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  #1  
Old 05-23-2012, 12:40 PM
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Default Pulse Delay on a PI is Less More?

By popular request, I am making this a sticky.

Originally Posted by Another Member View post
There is a lot of controversy over "Pulse Delay," and its importance to depth and small target sensitivity. Unfortunately, most people don't understand that a setting of 10us pulse delay, is very noisey and unsteady on wet ocean sand. So even though you may be getting more sensitivity to smaller targets, you might not hear them.

Now, move your coil from the wet sand and into the water and you will have to increase your pulse delay to at least 15us to smooth out the threshold. By the time you have reached a depth of five-feet in saltwater, you'll need a pulse delay of about 20us.

...

The pulse delay must be coupled with power to the coil, and circuitry that filters and reads the return signal. The performance of a machine is NOT based soley on pulse delay. That is my opinion, and why I think I would rather have a set pulse delay of 20us, than 15us.
...
Whoever made a claim that shorter pulse delays lead to more depth lacks an understanding of physics, geophysics and electronics.

Having said that, a detector capable of stably operating at shorter pulse delays will, everything else being equal, be more sensitive to small gold items and other so called "low conductors". Target signals are spread in time from the transmit pulse shut off. The signals will reach a peak sometime after the transmit pulse is turned off and this time is given by the geometric shape of the target, its mass and its admittance (a vector quantity combining its conductance and reactance). The signals from low conductors (including ground mineralization) are the first ones to decay after the transmit pulse is turned off. High conductors --such as silver-- peak later and will hang around a lot longer because their high conductivity do a better job of supporting Eddy currents for a longer time. Ideally, for maximum pickup, we would like the detector's sampling delay to not happen after the target's signal has totally decayed. One could envision the delay time acting as a sort of discriminator, progressively discriminating out lower conductors as we increase the delay time.

Now, we can't start sampling immediately after removing the transmit pulse. Inductors (which is what the coil is) resist an instantaneous change in current flowing through them and will respond with a very large transient voltage spike, trying to keep the current through them flowing. During this voltage spike the detector can't sample anything, it is blinded by said transient.

We may ask what determines the length of time this transient condition exists. Although there are things an engineer can do to minimize the length of time the transient takes e.g. clamping diodes, damping resistors, etc., the coil's inductance and parasitic capacitance play a large role. For depth and reception sensitivity, we want a large inductance, but how large is limited by things like coil wiring resistance, power source, design target pulse repetition rate, etc. Parasitic capacitance is something we can do without. Certainly the coil's self-resonant frequency needs to be dealt with, but in general, a larger than necessary capacitance is undesirable and may lengthen the time at which the detector is able to sample. In this regard it should be noted that --for the same inductance value-- a coil made from a spiral PCB trace will have a significantly larger parasitic capacitance than a well designed wire coil. This is plain physics and it's due to the much higher dielectric constant (er) of the PCB substrate (usually an FR4 type copper clad material), compared to enameled wire and air as the dielectric. A detector with a PCB coil can not in general be used at lower sampling delay values as one made with a well engineered wire coil. It does however have a cost advantage as PCB spiral coils can be manufactured in a batch process and each unit has a near identical behavior, cutting down on manual tuning.

Wether a particular ground/gain combination allows the delay to be set at 10Ás is immaterial. It could well allow a delay of 15Ás and pick up a small gold item that may not be "seen" by a detector with a longer delay, simply because the signal has peaked and has practically decayed to nothing before the longer delay machine gets to sample the signal. Perhaps this is the reason why you may have heard some people say the shorter delay machines go deeper?

As you said, pulse delay is but one parameter of a machine's performance. Filtering, good low noise amplifiers on the receive side, peak coil current (related to pulse width and pulse repetition rate as well as design constraints) are also important, as are others not even mentioned.

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  #2  
Old 05-23-2012, 01:24 PM
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Thanks for posting Rudy. I hope after reading it about 50 times I will understand it! This should get interesting.

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Old 05-23-2012, 01:36 PM
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Great post, the only thing that confuses me is the first sentence...

