Equinox Tid Value / Target Frequency Calibration Table

[PimentoUK]

Many of you will be aware that the 'Target ID' number given by a detector is related to the 'Target frequency'. 'High frequency' targets read low-down on the ID scale, typically because they are small, thin, or made of a metal that's a poor electrical conductor. Conversely, 'low frequency' targets tend to be physically larger, and more likely to be composed of better conducting metals.
By careful measurement, it's possible to calibrate the ID scale of a given detector. I've done this for my Fisher F75 previously, it has a wide non-ferrous ID scale, from 16 -> 99, so it can give quite precise target freq. values.

I finally got round to properly working out the calibration of the Equinox 00 -> 40 scale. I have quite a varied selection of test targets that I've used for this calibration. Some of this as a result of work done on the Geotech1 forum, where both PI and VLF's were used for tests. The real physical targets include squares cut from aluminium drinks cans, coins, precisely-made copper rings. The best target is a 'synthetic' one: a coil of wire, with a selected resistor as a load. Knowing the inductance and resistance lets you calculate the time-constant / frequency.

Having taken a mountain of readings, it became apparent what Minelab have devised to create the scaling of the Equinox. It's actually very nicely done, and probably only possible as a result of using a high performance microprocessor. Each ID number step represents a fixed frequency multiple change from the previous number. This applies over the entire range from '02' up to '39' , covering a range from 80 kHz to 0.5 kHz. If you plot 'Target frequency' against 'ID number' on log/lin graph paper, you get a perfect straight-line graph.

Here is some nerdy maths for you:
The ratio from one step to the next is 1 : 1.1472 , so every 8 ID numbers represents a target freq change of 1:3 . An increasing ID number means a lower target frequency, (and a larger target time-constant).

Target time-constant and target frequency are related by:

2 * pi * Freq * time_constant = 1

which may be useful to PI users, nugget-hunters.

The designers obviously have to choose some particular ID value as their 'reference', and every other ID relates to this. It appears Minelabs engineers have chosen 'centre-scale' to roughly match the 7.8kHz operating frequency ( which dominates the Park1 / Field1 modes). A 7.8kHz target would actually read mid-way between '18' and '19', so technically they were 2 digits out, as it would ideally be 20/21.

( The numbers in the table are calculated. I chose ID ='20' as being a 25 microsecond target, and derived my figured from that.)

A few things to note:
This ID calibration is done for 'Park1' mode; 'Field1' should be the same. If you're using modes with a higher freq bias ( eg. Park2/Field2 ), the numbers can differ by a point, as you will have no doubt observed.

The frequencies shown are for the middle of that ID value range. So for example, TID = '16' actually encompasses targets from 10.3 kHz -> 11.8 kHz

Frequencies for ID values over '33' are estimated mathematically. I didn't have any useful test targets reading that high up the scale to verify the numbers.

To get a true measurement of a test target, you need to measure it at a test frequency close to that of the target. If you use a different measurement frequency, particularly one higher than the target, you can get errors due to 'skin effect' , where not all of the sample's metal is being measured. So, for example a US silver Dollar has a target freq of something like 0.85 kHz. But measure it with a typical 10 kHz detector, it looks like a 1.2 kHz target.

 

[mh9162013]

Looks rather interesting. I look forward to reading more.

But how can we use this calibration table to more accurately ID targets in the ground (or understand what's going on when our VDI numbers seem to be "off?")

[PimentoUK]

"But how can we use this calibration table to more accurately ID targets?"

It's no use for that.

It has academic interest, and it removes the mystery of 'what the numbers mean'.

It allows you to in turn calibrate the ID scale of any other 'ID' detectors you have.