Vallon VMH3CS Mine Detector

[STAVROSNU]

Dear Eric,

please read your private messages.



Best Regards

Stavros

[Ferric Toes]

Eric maybe it was at "Beggary Hill" at a guess.

Hi Gary, That's the place.

[Ferric Toes]

Eric, can you give a relative rate of "rapid change of amplitude" that you experienced? I have assumed the fastest rate to be more than 10 seconds, but this is a total "assumption". This is an area where I have no experience or data to use as a basis for comparison.

Difficult to answer, but at a sweep speed of 1m/sec the coil could encounter, within the 1m, say, three high viscosity areas and three low. The 'rapid change of amplitude' is observed at the output of the detectors first amplifier. This is then sampled, integrated, balanced, and subtracted to give a dc output that does not change throughout the sweep. This is similar to earth field cancellation where you are sweeping over magnetised rocks. A good detector will give no output but if you observe the first RX the signal is leaping about quite dramatically. I would say milliseconds rather than seconds depending on pulse rate and coil diameter.

Eric.

[garyq]

Hi Eric,

OK - I did a bit more searching, and here it is ->
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"For some years now, I have used readily available coins as a means of comparing detection ranges both for air and in ground tests. Depending on the application, different coins can simulate different types of non-ferrous target. For example a US 5c coin has similar electrical characteristics to a medium 14k gold ring. Hence a detector that gives a good range on a US 5c will be efficient at finding a wide range of rings. In the UK a similar coin in size and electrical conductivity would be a 20p piece. We also have a smaller 5p coin which is useful for testing the ability to find lower conductivity items. Recently, there was suggestion from an Australian detectorist about using their 5c coin as a standard test object for gold nugget detectors. I had an initial look at the coins to see how they compared in size, thickness and weight. Here are the results.
US 5c, 21.13mm diameter, 1.9mm thickness, 5.00gm weight
UK 5p, 18.01mm diameter, 1.72mm thickness, 3.2gm weight
Aus 5c, 19.41mm diameter, 1.38mm thickness, 2.8gm weight
I have not yet had a look at comparative detection ranges but the Australian 5c looks the most difficult target as it is the thinnest. I dont know how the alloys compare, but the Aus 5c is 75% copper and 25% nickel. Eric."
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With a PI detector I have air tested an Australian 5c coin and a US 5c (Nickel) coin as both have the same 75% Copper and 25% Nickel content and the US 5c coin produced an extra 1" of air depth above the Australian 5c coin.

[garyq]

Hi, I believe it was the area where there are vertical reefs/dykes of ironstone interspersed with layers of less mineralised material, Can't remember the name of the place though.
The rapid change of amplitude as you scan over the ground is certainly a good test of your GB method.

Eric.

Eric maybe it was at "Beggary Hill" at a guess.

[KingJL]

The rapid change of amplitude as you scan over the ground is certainly a good test of your GB method.
Eric.

Eric, can you give a relative rate of "rapid change of amplitude" that you experienced? I have assumed the fastest rate to be more than 10 seconds, but this is a total "assumption". This is an area where I have no experience or data to use as a basis for comparison.

[Ferric Toes]

Hi Eric do you remember from what area you took your sample ?

Hi, I believe it was the area where there are vertical reefs/dykes of ironstone interspersed with layers of less mineralised material, Can't remember the name of the place though.
The rapid change of amplitude as you scan over the ground is certainly a good test of your GB method.

Eric.

[6666]

while the same sample size from Wedderburn, Victoria, is over 1000.

Hi Eric do you remember from what area you took your sample ?

[Qiaozhi]

I don't know where to look to find the information on the earlier nickel and 5p tests but I will do a 5p test for comparison and also a selection of nuggets up to 10gm.

Eric.

