A comparison of magnetic and magnetically permeable weights

Transcription

1 A comparison of magnetic and magnetically permeable weights Stuart Davidson Mass Group, Industry and Innovation Division National Physical Laboratory EMAN (Engineering Measurement Awareness Network) Weighing and Density Special Interest Group 24th September 2007

2 Background OIML R111 gives limits for polarisation and magnetic permeability for classes of mass standards Induced error assumed to be <1/10 th of MPE R111 also states: The magnetic properties (magnetization and susceptibility) of the mass standards should be determined before the mass calibration to ensure that the magnetic interactions are negligible. A weight that fails the magnetism test should not be calibrated. Potential methods outlined include: Gaussmeter (permanent magnetisation) Susceptometer (susceptibility and permanent magnetisation) Attracting method (susceptibility) Fluxgate method (susceptibility)

3 Objectives A comparison among UK mass calibration labs to assess the effect of magnetic and magnetically permeable weights on calibration results Assess the effect of such weights on a range of balances and weighing techniques Validation (or otherwise) of OIML recommendations Assessment of requirements to make measurements of magnetic properties Validation of accredited laboratories BMCs

4 Results Magnetic properties of transfer standards Mass Standard Measured magnetisation (µt) Measured Susceptibility OIML weight Class Maximum magnetisation (µt) Maximum Susceptibility (> 20g) 1 OIML E cylinder ASTM (C&CA) X cylinder E F F

5 Results 1 OIML Relative mass for OIML Relative mass (mg) N1 D G1 G2 E A H C B1 B2 B3 F N2

6 Results 52 Cylinder Relative mass for Relative mass (mg) N1 D G1 G2 E A H C B1 B2 B3 F N2

7 Results ASTM C&CA Relative mass for C&CA Relative mass (mg) N1 D G1 G2 E A H C B1 B2 B3 F N2

8 Results Cylinder X Relative mass for X Relative mass (mg) N1 D G1 G2 E A H C B1 B2 B3 F N2

9 Results Balance Performance Difference to median (mg) Balance Description OIML Weight 52 Cylinder ASTM X Cylinder Uncertainty [k= 2] (mg) HK1000 electronic/mechanical Mettler H315 electronic/mechanical Oertling SC121 unknown LA2200S top pan electronic CC2001 top pan electronic Mettler PR2004 top pan electronic unknown unknown unknown unknown Twin pan Twin pan Sartorius CCE5004 top pan electronic Mettler AX 1006 top pan electronic Sartorius Z086 top pan electronic HK1000 electronic/mechanical

10 Results Effect of weighing method Balance Conventional mass [Upright] (g) Conventional mass [Inverted] (g) Conventional mass [Spacer] (g) Conventional mass [Spacer & Inverted] (g) HK Mettler H Oertling SC LA2200S CC Mettler PR Twin pan Sartorius CCE Mettler AX Sartorius Z HK Median

11 How can we measure magnet properties Severn bridge permeability meter Gaussmeter Susceptometer Compass

12 Measurement of Magnetic Properties Mass Standard NWML maximum NPL maximum compass deflection compass deflection 1 OIML No deflection No deflection 52 cylinder 2 degrees 5 degrees ASTM (C&CA) 8 degrees 20 degrees X cylinder 10 degrees Full rotation X cylinder (inverted) 14 degrees Full rotation

13 Conclusions With the exception of one result all measurements are agree with the reference value OIML limits for magnetic properties are conservative Results are balance dependant Weights with high susceptibility will become magnetised when exposed to a magnetic field Routine determination of magnetic properties is not feasible (necessary?) for mass calibration Compass method is sensitive, easy to implement and provides useful data on susceptibility AND permanent magnetisation

14 Next Steps Circulate draft A report All participants approve draft A Circulate draft B report (participants identified) Participants approve draft B Published as NPL internal report Published as journal paper

15 The End Thank you for your attention