with the units they <EM>might</EM> work best with. Two of these units were not able to be mated with a good-functioning 4" coil.
Note that all of the models we were trying coils on were designed with a factory preset GB and it is quite common for them to calibrate a little too negative.
With the Tesoro line, in all the years I have used and sold Tesoro's and worked with people and their detectors, I have only had one unit <EM>not</EM> function with a smaller-than-stock coil. That was a Silver Sabre II I sold a fellow and while the detector's calibration was ever so slightly positive with the stock 8" coil, it was too negatiove with the 7" concentric I sold him. We were on an archaeological project site with the Forest Service and didn't have time to try different coils. It took maybe 15 minutes to open it up, then carefully set the GB trimmer so that both coils worked just fine. Other than that, I haven't see a problem going from a stock coil to a 4" or 7" concentric coil.
I have seen larger coils work 'OK' on many models, but it is the larger coils that are more prone to needing attention.
<STRONG>"Also, does a ground balancing feature on a detector optimize it for each coil? That is if the ground balancing effected both all metal and disc modes."</STRONG>.... Yes, that will usually take care of the differences in coils. As you note, that is IF the external manual GB works on the Disc. mode (Bandido's, Eldorado, Tej
