Weighing magnetic masses or in magnetic fields: A cautionary note

Geochemistry Geophysics Geosystems

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Technical Brief

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Volume 3, Number 11 16 November 2002 1066, doi:10.1029/2002GC000328

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

ISSN: 1525-2027

Weighing magnetic masses or in magnetic fields: A cautionary note P. Rochette CEREGE University of Aix-Marseille 3, BP 80, Europoˆle de l’Arbois, 13545 Aix en Provence Cedex 4, France ([email protected])

[1] The magnetization of a weighed sample can influence the magnetic sensor of standard electronic balances. The corresponding weight bias is demonstrated on several meteoritic and terrestrial basalt samples previously exposed to large fields, as well as on small permanent magnets in the range 10–100 g. The mass bias is only a few mg (except for magnets which should never be brought near to an electronic balance), but it can be troublesome in case of differential weighting. Moreover, a similar magnetic bias is observed near laboratory variable field sources with significant gradient (such as electromagnets). Components: 2377 words, 1 figure, 1 table. Keywords: Electronic balance; weight bias; magnetization; meteorite; magnet. Index Terms: 1094 Geochemistry: Instruments and techniques; 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 3662 Mineralogy and Petrology: Meteorites. Received 13 February 2002; Revised 9 July 2002; Accepted 2 August 2002; Published 16 November 2002. Rochette, P., Weighing magnetic masses or in magnetic fields: A cautionary note, Geochem. Geophys. Geosyst., 3(11), 1066, doi:10.1029/2002GC000328, 2002.

1. Introduction [2] Modern electronic balances bring a precision, rapidity and ease of use in weighing samples compared to traditional mechanical scales. However, this progress is associated with a possible artifact that was not exhibited by old systems and which remains unknown to most users. As the sensor of an electronic balance is based on a magnetic force measurement, it is sensitive to the ambient field generated by magnets surrounding the balance or by magnetization of the weighed mass itself. This effect is known by balance technicians; we first became aware of it when our rock magnetic laboratory was contacted to check the magnetization of some of their calibration masses that were giving unreliable results. Copyright 2002 by the American Geophysical Union

[3] The magnitude of the effect depends on the model of the scale, and on the shape and orientation of the magnetic sample, and it can very easily be observed by bringing a magnet near to the balance. In fact, weighing a magnet on an electronic balance may produce irreversible damage to the sensing device, and should be avoided. The purpose of this short technical note is to warn all users of electronic balances of this effect, particularly those weighing meteorites, some of which can consist of significantly magnetic materials. [4] Various tests on the samples described in following section suggest that magnetization induced by the weak terrestrial field (25–50 mT), as well as usual weak natural remanent magnetization (NRM), makes a negligible effect. On the other hand, large 1 of 5


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rochette: weighing magnetic masses

Table 1. Characteristics of the Samples Measured Sample

1. 2. 3. 4. 5. 6. 7. 8.

NdFeB magnet Ferrite bar with Teflon coating Sahara full stone (H chondrite) Gao full stone (H4-5 chondrite) Alcaline basalt (0.1 Ma France) Tholeitic basalt (30 Ma Ethiopia) Ghubara slice (L5 chondrite) Sikhote-Alin full stone (iron meteorite)

Actual Mass, g

Shape

Irm, kA/m

Dm, mg

d, mm

20.3 4.2 98 160 101 31.7 31.6 52

flat cylinder 2 cm long rod irregular rounded irregular flat irregular flat cylinder 3 mm thick slice irregular rounded

1500 400 1.6 1.8 0.7 0.7 0.6 n.m.

110.3 27.7 4.7 4.1 2.8 1.2 1.2 0.0

32 20 15 15 10 11 2 8

All have been saturated in a 1 T field except the magnets. *IRM values, per unit volume, are not measured on the actual piece but average from same class after [Terho et al., 1993]. The maximum weight bias Dm measured is reported, together with the distance d of sample center to top of scale for this weighing.

isothermal remanent magnetization (IRM, two or more orders of magnitude larger than usual NRM and induced magnetization) are produced when a coherent material containing ferromagnetic fine grains is exposed to a high field such as produced by exposure to a magnet or in nature by lightning. Unfortunately, most meteorites found in the desert [see Bischoff, 2001, Figure 5; Rochette et al., 2001a] have been exposed to hand magnets, producing at their contact a field of the order of 60 mT (traditional ferrites) to 300 mT (rare earth magnets). This is also true for the Antarctic meteorites collected by Euromet (L. Folco, personal communication, 2001). Even falls have sometimes been exposed to magnets while handled in museums [Collinson, 1997, Wasilewsky and Dickinson, 2000]. [5] A typical highly magnetic terrestrial rock like basalt yields an IRM in the range 0.1–1 kA/m. Ordinary chondrites have IRM between 0.1 to 10 kA/m. CR and E chondrites, ureilites and other high metal classes are within the same range [Terho et al., 1993, Rochette et al., 2001b]. On the other hand, iron meteorites have rather low IRM due to the massive state of iron. For comparison, ferrite and NdFeB magnets have remanent magnetizations of 400 and 1500 kA/m, respectively.

