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The MFAM Module is slightly magnetic. That it is why it is separated from the MFAM sensors by the 20 inch flex circuit. If the MFAM module is brought closer to the sensors this is what will be the effect on the measurement (peak to peak) as the MFAM module is rotated in all orientations by a stationary sensor:
24 inches: < 0.25 nTpp
12 inches: 1.5 nTpp
6 inches: 7.0 nTpp
3 inches: 50 nTpp
Note that these measurements were made using another magnetometer on a special magnetic Signature measuring table to measure the MFAM module Signatures at the specified distances. That is how we were able to measure the Signature at 24 inches while the MFAM module to MFAM sensors distance is only 20 inches.
In general, avoid stainless steel parts. If you have to use stainless steel parts, make sure it is made of SS316.
To make sure that the parts are magnetically clean, you can set up the MFAM Dev Kit in the gradiometer mode.
1. Have the two sensors separated by 20cm or more.
2. Turn on the gradient reading (green curve) in MagViewMFAM.
3. Place the part 10cm to one sensor and 30cm to the other.
4. Rotate the part while monitoring the gradient curve. Make sure you don't have any magnetic parts (such as keys, cell phone, watch) with you while doing this.
5. The peak-to-peak difference in the gradient curve (you may have to adjust the gradient scale if the curve wraps on screen) is the magnetic Signature of the part at 10cm.
6. For the magnetic Signature at different distances, scale the reading as 1/R^3. For example, if the part will be 20cm away, its magnetic Signature should be 1/8 of of the reading at 10cm.
The MagArrow uses a 3 cell Lithium Polymer battery to power the MagArrow during surveys. The two main requirements for the battery are that it must fit into the battery compartment, and it must be nonmagnetic.
Non-Magnetic Batteries:
Some types of Lithium Polymer batteries are extremely magnetic. This is because the cell-to-cell connections are made with nickel strips (nickel is extremely magnetic). This makes them unsuitable for use in the MagArrow since they will interfere with the background magnetic field that is being measured. Whether or not the batteries are magnetic is not something that appears on the data sheet, so it is important to choose batteries of a particular construction form factor that in practice has been shown to have a very low magnetic Signature. Examples of this battery type will be shown below. There are many brand names for this battery type, and the brand names seems to change frequently.
Evaluating the Magnetic Properties of a Battery:
Batteries should be measured for magnetic Signature before using them. This is especially true when trying a new battery brand just to be sure the battery is not going to affect the survey data. To perform this test you will need to start a survey with a stationary MagArrow pointing north-south on a nonmagnetic platform (wooden sawhorses, cardboard box, etc). Hold the battery to be tested immediately over the battery compartment and rotate it in all orientations. Download the data and look for variations in the magnetic field that correlate with the battery rotation. There shouldn't be any correlation above 1 nT peak to peak. Make sure the operator is nonmagnetic when doing this test (shoes, belts, watches, cell phones, keys, etc. can all corrupt the results).
Battery Size and Shape:
The correct batteries are rectangular in shape and measure roughly 105x34x24mm. They are made from 3 flat cells stacked up measuring 11.1 volts nominal. They should be between 1800 and 2200 mAh (milliamp-hour). Higher capacity batteries will not physically fit in the battery compartment. Lower capacity batteries will work, but with a reduced run time. One 1800 mAh battery will run the MagArrow for about two hours. The MagArrow power connector is XT-60 so the battery must match. There are other power connector types, but XT-60 is commonly used. The 4-pin balance port connector is a JST-XH4 connector (though this is standard on most batteries).
Where to Find Batteries:
If you are in an area that doesn't have strict controls on shipping Lithium Polymer batteries, then Amazon.com is a good source. Another good source is hobby stores, or anyplace that sells radio-controlled toy cars, boats, or airplanes. This is typically where this style of battery is used the most.
What do the Battery Specifications Mean?
3S: This means it is a stack of three Li-Po cells
Voltage: A fully charged 3 cell Li-Po battery measures 12.6 volts. A depleted battery will measure 9.6 volts. Thus, the voltage for this battery is typically labeled as 11.1 volt (the average of 12.6 and 9.6 volts.
35C (or any other "C" value): This is a rating on how much current can be safely drawn from the battery. To get the value in amps, take the milliamp-hour rating and divide by 1000 (to get amp-hours), and then multiply by the "C" value. For a 2200 mAh battery with a 35C rating multiply the 2.2 amp-hour capacity (2200 mAh / 1000) times the C value of 35, which gives a maximum discharge current of 77 amps. The MagArrow draws about 0.6 amps, so any C value is fine - even if is down to 0.5.
