Whites TDI SL User manual

TDI SL
White’s Electronics, Inc.
Sweet Home, Oregon USA
Building the World’s Finest
metal detectors for over 60 years.

Page 2
TDI SL Owner’s Manual
CONTENTS
Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Batteries and Charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
QuickStart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Principles of Pulse Induction . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Controls
Threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Power/gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Pulse Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Target Conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Ground Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
TDI In Depth
Ground Balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Audio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Pulse Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Dual-field coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Dealing with Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Code of Ethics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Back Cover

Page 3
TDI SL Owner’s Manual
Assembly
1. Remove all parts from the shipping carton and check assem-
bly diagram to ensure all parts are present.
2. Install black rubber washers on fiber clevis/lower rod, insert
clevis lower rod onto loop ears. Use only nonmetallic wash-
ers, fiber bolt, and fiber thumb nut to secure loop/search coil
to clevis/lower fiber rod.
3. Unlock “S” rod cam-lock and insert clevis/lower rod into
curved “S” rod so that stainless steel spring clip buttons line
up and lock into one of the adjustment holes in the curved
“S” rod. Turn cam-lock to secure. The second or third
adjustment holes are suitable for average sized adults.
4. Wind the loop cable around the rod assembly, first revolu-
tion over the top of the rod, all the way to the top of the
curved “S” rod, about five revolutions. Use the black cable
retainers, one near the loop, and one near the top of the
curved “S” rod, to hold the loop cable in place. Plug the
cable into the jack on the front of the control box.
5. Grip the instrument by the handle, with your arm in the
elbow cup with strap and sweep the loop/search coil over
Search coil
Lower rod
Arm cup
Battery door
Clevis
Rubber
washers (2),
fiber bolt
Cam lock

Page 4
TDI SL Owner’s Manual
the floor. If the instrument fit feels uncomfortable, readjust
clevis/lower rod length with spring clip button and cam-
lock so that search coil can be held near the floor without
stooping over.
6. Remove the protective paper from the two black arm cup
foam pads. Carefully align pads on the inside of the elbow
cup, one on each side of the center rod, and press firmly into
place.
7. Adjust the elbow cup strap so that it is loose enough for you
to slide your arm in and out without loosening each time
you want to set the detector down. The elbow cup strap pro-
vides extra leverage and control. However, some prefer not
to use it.
8. Install the battery pack; see the next section for information
on batteries and charging.

Page 5
TDI SL Owner’s Manual
Batteries and Charger
The TDI-SL includes a rechargeable NiMH battery pack as
the primary power source, and a standard alkaline battery pack
as a backup. Fully charged, they supply a nominal 12 volts (the
NiMH may be slightly less) and will power the TDI-SL for
about 4-5 hours. The TDI will operate until the batteries drop to
about 8.5 volts, at which point the low battery LED will turn on.
A drop-in charging cradle (Part Number 509-0036) is used
for recharging the NiMH battery. With the wall transformer
connected to the cradle and plugged into a wall socket, simply
drop the NiMH pack into the cradle. An LED indicates the sta-
tus of the charge cycle; when the LED turns off, the charge
cycle is complete and the charger will automatically switch to a
trickle mode. A typical recharge time of a depleted NiMH pack
is 1-2 hours. It is normal for NiMH batteries in storage to
slowly lose their charge over time; be sure to top off them off
before going out.
The alkaline pack is designed so that it cannot be used with
the charger, and trying to recharge alkaline batteries can cause
them to burst and leak. Individual NiMH batteries can be used
in the alkaline tray; it is recommended that the batteries be
removed and charged using the manufacturer’s charging sys-
tem.

Page 6
TDI SL Owner’s Manual
QuickStart
1. Set the Conductivity switch to ALL.
2. Set the Frequency offset to the midpoint.
3. Set the Ground Balance to ~8.
4. Turn on the power and adjust the Gain to “2.”
5. Adjust the Threshold to achieve a faint background hum.
6. With the coil on the ground (no motion) increase the Gain
until the audio begins to chatter. Back off slightly for a rea-
sonably smooth threshold.
7. Optional: adjust the Frequency offset to minimize the
amount of chatter.
8. Using a spot free of targets, bob the coil up & down and
adjust the Ground Balance control to minimize the audio
response.
9. For most nugget hunting, set the Conductivity switch to
LOW.