2. Examples of Magnetic Bias on Measured Masses [6] A few tests on representative samples (Table 1 and Figure 1) were conducted using a new Sartorius brand electronic balance, internally calibrated

with a sensitivity of 0.1 mg and maximum range of 120 g. One heavier sample was measured on another older Sartorius balance going up to 210 g. Except for the two magnets, a saturation IRM was imparted by exposing the samples to a 1 Tesla field along the cylinder axis or along the short axis for the slice or irregular samples. The same protocol was used for all samples: the balance was zeroed with the sample on the scale with its magnetization vertical, then the ‘‘magnetic bias’’ was obtained by subsequent weighing the sample upside down. Increasing distance between sample and scale was obtained by putting the sample on top of 1 to 6 low mass plastic boxes. Reproducibility of the difference was tested by reweighing in the initial position (checking that a zero mass was obtained) or by repeating the whole procedure. Sometimes for large magnetic bias a slow drift was observed (of less than 0.5 mg) either due to the adaptation of the magnetic sensor to a changing environment or to a drift of sample magnetization, the so-called viscous magnetization in the Earth field (VRM). However, this second hypothesis is a priori unlikely as VRM should be negligible compared to IRM. [7] For the basalt and meteorite samples the magnetic bias in natural state (i.e. before imparting an IRM) was found to be undetectable. For the Gao sample the stability through time of the bias was tested: it remains unchanged when remeasured one hour and one day after saturation, showing that viscous decay of IRM does not play an important role. Interestingly, a Sikhote-Alin (massive iron 2 of 5


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magnetic mass bias (mg)

1000

1 100

2 10

3 5

4 6

1

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0.1 10

100

distance to scale top (mm) Figure 1. Magnetic bias (difference between readings with upward and downward remanence) on mass in 10 4 mg, versus distance between geometric center of sample and top of scale in mm. Lowest distance correspond to sample directly on the scale, except for magnets (1 and 2).

meteorite) piece of 52 g gave weight bias neither in natural state nor after imparting an IRM: selfdemagnetization of massive iron seems to prevent a large IRM [Terho et al., 1993]. [8] The magnetic bias decreases rapidly with distance, less for large pieces (4–5). Magnets create a strong bias, even at a large distance from the balance and for the small sized ferrite (whose tabulated weight includes the Teflon coating). If extrapolated to the balance surface, the rare earth magnet creates a bias of about 10% of its mass. [9] The upside-down flip of the weighed sample appears as an efficient way to detect the magnetic bias. Can it be used to obtain the true mass of the sample by averaging the up and down readings, in the case of a vertical magnetization? Sample 6 (Ethiopian basalt cylinder) was first demagnetized

in an alternating field of 100 mT to demonstrate that this can erase the magnetic weigh bias. Indeed, the mass readings are exactly identical in the up and down position of the cylinder, thus giving an unbiased mass. Then the sample remanence is resaturated along the cylinder axis and weighed in up and down positions. The average of these two readings, as well as the reading obtain with the remanence horizontal, is equal to unbiased mass previously measured within half a digit (0.05 mg). To check that the actual mass was not varying (either due to balance drift or manipulation of the sample, the sample was again demagnetized and reweighed: the exact initial unbiased mass was obtained. Therefore the 180C flip of the sample with vertical remanence, or the horizontal orientation of remanence, is an efficient procedure to retrieve unbiased mass, paying attention to center 3 of 5


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the sample on the scale. This procedure is however only applicable for sample of simple symmetric shape with known orientation of remanence. In more complex cases (irregular samples with unknown remanence orientation), the only way to eliminate the magnetic bias is to demagnetize the bulk sample, either using alternating fields or by crushing it.

