FGM-3 or FGM-3h for Carl Fluxgate Magnetometer?

[Alex]

I intend to buy a kit to make a Carl gradiometer.I see another sensor FGM-3h who is 2 times and half more sensitive than FGM-3 ,but has more problem
with the direction (EAST-WEST),and fluctuation of power supply.
CAN I use this FGM-3H or no,and what are the changes to do??

Many thanks
Alex

[Geophyz]

No. this has been discussed on these and other forums at length. Contact Fat Quarters and they tell you the same thing. Use the FGM-3.

[schatzsucher]

...and my opinion is: YES!

[Alex]

Hi Bobo

I already ordered the kit with FGM-3 sensors but this not change my question.I'm always for do something better so Can you share with us your positive opinion ?(even in your english don't worry I,m not a english speaker too-like many of the guys here)
Really thanks.
Alex

[Carl-NC]

The biggest problem with any fluxgate design is aligning the sensors. The FGM-3's are already difficult, so the 3H sensors will be extremely difficult.

The FGM-3 sensors have a built-in feedback coil that is unused in my design. The next step I had wanted to take is to make use of the feedback coil in balancing the sensors. This could be done in a one-time cal, or on a continuous basis to make a sort-of motion magnetometer. Unfortunately, I have no time to pursue this. But I think it would be a better strategy than trying to use the 3H sensors.

- Carl

[Alex]

Thanks Carl

When I will receive a kit I will use FGM-3 as I ordered.After a while (and some
experience in alignement the sensors) I will try maybe with fgm-3h .I will let you know.
Thanks again.
Alex

[Alex]

You're right Carl !

PHYSICAL CHARACTERISTICS
Sensor Outline - FGM-3

The sensor has been made with a cylindrical form in order to facilitate the overwinding of external
feedback coils. Alternatively, it can be readily inserted into a separately fabricated coil on a tubular core.
As a simple guide, for example, a single layer of 0.2mm wire ( 0.25mm overall say ) wound over 60mm
of the sensor length will give the equivalent of 4000 turns/metre. Since 1 oersted is approximately 80
ampere-turns/metre, such a winding will produce a field of around 50 oersted/ampere. Thus with up to
100mA flowing it is possible to offset fields of

±5 oersted, increasing the range of the sensor by x10........FANTASTIC!!
Negative feedback also brings all the usual benefits of improved linearity and stability, of course.
CAN YOU EXPLAIN MORE ABOUT THIS CARL?
Praticaly what can I do ?
The external windings must be conect to one sensor pin or let it free?
Thanks
Alex

[Alex]

YOU'RE RIGHT CARL

I 've found this in sensors data sheet:

PHYSICAL CHARACTERISTICS
Sensor Outline - FGM-3

The sensor has been made with a cylindrical form in order to facilitate the overwinding of external
feedback coils. Alternatively, it can be readily inserted into a separately fabricated coil on a tubular core.
As a simple guide, for example, a single layer of 0.2mm wire ( 0.25mm overall say ) wound over 60mm
of the sensor length will give the equivalent of 4000 turns/metre. Since 1 oersted is approximately 80
ampere-turns/metre, such a winding will produce a field of around 50 oersted/ampere. Thus with up to
100mA flowing it is possible to offset fields of

±5 oersted, increasing the range of the sensor by x10. Fantastic no?
Negative feedback also brings all the usual benefits of improved linearity and stability, of course.
CAN YOU EXPLAIN MORE ABOUT THIS CARL?
THE EXTERNAL WINDINGS MUST BE CONECT TO ONE SENSOR PIN ,OR LET IT FREE?
THANKS A LOT
Alex

[Carl-NC]

I think you would just pump a current into the feedback coil to create a magnetic field. Most likely, for a gradiometer you would need to do this only for one sensor, and (most likely) it will be sensitive to directional orientation (e.g. north/south).

