New 705 owner - coin ids
Posted by: jjbunn [ Send a Message ]
Date: April 03, 2019 08:08PM Registered: 3 months ago
Posts: 3
I just received a 705, and have been testing it outside against a set of old British coins laid on the ground. I first did the auto ground balance. I'm a bit puzzled by a couple of results, shown in the table below:
• 1941 Farthing (Bronze) ID = 28
• 1885 Florin (Silver) ID = 42
• 1797 Cartwheel Penny (Copper) ID = 44/46
• 1965 Penny (Bronze) ID = 42
• 1945 Threepence (Brass) ID = 34/36
• Gold Ring ID = 18
• Iron Nail ID = -6
So, the two bronze coins show very different ID values (28 and 42) ... is this a measurement error on my part, or a usual variation? I'm also surprised the silver florin IDs the same as the bronze penny - again, maybe my measurement error?
Comments on these measurements would be welcome
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Hello UK jjbun....
Xterra 705 .....Alas, no indication of what search-coil used, so we do not know at what frequency the testing was done....but the listing is nicely presented and from that I will assume it was a 7kHz coil ...?
Based on that assumption, I will estimate the theoretical VDI :-
The 1941 farthing, quoted as ‘bronze’; is normally 20 mm in diameter.
If it were ‘pure’ annealed copper, its relative conductivity would be 100.
‘Bronze’ as an alloy of copper, and the 1941 farthing I approx. rate at 88.
What then do I theoretically estimate its VDI ?
Theoretical calculations produce, 27.6 Rounded-up, 28 QED.
Now for the 1885 Florin, (Silver) ID = 42
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1885 Florin..’Silver’......but we ask, “What specific quality of silver dose that equate to?”
Web search:- % silver 92. Diameter = 30 mm.
My estimation of VDI using X-terra 705 @ 7.5KHz. = 43.3...rounded-down....43 QED.
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Now for the 1797 Cartwheel Penny; History and Inflation.
By Ozcopper
The 1797 cartwheel penny is an important coin because it is the first coin made using a steam driven stamper, They were made by Matthew Boulton’s famous Soho Foundry in Birmingham.
Each coin contained an (Av) ounce of near-pure .999 copper.
Thus we know copper was worth a penny an ounce in 1797. The weight of these coins also made them ideal as substitutes for weights in measuring produce, a task for which they were intentionally designed.
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So, they were ‘pure’ copper ! (99.9%) and 36 mm in diameter.
My estimated VDI then is approx. 47.1 Rounded-down = 47. QED.
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Now for the 1965 penny; (Bronze) ID = 42.
Dates Composition Mass (grams) Diameter mm)
1925–1943, 1945–1954 Bronze (95.5% Cu, 3% Sn, 1.5% Zn)
9.4 31
1944, 1961–1970 Bronze (97% Cu, 0.5% Sn, 2.5% Zn)
9.4 30
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The approximate calculations for the 1965 penny initially indicated an ID of 46, but then I noticed an increased Zn content of 2.5, which reduced the ID to 41.5
So rounding that up, gives us the same figure quoted by you 42. QED.
Well JJ..I think that brief assessment of your ‘IN-AIR’ tests demonstrate the significance of the process AND your competence of doing it correctly.
”Well done”
With regardes to the ‘sameness’ of ID’s for differing targets…..
That’s the enigma of single-entity identity limitations method, for the majority of hobby-detectorists who don’t have the deeper, technical background qualifications or interests. That’s not a critisimn; simply a fact of life!
The FREQUENCY factor involved in all these complex calculations plays a major role. If you use a relatively lower frequency, then depending on the ‘other’ factors involved, the target’s ID significance will be ‘more individualistic.’
Because you GB’d initially, and coins were on its surface; its contribution was negligible.
Perhaps you can therefore appreciate the ‘benefits’ of the E-Trac’s 2 dimensional VDI methodology.
I hope you enjoy your Xterra…its lightweright…its options…..and its limitations.
At 18 KHz, its gobbles up the thinner class of coins, rings etc ( 1 mm or less)
……matt