AlphaLab Gaussmeter Model GM2 -- Instructions

This meter measures magnetic flux density in one axis from -29,999 to +29,999 gauss (2.9999 teslas) and it indicates the polarity (a "minus" sign displays if polarity is negative). At fields below 20,000 gauss, an additional digit appears (e.g., “19867.3” gauss). It also measures the peak DC value (2 millisecond capture window) and can display the AC field (10-1500 Hz) up to 10,000 gauss. An alarm with two modes of operation can be set to sound either if the field is between two limits (whether + or -polarity) or if the field is above a threshold. The user can set these limits/threshold at any time.

Operation: Plug in the probe on the top edge of the meter. Turn the meter on (“DC”). There are several probe styles available. The most popular is the universal probe, which is described in this paragraph. Note that the last few millimeters at the (black) end of the universal probe has a square bulge on one side; the other side is flat. The center of that square bulge is the location of the Hall-Effect sensor, which is very small (0.2 x 0.2 mm). Place the flat side of the probe’s end (not the square bulge side) against the surface to be measured. A negative sign indicates that the probe is touching the south pole of a magnet; in contrast, a north pole will read positive (“positive” is indicated by the absence of a polarity sign on the display). The actual center of the sensor is 0.75 mm above the flat surface or 0.6mm below the top of the square bulge) and is centered in the center of the square bulge.

An axial probe is available as an option. It is 6.6 mm (0.25”) diameter and 150 mm (6”) long on a 100 cm cable. It detects the field component in the same direction as its 150 mm axis. A rigid transverse probe is also available. It is 0.65 mm thick (= direction of magnetic axis) x 3.25 mm wide x 75 mm long on a 100 cm cable. All sizes and lengths can be custom-modified. A high-stability probe is another option. When this probe is plugged into the meter, the display changes so that two digits, instead of one, are to the right of the decimal point. Also a triangle appears under “HS probe (gauss)”. Then the display reads up to +/-799.99 gauss, and shows overrange above that field strength. This probe is stable down to +/-0.03 gauss.

Below 10 gauss, only two digits (such as “3.7”) will be displayed. For stronger fields, more digits will appear. For fields stronger than +/- 9999.9 gauss, the extreme left digit will display. (It will be a “1” or “2”). Very few magnet assemblies have a field this strong, which is usually found only in the gap between two rare earth magnets or in a high-power electromagnet. If the field is stronger than +/- 19,999.9 gauss, the display will read switch to the 29.999kG range. (A triangle will appear under the word “Kilogauss” and a decimal point will appear after the first two digits.) When the field decreases below 18.000 kG, the display will switch back to the gauss range. If the field is over 29.999 kG (unlikely to be encountered except with a superconducting magnet), it is overrange and will display “1- - - - -”. These high fields will not harm the meter, although some styles of batteries may malfunction if exposed to such high fields. (This malfunction would only occur if the battery itself is exposed to the high field.)

If the display reads “LO BATT”, there is about one hour of battery life remaining. Remove the soft bumper (the “boot”) if the bumper is present, and then slide off the battery door on the back side. Replace with a common 9-volt rectangular battery. Alkaline is preferred. Current drain is 15ma and the LOBATT reads if battery voltage remains below 6.8V for at least one minute. (Accuracy errors will occur only below 5V). The AC adapter can be substituted for the battery. When plugged in (left edge of enclosure), the adapter disconnects the battery (does not charge the battery).

System Reset: (Restores the offset to the original value; can also be used in the event the meter freezes up). Turn the meter on and plug the AC adapter cord without the AC adapter being connected to power. Then turn the meter off and disconnect the cord from the meter. System reset can also be done by removing the battery while the meter is on, then turning the meter off and replacing the battery. The system resets because power is interrupted. This includes interruption of internal capacitor power, which has enough energy after the battery is disconnected for 30 seconds while the meter is off, or 0.01 sec while it is on.