Page 7
TDI SL Owner’s Manual
Principles of Pulse Induction
Pulse induction (PI) detectors are very different from tradi-
tional induction-balance (VLF) designs. Instead of transmitting
a continuous-wave magnetic field, the PI detector periodically
transmits brief impulses of magnetic energy. The PI receiver
then looks at the impulse decay for target (and ground) informa-
tion.
The use of high-powered impulses results in better ground
penetration than VLF, especially in highly mineralized soil.
This makes the PI detector the best choice for certain kinds of
hunting in bad ground where many VLFs simply won’t work.
There is no traditional discriminator in PI detectors,
although there is some ability to discern certain types of metals.
The decay response varies with target conductivity (plus other
characteristics) so that low conductive targets have a fast decay
and high conductive targets have a slow decay:
The Pulse Delay control, which determines the point where
the decay is sampled, can be used to eliminate low conductive
targets such as thin foil. Further increasing the delay can knock
Exponential decay
Pulse Rate
TX
RX
No target
High-conductive
target, long decay
Low-conductive
target, fast decay
Raw Response

Page 8
TDI SL Owner’s Manual
out other minor trash targets at the expense of eliminating
potentially good targets such as jewelry, but this method is very
limited in the range of trash targets that are rejected.
Furthermore, the ground balancing method used by TDI has
a side affect with target responses. Target conductivities below
the ground balance point produce a positive response for which
TDI assigns a high tone. These targets are generally low con-
ductive such as small nuggets, jewelry, and small iron. Target
conductivities above the ground balance point produce a nega-
tive response for which TDI assigns a low tone. These targets
are generally high conductive such as silver coins, relics, and
large iron.
The Target Conductivity switch can be used to eliminate either
class of targets, or to accept them all. Care should be used with
this feature as it is not always obvious as to how a target might
respond. Testing with known targets is the best way to learn the
responses, both with delay settings and conductivity mode.
No target
Differential Response
High-conductive target
Low-conductive target
Ground balance point

Page 9
TDI SL Owner’s Manual
Controls
Threshold
The Threshold control sets the level
at which the background audio tone is
heard. In most cases, faint target sig-
nals are easiest to hear when the thresh-
old is set to a low but clearly audible
level. A higher threshold level can
result in audio fatigue. Too low of a
threshold level means that a stronger
target signal is required to produce an audible change. Thresh-
old is best determined at a low gain setting where there is no
chatter.
The threshold can be set low enough to effect a “silent
search” audio. Some people prefer silent search to prevent
audio fatigue. If you choose to do this, increase the threshold to
the point where you can hear the threshold tone, then slowly
back off until it is barely inaudible. Reducing the threshold any
more than necessary requires stronger target signals to produce
an audio response. Other controls — including Gain, Ground
Balance, and Frequency offset — are best adjusted with an
audible threshold.
Power/Gain
The Gain control increases or
decreases the amplification of the
receive signal. Normally, this deter-
mines how deep the detector will “see”
a target.
However, increasing the gain not
only increases the amplification of tar-
get signals, but also that of ground signals and electromagnetic
interference (EMI) as well. A common mistake is to assume

Page 10
TDI SL Owner’s Manual
that higher gain will always give better depth. In air tests this is
generally true, but ground results can depend on the amount and
type of mineralization.
With the coil on the ground, increase the gain until the
detector exhibits chatter, then back off slightly for a reasonably
steady threshold tone. Use the Frequency offset control to help
deal with EMI-related chatter, and proper ground balancing will
minimize chatter due to mineralization.
The Gain control is also used to turn the detector on and off.
Pulse Delay
TDI transmits a pulse and then,
after a short time delay, samples the
received signal. Pulse Delay allows this
time delay to be adjusted from 10s
(microseconds) to 25s.
In response to a transmitted
impulse, all targets exhibit a response
with an exponential decay. Regardless of conductivity, the
response is strongest at low sample delays and weaker at higher
delays. However, targets of low conductivity or that are thin
will decay faster than highly conductive or thick targets. See the
Principles of Pulse Induction section for more details.
Smaller gold nuggets fall in the former category, so nugget
hunters will want the Pulse Delay set as low as possible. Coin or
relic hunters may want to increase the delay to knock out thin
foil trash and still detect desirable targets. Most iron (especially
larger) responds with a long decay and is usually not very
responsive to the delay setting.
Conductive salts also respond with a fast decay and can be
detected at low delay settings. If beach hunting in wet salt sand,
you may need to turn the delay up to 15s to eliminate the salt
response. Dry salt is usually not a problem, so as you move out
of the wet sand into dry sand remember to turn the delay back
down to better detect small jewelry items.