Battery Chargers:
Most battery chargers being sold now are universal chargers which support a variety of rechargeable battery chemistries and output connectors. They come in many sizes and shapes, but most of them operate identically because the internal circuitry is the same. Most chargers will charge at a much faster rate than the MagArrow discharges them, so you technically only need two batteries in the field. A nice feature to look for is the ability to power the charger off 12V as well as with AC power. This will allow charging in the field off a car battery.
Be sure to charge in batteries in "Balanced Charge" mode using the battery balance JST-XH connector. This allows more charge current into cells that are more deeply discharged than the others and ensures that the battery gets all three cells completely charged.
Battery Safety:
Lithium Polymer batteries are small and light but store a tremendous amount of energy inside. This is good for running equipment for long periods of time between charges, but it also means that if something goes wrong and it releases all its energy at once it can be a serious fire hazard.
Never charge a lithium battery unattended, charge only in a fireproof location.
Batteries that are swollen or damaged should not be used. Dispose of these per local regulations.
Be sure to follow all regulations for shipping or hand carrying Li-Po batteries. This may include packaging and labeling requirements, limiting the number of batteries, and discharging the batteries to 30% capacity before shipping.
Do not discharge the battery below 9.6 volts (3.2 volts per cell). This damages the battery and could result in destructive decomposition and fire. If a battery that is discharged below a safe level is placed on the battery charger it will refuse to charge it. Batteries that are discharged below 9.6V should be removed from service and disposed of according to local regulations.
To download a copy of this document as a PDF, click here.
Some example batteries are shown below:
The MagEx uses a 3 cell Lithium Polymer battery to power the MagEx during surveys. The two main requirements for the battery are that it must fit into the battery compartment, and it must be nonmagnetic.
Non-Magnetic Batteries:
Some types of Lithium Polymer batteries are extremely magnetic. This is because the cell-to-cell connections are made with nickel strips (nickel is extremely magnetic). This makes them unsuitable for use in the MagEx since they will interfere with the background magnetic field that is being measured. Whether or not the batteries are magnetic is not something that appears on the data sheet, so it is important to choose batteries of a particular construction form factor that in practice has been shown to have a very low magnetic Signature. Examples of this battery type will be shown below. There are many brand names for this battery type, and the brand names seems to change frequently.
Evaluating the Magnetic Properties of a Battery:
Batteries should be measured for magnetic Signature before using them. This is especially true when trying a new battery brand just to be sure the battery is not going to affect the survey data. To perform this test you will need to start a survey with a stationary MagEx pointing north-south on a nonmagnetic platform (wooden sawhorses, cardboard box, etc). Hold the battery to be tested immediately over the battery compartment and rotate it in all orientations. Download the data and look for variations in the magnetic field that correlate with the battery rotation. There shouldn't be any correlation above 1 nT peak to peak. Make sure the operator is nonmagnetic when doing this test (shoes, belts, watches, cell phones, keys, etc. can all corrupt the results).
Battery Size and Shape:
The correct batteries are rectangular in shape and measure roughly 105x34x24mm. They are made from 3 flat cells stacked up measuring 11.1 volts nominal. They should be between 1800 and 6000 mAh (milliamp-hour). Higher capacity batteries will not physically fit in the battery compartment. Lower capacity batteries will work, but with a reduced run time. One 1800 mAh battery will run the MagEx for about two hours. The MagEx power connector is XT-60 so the battery must match. There are other power connector types, but XT-60 is commonly used. The 4-pin balance port connector is a JST-XH4 connector (though this is standard on most batteries).
Where to Find Batteries:
If you are in an area that doesn't have strict controls on shipping Lithium Polymer batteries, then Amazon.com is a good source. Another good source is hobby stores, or anyplace that sells radio-controlled toy cars, boats, or airplanes. This is typically where this style of battery is used the most.
What do the Battery Specifications Mean?
3S: This means it is a stack of three Li-Po cells
Voltage: A fully charged 3 cell Li-Po battery measures 12.6 volts. A depleted battery will measure 9.6 volts. Thus, the voltage for this battery is typically labeled as 11.1 volt (the average of 12.6 and 9.6 volts.
35C (or any other "C" value): This is a rating on how much current can be safely drawn from the battery. To get the value in amps, take the milliamp-hour rating and divide by 1000 (to get amp-hours), and then multiply by the "C" value. For a 2200 mAh battery with a 35C rating multiply the 2.2 amp-hour capacity (2200 mAh / 1000) times the C value of 35, which gives a maximum discharge current of 77 amps. The MagEx draws about 0.6 amps, so any C value is fine - even if is down to 0.5.