Offset adjustment (only necessary if measuring weak fields): On “DC”, pressing the “Relative Zero” button will subtract the present field from the display, causing it to read zero. Then if the field increases to higher than that value, the display will become a positive number and vice-versa. This subtraction of the field at the time (“Relative Zero” was pressed) will continue until “Relative Zero” is pressed again. When this subtraction is in effect, a triangle will appear over the label phrase “Relative Zero”. When not in “Relative Zero” mode, the OFFSET knob allows you to add or subtract any number from the displayed number of gauss. (Turning the OFFSET knob will not change the offset when “Relative Zero” is displayed.) Clockwise increases the number and vice-versa. This offset stays in effect even after you turn the meter off and then on, until you change the offset again or do a system reset. Then you bring in a magnetized object to measure. This is the magnetic equivalent of adjusting the “tare” weight of a weight scale, because there may be an ambient magnetic field that you’ll want to subtract out. Note that the OFFSET knob is 2.4 gauss per rotation. At any time, the offset can be returned to near zero by momentarily pressing down on the OFFSET knob. You can perfectly adjust the offset to zero if you do one of two things, either:

  1. Place the sensor in a “zero gauss chamber” (not supplied with this meter) and then either turn OFFSET until the display shows a zero reading or press “Relative Zero”. (However, a “zero gauss chamber” is sometimes accidentally magnetized, so it is not always reliable.) or
  2. Place the sensor end flat on a non-magnetic table or desk with the square bulge facing up. In this orientation in the northern hemisphere, the meter will read the upward-pointing component of the magnetic field. (If the field is instead downward-pointing there, the meter should then read a negative number.) Note the number on the display. Then flip the sensor so the square bulge is facing down. This 2nd reading should be the negative of the 1st reading. That is, if the 1st reading was -0.4 (gauss), then the 2nd should be 0.4 (gauss). If the two readings are not the negative of each other, then the OFFSET is not adjusted perfectly for a zero reading in zero field. For example, if the 1st reading is 1.0, and the 2nd reading is 0.0, then the OFFSET is off by 000.5 gauss (the average of the two readings). Generally, this fine level of adjustment is not necessary, because it makes a difference of only a fraction of a gauss when measuring a multi-thousand gauss magnet. This method however, is more reliable than using a “zero gauss chamber”.

AC measurement: Set the knob to “AC”. The display will show AC (pseudo-RMS, +/-3% of reading +/-10 counts, from 45 to 800 Hz; 3 dB corners are at 11 to 1500 Hz). When using an AC adapter, a significant background AC reading may appear; it can be removed by pressing “relative zero”. The OFFSET does not affect the AC reading.

Peak Hold: The highest value of the DC field (whether + or -polarity) since the last time the “Peak Reset” button was pressed will be held in memory. This value has a capture time of only 2 milliseconds, so even a brief spike will be detected. It can be displayed at any time by switching the knob to “Peak Hold”. The peak hold detection circuitry continues to operate regardless of the setting (DC, AC, or Peak Hold), and is retained in memory even when the meter is off. Therefore, in order to clear the previous peak hold value, press “Peak Reset”, which will return the peak hold number to within about one gauss of the present field. (Noise at this high sampling speed will cause the peak hold displayed to be about one gauss higher that the actual value.)

Alarm: The alarm will sound when field either exceeds a threshold that you set or is between two levels that you set. The alarm will never sound or light unless you set it and it will not sound when muted (by setting it to 0.0). It will also become muted whenever the meter is turned off and back on. To set a threshold, Press and continue to press “Alarm Set”. The threshold will be displayed (initially 0.0) and can be changed by turning the OFFSET knob. Clockwise increases the number; counter-clockwise decreases it (a few clicks CCW from 0.0 is the highest alarm value: 19000.0 gauss; more clicks CCW will decrease that value). At any time, the value can be directly reset to 0.0 (muted) by pushing down on the OFFSET knob while pressing “Alarm Set”. Once “Alarm Set” is released, the value will be remembered and can be modified later by pressing and holding “Alarm Set” again. There are 94 different alarm values that can be selected, from 1.0 to 19000.0 gauss (these values will be obvious). The alarm will sound only during the times that the field exceeds the threshold you had set, and a delay of about 2 milliseconds is required for the alarm sound to begin. There is about a 2 millisecond lag (after the magnitude drops below the alarm level) for the sound to stop. This mode of the alarm is often used to check for the presence of magnetized objects. Typically the threshold is set at about 30.0 gauss. Then an area can be rapidly scanned. By listening for the alarm, the area of strongest magnetization can be quickly found.

