PI questions (long post)

[Anonymous]

Hi all,
I have some questions. In the last 2 threads, Eric stated that conductive ground may cause you to have to reduce sensitivity or adjust SAT. What would the symptoms be and how would you know which adjustment would be required? Mr Bill states that if the surfmaster pi hits some black sand soil, you may have to reduce sensitivity by increasing the pulse delay. If your detector is misbehaving because of a highly conductive ground, then does increasing the pulse delay ( on the surfmaster pi)also counteract this problem as does reducing a sensitivity control (op amp gain I assume) on the aquastar? OK, now the Impulse is designed to go down to 250 feet in salt water. I would think that there would be more conductivity in salt water than in fresh water. It has no user adjustable pulse delay or sensitivity controls. The only control is a threshold control. How does this figure into dealing with high conductivity envioronments?
Summary, how does a detector behave that you would know that you are in an exceptionally high conductive soil or salt water effects? Will any of these 3 or 4 parameters able to deal with it? The detector I'm building has a threshold control and pulse delay but I didn't put a sensitivity control on it, it runs wide open. I'm planning to take it to a saltwater beach in the next couple of weeks. Do I need to put a sensitivity control on the final op amp or will I be able to deal with the conductivity by increasing the pulse delay or increasing the threshold?
Thanks for your help,
Fred

 

[Anonymous]

Fred,
I probably took the word "Sensitivity" out of context a bit using it like I did for the Surfmaster. For the Surfmaster it should have been Pulse Delay control. We were talking about black sand in a very general way, not cast in stone. The best way to understand when your detector is acting up from mineralization, is familiarity of the detector. If the detector was responding to the earth because is pulse delay was too fast, then slowing it down would stabilize the detector.
On different PI

 

[Anonymous]

The Impulse has a fairly short and non-adjustable pulse delay. However, its ramp type transmit-flyback waveform (as shown in the patent) reduces the eddy current induced in the salt water so that ordinarily there's not too much salt pickup. It can be noisy when used in the shallow surf, esp. if accompanied by black sand; and, when doing underwater scuba work, you may hear the threshold tone rise and fall slightly with the waves overhead and with changes in your depth in the water relative to the surface and to the bottom.
A longer pulse delay would have reduced the salt pickup, but would have also reduced sensitivity to jewelry items. An SAT (autotune) system would have made it a little better on the beach, but would have cost sensitivity for underwater salvage work where divers are often looking for large objects several feet away. What you have in the Impulse is what we believed to be a good compromise for the various conditions and uses to which it would be put.
--Dave J.

 

[Anonymous]

Hi Fred,
There are usually three things in a PI receiver that affect the detection range and/or sensitivity. Firstly, the sample pulse delay. The shorter this is, the more the detector will respond to lower conductivity items. There are certain items in this category that we want to detect, such as thin low carat gold rings, platinum rings, gold chains, ear rings etc., However, a salt wet beach constitutes a very large low conductivity object and will give a signal which increases rapidly as the sample pulse delay is shortened. Near, and just in, the water, 10uS is about as short as you can go with standard coil and electronic arrangements, but even at 15

 

[Anonymous]

Excellent responses, thanks everyone.
Fred