For those who are building their own detectors and coils
Earlier in the week I started building a different type of coil for my pi called a flat spider coil. It is a timeous job and took a couple of days until such time that it can be handled.
The coil winder almost took as long to make as the coil
it has an inner diameter of 22cm and outer of 23.5cm. I used 0.40 enameled wire. 23 turns. 280uH
these coil are known to have the lowest 'Q' (interwinding capacitance) of all coils and for that reason I gave it a try
Did a bit of comparative testing in the workshop this morning with my 10 inch round and the 9 inch spider.
Although these test dont mean too much right now I cant wait to built a housing for the spider and test it in the real world against the same coil on wet beach
10" spider
50c coin 20cm 22cm
3" nail 24 29
5mm 9ct ring 21 25
2mm 18ct ring 22 23
2mm silver ring 21 22
5mm silver chain 18 19
8mm 9ct ring 24 26
the tests were all run with sensitivity at 5/10. at a higher sensitivity the depth would increase slightly