dsp@oakhill.UUCP (Generic DSP account) (09/10/88)
This message is for Bob Parnass and anyone else wondering about
metal detectors.
I just bought one of those $89 VLF discriminator metal detectors from
Radio Schmuck. It works quite well although it gets heavy to hold after
a couple of hours of searching. The thing is capable of detecting a
dime at 6" underground. While using the detector I discovered a new
form of pollution on the earth: pulltabs.... the thing picks 'em up
like gangbusters. What is needed is a pulltab detector to go along
with your metal detector...when you find something you would use the
less sensitive but discriminating pull tab detector to figure out
whether you should dig/scrape. The only problem is that a pull tab
ring is about the same size as a diamond ring!
Any ideas? perhaps an imaging metal detector that lets you *look* at
your find.
-Charlie Thompson
ut-sally!oakhill!radio
parnass@ihuxz.ATT.COM (Bob Parnass, AJ9S) (09/10/88)
x
Charlie Thompson, at ut-sally!oakhill!radio, wrote about
the need for metal detectors to indicate or reject metal
pulltabs.
Since my last posting a month or so ago, I bought a White's
6000 Di Pro Plus metal detector. This heavy, but well
built unit incorporates adjustable discrimination and a
multiple scale meter, which can read type of object or coin
depth. In addition to silver, nickel, gold, etc., the
"object" meter scale is calibrated to read things like:
nail, nickel, tab, penny/dime, quarter.
If your Radio Shack unit is anything like this White's
model, it will take several outings to really get to know
your detector's behavior and how to exploit it to best
advantage. My detector has 4 modes, but I've concentrated
on using only one so far - GEB DISC, which is automatic
ground balance with discriminator.
Someone gave me good advice: if you're just starting out
with a new detector, dig every target, and compare them to
what your your detector said they were. Think of this as
"calibration."
After several trips, on which I do the searching and my
wife does the digging, I can now tell with about 75% accu-
racy when an object is a coin or trash. At first, my score
was only about 40%.
Some of the more complex situations are:
1. When several objects, usually made of different
metals are buried close together. Sometimes the
detector will behave differently, depending on
whether you swing the searchcoil from the left or
right over the target(s).
2. When a piece of a pulltab is buried, and produces a
false reading.
3. A beer can at 4 or more inches can produce the same
reading as a quarter at 1 inch. Since we're looking
for coins, we won't dig past twice the depth indi-
cated on the depth meter.
4. A target sounds and "looks" pretty good on the meter,
but when some of the dirt is removed from the area,
the meter begins to indicate junk. When this hap-
pens, we assume it's junk and stop digging.
On most detectors, if you set the discrimination high
enough to reject pulltabs, you will reject nickels and some
rings as well. I try to use the meter and the "shape" of
the target to determine if it's a pulltab, but I need more
practice so I won't mistake nickels or rings for pulltabs.
In the past few weeks, we've found well over 100 coins
(quarters, dimes, nickels, and pennies) and 4 Matchbox-like
toy cars. Our searches have been restricted mainly to
schools, town squares, parks, etc. At my parents' house,
we found our ex-dog's license tag from 1971, and coins
dating back to 1930. Among last week's more interesting
finds were a 1919 wheat cent, a ring, and a sterling silver
pin.
Searching along the beach, or near swimming holes, is sup-
posed to yield the most jewelry, but we haven't tried yet.
Our searches in town squares, and along sidewalks adjacent
to grade schools were most fertile.
Have fun!
> This message is for Bob Parnass and anyone else wondering about
> metal detectors.
>
> I just bought one of those $89 VLF discriminator metal detectors from
> Radio Schmuck.......
> While using the detector I discovered a new
> form of pollution on the earth: pulltabs.... the thing picks 'em up
> like gangbusters. What is needed is a pulltab detector to go along
> with your metal detector...when you find something you would use the
> less sensitive but discriminating pull tab detector to figure out
> whether you should dig/scrape. The only problem is that a pull tab
> ring is about the same size as a diamond ring!
>
> Any ideas? perhaps an imaging metal detector that lets you *look* at
> your find.
--
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Bob Parnass AJ9S, AT&T Bell Laboratories - att!ihuxz!parnass - (312)979-5414