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How and Why to Measure Magnetism Accurately
by Paul I. Nippes* and Elizabeth N. Galano+
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There are often many ways to look at things so that they make sense to us. Some ways work for us but not for others. Here is a different way to look at magnetic testing.
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Introduction
Magnetism is everywhere, no matter where you are on this earth and possibly in the universe. Magnetism originating from within the center of the Earth permeates your body, your flesh and bones. The first recorded observation of magnetism dates to AD 1200 when, in Magnesia (a part of Asia Minor), it was observed that a certain iron ore, called lodestone, was observed to have peculiar properties, henceforth referred to as magnetism.Magnetism requires proper instruments and techniques for its accurate measurement. These are necessary for both the utilization of magnetism and consideration of the negative effects that magnetism can produce. Because magnetism is very difficult to understand and test, we offer some guidelines to aid the technician in making and recording magnetic measurements.
Source of Magnetism
The very basis of all magnetism is electron spin and rotation, as depicted in Figure 1. In a free state, atoms have random orientations of electron spin and rotation. If an object is susceptible to magnetism and is placed into a magnetic field, its spins and rotations align themselves to be coherent with the applied field to a limit of intrinsic saturation, a condition where magnetism increases further only at the permeability of air.
Magnetic surveys should be performed at incoming testing and prior to and during parts assembly.
Direct Component of Magnetism
The direct component of magnetism is either artificially induced or natural. Intense magnetism in machinery and equipment is produced by us for our own purposes. Just how much magnetism is required for a certain purpose and how to measure and control it is of great importance. When the magnetizing force is removed, the remaining magnetism is referred to as residual magnetism.Magnetic flux density is expressed in teslas (volt seconds per square meter). The measurement is often still expressed in gauss (corresponding to the number of lines of magnetic flux per square centimeter). The magnetizing force is expressed in amperes (gilberts or ampere turns per centimeter in the centimeter/gram/second measurement system). Figure 2 shows the character of a material having high magnetic retentivity (a magnetically hard material). Increasing magnetizing force H, the magnetic flux density B will increase along the dashed line of Figure 2, reaching an upper point a when H reaches value g. Then, as the magnetizing force is reduced, the flux density B, tending to retain its magnetism, falls along line d to the residual value Br as the magnetizing force H reaches zero. With reverse application of the magnetizing force -H, B drops to zero when the magnetizing force reaches Hc. A pattern is followed as it continues, first in the negative direction, then in the positive direction, to trace out the hysteresis curve of Figure 2. The remaining residual magnetism could become a nightmare for the owner or user unless it is removed by downcycling demagnetizing. An example of this is shown in Figure 3, where progressively smaller magnetizing forces are applied at alternating polarities, reducing the residual magnetism to zero.
If the part has low magnetic retentivity (is magnetically soft), its residual state would be br and hc, as is depicted in Figure 4, having considerably less magnetism than the magnetically hard part. Only if the object is very large or long, such as part of a pipeline, would residual magnetism pose a problem. Other sources for direct current magnetism are the earth's magnetic field and objects on the earth, such as lodestone. The earth's magnetic field performs as if there were a large iron bar extending between the South and the North Poles, as shown in Figure 5. Measuring low level magnetic fields in objects can very easily be affected by the earth's magnetic field, especially near large steel structures.
Measuring the Direct Component of Magnetism
The generic name for meters used to measure low levels of magnetism and, notably, the earth's field is magnetometer. The simplest and cheapest is the dial indicator. Its accuracy is always in question since it can easily be damaged, either mechanically from being dropped or from exposure to excessive magnetism. It can measure only fields directed upward into the case bottom and it cannot be inserted into a gap or crevice where the most intense fields occur.An electronic meter (Figure 6) obtains its direct component of magnetism input signal from a hall sensor. It is the most universally employed type of sensor for measuring magnetic fields in the range of 0 to 1 T (0 to 10 000 G). All hall probes should have at least a 152 mm (6 in.) reach and be durable, with an interconnecting cable 0.9 to 1.2 m (3 to 4 ft) long to provide access to all areas where readings are to be made.
Auto ranging of the display range is necessary for ease of operation and the display should be intuitive so that field scanning and rough surveying are possible, with no confusion or digital jump in making readings. The meter should withstand mechanical shocks and abuse and should perform in the intense fields used in magnetizing and demagnetizing without damage and with no effect on readings or calibration.