Hi Eric,

OK - I did a bit more searching, and here it is ->
-----------------------------------------------------
"For some years now, I have used readily available coins as a means of comparing detection ranges both for air and in ground tests. Depending on the application, different coins can simulate different types of non-ferrous target. For example a US 5c coin has similar electrical characteristics to a medium 14k gold ring. Hence a detector that gives a good range on a US 5c will be efficient at finding a wide range of rings. In the UK a similar coin in size and electrical conductivity would be a 20p piece. We also have a smaller 5p coin which is useful for testing the ability to find lower conductivity items. Recently, there was suggestion from an Australian detectorist about using their 5c coin as a standard test object for gold nugget detectors. I had an initial look at the coins to see how they compared in size, thickness and weight. Here are the results.
US 5c, 21.13mm diameter, 1.9mm thickness, 5.00gm weight
UK 5p, 18.01mm diameter, 1.72mm thickness, 3.2gm weight
Aus 5c, 19.41mm diameter, 1.38mm thickness, 2.8gm weight
I have not yet had a look at comparative detection ranges but the Australian 5c looks the most difficult target as it is the thinnest. I dont know how the alloys compare, but the Aus 5c is 75% copper and 25% nickel. Eric."
-----------------------------------------------------

[Ferric Toes]

Hi Eric,

I'm still curious about the Vallon's ability to detect a 0.3g gold nugget at 2 to 3" when it has such a late sample time. In the past (if I remember correctly) you once did some tests using standard coins, such as a US nickel, and a British 5p, etc., as alternatives to using actual gold nuggets; but I cannot seem to find that information in my archive.

Yes, it is still unclear to me. All I can do at the moment is connect a wide band amplifier across a Vallon coil being driven by the Vallon TX. That way I can see the recovery waveform and the point at which the earliest sample can take place. I will post a picture of this waveform shortly and see what readers of this thread make of it.

I don't know where to look to find the information on the earlier nickel and 5p tests but I will do a 5p test for comparison and also a selection of nuggets up to 10gm.

Eric.

[Qiaozhi]

Hi Chet,
I don't anticipate any problem with the salt as it appears that the sampling in the Vallon is quite late in time i.e 20 - 30uS.

Hi Eric,

I'm still curious about the Vallon's ability to detect a 0.3g gold nugget at 2 to 3" when it has such a late sample time. In the past (if I remember correctly) you once did some tests using standard coins, such as a US nickel, and a British 5p, etc., as alternatives to using actual gold nuggets; but I cannot seem to find that information in my archive.

[Ferric Toes]

Hi Eric

This is an interesting operating concept. I am looking forward to your beach hunt results. I am very interested in how well the small circular area of the probe responds to the salt and how well it ground balances.

Hi Chet,
I don't anticipate any problem with the salt as it appears that the sampling in the Vallon is quite late in time i.e 20 - 30uS. It it only when you go to <15uS that the salt signal becomes significant. At 5uS a PI detector is barely useable in wet sand.
Iron mineralised ground is an unknown at the moment. The Vallon appears to be good with standard open coils but there comes a point in magnetic field strength where the signal departs from the 1/t^-1 law that ground balance relies on; which may occur if the probe tip contacts the ground. The end of the ferrite rod is about 1/8in in due to the thickness of the plastic bung in the tube end. I would think that scanning with the probe an inch or two off the ground is OK, but for down hole pinpointing it would be nice is there was no such non-linearity. The Victoria goldfields in Australia would be the best test as, in some areas, the ground is the most iron mineralised that I have come across. Soil from Red Hill, Virginia, USA, reads about 300 for a 10gm sample on the viscosity meter, while the same sample size from Wedderburn, Victoria, is over 1000.

Eric.

[Chet]

Given the good ranges on coins and rings I am going to do a beach search just using the probe with the 6in core. That way, as I dig, I can go right down in the hole without need for a pinpointer. In sand, a lot of time is wasted just chasing the target around so I hope this will speed up recovery.

[ATTACH]36440[/ATTACH]

Eric.