The second alarm mode allows you to determine whether a magnet is within specifications. To clarify this use, suppose a magnet is acceptable if its field if over 1700 gauss. If the alarm were set in such a way that it sounds any time the field is below 1700, then it would sound if the magnet were for example 1682 gauss (out of spec). Unfortunately, it would also sound whenever there is no magnet nearby, meaning that it would be sounding unnecessarily. To avoid this problem, the second alarm mode only sounds when the field is between an upper level (e.g., 1700) and a lower level you select (e.g., 100 to prevent unnecessary noise). To set the upper level, press and continue to press “Alarm Set” (as before) while rotating OFFSET (during this time, a triangle appears above the number displayed). To set the lower level, press and release “Data Option” while still pressing “Alarm Set”. Then the triangle will appear below the number displayed. While still pressing “Alarm Set”, rotate the OFFSET to the lower alarm level. (You must set the level to at least 1.0 gauss to activate this mode.) If this “lower” level is set higher than the “upper” level, the alarm will still only sound if the field is between the two levels.

To activate a digital output that will go high (+3.5V) when the alarm sounds, press “Alarm Set” and while holding that button press “Relative Zero”. You
will notice a flag turn on at the bottom of the display telling you the output jack is now a logic signal. To return to a linear output, repeat the procedure.

Alarm notes: The alarm can be set whenever the meter is on (DC, AC, or Peak Hold). It detects the DC signal when on “DC” and the AC signal when on “AC”. When the left knob is set either to “DC”, or “AC”, the alarm operates based on the displayed number, not the actual field, and the alarm ignores the negative sign, if present. Therefore, if the alarm is set to sound over 500 gauss, and the actual field displayed is -600 gauss (DC), the alarm will sound. If the “Relative Zero” button is then pressed, 600 gauss will subsequently be added to the displayed number so it will drop to approximately 0.0, and the alarm will stop sounding. When set to “DC”, any strong oscillating magnetic field (AC field) may cause the alarm to alternate rapidly between sound and no sound. On “AC” this effect does not occur.

Output: The output jack (left edge of meter) represents the actual instantaneous magnetic signal. It is unaffected by the choice of “DC”, “AC”, or “Peak Hold”. With the standard (not high-stability) probes, on the gauss range, the output is 1 volt per 10000 gauss (maximum is +/-2 volts). On the kilogauss range, which the meter automatically switches to at 20000.0 gauss, the output becomes 0.1 volt per 10000 gauss (10.000 kilogauss). Maximum in that range is +/-0.3 volt. Output example: if the actual field is a 10000 gauss (pp) sine wave, the output voltage will be a 1 volt (pp) sine wave, regardless of the left knob setting. When a high-stability probe is being used, the output is 0.1 volt per 100 gauss up to +/-0.8 volt (800 gauss).

Measurement of magnets: Most magnet materials have a published “remnant magnetization” or “internal flux density”. This is typically as high as about 5000 gauss for ceramic magnets and up to about 14,000 gauss for some rare earth magnets. The actual field (technically, “flux density”) on the surface of a single magnet is at most half this number, and it’s only that high if the magnet is long compared to its diameter. A stubby magnet, such as a disk or pill-shaped magnet will have a surface flux density that is even lower than half the published remnant magnetization. You will see the highest number of gauss (for a given type of magnetic material) in the gap between two long, thin magnets if the poles of each magnet are separated from each other, but the north of one magnet is almost touching the south of the other, as illustrated.

If the gap spacing is much less than the diameter (or width) of the magnets, and the length of each magnet is much greater than the diameter, then the reading in the gap (with the thin dimension of the probe slipped into the gap) should be almost as high as the published remnant magnetization number.

