BS EN 71-1:2005+A8:2009 EN 71-1:2005+A8:2009 (E) is a standard concerned with the safety of Toys for use by children. The standard considers ( amongst other items ) the safety of magnets and magnetic components used in toys.
The standard lays down a procedure for determining the “magnetic flux index” of magnets. This procedure requires the use of a gaussmeter.
The “specification” stated for the gaussmeter is as follows:-
“18.104.22.168 Direct current field Gauss meter which is capable of determining the field to an accuracy of 5G.
The meter shall have an axial type probe with
- an active area diameter of (0,76 ± 0,13) mm
- a distance between the active area and probe tip of (0,38 ± 0,13) mm”
A more accurate technical description of this would be:- A Hall effect gaussmeter capable of determining the magnetic flux density (B) with an accuracy better than ± 0.5 mT ( milli Tesla) [± 5 gauss] shall be used.
The gaussmeter shall be used in conjunction with an axial Hall probe with the following characteristics:-
- The active area of the Hall probe sensor within the probe will have an area no greater than 0.622mm2. The profile of the active area will not exceed that describe by a circle of with an absolute maximum diameter of 0.89mm.
- The position of the active area of the Hall probe sensor below the surface of the axial Hall Probe, in contact with the magnet, will be 0.38mm ± 0.13mm. The active area of the Hall probe sensor will be assumed to have negligible thickness compared to the dimensions describing the active area.
The Hall probe used in the determination of the magnetic flux index is intended to detect the maximum field at the surface of the magnet under consideration. In a homogeneous field, the dimensions and position of the Hall sensor active are will be unimportant. In a non homogeneous field, as with measurements at the surface of a magnet in a magnetically open circuit, the dimensions and position of the active area become important. While measurements take in this way can be considered to give a useful measurement indication, it cannot be considered a traceable measurement due to the in homogeneous nature of magnetic fields being measured.
The practicalities of Hall sensor technology is such that the active area of the Hall probe sensor cannot be exactly at the surface of the axial Hall probe. It must be at some distance from the surface, to permit a protective layer to be in place.
If the distance from the surface of the Hall probe, to the active area of the sensor within the probe, is too great, then measurements taken will be lower than which would be considered correct. In reality 0.3 – 0.6 mm is a typical distance.
If the active Hall sensor active area is large, the effect will be to average out local high spots of magnetic flux density with adjacent lower areas of magnetic flux density. This will give rise to false low readings.
Where the area is small, and similar to the distance of the Hall sensor’s active area below the surface of the probe, the readings will be, to all practical purposes, identical.
It would appear that the “specification” described in BS EN 71-1:2005+A8:2009 EN 71-1:2005+A8:2009 (E) is that adopted from the technical specification of an individual model of an Axial Hall probe. This represents a particular embodiment of an axial Hall probe which is not optimised for this application.
The area described by a circle of diameter of 0.76mm is 0.4537mm2
The area described by a circle of diameter of 0.76mm -0.13mm (0.63mm) is 0.3117mm2
The area described by a circle of diameter of 0.76mm +0.13mm (0.89mm) is 0.6221mm2
This means that the upper limit on Hall probe active area specifications stated in the standard is 100% bigger than the lower limit.
Typical modern Hall sensors, based upon materials such as Gallium Arsenide etc. are typically ~ 0.07 x 0.07mm to 0.3 x 0.3mm.
Hall sensor active areas, positioned 0.38mm from the surface of a magnet, and having an active area of 0.2 x 0.2 mm to 0.3 x 0.3 mm would not be considered in this application to have a significantly different response to that of Hall sensor active area, at an identical depth with dimensions of 0,76 ± 0,13 mm.
If any differences were to be detected, then the smaller active areas would, if anything, give a very slightly higher reading, but well within the limits determined by the variations in Hall sensor depth as stated in the Standard.(0.38mm ± 0.13mm)