CHAPTER IX
TESTING & MEASUREMENT
Not just engineers, scientists, inventors, but educators and homeowners in over 80 countries have used our Lab Kits and shielding materials for hands-on evaluation. Our Lab Kits, which contain various samples of our proprietary shielding alloys and technical information, have helped many individual consumers and technical professionals solve unwanted magnetic interference from low frequency AC source fields (Ø Hertz to 100 kilohertz). Although the evaluation process can vary by application, the following steps are generally followed by all, for “hands-on” evaluation.
1. Determine Source of Interference
Internal interference is created by operating components within a system, affecting each other. This can be neutralized by shielding the individual components and confining the magnetic forces they create. External interference is created by a source outside the system which may affect many components or a large area. In this case, the entire unit being affected must be shielded by an enclosure, a chamber, or walls. Many people ask “Should the source of interference or the sensitive device be shielded?” The answer to this question depends on several factors. Shielding the source may involve stronger fields, and therefore thicker materials. One must be sure that all interference sources are shielded, or the sensitive device will still be affected. The usual approach is to shield the sensitive device. This prevents interference from both present and future sources.
2. Locate and Measure Magnitude of Interference
Locating internal or external interference and determining the magnitude (strength) of the interfering field are important in shielding alloy selection. How do you measure magnetic fields? By using a Gaussmeter, the field strength is measured in “Gauss”, and output into numeric values. For AC fields, use one of our Magnetic Pickup Probes with a digital volt meter (DVM) or oscilloscope, or use one of our Gaussmeters to measure the interfering field strength. These measurement devices are included with certain Lab Kits, or sold separately. Field mapping, or plotting of these measurements, will provide a baseline and help determine effectiveness of your magnetic shield. The traditional CGS units for measuring magnetic fields are Gauss and Oersted. Magnetic flux density is measured in Gauss, while magnetic field intensity is measured in Oersted. The ratio of B, magnetic flux, in Gauss, to H, magnetic field, in Oersted, is defined as permeability, "µ". The B/H ratio, or "µ", is a measure of the material's properties. It is high for ferromagnetic materials. In air, however, Gauss and Oersted are identical numerically. The modern S/I system prefers the Tesla (T) and Ampere-turns/meter (A/m) units for magnetic flux density and magnetic field intensity, respectively. Conversions are shown in the table below.
Magnetic Properties Conversion Table
Not just engineers, scientists, inventors, but educators and homeowners in over 80 countries have used our Lab Kits and shielding materials for hands-on evaluation. Our Lab Kits, which contain various samples of our proprietary shielding alloys and technical information, have helped many individual consumers and technical professionals solve unwanted magnetic interference from low frequency AC source fields (Ø Hertz to 100 kilohertz). Although the evaluation process can vary by application, the following steps are generally followed by all, for “hands-on” evaluation.
1. Determine Source of Interference
Internal interference is created by operating components within a system, affecting each other. This can be neutralized by shielding the individual components and confining the magnetic forces they create. External interference is created by a source outside the system which may affect many components or a large area. In this case, the entire unit being affected must be shielded by an enclosure, a chamber, or walls. Many people ask “Should the source of interference or the sensitive device be shielded?” The answer to this question depends on several factors. Shielding the source may involve stronger fields, and therefore thicker materials. One must be sure that all interference sources are shielded, or the sensitive device will still be affected. The usual approach is to shield the sensitive device. This prevents interference from both present and future sources.
2. Locate and Measure Magnitude of Interference
Locating internal or external interference and determining the magnitude (strength) of the interfering field are important in shielding alloy selection. How do you measure magnetic fields? By using a Gaussmeter, the field strength is measured in “Gauss”, and output into numeric values. For AC fields, use one of our Magnetic Pickup Probes with a digital volt meter (DVM) or oscilloscope, or use one of our Gaussmeters to measure the interfering field strength. These measurement devices are included with certain Lab Kits, or sold separately. Field mapping, or plotting of these measurements, will provide a baseline and help determine effectiveness of your magnetic shield. The traditional CGS units for measuring magnetic fields are Gauss and Oersted. Magnetic flux density is measured in Gauss, while magnetic field intensity is measured in Oersted. The ratio of B, magnetic flux, in Gauss, to H, magnetic field, in Oersted, is defined as permeability, "µ". The B/H ratio, or "µ", is a measure of the material's properties. It is high for ferromagnetic materials. In air, however, Gauss and Oersted are identical numerically. The modern S/I system prefers the Tesla (T) and Ampere-turns/meter (A/m) units for magnetic flux density and magnetic field intensity, respectively. Conversions are shown in the table below.
