NEW SUSCEPTOMETER AT NMISA

Speaker/Author: RT Mautjana

National Metrology Institute of South Africa

Private Bag X 34, Lynnwood Ridge, Pretoria, 0040, South Africa

Phone: 012 841 4798 Fax: 012 841 2131

Abstract

NMISA has established capabilities to measure magnetic properties of mass standards by acquiring a susceptometer. Measurements of magnetic properties are important in mass calibration because electronic balances have permanent magnets in their weighing cell which generate magnetic fields near the pan. There is also an earth magnetic fieldof ~ 30 µT and if the unit under test is magnetic, the balance reading may be incorrect because of the magnetic force between the unit under test and the fields due to earth and balance. The current susceptometer can measure susceptibility and permanent magnetization of materials for mass standards from 1g to 50kg.A susceptibility standard is used to check the process to confirm that the system is giving valid results. The paper present some typical values (results) obtained at NMISA for OIML shaped weights.

  1. Introduction

Magnetic susceptibility is an important parameter to indicate the quality of mass standards made from various alloys of stainless steel.NMISA has established capabilities to measure magnetic properties of mass standards by acquiring a susceptometer.The current susceptometer can measure susceptibility and permanent magnetization of materials for mass standards from 1g to 50kg.

  1. Basic Principles

The magnetic susceptibility is a dimensionless constant that indicates the degree of magnetization of a material in response to an applied magnetic field [2]. It is amechanical effect that occurs in all materials. It is the property of materials that causes it to create a magnetic field in opposition to an externally applied magnetic field. Materials which are generally non-magneticaresaid to be diamagnetic. They have a relative magnetic permeability of ≤ 1, and therefore a magnetic susceptibility ≤ 0, since susceptibility is defined as χ = μ – 1, whereχ = magnetic susceptibility and μ = permeability [5]. This means that diamagnetic materials are repelled by magnetic fields. On the other hand some materials are attracted to the applied magnetic field (paramagnetic) therefore they have a relative magnetic permeability ≥ 1 (i.e. a positive magnetic susceptibility)[5]. Generally, non-magnetic materials are said to be diamagnetic because they do not possess permanent magnetization without external magnetic field.

In mass determination, nowadays mainly electronic balances and comparators are used to measure the downward (weight) force which is converted into mass of the object [4]. Electronic balances have permanent magnets in their weighing cell which generate magnetic fields near the weighing pan. The Earth also has magnetic field of about 30 µT and if the unit under test is magnetic, the balance reading may be incorrect [3]. The magnetic field between the balance and the earth induces a magnetic force which will pull or push on the unit under test and make it appear either heavy or light. These forces can also arise from interaction between the mass standards and the balance/comparator used and other magnetic objects within the vicinity of weighing [1]. The magnetic forces if undetected can therefore adversely affect the weighing process in the determination of mass [4].

Anelectronic micro balance with a pedestal for a magnet and a bridge (susceptometer) is used to detect the magnetic forces or the effect of magnetization in mass measurements see Figure 1 and schematics of susceptometer in Figure 2 below.A susceptibility standard traceable to NPL is used to check the process to confirm that the system is giving valid results

Figure 1: View of Susceptometer Figure 2: Schematics diagram for Susceptometer [2]

Figure 2 is the schematic diagram of Figure 1 to illustrate the critical components of a susceptometer.On NMISA’s system gauge blocks are added between the top of the bridge and the vertical supporting pillars to increase the height (distance) while at the bottom is the adjustable screwsalso for height adjustments. Susceptometerworks by measuring the force on the weight under test when subjected to a magnetic field. It uses small permanent magnets on top of a pedestal on the balance weighing pan at a defined distance (Z0)below the weight under test. The measurements are repeated with the reversed field, when the magnet is inverted and permanent magnetism and magnetic susceptibility of the weight are calculated from the two forces.


Critical to determining the force on the weight subjected to a magnetic field is the knowledge of the distance between the bridge and magnet. To establish the distance one of the two methods isemployed: one using the reference susceptibility standard and/or secondusingvernier caliper to measure the distance (Z00)from the pedestal (base of magnet) to the base of the weight under test [2]. See Figure 3 below depicting the use of vernier caliper to establish the distance Z00.