Say you have a target that is a whisper at 10uS, then you turn the delay up to 15uS, wouldn't the whisper target disappear? The target that was barely detectable at 10uS would surely have decayed to 0 in another 5us, no? That would equate to more depth if that is the case. I know on my Infinium, I get more depth across the board at lower pulse delay vs higher pulse delay, but the depth gain is much more obvious on small gold vs large gold. For example I may lose an inch of depth on a nickel going from 10uS to 25uS, but I cant hear a .5 gram ring at all at 25uS, where as I get 5-6 inches on the same ring at 10uS.

Not questioning your expertise, just curious.

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Old 05-23-2012, 03:32 PM
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Great idea Rudy! Now we can lay all the speculation to rest and get on with finding treasure!
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Old 05-23-2012, 05:56 PM
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Originally Posted by Crumble View post
Great post, the only thing that confuses me is the first sentence...

Say you have a target that is a whisper at 10uS, then you turn the delay up to 15uS, wouldn't the whisper target disappear? The target that was barely detectable at 10uS would surely have decayed to 0 in another 5us, no? That would equate to more depth if that is the case. I know on my Infinium, I get more depth across the board at lower pulse delay vs higher pulse delay, but the depth gain is much more obvious on small gold vs large gold. For example I may lose an inch of depth on a nickel going from 10uS to 25uS, but I cant hear a .5 gram ring at all at 25uS, where as I get 5-6 inches on the same ring at 10uS.

Not questioning your expertise, just curious.
Hi Crumble,

The target response "width" (ie. how long they exist and are therefore detectable) vary somewhat. Some can be very "narrow" (for example, a broken ring), others can be wider. In general, the more conductive targets tend to be wider than an equally shaped --less conductive-- target of the same mass. When in time --relative to the turn off of the transmit pulse-- is another matter. As I mentioned earlier, the time at which the signals peak are proportional to their admittance, which for this illustration you can think of only the conductance part. Low conductors such as gold peak soon after the end of the transmit (short delay) pulse and high conductors peak much later (longer delay).

Another thing to keep in mind is that the detector incorporates a filter --a low pass filter or integrating filter to be more accurate-- that maintains an average level at its output. This helps filter out some of the fine grained noise that would otherwise drive anyone crazy. What this also means is that a very small signal acquired on a single pass may not have enough oomph to get through the filter and make its presence known. It is only through repeated passes of the coil that the signal will charge up that low pass filter and make its presence known. One very important reason why overlapping sweeps is important to detecting the deepies. You can also see this in action on videos where the detector depth is being tested by moving a target near the edge of detection. Notice how many times the target needs to be waived in front of the coil before the detector picks it up?

So, to get back to your question, yes, oftentimes a shorter delay makes it look like you are getting more depth, if the signal in question is occurring near the end of the transmit pulse, as would be the case with a low conductor. A shorter delay means that you are able to sample more of the signal energy and charging up the filter to where the signal is detectable, with fewer sweeps of the coil. You may still pick it up with a slightly longer delay setting, as long as the signal is still present when the delay ends, but you won't pick up as much energy from it (because the signal had started before the delay had expired), but if you re-sweep several times, you may still pick it up.

This leads many to think that shorter delays let you go deeper, but what is happening is that your delay is set up to more optimally sense the signal from low conductors. To verify this, use a high conductor target like a silver quarter. Try picking it up with various delay settings. You'll see very little difference in detection depth until you get to some really long delays.

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Old 05-25-2012, 02:02 PM
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Default interesting topic

i need to rethink maybe how i run my duelfield . i run it hot without a smooth threshold , somtimes with pulse delay off. could i be losing depth with delay turned way down or off and should i aim for a smooth threshold.
would appreciate any advice. btw the duelfield is the deepest md i have ever used jim
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Old 05-25-2012, 04:35 PM
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Originally Posted by mooncusser View post
i need to rethink maybe how i run my duelfield . i run it hot without a smooth threshold , somtimes with pulse delay off. could i be losing depth with delay turned way down or off and should i aim for a smooth threshold.
would appreciate any advice. btw the duelfield is the deepest md i have ever used jim
Try running the gain at 2pm with the pulse delay at 7am it still crackles a little but allows you to hear the deepest whispers that you can miss if you run hotter.