Magnets produce an additive field in general. This fact can be demonstrated from the gap magnet setup illustrated. If the meter reads 13,000 gauss in the gap, and then you take away one of the magnets, the new reading will be about 6500 gauss, or half as much. The field from each permanent magnet adds together, because the two magnets do not alter each other’s field. (If iron or steel is used as part of a magnetic structure, then it’s a different story, because the permanent magnets in the structure do magnetize the iron or steel.

Magnets may lose strength because of overheating (100°C will demagnetize some types of FeNdB magnets, but over 800°C is required to demagnetize some types of ceramic magnets). They may also partially demagnetize if struck hard or exposed to a sufficiently strong reversed magnetic field, either from coils that carry electric current (as in a permanent magnet electric motor) or from another strong permanent magnet. In any case, it’s easy to detect demagnetization: when a new magnet comes in, measure the number of gauss in one or more critical spots and record this. Then compare these to later measurements. There is one warning: all magnets have a slightly lower field when they’re warm than when they’re cool, so try to standardize the temperature at which you measure. (Each type of magnetic material has a published percent change/°C temperature sensitivity. Note that magnets are not permanently demagnetized by going through repeated warming and cooling cycles. They only permanently demagnetize if heated above their maximum allowed operating temperature.

Measurement of residual magnetism: This type of meter can also be used to check residual (accidental) magnetization of parts. In general, this accidental magnetization is perpendicular to the surface of the part, so the probe can be placed flat against the part, and this is the correct direction for detection of that field. You may need to scan the probe across the surface to find the highest number. It is helpful to set the alarm at a threshold of about 10 gauss for this. This highest reading is usually found at the ends or sharpest points of the part.

There are some peculiarities of measuring residual magnetization. Long, thin steel parts will often “amplify” the Earth field by a factor of 10 or so, at the ends of the part. If the long-axis of the part is pointed east-west, or perpendicular to the local indoor field, this is not a problem. The strength of the Earth field is about 0.5 gauss, so you may see up to about +/-5 gauss at the end of a properly demagnetized steel rod if the rod is pointed in the direction of the Earth field. The north pole of the earth is not horizontal in most locations. In most of Asia, the magnetic field direction is within about +/-20°of horizontal. In North America, if you face north and then look downward from horizontal 20°(Central Mexico) to 55°(Northern US) to as much as 90°down in parts of Canada, that is the direction of magnetic north. You can detect the field strength and direction with the meter. If the offset is properly adjusted, then with the bulge in the probe pointing toward the Earth north, you will read a positive number, because it's pointing toward the south pole of a magnet. If you flip the sensor 180° so that the flat part is facing Earth north, you will read a negative number, of course.

Specifications:In the 0.0 to +/- 19,999.9 gauss range, accuracy is +/- 1% of the reading at room temperature (16C to 29C or 61F to 84F), and +/-2% of reading in the full temperature range of -4C to 65C (25F to 149F). Add +/- 1% to the error above 20.00 kilogauss. Offset drift is less than 0.1 gauss/deg C at room temperature with the standard probe series. Offset can be adjusted by the user at any time. The probes are interchangeable, including an optional high-stability probe (+/-799.99 gauss range--specifications are included with the high-stability probe, if that probe was ordered). Standard probe is 1.1 mm thick x 4.3 mm wide, with the sensitive area 1.5 mm from the probe tip. Magnetic sensitivity direction is parallel to the 1.1 mm thickness direction. The included standard 9-volt battery lasts about 50 hours of "on" time (drain = 6 ma). LOW BATTERY indicator shows when about one hour of battery life remains. Weight is 260 grams (0.6 lb.) Meter dimensions are 14cm x 9cm x 4.5cm thick (5.5" x 3.5" x 2.75"). The model GM-2 is warranted for one year. (During that time, AlphaLab Inc. will pay for standard shipping back to us and for return shipping to you. The warranty time is then extended by the amount of time the meter was not in your possession.) A calibration certificate is included. Recalibration is normally not required within 10 years, but we can perform a recalibration on this meter if required (fee applies).

Manufactured and calibrated in USA by AlphaLab Inc., 3005 South 300 West, Salt Lake City, UT 84115 USA. Tel 1-801-487-9492. See DC Gaussmeter Model GM2 for any updates.




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