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**rick** · 05-10-1999, 07:59 AM

Re: coils still- but not forever

thanks to respond too jimi...

more thoughts the better....

presently use sheet acrylic to make bobins for

permanamet form----

example--take single strength sheet and cut out

3 "donut" shapes--say 2)5"od x 4"id and

1)4.25"od x 4"id////sand smooth edge etc.

now use acrylic cement to make sandwich assembly

(of coarse there are certain technics to follow

such as clamp 2x4 in vice with hole in it for jig

saw blade to keep plastic flat--no bounce cut slow

at slow speed else plastic melt tablesaw surface

flat reference when cement together sandwich ultra

clean less dust make surface not touching---)you do

correct and smile---now in this case bobbin

round (can be ellipse or any shape) 5"od x 4.25id

hence a channel 3/8"deep x ~1/8"wide (use 1/4" or

??? for center ) you smile----line with al. foil

for channel---leave space less 1 turn winding

shunt field........cut 2 plastic face --to make mount

shaft etc....

**Patrick Duff** · 02-20-2000, 08:08 AM

Re: My opinion of the Dalton unit.

I just wanted to comment a little further on Jim's post. If anyone

is considering Dalton's unit for marine use you will no doubt need a

chart recorder. Dalton sells one for around $500 for his system.

As Jim mentioned, you will have difficulty hearing it with boat

motors running. In addition, it will be very difficult to remember

where/when the sound was loudest.

The chart recorder provides a history that your mind will quickly

forget. Especially if you are operating the boat with limited help.

Phil, do you know if anyone has hooked a chart recorder up to your

unit?

Patrick

Jim Koehler wrote:

The Proton Mag ForumHi guys,

The Dalton mag is a gradiometer - that is, it is sensitive to the

difference between the two sensors.The Geometrics unit Patrick is

referring to is, I believe, a single magnetometer only. The big

advantage of a gradiometer is that it is not affected by general

changes in the earth's magnetic field due to ionospheric currents.

At low magnetic latitudes, the local magnetic field varies fairly

slowly and not very much. At high magnetic latitudes, you can get

very rapid and very large changes in the magnetic field on the

ground. Up at Churchill, Manitoba, at the rocket range, I have

seen changes in the magnetic field (on the ground) of more than

1000 nT over a time period of about 5 minutes. Using a single head

mag. like the Geometrics (or any other single sensor mag.), you

can't tell if the change you are seeing is caused by something

you're going past or by the ionospheric currents overhead.

I have seen a Dalton mag. owned by one of my neighbours down here

in Arizona and, in my opinion, it is not worth the money. It is

essentially identical to the gradiometer that Phil Barnes

describes with one small difference. Dalton passes a small DC

current through one of his sensors during the 'readout' phase so

that you get a slow beat even if there is no magnetic material in

the vicinity. Changes in the local magnetic gradient due to nearby

magnetic materials then cause a change in the period of the beat.

He calls this method the "delta-squared' method because he

observes this 'change in the difference' - the difference being

due to this small DC current. This helps but, in my opinion, only

slightly. Unless you are going to invest in a portable chart

recorder, it is very difficult to hear, through earphones, any

meaningful change due to small gradients in the field. I must add

the caveat that I've never taken the Dalton unit apart or looked

at the insides. I've just seen the unit, seen it operating and

read the literature he provides. The unit I saw was built more

than three years ago and I have no idea how much, if any, he has

improved it since then.

All things considered, I'd just buy Phil Barnes excellent

construction CD (http://members.aol.com/phil770/index.htm)and

build the Phil Barnes unit. It works, has readily available

components (most of these can be bought at a local Radio Shack

outlet), is simple and inexpensive, and is essentially the same

thing as the Dalton unit. The sensitivity is, as I've described in

my overall document, about 25 nT, give or take. For a gradiometer

of this type where you are listening to the beat, you want the

signal to decay very slowly and hence probably want to use

distilled water as the liquid since it has a decay time constant

of ~3 seconds. Kerosene has a decay time of ~0.5 seconds so

wouldn't be usable in this type of mag. - the signal just fades

out too rapidly - you need to measure the frequency electronically

over this short a period.

The 'stand-alone' unit I've described in my most recent addition

to my Web pages can use any liquid and gives a digital readout and

has a resolution of about 0.1 nT. However, it is a single sensor

unit, like the Geometrics, and has the failings of a single sensor

unit. For prospecting on the ground, you probably will want a

gradiometer and that requires two single magnetometer units. I've

finished my gradiometer but am making minor changes in it and am

not ready to release the plans till I'm sure all the i's have been

dotted and the t's crossed.

My 'stand-alone' mag. can probably be built for approx. $100 -

that's the cost of the parts and assuming that the labour,

provided by the builder, is free. The gradiometer requires two of

these units plus an additional few parts. I'd estimate a total

cost of about $300.

JimJ.A. Koehler

3290 N. Koehler Rd.

Florence, AZ, 85232

Ph: (520) 868-1156

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Re: My opinion of the Dalton unit.

Re: My opinion of the Dalton unit.

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