To measure something that is 0.2% of the earth magntic field is much more of a challenge.

First, if you want to cancel out the Earth magnetic fields by using a triaxial Helmholtz coil, then you will have 2 problems: 1) you would need to have a magnetometer that is much more sensitive than the measurement you want to make in order to actually being able to measure if you indeed have cancelled the earth magnetic field to a level that is low enough that it would allow you to measure such a small mangetic field coming from the device you are describing. (Hall effect probes usually have temperature dependent variabilities alone that will generate noise that is above the levels you describe)

Second, the power supplies utilized for the coils must be have an unusually small amount of noise in their DC signal in order not to generate extra and unpredictable mangetic fields that would interfere with the measurement.

Third, other devices, as well as solar actuvity can generate fluctuations in the environment DC mangetic field of magnitudes close to the one you want to measure and the helmholtz coil approach is therefore inapropriate (or at least incomplete).

Therefore, this specific measurement must first be decoupled from the environment. The way to do this is with a high permitivity enclosure (m-metal is a typically used alloy) however, this shield will be expensive and hot straight forward to manufacture in order to achieve the levels of shielding you are looking for. This can be in the order of thousands of dollars for volumes of a few litters (designign such enclosures was part of my thesis project).

I think the best solution would be not to build such structure, but to find an MEG facility near you (if possible) and make the measurement in there if such device is available in your country.

REgarding the magnetometer to use, you have not provided the size of the device you want to measure, or the size from where the magnetic field is emanating. I assume this device is not big, therefore you want the probe to be as small as possible. Fluxgate magnetometers are relativvely afordable and their probes are of medium size and able to measure such low fields accurately. you can get them easily online. They were first used to measure the fluctuations in the Earth magnetic field. Of course you can also use squids...but you can consider this a non-viable option...these devices need strict temperature control and you will not get them online easily and are usually not affordable.

Hope this helps.

I don't have experience with such low magnetic fields.

I did notice in my lab that my rather sensitive Hall Effect Probe could detect not only the magnitude but the orientation of the Earth's magnetic field, which descends at a rather steep angle here in Norway. Turned at right angles, it of course read zero.

So I would assume if you were to align your equipment at right angles to the Earth's field you could get an accurate measurement.

However, depending on what experiments you are trying to do, I would tend to assume you will need a different solution to avoid the interference of the much more powerful field of the Earth (and probably nearby electronics!).

Regards,

Mark.

Well, I think the best thing would be to shield your measurement environment using

a mesh of iron around the apparatus. You need something like the anacheoic chamber but for the earth magnetic field.

You can measure it with Narda sytems. You can find the details of the device from this page: http://www.narda-sts.com/pdf/niederfrequenz/DS_THM1176-PDA_EN.pdf

I hope this helps you.

Best...

http://www.narda-sts.com/pdf/niederfrequenz/DS_THM1176-PDA_EN.pdf

I would suggest a wheatstone bridge, where the Earth's contribution is cancelled out.

Hi Maher, in my opinion the NARDA system is not applicable (it's made for safety , that is, to test the compliance of magnetic fields with limits imposed for safety purposes).

You can use anisotropic magnetoresistive sensors, you can find them , for example , at Honeywell site.

The Earth field is strong but fixed ,so it can be eliminated by som sort of calibration. See Honeywell application notes.Bye

You can try to use a system of Helmholtz coils to compensate for the vertical and horizontal component of the Earth's magnetic field. I think that full compensation can not be achieved.

Best regards,

Tatiana.

In SI units, magnetic field is Tesla (T). In CGS units, magnetic field is Gauss. Important classes of magnetometers including Hall Effect magnetometers, Anisotropic Magnetoresistive (AMR) sensor (Honeywell Inc.), and SQUID magnetometers. Between them, the SQUID is a very sensitive vector magnetometer, it is most commonly used to measure the magnetic fields produced by brain or heart activity. However, it is very expensive and needs of magnetically-shielded room of permalloy or aluminum. You can use two possible solution for your question: i) A system, as coil, to compensate the Earth's magnetic field associated with Hall Effect or AMR magnetometer; or ii) Measuring the small magnetic field inside a shielded room or chamber associated with all kind of magnetometers. I have good experience using AMR sensor of the Honeywell (HMC1001 - magnetic field range -2 to +2 Gauss and resolution of 27 microGauss) as a cheap solution.

Regards,

Fabiano Paixao

To measure something that is 0.2% of the earth magntic field is much more of a challenge.