Page 11
TDI SL Owner’s Manual
Pulse Delay affects the ground balance point, so whenever
the delay is changed the ground balance should also be checked
and re-adjusted. Higher pulse delays require a lower ground
balance setting.
Target Conductivity
The Target Conductivity switch
allows you to select between high-only,
low-only, or all target responses (see
the Principles of Pulse Induction sec-
tion for details). There are two caveats
with this switch. First, the two-tone
response, and therefore the functional-
ity of this switch, are disabled when ground balance knob is
turned to the OFF position. Second, in order to ground balance
the TDI this switch must be set to the ALL position.
Be aware that target conductivities can vary considerably,
even targets of the same metal. Small gold nuggets are usually
low and large gold nuggets are usually high, but where the tran-
sition occurs depends on the composition of the gold alloy. Be
mindful that searching in low-conductivity mode could elimi-
nate very large nuggets, if any happen to be in the area1.
The same is true for relics and jewelry. Many bullets, but-
tons, and most buckles register as high conductors, but cuff but-
tons often respond as low conductors. Most small jewelry
responds as low conductors, but larger men’s rings can easily
fall in the high-conductive range. Most coins fall in the high
range, except for e.g. US nickels and small or thin coins like
hammered silvers or very small bronze coins.
1. The potential for Very Large Nuggets depends on where you are
detecting. In most gold-bearing regions of the US, detectable
gold is found as small placer nuggets up to multiple grams in
size which register as low conductive targets. Alaskan and Aus-
tralian gold is still being found in multiple ounce-sized nuggets,
which may respond as a high conductive target.

Page 12
TDI SL Owner’s Manual
Ground Balance
This control is used to balance out
the signal from ground mineralization.
There is an OFF position for which TDI
runs in straight PI mode with no
ground balance.
To ground balance TDI the Target
Conductivity control must be set to ALL
in order to hear both conductivity tones. Set the Gain to about the
mid-point or until the audio just begins to chatter. Pump the
search coil up and down from about one inch off the ground up to
6-8 inches above the ground while listening for a change in the
audio. If the GB is too low, the audio will have a high tone when
the coil is lowered to the ground; if the GB is too high, the audio
will have a low tone. Adjust the Ground Balance control for min-
imal audio changes. For most moderate mineralization, this will
be around “8”. A simple technique is to start with the GB set
intentionally too low (say, 5), then pump the coil and increase the
GB until the high tone begins to transition to a low tone.
If the Pulse Delay control is changed, you will probably
need to re-adjust the Ground Balance setting. In addition, if you
begin to hear ground noises (false signals) as you sweep the coil
you should check the ground balance.
There are some cases where a particular target will fall
exactly on top of the ground balance point and TDI will not be
able to detect it at all. Targets that are close to the ground bal-
ance point may have their response shifted if the Ground Bal-
ance control is adjusted. For example, a given target may have a
low conductor response with GB=9 but a high conductor
response at GB=5.
Turning the Ground Balance to the OFF position disables
the ground balance and TDI will run as a straight PI detector.
This gives a slight improvement in depth but, of course, will
respond to mineralization and eliminates the ability to discern
conductivities.

Page 13
TDI SL Owner’s Manual
Frequency
This control makes small adjust-
ments to the transmitter pulse rate. This
is used to counteract any interference
which might be due to outside electro-
magnetic sources, such as radio sta-
tions, microwave, short wave, electric
fences, power lines, lightening, electri-
cal storms, or other metal detectors being used nearby. This
interference is recognized by a warbling or repetitive pulsing of
the audio threshold, and can mask target signals.
If you are experiencing interference, adjust the Frequency
control for minimal chatter. Adjustments should be done in
small steps, with a few seconds between each adjustment to see
if the TDI settles down. If you cannot deal with interference
with the Frequency control, then reduce the Gain and/or Thresh-
old settings to obtain a reasonably stable operating point.