Battery Chargers:
Most battery chargers being sold now are universal chargers which support a variety of rechargeable battery chemistries and output connectors. They come in many sizes and shapes, but most of them operate identically because the internal circuitry is the same. Most chargers will charge at a much faster rate than the MagEx discharges them, so you technically only need two batteries in the field. A nice feature to look for is the ability to power the charger off 12V as well as with AC power. This will allow charging in the field off a car battery.
Be sure to charge in batteries in "Balanced Charge" mode using the battery balance JST-XH connector. This allows more charge current into cells that are more deeply discharged than the others and ensures that the battery gets all three cells completely charged.
Battery Safety:
Lithium Polymer batteries are small and light but store a tremendous amount of energy inside. This is good for running equipment for long periods of time between charges, but it also means that if something goes wrong and it releases all its energy at once it can be a serious fire hazard.
Never charge a lithium battery unattended, charge only in a fireproof location.
Batteries that are swollen or damaged should not be used. Dispose of these per local regulations.
Be sure to follow all regulations for shipping or hand carrying Li-Po batteries. This may include packaging and labeling requirements, limiting the number of batteries, and discharging the batteries to 30% capacity before shipping.
Do not discharge the battery below 9.6 volts (3.2 volts per cell). This damages the battery and could result in destructive decomposition and fire. If a battery that is discharged below a safe level is placed on the battery charger it will refuse to charge it. Batteries that are discharged below 9.6V should be removed from service and disposed of according to local regulations.
Some example batteries are shown below:
I agree with Peter that it will be very challenging to locate the MagArrow II or the drone using the magnetic method. You will need to get to <0.2m from the MagArrow II to detect any significant magnetic Signature. As for the drone, the detection range is probably < 1 m because the 3 m suspension cable is far enough to make the drone "invisible" to the MagArrow. As Peter pointed out, vision method probably works better than the magnetic method in this case.
Wish you the best luck!
Magnetic contamination can be a problem, though rarely. Most often this is caused when a “permanent” magnetic component was accidentally attached to or close to one of the sensors. This can cause big shifts randomly in the data from one or both sensors. Check by removing the sensor door and visually inspect the two MFAM sensors and their surrounding areas for anything unusual.
If you have another magnetometer, like the G-864, you can also measure the magnetic Signature of the sensor part of the MagArrow with the following instructions.
Turn on the magnetometer
Wave the sensor part of the MagArrow above the magnetometer (as close as possible but not touching)
Wave in both west and east directions
Check whether this is any magnetometer reading change when the MagArrow passes by
If reading changes are observed, there must be some contamination.
The batteries used in the portable magnetometer instruments are lead-acid gelled electrolyte batteries. The choice of this type of battery was dictated by their non-magnetic internal construction. We “magnetically compensate” these batteries to further reduce their magnetic Signature. We do this with bucking coils which are mounted against the battery surfaces and then an external wrap applied.
The batteries should be charged using the charger furnished with the instrument. These chargers are fully automatic and designed to do the best job of charging and maintaining the batteries for long life. All of the chargers are equipped with lights indicating when the battery is being charged and when the charging cycle is completed.
The battery packs will provide the most operating cycles when they are fully charged after each use. The number of operating cycles can vary from 250 cycles to above 1000 cycles depending on how deep the discharge was and how soon the battery is charged after use.
A 30% discharge per cycle may result in a lifetime of 1000 cycles or more, whereas a 100% discharge per cycle can result in only 250 cycles. As a rule the magnetometer will shut down when the battery is discharged to about 20% of full voltage. This is to ensure proper shutdown of the instrument.
It is very important to recharge the battery as soon as possible after use so the maximum life can be expected from the pack. If the discharged pack is left to charge “when we get back from the field” the pack can suffer from “sulphation”. This is a high-resistance buildup in the battery which may render the battery unusable.
If a battery of this type must be stored for an extended period, it must be stored in a fully-charged condition. If such a battery is stored discharged and subject to below-freezing conditions, it is likely to freeze and be subsequently unusable.
All Lead-Acid batteries must be maintained when in storage. This means that the user must recharge each pack at least once a month. Lead Acid batteries will self-discharge due to stray internal resistances, causing very small drain currents. Thus the maintenance requirement for monthly recharging is critical to long battery life. Do not leave the charger on all the time during storage. Also it is very important to use discharge the batteries on a regular basis otherwise the lifespan will be severely shortened. For more information contact our Support Department.