Magnetic Properties Conversion Table
Often prefixes are used to make the quantities more manageable. For instance, we may speak of magnetic fields in milliGauss, where 1000 milliGauss (mG) are equal to one Gauss. Because a Tesla is a large amount of magnetic flux, fields are often described in mT (milliTesla) or µT (microTesla). 10 milliGauss are equal to one micro-Tesla. You may notice that the magnetic fields are sometimes described in technical literature as fields and sometimes as magnetic flux. In air, the magnitudes of magnetic field (in Oersted) and magnetic flux (in Gauss) are numerically equal, so the terms are sometimes used imprecisely, leading to such confusion. In air, relative permeability, µr, is equal to one, so the numerical magnitudes are the same.
3. Calculate Shielding Requirements
Formulas are used to determine which materials and thicknesses will provide the most effective shielding. The source (interfering) field known as Ho is measured in Gauss. Knowing Ho, and estimating the approximate size of your shield, shield thickness can be determined. Certain characteristics of the shielding alloy are given, such as permeability (μ), saturation induction, and flux density (B). Using these formulas can provide theoretical values.
To simplify these formulas, we have developed the Co-Netic Slide-Rule Calculator. Our easy to use calculator provides a quick reference for comparing thickness of Co-Netic alloy required to effectively shield the source field (Ho) vs. diameter (size) of the magnetic shield you will use (calculated as a theoretical cylinder). The calculator compares the source field (Ho) to the attenuated field (Hi) in Gauss, for both DC and 60Hz AC fields, and has a scale that covers most common requirements and applications we’ve seen over past decades. A Slide-Rule Calculator is included with each Lab Kit to aid in material thickness selection. Problem solving examples and instructions for the calculator are located on the "Resources" page of this site.
Saturation of Shielding Materials
Saturation is the point at which a shielding material is full. No additional flux can be absorbed. In calculating shield design, the flux density (B) should be less than 7,500 Gauss for Co-Netic and less than 21,000 Gauss for NETIC to avoid saturation. If the flux density is greater, thicker material or multiple layers are required. For high attenuation of low to moderate strength fields, a single layer of Co-Netic alloy is normally adequate. In high strength fields, light gauge Co-Netic may saturate and become less effective. If layers of NETIC and Co-Netic are used in combination, to increase the flux capacity, the NETIC material should always be placed nearest to the source of interference.
· FLUX DENSITY~ Gauss(B)
· Flux density of the shield material in Gauss (ref. Saturation of Shielding Materials above)
· B = (1.25 * D * Ho) / t
· D = diameter of shield (approx. cylindrical size required to shield affected component)
· HO = source (interfering) field in Gauss (measured at the proposed shield location)
· t = thickness of shielding alloy (material selected from the Lab Kit)
Attenuation Ratio of Designed Shield
The shielding efficiency of a magnetic shield is specified as the Attenuation Ratio (reduction ratio). This is the ratio of measured field before shielding to that measured after shielding. Attenuation, measured in decibels (dB), is simply 20 times the logarithm to the base 10 of the shielding ratio.
A properly designed single layer Co-Netic shield of small size will easily provide 30 to 40 dB attenuation. Attenuation of 60 dB or more is attained by utilizing multiple layer shielding, such as found by using our three-layer Zero Gauss Chambers.
Typically, attenuation decreases in shields of large volume, unusual shape/configuration, or with large openings (leak points).
A simple formula for estimating attenuation ratio of a cylindrical shield is:
· A = (μ * t) / D
· μ = known permeability of the shielding alloy (at flux density B)
· t = thickness of shielding alloy (material selected from the Lab Kit)
· D = diameter of shield (approx. cylindrical size required to shield affected component)
Building A Prototype Shield
Using our Lab Kits, shielding can be prototyped with everyday common hand-tools, or bench-top and model-shop equipment. Layers of material from a Lab Kit can be added until the unwanted field is reduced (attenuated) to the desired level. Your prototype then serves as a production model. Further changes in layout, field intensity, or component orientation can be easily evaluated without relying on imprecise theoretical formulae. Shield design should consider the following:
Optimal Shapes& Size:
· Completely enclosed spherical configuration is theoretically ideal
· Effective shapes are cylinders, cans and 5-sided boxes
· Flat shields are effective if L x W extend beyond the field area and the affected component
Effective Shield Design:
· Provide ¼" gap (air space) between the shield and component to be shielded
· Enclosure of the component by the magnetic shield should be as complete as possible
· Shields should be 5 or 6-sided until it can be determined that less enclosure is effective
Layout and Patterns:
· Cardboard or paper templates can be used to check fit before cutting the shielding material
· Seams and joints should be held to a minimum
· Overlap material ½" to ¾" at each joint to simulate welded or fabricated joints
Forming a Prototype:
· Hand forming of shaped shields can be done easily with foil
· Roll forming may be necessary for cylindrical shapes
· Right-angle bends should have a min. inside radius two times metal thickness to avoid work-hardening & preserve shield permeability
Joining Methods:
· Soldering overlapped seams may create a non-magnetic gap
· Spot welding is effective to fuse the alloy, making a stronger magnetic path
· Heliarc welding is optimal. Butt-joints are clean & reduce material use
· Welding must be followed by re-annealing
· Pressure Sensitive Tape (included in Lab Kits) is used to join shields or adhere shield to components
Measure and compare the resulting field strength to your initial results (field mapping). Remember, additional layers of material may be added until the unwanted field is attenuated to the desired level.