- Carl

[schatzsucher]

My opinion is, that the feedback-coil added to the source-coil inside the fluxgate-system.
Because, you can control the trigger-frequency from the sensor and electric adjust the sensor-orientation.
But, i am not sure of the electrical data differnece from sensor´s

[archaeofreak]

hi. I am an archaeology grad student. We use a geoscan FM36 gradiometer for survey work in archaeology. is anyone familiar with the geoscan FM36 fluxgate grad? The FM36 has a data storage resolution of .05 nT, which is basically the sensitivity. the fgm3 sensitivity is only 50 microT, correct? Thats 50000 nT, or 500000 times too coarse for archaeological work!

Does anyone know how to build these sensors themselves, rather than buying very cheap 15 dollar sensors used in boat compasses and the like? Just curious. Or is there a source available for the sensor that geoscan uses? They probably make it in house, I would guess, however....

[Geophyz]

hi. I am an archaeology grad student. We use a geoscan FM36 gradiometer for survey work in archaeology. is anyone familiar with the geoscan FM36 fluxgate grad? The FM36 has a data storage resolution of .05 nT, which is basically the sensitivity. the fgm3 sensitivity is only 50 microT, correct? Thats 50000 nT, or 500000 times too coarse for archaeological work!

Does anyone know how to build these sensors themselves, rather than buying very cheap 15 dollar sensors used in boat compasses and the like? Just curious. Or is there a source available for the sensor that geoscan uses? They probably make it in house, I would guess, however....

Archaeofreak,
The sensitivity of a gradiometer like the Geoscan or the Bartington Grad601 is highly dependent on the mechanical alignment of the two fluxgate sensors. The better the sensors are aligned, the better the " data storage resolution" will be. Same deal with the FGM-3 gradiometer design. The English style fluxgate systems have very precise alignment and low system noise to extend the resolution of the fluxgate sensors.

I guess my point is that if you can mechanically align your FGM-3 sensors with a high level of accuracy, then you can definitely use that mag for archaeology. Soldering the electronics to the circuit board is the easy part, but aligning and adjusting the sensors is art!

[archaeofreak]

ok, yes you have to mechanically align the sensors of the FM36; but that doesn't change the fact that the sensitivity of the FGM 3 is only capable of a 50 uT resolution, does it? I can see that 50 uT may be sensitive enough to require alignment with the local magnetic field, but if it is only capable of 50 uT, NOT .1-.05 nT, like the FM36, of what use is it to archaeologists? Not trying to argue with you, I just dont understand how it could be useful for arch if its such a coarse sensitivity.

Are you saying that the FM36 sensor sensitivity is not really .05 nT, but that that sensitivity is a function of correct alignment?

[Geophyz]

Are you saying that the FM36 sensor sensitivity is not really .05 nT, but that that sensitivity is a function of correct alignment?

Exactly. Sensor alignment in a fluxgate gradiometer determines sensitivity to a large degree. Most fluxgate sensors are limited in their resolution as stand-alone sensors. I do not know the exact resolution of a single Geoscan sensor but i would bet the farm it is not .05nT. But I would also bet the farm that it is much better than the FGM-3.

See, all that the "English fluxgates" are doing is subtracting the botton reading from the top. If both sensors are exactly aligned and reading the same vertical field, then heading error is negated. You are able to record to .05nT resolution by simple subtraction, not because that is the absolute resolution of each sensor. The whole is greater than the parts.

Read the chapter of "Seeing Beneath the Soil" on the subject again. I believe Clark talks about abandoning 1meter sensor separation and working on eliminating system noise and better sensor alignment. It was a tradeoff. Like I said, fluxgate gradiometry is part art and part science!

The Bartington grad601 is unique in that it uses space age materials in tube construction (enabling 1 meter sensor separation) and a digital sensor balance system that is software controlled.

[Geophyz]

EPE design logs 1nT resolution

Archaeofreak,
As an example of our discussion, the EPE fluxgate gradiometer design logs the data down to 1nT resolution with the Fat Quarters FGM-3 sensor.