Capabilities & Limitations - Fisher Labs F 75 User Manual

F75
Comprehensive Operating Manual & Guide to Metal Detecting
Capabilities And Limitations (continued)
STEEL BOTTLE CAPS & FLAT IRON TRASH
Modern motion-type target ID metal detectors usually have difficulty consistently identifying
steel bottle caps and other flat iron trash objects. Double-D searchcoils also have a reputation
for having difficulty distinguishing steel bottle caps from coins, and for being unable to
eliminate steel bottle caps from detection. If you are searching in an area where there are
many steel bottle caps or other flat iron trash targets, minimize the amount of unnecessary
digging with the following methods:
1. SEARCH WITH THE bc (BOTTLE CAP) PROCESS using the PROCESS # feature.
This method calculates visual ID differently in order to cause steel bottle caps to read
lower on the scale, and to register less consistently. A desirable object such as a coin
will usually produce numeric values that are fairly consistent in both directions of
sweep. See PROCESS # under the DISCRIMINATION Mode section of this manual.
2. SEARCH WITH THE dP PROCESS. This method calculates visual ID differently in
order to cause steel bottle caps to ID lower on the scale and to register less
consistently. It also produces more audio clues to the character of the target.
3. LIFT THE SEARCHCOIL. Within 2 inches of a Bi-Axial searchcoil, the crossed magnetic
fields of the Double-D construction can produce anomalous responses. If the object feels
shallow (strong signal, narrow response, or multiple responses in a single sweep) and is
giving consistent high readings like a coin, raise the searchcoil 2 to 3 inches and try again.
A coin will almost always continue to give consistent readings unless it is right next to an
iron object. A steel bottle cap that is at least 3 inches away from the searchcoil will
usually produce readings that bounce around from medium to low numbers.
4. SWEEP THE REAR OF THE SEARCHCOIL OVER THE CENTER OF THE TARGET,
OR SWEEP RAPIDLY.
a. If the ID# is repeatable in the range of 68 to 72, when passing the
center of the searchcoil over the target at a normal speed, then the
target is probably a dime or copper penny.
b. If the ID# is not in the range of 68 to 72 then:
i. Sweep the back end of the searchcoil over the target. If tones
change from high to low, the target is probably a bottle cap.
ii. Sweep the center of the searchcoil rapidly across the target.
1. If tone and ID# drop, it is probably a bottle cap.
2. If a bottle cap, then the faster you sweep, the
5. USE THE CONFIDENCE INDICATOR
Coins will usually produce a high
confidence level, whereas steel bottle caps
and other trash will usually indicate a low or
erratic confidence, even if the ID numbers
are consistently in the range of a coin.
Confidence level is useful even if not
using the dP or 3b processes.
30
lower the tone.
Sweep back end
of searchcoil over
suspected bottle cap.
(Low Tone = Bottle Cap)