Page 14
TDI SL Owner’s Manual
TDI In Depth
Ground Balancing
One of the key features of TDI is its ability to balance out
ground mineralization while maintaining pulse induction
depths. This is achieved using a differential technique whereby
two decay samples are compared; the Ground Balance control is
adjusted until the two samples have the same response, thereby
canceling the ground signal.
Targets below the GB point produce an overall positive
response, and targets above the GB point produce an overall
negative response. Broadly, the two ranges of targets depend on
conductivity, which gives TDI some ability to discriminate. In
rare cases a target might fall so close to the GB point that it
gives either no response at all or a double-tone response.
Because ground balance is achieved using a subtraction
method, there is a slight loss of depth any time ground balance
is on. Some ground, notably neutral beach sand, is mild enough
that no ground balance is needed. In such cases, Ground Bal-
ance can be switched off and TDI will run in straight PI mode.
This will slightly improve depth, but loses the ability to distin-
guish between low and high conductivity targets.
Audio
TDI has the following audio tonal responses:
Low
Conducto
r
High
Conductor
On High ‐‐‐ Low
On All High Low
On Low High ‐‐‐
Off ‐‐‐ High High
AudioTones
Ground
Balance
Conductivity
Mode

Page 15
TDI SL Owner’s Manual
With Ground Balance = On and Conductivity Mode = All, all tar-
gets will give an audio response, either a high or low tone. New
users are encouraged to use this setup and practice with various
targets to learn their audio responses. A US nickel or small gold
nugget works well for a low conductive target, and a US quarter
makes a good high conductive target. TDI is a motion detector,
so target responses depend on the motion of the coil. If you hold
the coil steady over a target, the audio response will fade back
to the threshold tone.
In some cases there is not enough mineralization to require
ground balancing (e.g. non-mineralized beaches), and Ground
Balance may be turned off. In this mode the audio response is
high for all targets.
In cases where you have determined that you definitely
need only one of the conductivity modes and not the other, you
will often find that switching to that mode reduces the amount
of audio noise (chatter) and allows you to run a higher gain. To
better illustrate this, let’s use a strictly inaccurate but useful dia-
gram. The following is a representation of the audio response,
including a low conductor response, a high conductor response,
and random noise:
The ideal no-noise response is what you would get if you dis-
connect the search coil. With Target Conductivity = All, any-
thing above the ideal response will have a high tone and
anything below the ideal response will have a low tone. If there
is a lot of EMI or the Gain is set aggressively high this can result
in a lot of audio chatter.
Low-conductor
response
High-conductor
response
Noise
Ideal no-noise
response
Target Conductivity = ALL

Page 16
TDI SL Owner’s Manual
Setting the Target Conductivity to either Low or High will cut
out half of the response, as shown in the next diagrams. The
noise, which tends to be equally distributed between the high
and low tones, is cut in half. However, you also lose one of the
conductivities. If you know you primarily want to hunt small to
medium nuggets or fine jewelry, then you will not lose much
running in the Low mode. If you primarily want to hunt coins or
relics, then you will not lose much running in the High mode.
Be mindful that “All” is required for ground balancing, because
that is the only mode that gives you both tones.
Threshold also has an effect on how much noise gets
through to the audio. A moderate threshold lets some of the
noise through (anything in the gray region is suppressed):
In this case you will hear a moderate amount of chatter, depend-
ing on the Gain setting. Weak target signals may come through
but may be indistinguishable from noise.
Target Conductivity = HIGH
Target Conductivity = LOW