3. Calculate Shielding Requirements
Formulas are used to determine which materials and thicknesses will provide the most effective shielding. The source (interfering) field known as Ho is measured in Gauss. Knowing Ho, and estimating the approximate size of your shield, shield thickness can be determined. Certain characteristics of the shielding alloy are given, such as permeability (μ), saturation induction, and flux density (B). Using these formulas can provide theoretical values.
To simplify these formulas, we have developed the Co-Netic Slide-Rule Calculator. Our easy to use calculator provides a quick reference for comparing thickness of Co-Netic alloy required to effectively shield the source field (Ho) vs. diameter (size) of the magnetic shield you will use (calculated as a theoretical cylinder). The calculator compares the source field (Ho) to the attenuated field (Hi) in Gauss, for both DC and 60Hz AC fields, and has a scale that covers most common requirements and applications we’ve seen over past decades. A Slide-Rule Calculator is included with each Lab Kit to aid in material thickness selection. Problem solving examples and instructions for the calculator are located on the "Resources" page of this site.
Saturation of Shielding Materials
Saturation is the point at which a shielding material is full. No additional flux can be absorbed. In calculating shield design, the flux density (B) should be less than 7,500 Gauss for Co-Netic and less than 21,000 Gauss for NETIC to avoid saturation. If the flux density is greater, thicker material or multiple layers are required. For high attenuation of low to moderate strength fields, a single layer of Co-Netic alloy is normally adequate. In high strength fields, light gauge Co-Netic may saturate and become less effective. If layers of NETIC and Co-Netic are used in combination, to increase the flux capacity, the NETIC material should always be placed nearest to the source of interference.
· FLUX DENSITY~ Gauss(B)
· Flux density of the shield material in Gauss (ref. Saturation of Shielding Materials above)
· B = (1.25 * D * Ho) / t
· D = diameter of shield (approx. cylindrical size required to shield affected component)
· HO = source (interfering) field in Gauss (measured at the proposed shield location)
· t = thickness of shielding alloy (material selected from the Lab Kit)
Attenuation Ratio of Designed Shield
The shielding efficiency of a magnetic shield is specified as the Attenuation Ratio (reduction ratio). This is the ratio of measured field before shielding to that measured after shielding. Attenuation, measured in decibels (dB), is simply 20 times the logarithm to the base 10 of the shielding ratio.
A properly designed single layer Co-Netic shield of small size will easily provide 30 to 40 dB attenuation. Attenuation of 60 dB or more is attained by utilizing multiple layer shielding, such as found by using our three-layer Zero Gauss Chambers.
Typically, attenuation decreases in shields of large volume, unusual shape/configuration, or with large openings (leak points).
A simple formula for estimating attenuation ratio of a cylindrical shield is:
· A = (μ * t) / D
· μ = known permeability of the shielding alloy (at flux density B)
· t = thickness of shielding alloy (material selected from the Lab Kit)
· D = diameter of shield (approx. cylindrical size required to shield affected component)
Building A Prototype Shield
Using our Lab Kits, shielding can be prototyped with everyday common hand-tools, or bench-top and model-shop equipment. Layers of material from a Lab Kit can be added until the unwanted field is reduced (attenuated) to the desired level. Your prototype then serves as a production model. Further changes in layout, field intensity, or component orientation can be easily evaluated without relying on imprecise theoretical formulae. Shield design should consider the following:
Optimal Shapes& Size:
· Completely enclosed spherical configuration is theoretically ideal
· Effective shapes are cylinders, cans and 5-sided boxes
· Flat shields are effective if L x W extend beyond the field area and the affected component
Effective Shield Design:
· Provide ¼" gap (air space) between the shield and component to be shielded
· Enclosure of the component by the magnetic shield should be as complete as possible
· Shields should be 5 or 6-sided until it can be determined that less enclosure is effective
Layout and Patterns:
· Cardboard or paper templates can be used to check fit before cutting the shielding material
· Seams and joints should be held to a minimum
· Overlap material ½" to ¾" at each joint to simulate welded or fabricated joints
Forming a Prototype:
· Hand forming of shaped shields can be done easily with foil
· Roll forming may be necessary for cylindrical shapes
· Right-angle bends should have a min. inside radius two times metal thickness to avoid work-hardening & preserve shield permeability
Joining Methods:
· Soldering overlapped seams may create a non-magnetic gap
· Spot welding is effective to fuse the alloy, making a stronger magnetic path
· Heliarc welding is optimal. Butt-joints are clean & reduce material use
· Welding must be followed by re-annealing
· Pressure Sensitive Tape (included in Lab Kits) is used to join shields or adhere shield to components
Measure and compare the resulting field strength to your initial results (field mapping). Remember, additional layers of material may be added until the unwanted field is attenuated to the desired level.