3. Magnetic Field Effect on the Balance [10] The magnetic field generated by basalt sample 6 at 3 cm from its base, i.e. estimated position of the balance sensor, is only 7 mT (as measured with a 3-axis fluxgate). Given that such a low field generates a 1 mg bias, one needs to worry about bias induced not only by the sample magnetization but also by field sources surrounding the balance. If the magnetic field is stable (e.g., due to fixed iron masses) this effect may be cancelled by calibrating the balance, but variable field (e.g., locating the balance near a mass spectrometer) could severely bias the measured weights of small masses and eventually damage the balance. To test briefly this question, our balance was installed in the vicinity (about one meter) of a two inches polar pieces electromagnet (from the Micromag magnetometer). After zeroing the balance with no current in the electromagnet, magnetic fields of +1, 1 and 0 T were successively applied within the electromagnet poles gap. Mass reading (after an excursion toward higher values) stabilized to –0.6, + 1.3 and + 0.4 mg, respectively. Meanwhile the ambient vertical magnetic field measured at 5 cm from the scale was –79, 130 and 13 mT, respectively (total field: 89, 134 and 33 mT). This test demonstrates that laboratory variable fields do affect mass reading, with irreversible effects. However, the weight bias may be linked to the field gradient rather than to the field itself or a function of both, so that our measurements cannot directly be used to quantify the effect of variable ambient fields. The necessary information to model the magnetic effect (actual design of the sensor magnetic parts, shape and three-dimensional magnetization distribution within the sample, exact position on the scale with respect to the sensor) is quite difficult to obtain. Such modeling is beyond the scope of this note and

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the empirical approach presented here is better suited to address the bias problem on various balances and various irregular and heterogeneous samples. Figure 1 shows that mass bias and distance to scale top (d) are not related by a power law. This is understandable as both the balance sensor and the sample do not see each other as a point source, and the distance to scale top is different from distance to sensor. However, for sample 1 at large distance, the relation is closer to d 4 than d 3, suggesting that the field gradient dominates in the effect. This is confirmed by the small size of the mass bias due to the rather high field in the electromagnet experiment. A final experiment, using a pair of large Helmoltz coils to generate fields similar to the ones measured in the electromagnet case, confirms this contention. No mass bias was observed in vertical magnetic fields from –79 to 130 mT, thanks to the very small gradient generated by these coils, compared to the electromagnet experiment.

4. Conclusions [11] The examples provided show that the magnetic bias on the weight measured on an electronic balance is strong for magnets (which should in fact never be put on an electronic balance because of large bias and potential damage to the sensor), and detectable for basalts and ordinary chondrites bearing a saturation remanence. Relative to total weight the effect may appear negligible (of the order of 10 4) but it could be worrying in case of differential measurements (e.g., when comparing dry and wet masses or change of weight through time). Therefore, when a high precision weight is needed on a potentially magnetic sample (such as the majority of meteorites, as well as basalts) one should check for this effect by comparing the measured mass with different orientations of the sample. Variable ambient laboratory magnetic field gradients may also give measurable bias on the balance. This effect, presently detected in the vicinity of an electromagnet, is of serious concern when measuring low masses. Therefore one should control the stability of the magnetic environment of the balance for high precision weighing. 4 of 5


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Acknowledgments [12] I am grateful to G. Consolmagno, E. Lewin and B. Weiss for their help in improving the original manuscript, as well as to Mr Bouillon from CT2M company, for pointing out the existence of magnetic weight bias.

References Bischoff, A., Meteorite classification and the definition of new chondrite classes as a result of successful meteorite search in hot and cold deserts, Planet. Space Sci., 49, 769 – 776, 2001. Collinson, D. W., Magnetic properties of Martian meteorites, Meteorit. Planet. Sci., 32, 803 – 811, 1997.

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Rochette, P., J. P. Lorand, G. Fillion, and V. Sautter, Pyrrhotite and the remanent magnetization of SNC meteorites: A changing perspective on Martian magnetism, Earth Planet. Sci. Lett., 190, 1 – 12, 2001a. Rochette, P., L. Sagnotti, G. Consolmagno, L. Folco, A. Maras, F. Panzarino, L. Pesonen, R. Serra, and M. Terho, A database of magnetic susceptibility of stony meteorites, Quaderni de Geofisica, 18, 30 pp., 2001b. Terho, M., L. J. Pesonen, I. T. Kukkonen, and M. Bukovanska, The petrophysical classification of meteorites, Stud. Geophys. Geod., 37, 65 – 82, 1993. Wasilewsky, P., and T. Dickinson, Aspects of the validation of magnetic remanence in meteorites, Meteor. Planet. Sci., 35, 537 – 544, 2000.

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