Page 17
TDI SL Owner’s Manual
A lower threshold makes the TDI quieter and less chattery
but also requires a stronger target signal to produce a tonal
response:
A lower threshold can be played off against a higher Gain set-
ting, although you may find that the net outcome is about the
same.
Setting the threshold exceptionally low can result in a
“silent search” audio, whereby there is no audio sound until a
target is detected:
The drawback is that sensitivity can be markedly reduced and
weaker targets completely eliminated. However, Gain can be
increased to counter this, though the net result is still likely to
be less sensitivity than an optimally adjusted threshold.
Pulse Delay
Sampling as close as possible to the end of the transmitted
pulse gives the highest sensitivity to all metal targets regardless
of their size or conductivity. As the Pulse Delay is increased
toward 25s low-conductive targets will drop out, leaving only
the more high-conductive targets.
Conductivity is not the only factor that determines how tar-
gets respond; size, thickness, and shape also have a strong influ-

Page 18
TDI SL Owner’s Manual
ence. Sub-gram gold nuggets are often only detectable at the
lowest delay setting while larger nuggets, even of the same
alloy, can be detected at higher delay settings. This is largely
due to skin effect, whereby the response of small thin targets
dissipates quickly.
A good exercise is to experiment with different targets at
different Pulse Delay settings. This is best done with the Ground
Balance switched off so that all targets sound the same (no two-
tone response) and the sensitivity to the target can be more eas-
ily observed. After you determine how the Pulse Delay affects
the signal, you can try the same tests with the Ground Balance
turned on and the Target Conductivity to All to see whether the
target produces a high tone or a low tone. You can then choose
Low or High to eliminate or accept a target.
In some soils the minimum pulse delay cannot be used.
Notably wet salt soil (such as a saltwater surf zone) behaves
like a low-conductor target and will respond regardless of the
Ground Balance setting. Increasing the Pulse Delay to about
15s will eliminate wet salt response. However, as you move
out of the surf zone onto dry sand remember you can reduce the
Pulse Delay to maximize sensitivity as dry salt sand does not
cause a problem. Areas with high concentrations of black sand
may also require an increase in the Pulse Delay.
Any changes to the Pulse Delay setting affects the ground
balance point so the TDI will need to be re-balanced.
Dual Field Coil
With any metal detector, the size of the search coil presents
a trade-off between depth and sensitivity. A larger coil offers
better depth on larger targets at the expense of small-target sen-
sitivity, and a smaller coil has better sensitivity to small targets,
but at a lesser depth.
Detectable gold nuggets can range from several grains to
multiple ounces, so a coil that offers both depth and sensitivity
would be ideal. TDI uses a patented dual field (DF) coil which

Page 19
TDI SL Owner’s Manual
literally gives you two different size coils running simultane-
ously: a larger outer coil for large target depth, and a smaller
inner coil for small target sensitivity. As such, even the 12” DF
coil, with its 6” inner coil, can pick up nuggets of only a few
grains. The 7” DF coil, with its 3.5” inner coil, is even better at
picking up grain-sized nuggets, with little compromise on large
target depth.
Proper sweep technique is important with any detector, and
the DF coil adds a slight twist. Round coils tend to have a coni-
cal pattern of sensitivity as illustrated below. As such, coverage
at maximum depth is less than the size of the coil, so it is always
recommended that successive sweeps are overlapped by about
50%.
The DF coil has an additional inner coil which has its own
conical detection pattern. Since this coil is most sensitive to
shallower tiny targets that the larger coil may miss, in order to
get the best ground coverage on these targets it is recommended
that the 50% overlap be applied to the inner coil, as illustrated
below.

Page 20
TDI SL Owner’s Manual
Besides overlapping each sweep, it is important to keep the
coil close and parallel to the ground. Pendulum-swinging the
coil results in loss of depth, and if the ground balance isn’t per-
fectly set the variation in loop height will create a ground
response. The following illustration shows improper and proper
coil sweep motion.
Dealing with Noise
Because PI detectors have wideband receivers they are
more prone to noise and interference than narrowband VLF
designs. There are a few strategies for dealing with noise. First,
TDI has a Frequency Offset control that slightly adjusts the
overall pulse rate. With the Threshold and Gain turned up to the
point of mild chatter, make very small incremental adjustments
to the Frequency control to minimize the chatter.
Some sources of noise cannot be dealt with electronically,
such as electric fences. Try to identify the noise source and dis-
able it. Indoor testing is notoriously difficult to do with PI
Wrong Right