First, if you want to cancel out the Earth magnetic fields by using a triaxial Helmholtz coil, then you will have 2 problems: 1) you would need to have a magnetometer that is much more sensitive than the measurement you want to make in order to actually being able to measure if you indeed have cancelled the earth magnetic field to a level that is low enough that it would allow you to measure such a small mangetic field coming from the device you are describing. (Hall effect probes usually have temperature dependent variabilities alone that will generate noise that is above the levels you describe)

Second, the power supplies utilized for the coils must be have an unusually small amount of noise in their DC signal in order not to generate extra and unpredictable mangetic fields that would interfere with the measurement.

Third, other devices, as well as solar actuvity can generate fluctuations in the environment DC mangetic field of magnitudes close to the one you want to measure and the helmholtz coil approach is therefore inapropriate (or at least incomplete).

Therefore, this specific measurement must first be decoupled from the environment. The way to do this is with a high permitivity enclosure (m-metal is a typically used alloy) however, this shield will be expensive and hot straight forward to manufacture in order to achieve the levels of shielding you are looking for. This can be in the order of thousands of dollars for volumes of a few litters (designign such enclosures was part of my thesis project).

I think the best solution would be not to build such structure, but to find an MEG facility near you (if possible) and make the measurement in there if such device is available in your country.

REgarding the magnetometer to use, you have not provided the size of the device you want to measure, or the size from where the magnetic field is emanating. I assume this device is not big, therefore you want the probe to be as small as possible. Fluxgate magnetometers are relativvely afordable and their probes are of medium size and able to measure such low fields accurately. you can get them easily online. They were first used to measure the fluctuations in the Earth magnetic field. Of course you can also use squids...but you can consider this a non-viable option...these devices need strict temperature control and you will not get them online easily and are usually not affordable.

Hope this helps.

You might be able to surround the measurement space with some silicone steel they use for high power electrical transformer core.... If that space is not that large...just a thought...would silicone steel shield that much of field? You need to check on that...

1 Gamma = 10^-5 Gauss. Well, commercial SQUID magnetometers can measure magnetic moments anywhere between 10^-6 to 10^-8 gauss. They are designed to detect fields this small. As a consequence, who would spend the money on their system if environmental stray fields affected the sample space? I think most commercial SQUID magnetometers have shielded sample spaces. But, the ability to apply an external 1 Gamma field uniformly without signal noise could be problematic. However, I imagine this issue has already been considered and addressed by highly reputable commercial magnetometer companies. I would suggest you talk to some commercial magnetometer companies. I have dealt with Quantum Design (HQ in La Jolla, California, USA) in the past. Very good customer service and highly skilled, knowledgeable staff. www.qdusa.com. You might also just 'google' magnetic shielding. I did and one of the first links was: "http://www.magneticshielding.com/" It is a company in New Hampshire, USA that specializes in magnetic shielding. I am sure they can very specifically address your needs.

Thank you all for your answers, i think i will try to find MEG facility near by me for magnetic field shielding . but regarding the magnetometer i found 2 and their probes can be used for my project

First magnetometer:

http://www.trifield.com/content/dc-milligauss-meter/

=========================================

An improvement on the flux gate magnetometer, the DC Milligauss Meter measures magnetic fields (technically “flux density”) up to several times the strength of the Earth field. It has a resolution of 0.01 milligauss (1 nanotesla) and a range of +/-2000 milligauss (200 microteslas). The meter's magnetoresistive sensor is a major improvement over an uncompensated fluxgate magnetometer both in cost and stability. In fact, this sensor approaches a proton precession magnetometer in temperature stability.

==========================================

Second magnetometer:

GEOMAGNETOMETER

http://www.lessemf.com/dcgauss.html#3100

============================================

which one is the best in terms of company reputation and sensor sensitivity ?

Thank you for your time

In my opinion, You can measure DC Magnetic Fields as colleague Arzu Firlarer. The earth static magnetic fields it is impossible to eliminate but near "strong" (10mT +) men made sources of static Magnetic Fields (artificial magnets, MRI etc) THM1176-PDA very useful is.

Regards,

Bane V.

Hi Maher,

try to take a look to this papers and its references:

Bernarding J, Buntkowsky G, Macholl S, Hartwig S, Burghoff M, Trahms L.,"J-coupling nuclear magnetic resonance spectroscopy of liquids in nT fields," J Am Chem Soc. 2006 Jan 25;128(3):714-5.