Figure 3: Calibrating the bridge mechanically[2]

The measured distance Z00subtract half height of the magnet result in Z0. However, using the reference standard the total distance is given by Z0 = ZB +Z00, whereZB is the gauge block distance and Z00 the calculated distance [2].

  1. Measurements Results

The NMISA susceptometer consist of a balance with a resolution of 0.1?g, cylindrical permanent magnet, pedestal, non-magnetic aluminum bridge, gauge blocks and cylindrical susceptibility standard. The procedure used in determining magnetic susceptibility and permanent magnetizationis described in [1], page 35 B.6.4 to page 38 B.6.4.8.The geometry correction factorsfor the weights are estimated from their shape.The distance between the magnet and the bridge Z00was determined using a magnetic susceptibility reference standard (traceable to NPL) and gauge blocked (ZB) were added to increase the total distance (Z0).Table 1 below presents the laboratory constants used during measurements.

Table 1: Laboratory constants used during measurements

Data
Total distance Z0 during measurement / 21.16 mm
Local gravitational acceleration / 9.7861 m/s2
Magnetic moment / 0.01078 Am2
Local vertical earth magnetic field / 30.00 ?T

Table 2 below presents the magnetic susceptibility and permanent magnetization for a sample of OIML shaped weights.

Table 2: Results as measured at NMISA for OIML Class E1 and E2

Nominal weight / ID/Class / Magnetic Susceptability / Maximum
OIML R111 / Magnetization
?0M (?T) / Maximum
?0M (?T)
OIML R111 / Comments
1kg / E2 / 0.00428 / 0.07 / -0.36 / 8 / √
500g / E2 / 0.057585 / 0.07 / 26.613 / 8 / X
200g / E1 / 0.00301 / 0.02 / -0.2115 / 2.5 / √
100g / E2 / 0.00061 / 0.07 / -0.090 / 8 / √
40g / Reference / 0.00528 / - / 8.498 / - / √
40g / Reference / 0.00532 / - / 8.345 / - / √
5g / E2 / -0.002815 / 0.18 / 0.529 / 8 / √

NB: Magnetic permeability 1.005293 with a relative uncertainty of ± 0.002%

Table 2 depicts the measured susceptibility and magnetization as found for 1kg, 500g, 200g, 100g and 5g weights together with the maximum limits as set in [1] for each class. For a weight to pass the test its magnetic susceptibility and magnetization had to be below the maximum limits. If one or both of the limits is/are not met a weight fails the test and it gets rejected for calibration.The table shows that 500g failed the test because the detected magnetization exceeded the maximum limits. The results suggest the 500g weight could have been exposed to high magnetic field strengths [6]. Although non-magnetic materials such as stainless steel weights should have zero permanent magnetization, in fact, they are weakly magnetic and therefore can be magnetized [6]. According to [3] older weights are more likely to have poor magnetic properties.

It was found in [3] that for high quality (class E1 & E2) OIML shaped weights magnetic susceptibility rangedfrom 0.003 and 0.004 and our system confirms these findings. A 40g susceptibility reference standard was used to validate the result and the calculated magnetic susceptibility agreed well with the certified value of 0.005293.

  1. Conclusion

The NMISA Susceptometer wassuccessfully commissioned and already gives insight into expected values for magnetic susceptibility for OIML shaped weights. Standard weights with high magnetic susceptibility are more likely to be magnetized while the ones with low magnetic susceptibility are the most difficult to magnetize.

References

  1. OIML R 111-1, “Weights of classes E1, E2, F1, F2, M1, M1-2, M2-3 and M3 Part 1. Metrological and technical requirements”, Edition 2004 (E)
  1. Mettler Toledo GmbH, Business Area Metrology, “User manual, SusceptometerControl Version 1.3”, Operating Instructions, Greifensee, Switzerland, 2004
  1. C. M. Sutton, “MSL Technical Guide 6 Magnetic effects in weighing”, New Zealand, 2004
  1. J. Nava, R.S. Davis, and J. Moran, “Intercomparison between CENAM, BIPM and TROEMNER to determine the volume of magnetic susceptibility of a 100g weight”, 2002
  1. Wikipedia - accessed [12 August 2013]
  1. R.S. Davis,“Determining the magnetic properties of 1kg mass standards”, Journal of Research of the National Institute of Standards and Technology, Volume 100Number 3, May-June 1995,p 209 to 225