Use gaussmeters and fluxmeters for manufacturing quality
Gaussmeters (also known as teslameters) and fluxmeters are commonly used in industrial QC (quality control) environments for making magnetic-field measurements. Many technicians, however, have limited experience with these instruments and thus are unfamiliar with some of the recommended practices for using them effectively.
In Ensure magnetic components meet specs, I discussed the necessity for–and current lack of–magnetics knowledge among engineers and technicians. The article outlined some basic units and values of magnetic measurement. In addition, the article summarized various instrumentation options available for magnetic measurement, from expensive, higher-end systems such as magnetometers and B-H loopers to more basic instruments such as Hall-effect gaussmeters and fluxmeters.
Now, let's examine the benefits, selection guidelines, and proper use of gaussmeters and fluxmeters. While the theoretical and technical topics might be of some interest to those immersed in the study of magnetics, I will concentrate on practical aspects that affect most industrial users of the instruments on the shop floor.
The Hall-effect gaussmeter (Teslameter)
The Hall-effect gaussmeter (Figure 1) is the device selected most often when making general magnetic-field measurements. It's easy to use, has a variety of compatible probes, and is low cost.\
Gaussmeters features include:
- High accuracy (0.05% to 0.10%)
- Measures both AC and DC magnetic fields
- Wide range of instrument types available (handheld, bench top, systems-compatible)
- Broad measurable field range (0.5µT to 20T) in a single instrument
- Variety of probe types available
- Low to moderate price range
Features to Look for
- Accuracy (a test specification): While gaussmeters in general offer accurate measurements at an affordable price, some discrepancies between devices do exist. Less expensive gaussmeters tend to be slightly less accurate and offer fewer probe options. A reduced level of accuracy might suffice for some users but not for others, depending on the application. Testing magnet pole strength, for example, may require no better than 1% to 2% accuracy because external factors often control the final measurement. On the other hand, when calibrating magnetic equipment or sensors, you're better off with an accuracy of 0.1% or better.
- Field magnitude range (a design consideration): Though gaussmeters offer a very wide range of measurable magnetic fields, not all instruments do. General-purpose gaussmeters will measure fields in the 2G to 20kG range. If you need to make measurements below or above this range, then you must take more care in instrument selection. The more sophisticated gaussmeters may offer the ability to measure fields as low as tens of milligauss up to 200kG. Generally, DC field ranges are wider than AC, and the probe must have the necessary field-magnitude capability.
- Probe selection: A wide probe selection is essential for testing versatility. The more probe types and sizes available, the wider range of testing you can conduct. Various versions of transverse and axial versions compatible with the selected gaussmeter are a must when selecting a more sophisticated instrument. Fewer probe types normally suffice for low-end gaussmeters. Two configurations of probes are available for general testing:
- Axial: Axial probes (Figure 2) are normally cylindrical in shape with the sensor mounted in the tip for reading fields in line with the length of the cylinder. These probes are excellent for reading pole field strength and uniformity and field shapes in volumes.
Figure 2. Axial probes are ideal for reading pole field strength, uniformity and field shapes. These types of probes typically have diameters of 1.5 mm to 6 mm and can measure fields in solenoids. On an axial probe, positive (+) flux always enters from the front.
- Transverse: The sensing area of a transverse probe (Figure 3) is normally a flat blade with the magnetic field perpendicular to the flat surface. Transverse probes are ideal for reading narrow air gap fields or pole field strength where an axial probe will not physically fit.
Figure 3. Transverse probes are useful for reading narrow air gap fields or pole field strength where axial probes will not fit. They can sense fields in gaps as small as 0.5 mm. On a transverse probe, positive (+) flux enters on one side.
After selecting an instrument for the above requirements, then the test engineer can think about additional requirements such as:
- Environmental conditions (temperature, humidity, dust): Like other instruments, gaussmeters and their probes can be sensitive to heat and other ambient conditions, which could affect performance. Manufacturers typically post specifications on these environmental conditions.
- Front panel ease of use: This can be a consideration when using the instrument requires working in a small space or in a dimly-lit facility. Display options such as liquid crystal displays (LCDs) might be acceptable for brightly lit environments, but vacuum fluorescent or LED readouts tend to be more appropriate for dimmer workspaces. User-friendly, direct function key entry or menu selection is offered and may be a personal preference.
- Computer interfacing: This is a useful factor to consider when results must be stored or analyzed, or when you need automated testing. Most modern industrial test instruments computer interfaces such as GPIB, RS-232, USB, or, Ethernet.
- Anticipated future requirements: You should consider instrument options that might be useful in future applications. Features that might come in handy include programmability and interaction with other systems, AC field frequency, and different probe sizes. You should, though, always select the most robust probe for the immediate application. A user might choose a very thin or very small diameter probe to meet possible future requirements, but if that probe is too fragile for current applications, its lifetime may be reduced.