Hi Eric


This is an interesting operating concept. I am looking forward to your beach hunt results. I am very interested in how well the small circular area of the probe responds to the salt and how well it ground balances.


Have a good day,
Chet

[Ferric Toes]

The Probes are coming.

I have been trying various ferrite rods, probe configurations, and range comparisons with some interesting results. The first probe I made used a 4in x 1/2in diameter core which was housed in a Vallon telescopic shaft assembly from a broken coil. After experimenting with the wire type and gauge it worked fine and would detect a nickel at 8in. This is the full army green shaft below the yellow hand probe. The lower probe housing can be unplugged and changed for a standard Vallon elliptical coil or an 11in open round coil that I made.

The yellow short probe with the coiled cable is a hand probe that can be plugged into the control box instead of a standard coil. This probe has the same ferrite core and winding as the first probe.

The third long shaft probe uses a 6in x 1/2in core but with a series resistor, as explained later.

The fourth probe uses a 4in x 3/4in D core in a larger diameter outer tube. This can be interchanged with the probe head that has the 6in core for comparison of detection ranges.

All probes have been wound to an inductance of 1.8mH and a final resistance of about 3 ohms. The first probe was wound with Litz wire and gave about 3 ohms, but subsequent ones wound with 0.2mm Teflon wire only measured 2.3 ohms so a 1 ohm metal oxide resistor was put in the live side in series with the coil. The exception was coil on the 4in x 3/4 rod which came out at 3.3 ohms and didn't need any extra.

The VMH3CS is quite sensitive to coil characteristics and there is obviously a balance between inductance and resistance, either of which if changed by a fairly small amount, the startup check fails and the detector not operate. I also check the current draw at 4.5V by using a bench power supply. The Vallon ellipical coil give a draw of 315mA, so I check that the probes also draw that amount

Testing the range on a nickel and using the 4in x 3/4in core improved the range to 10in. I appeared to achieve the same range with the 6in x 1/2in core. The latter is probably of more interest as 3/4in cores are harder to find than 1/2in cores and the slimmer tube has a better appearance. Standard heavy duty pipe couplings were used to make the probes detacheable. Just a little machining was needed to suit the fibreglass tubing. Good job I still have my lathe.

Given the good ranges on coins and rings I am going to do a beach search just using the probe with the 6in core. That way, as I dig, I can go right down in the hole without need for a pinpointer. In sand, a lot of time is wasted just chasing the target around so I hope this will speed up recovery.

All the probes are (or will be) potted in polyurethane resin and the couplings are sealed, so immersion up to a couple of feet should be OK. Additionally there less water resistance than when sweeping with a standard coil.



Eric.

[Ferric Toes]

Hi Eric

I presume you are referencing the vast empirical knowledge and experience in electronics that you have acquired over the many years.

I found the websites below on the R1132 receiver. It has quite an interesting history. It looks like there were a number of manufactures.

Have a good day,
Chet

http://www.radiomuseum.org/r/mil_gb_receiver_type_87_r1132ar.html

http://www.vmarsmanuals.co.uk/archive/2113_AP2555H_R1132A.pdf

http://www.preloved.co.uk/adverts/sh...-receiver.html

Well done, you got it all correct. I've been in many branches of electronics before metal detection and often have a 'feel' whether something will work or not. Try it and see, then consider how to refine it, and the theory often comes last - if at all.
The radios are R1132A, both restored to working order together with the Type 3 power supplies. All mine were made by EKCO (E. K. Cole). The nearest with 1/4in thick aluminium panel is 1940 manufacture and the next is 1941 with 1/10in steel panel. Aluminium was needed for aircraft manufacture; also the chrome plated handles were no longer and they were just painted. They R1132s were ground station receivers for communication with Spitfires and Hurricanes, with the associated transmitter of course. The set connected to the detector coil is R1155A of Bomber Command and of 1940 manufacture by EKCO. Also restored and fully working off the Type 3 psu.

Eric.