ALTERNATIVE COMPONENT OF MAGNETISM
The alternating component of magnetism exists as a regular and repeatable pattern of oscillation and is artificially induced. It usually originates with electric generators and motors and it is strongest in and near electrical equipment such as power transformers and rotating electrical machinery. The frequency of alternating magnetism ordinarily bears some relation to the frequency of electrical power, the speed of rotating machinery, their harmonics or any combination of the three.
Measuring the Alternating Component of Magnetism
Tests made on hall sensors prove it to be inaccurate for measuring alternating component of magnetism fields and combined alternating and direct component of magnetism fields. Separate alternating and direct component of magnetism measuring probes, each feeding its normalizing circuitry, constitute the only accurate way of determining the true magnetic field magnitudes.
MEASURING TECHNIQUES
Accuracy and Precision
There is the mistaken understanding that the more digits in the display of a digital meter, the greater its accuracy. This is clearly not the case, as the accuracy of the field measuring probe, its calibration method and the instrument signal conditioning are what control the meter's accuracy, regardless of the number of digits in the display.
Polarity Determination
Polarity is the identification of magnetic north and south poles in the otherwise continuous invisible magnetic field lines, similar to the earth's magnetic field (Figure 5). Polarity should not be confused with the different magnetizing field orientations employed during magnetic particle testing - a necessary technique for locating imperfections in all directions at the surface and subsurface of an object being tested. Polarity often may not be important so long as magnetizing is followed by thorough downcycling demagnetizing using the identical coil setup used to conduct the test. Polarity is an important factor that must be taken into consideration when demagnetizing a component of unknown residual magnetism. The technician must understand this importance and be able to establish reference polarity if demagnetization is desired. This requires a working knowledge of the probe and meter to be employed in order to properly note and record accurate readings, employing a simple notation such as: +x, +yh, where x is the area reading as positive or a north pole. The value y is listed when there is a notable maximum reading in the zone of consideration. The letter h is added next to the y reading if there is a pronounced peak rather than a general area maximum value.
Compensating for the Earth's Field
The earth's magnetic field has little direct effect on us or our machinery, but it does affect the making of low level direct component of magnetism measurements. The earth's field, normally around 5 x 10-5 T (0.5 G), can reach 1.5 x 10-4 T (1.5 G) or higher, influencing magnetic surveys made in open air, especially on objects where the residual limit is 2 x 10-4 T (2 G). An object's true residual magnetism can be determined by compensating for the effect of the earth's field.Residual magnetism in assembled rotating machinery should be low enough to prevent the generation of stray voltages and currents capable of causing damage to bearings or seals with loss of production and operating efficiency. Magnetism of critical components when in open air should be reduced to 2 x 10-4 T (2 G) or less.
High Reading Zones
Magnetism is highest near cracks, partings and on outside edges and corners. Demagnetizing success is achieved when the maximum value meets the allowable (Figure 7).
Low Reading Zones
Low reading zones of magnetism can be deceiving, as they may fail to reveal high internal magnetism that can be very serious when the part is assembled or machined (Figure 8). Major causes for this are circular magnetization and improper welding techniques.
RECOMMENDED FOR MAGNETIC TESTING
Do not overmagnetize. When magnetizing for magnetic particle testing, increase the direct current force gradually, stopping when an optimum indication is obtained, usually around 3 x 10-3 T (30 G). Afterwards, demagnetize using the magnetizing coil. With the coil still in place, switch to the opposite polarity and set a program of auto downcycling. Confirm a final magnetism free object using a reliable meter. Do not employ the circular magnetization head shot technique for magnetic particle testing, as intense circular magnetism remains, can generate damaging electrical currents, cannot be measured accurately and is very difficult to remove. If using alternating current magnetizing for magnetic particle testing, do not switch on and off but increase and decrease the magnetizing force gradually.
CONCLUSION
Because magnetism is not visible and can be so damaging if left behind, an accurate meter should be employed to measure for safe levels following magnetic particle testing and welding. Magnetic surveys should be performed at incoming testing and prior to and during parts assembly. Allowable maximums of residual magnetism for each type of part should be specified and adhered to. Results of magnetic surveys should be recorded and maintained for future comparisons, as well as for establishing trends.
* Magnetic Products and Services, Inc., 2135 Highway 35, Holmdel, NJ 07733; (732) 264-6651; fax (732) 264-6876; e-mail <support@gaussbusters.com>.
+ Magnetic Products and Services, Inc., 2135 Highway 35, Holmdel, NJ 07733; (732) 264-6651; fax (732) 264-6876.
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