Coil Parameters and paracitic values

[ericwd9]

Paracitic Capacity, resistance and self inductance are not easily measured with acuracy. The resisitance goes up with frequency due to skin effect. The real paracitic capacity is rarely known. In the "suck it and see" world the measurments and calculations we make statically are "good enough" For those who would model search coils and strive for early sample periods here are some facts to digest.

http://www.cliftonlaboratories.com/s..._inductors.htm

http://www.g3ynh.info/zdocs/magnetic...s/self-res.pdf

http://meettechniek.info/passive/inductance.html

http://meettechniek.info/passive/parasitic.html

It would be nice to own all the equipment but a knowldge of the method gives some insight.

Eric

[Polymer]

Nice to know someone else on the forum is reading this stuff! I thought I was the only weirdo interested in this.
Especially the article by David Knight on interwire capacitance has me wondering about the "established" theories on this.

In German there's a saying enigineers use: "Wer viel misst, misst viel Mist"

It's sort a rhyme, roughly translated as: "Who measures a lot, measures a lot of cr*p" or "the more you measure the more **** you'll get"
Dunno if there's a direct analogy or saying in english to that.

I was looking up dielectric values of materials and was utterly amazed at the values for water & salt water.
They are ridiculously high and also very similar which i did not expect (somwhere around 80).

I assume that the effect of saltwater on vlf detectors as opposed to sweet water must be something
else like ions? Just poking in the dark here not sure what I am talking about.

Perhaps someone can clarify this

Thanks for posting the links!

[Qiaozhi]

I assume that the effect of saltwater on vlf detectors as opposed to sweet water must be something
else like ions?

The chemical name for salt is Sodium Chloride (NaCl), and causes a problem for metal detectors when wet. This is because ions are produced, and sodium (Na) is a metal. The sodium ions cause the salt to become conductive, allowing eddy currents to flow. The problem is much more pronounced with VLF detectors (multi-frequency types are an exception) than with PI, as the decay time is extremely short.

[Koala]

I did an experiment with a bowl of water. I kept adding salt to it to try and get my detector to false over it so I could try different settings. No matter how much salt I added the detector would not false even though my detector goes berserk on a wet beach. Gave up then and not tried again. Keep meaning to fetch some wet sand back but keep forgetting LOL

[crane]

A small container of wet sea side sand or salt water is too small to produce a measurable signal and it also has a very short time constant.

You might notice that the wet sand signal at the beach becomes more pronounced as you approach the water and becomes maximum somewhere between the ocean's surface and the sea floor. The detector will also handle the salt better if you swing it parallel to the sea because the salt water content and volume is more constant.

You would need a very large sample to produce a measurable signal on a pi detector.

[Koala]

Its a VLF I was testing. On the beach even the slightly damp area around pebble is enough to make the detector sound. In all metal mode even very small areas of salt water is enough to change the threshold volume. Gulleys have a very sharp defined edge between the dry edge and the river running down it even if its only a few cm wide. It was this I was trying to replicate.


Does not mater too much now as I have experimented and found the best settings through trial and error.