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Old 05-25-2012, 04:58 PM
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thanks will give it a go , ive been running gain at 3 with delay off. ive read many of your posts you seem to have df down pat. thanks jim
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Old 05-28-2012, 12:25 AM
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Originally Posted by Cfmct-PI View post
Try running the gain at 2pm with the pulse delay at 7am it still crackles a little but allows you to hear the deepest whispers that you can miss if you run hotter.

Will using this setting on the df help to eliminate the falsing that comes with each passing wave? I dont know enough of the df to use at other than preset settings. It really frustrates me that each wave takes two to five seconds off hunting while waiting for the df to recover. And when waves are on 10 second intervals, i start cursing the water and air and sand and that little floaty piece of grass that always seems to follow me....

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Old 05-28-2012, 12:40 AM
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I don't have that problem... but then again I'm usually on the wet sand or deeper water... I would think turning the gain a little below factory settings will help you out...

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Old 05-28-2012, 01:42 AM
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Thanks Craig.
I have it anywhere from ankle deep to chest deep though i usually hang in waist deep water. On the wet sand I usually have just a crackle when at the factory presets.

Is the water there heavily mineralized? Any idea how much mineralization has an affect on the stability of the df?

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Old 05-29-2012, 12:56 AM
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Originally Posted by MDMike View post
Thanks Craig.
I have it anywhere from ankle deep to chest deep though i usually hang in waist deep water. On the wet sand I usually have just a crackle when at the factory presets.

Is the water there heavily mineralized? Any idea how much mineralization has an affect on the stability of the df?


Yeah we have blacksand and hot rocks...some beaches are nosier than others but when that happens I just turn my gain down a little plus the pulse delay up a little bit... I usually hunt with lots of backround noise in the threshold but for me its normal and I can pick out the whispers regardless... I have been hunting with a PI since I started waterhunting I guess I am used to noise.. even when I run my GT I like the buzzing after it comes out of a null over a smooth threshold tone

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Old 06-22-2012, 08:33 PM
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thanks for the great information.

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Old 10-20-2012, 05:36 PM
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Originally Posted by MDMike View post
Will using this setting on the df help to eliminate the falsing that comes with each passing wave? I dont know enough of the df to use at other than preset settings. It really frustrates me that each wave takes two to five seconds off hunting while waiting for the df to recover. And when waves are on 10 second intervals, i start cursing the water and air and sand and that little floaty piece of grass that always seems to follow me....
I run mine at full gain with no pulse delay. When the waves do that to me I turn the pulse delay until it stops happening, usually not much. I have barely any idea what I'm doing but this seems to work. The dialog on this topic is way way over my head. My knuckles dragging on the sand help me grid off my search area

Detector seems to work great for me on the beaches of Puerto Rico but I have never had the benefit of hunting with a buddy or trying another machine.

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Old 02-24-2013, 12:26 PM
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http://www.nuggetshooter.com/article...Idetector.html

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Old 02-24-2013, 01:26 PM
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Originally Posted by Cfmct View post
http://www.nuggetshooter.com/article...Idetector.html
Excellent article! Thanks for posting it Craig.

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Old 03-01-2013, 10:30 AM
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This looks like a pretty interesting read...

http://www.treasurelinx.com/projects.html

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Old 03-23-2013, 01:12 PM
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Default yo R

From the land of the Bluenose....PI is the way to go on wet sand or in the salt water. Damn the trash and dig it all!

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Old 04-04-2013, 02:25 PM
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Originally Posted by Hotrod Tom View post
Thanks for posting Rudy. I hope after reading it about 50 times I will understand it! This should get interesting.


50 your a quick learner that's gonna take me as long to understand as it will to learn MY new PI machine LOL
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Old 06-30-2013, 12:18 PM
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