Best regards,

marco tannino

Dear Maher,

commercial fluxgate magnetometers by Bartington or Stefan Mayer can be used to measure such small fields even in a large background field. As other responses already mentioned it is important that such a magnetometer does have a sensitivity of several nT, however the accuracy is typically much worse.

Hence, you should be very careful in the setup of your measurement. Fix the probe in space. If possible shield the experimental space against outside field either with a compensation coil system, e.g. Helmholtz coils, or MuMetal/Iron shields.

Measure the change of the magnetic field with probe and without at a very well defined position (mark the position at he sub mm level).

If you now do this measurement repeatedly you will find a periodic change of your magnetic field sensor reading. This you can correct for drifts of the surrounding field, the repeated change is the field generated by you device in a given distance.

Good luck,

Philipp

Аccording to the experience that I have with experiments related to measurements of magnetic fields, I can offer you to using unechoed chamber (zero gaus chamber or Faraday cage), made by high conductive metals. Price of the metal is only limitation for quality of such a shilding device. When you use un echoed chamber intensity of earth magnetic field is zero (if your chamber is well insolated). Insulation inside of such a chamber is made by metalised poly-urethans or similar material.

Best regards:

Lubomir

There is no need to eliminate the constant geomagnetic field. The elimination of the magnetic field is very very expensive technique. You will only have sensitive magnetometer, which has a wide measuring range magnetic induction, but with the requisite precision that you would register a change of the magnetic field in the experiment.

Thank you all again for your answers.

Dear victor,

What do you mean by "which has a wide measuring range magnetic induction"?

do you have any suggestion for specific devices that has a small and thin probe ?

Thank you

Dear Maher A. A. Abdelsamie.

1. The wide range is about 5-6 orders of 10-9 - 10-3 tesla.

2. Usually Hall-sensors are thin and small in size.

For a preliminary assessment you can just take an ordinary smartphone, which has a Hall sensor, and see the variation of the magnetic field by means of the proposed software. If by the Android OS, the Sensor Kinetics is a simple and useful free utility. It is possible that the background variation of the magnetic field in the range of less than 1 Hz will be on the order of several microtesla. Then you'll understand what device you need for your measurements.

If you would be interested in low-frequency electromagnetic fields, you have to use other tools. In the range of 10 Hz - 20 kHz the background variations can be tens to hundreds of nanoteslas.

At the same time you will have a very noticeable spatial gradients of the geomagnetic and tecnogenous fields, if you have iron-concrete building filled with a variety of electrical equipment and cables.

In general, the measurement of magnetic fields is a special exciting "sport."

Victor

You can do these measurements with SQUID magnetometers that are designed to detect extremely small magnetic fields. Most commercial SQUID magnetometers have shielded sample spaces, including the Overhauser, optically pumped Potassium (K-Mag) and Proton Precession magnetometers.

Your task is very similar to that done in geomagnetic observatories around the world, where accuracy is paramount. You might wish to contact investigators at the Malaysian geomagnetic observatory:

http://eprints.uthm.edu.my/2798/

Pavel Ripka in Prague has written several good summary papers on magnetometer types.

Sensys, Billingsley and Ascension-Technology are three more producers of small fluxgate sensors:

http://magnetometer.com/ ;

http://www.ascension-tech.com/ ;

http://www.sensys.de/

The best geophysical sensors may be too large for your requirement.

Don't try to shield the earths field (it will help to have a mu metal shield) but it isn't necessary. You can't get around it by a sensitive magnetometer since the earth's field drifts on a day to day period (or hour to hour) due to solar wind pressure on the magnetosphere.

What you need to do is make a differential measurement. But two sensitive magnetometer above and next to your sample where the field is opposite in direction and look at the differential field.

Or do a measurement by moving the sample in and out of a coil and compare the result to a coil without the sample, use a lockin technique to see the signal. (i.e. use a Vibrating sample magnetometer look up a VSM).

Flux gate, if it is not too expensive. A good one cost a few k$. I have that works with 3 pT/Hz1/2 sensitivity in the Earth field. I did not look at stability issues for DC measurements, but it can be found from the vendors.

Dear Maher,

I agree with Philipp Schmidt-Wellenburg (see above). Anyway DC is more complicated as compared with e.g. low frequency AC. For the later, there are a couple of Hall, Fluxgate,and 3 dim-Induction (miniaturized) coil devices working in the nT-range on the market.

Regards

Frank

Thank you all for your answers