The Magnum is now operating on three continents.
The Magnum has appeared in the New Products section of Physics Today, August 2003
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Demonstrating nuclear magnetic resonance (NMR), observing variations in
the Earth's magnetic field, calibrating low field magnetometers, and
teaching modern signal processing, can all be done with a new instrument
costing only $3,400. The Magnum is a proton precession NMR instrument. It
can be used to calculate the nuclear magnetic relaxation times for different
liquid samples. In addition, daily variations of the Earth's field, seasonal
variations, and geomagnetic storms can be monitored. The Earth's magnetic
field is determined with this instrument by use of the fundamental NMR
formula B = (2*pi*f) / gamma where f is the resonant
frequency, and gamma the
gyromagnetic ratio of the proton.
The Magnum is, therefore, useful
for calibrating low field magnetometers. This instrument is also a tool by
which modern signal processing techniques can be taught. Included with the
Magnum is signal processing software for spectral analysis and digital
filtering.
Shown on the left is the Magnum in operation outdoors on a wooden stand, specially constructed with wood dowels and glue so as to contain no ferromagnetic materials such as nails or screws.
The operation of the Magnum is as follows. In a standard 2 ounce (59 ml) bottle, filled with a hydrogen rich liquid such as water or alcohol, the protons are polarized perpendicular to the Earth's magnetic field using several amps of current through the polarizing coil. When the current is cut off, the protons begin precessing in unison about the Earth's field. This momentary harmony induces a voltage in the signal coil. After going through a high gain amplifier, the signal voltage is digitized by a data acquisition card in a computer. The digitized signal is then processed, resulting in a precise determination of the Earth's total magnetic field, as well as providing information about the hydrogen rich sample used.
Here is the Magnum User's Manual in pdf format.
U.S. academic and research scientists may be able to get funding for the
purchase of the Magnum through the NSF Earth Sciences: Instrumentation and
Facilities Program. The website for this program is at
http://www.nsf.gov/pubs/2005/nsf05587/nsf05587.htm
Exstrom Laboratories LLC is interested in collaborating with scientists at universities and nonprofits for the development of innovative Earth sciences instrumentation. Contact: Richard Hollos, richard[AT]exstrom DOT com, (303) 678-1487.
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The polarizing coil is wound on an acrylic form using 14 AWG solid copper wire (wire diameter w/o insulation = 1.63 mm). The coil has an inner diameter of 11.4 cm and a length of 10.0 cm. There are 6 layers of wire with 38 turns/layer. The polarizing coil's inductance is 4.3 mH, and its resistance with 15 meters of line is 1.1 Ohm.
The sample and canceling coils are each wound on an acrylic form using 22 AWG copper magnet wire (wire diameter w/o insulation = 0.643 mm). The coils have an inner diameter of 3.8 cm and a length of 10.0 cm. There are 4 layers of wire with 138 turns/layer. The sample coil's inductance is 3.5 mH, and its resistance with 15 meters of line is 5.9 Ohms.
The tiltable platform on which the sensor is mounted consists of a 15 by 15 cm piece of white acrylic, 1.2 cm thick. This is mounted with acrylic hinges on top of another piece of white acrylic 15 cm by 28 cm. The angle of tilt can be adjusted from zero to 90 degrees in 5 degree increments by the use of brass supports on the sides of the platform. This allows the angle that the sensor makes with respect to the local Earth's magnetic field to be adjusted for maximum signal strength. The platform is made entirely of acrylic with a small amount of brass. It contains no ferromagnetic materials so that it does not affect the Earth's field.
The sensor and polarizing coil are connected to the electronics through 15 meters (50 feet) of cable. There are 6 sample bottles included with the sensor.
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A 150 Watt, 12 Volt power supply is used to provide several amps of current through the polarizing coil. A digitally controlled solid state switch is used to turn the current through the polarizing coil on and off. The switch is able to turn off the current in the coil in less than 200 microseconds. For a strong signal the polarization field must go to zero in a time less than the period of the NMR signal, therefore quick turn off of the current is essential. The switch is all solid state with no moving parts which should give it a long lifetime. The control and timing of the switch is handled by a programmable microcontroller in the switcher.
The data acquisition system consists of a PCI card in a computer with a 12 bit ADC that has a maximum sampling rate of 100 ksps. The computer (not included) requirements are listed in the FAQ. This is a general purpose data acquisition board with 16 single-ended or 8 differential analog input channels and 16 digital I/O lines. The board uses DMA (direct memory access) so that the computer's processor is not tied up during the data acquisition process. The PCI card connects to an external interface board via a 68 pin standard SCSI cable. The interface board consists of screw terminals for the analog inputs and the digital I/O lines.
Included is software for controlling and acquiring data through the data acquisition system, as well as signal processing software: spectrum analysis and digital filtering. The data acquisition and control software can perform automatic signal averaging and real time digital filtering of the data.
See the Magnum FAQ for more information.
Below is a graph showing some Magnum results. The proton NMR peak of distilled water in the Earth's field at Longmont, Colorado on March 10, 2003 14:00 MST is shown, with 20 times signal averaging, and using a digital bandpass filter. The peak is at 2266.3 Hz, which corresponds to 53,228 nT (nanoTesla or gamma).
This measurement was taken outdoors, with the Magnum sensor on a wooden stand 5 feet (1.5 meters) above the ground, and about 20 feet (6 meters) from the nearest building (wood siding).
The plot below is a magnification of the peak, with a 50 point high resolution Fourier transform ( "C Program Magnifies Spectrum When An FFT Can't Hack It", Stefan Hollos and Richard Hollos, Electronic Design, Aug 18, 2003 ) superimposed on the original. In this case the peak happens to fall near the center of the regular FFT bin, which results in an improvement of less than a nanoTesla. A better improvement can normally be expected.
Below is a plot of the filtered NMR data whose spectrum is shown above. The free induction decay is clearly visible here which represents approximately 2 seconds of data.
Would you like to hear the free induction decay? Click below to hear a wav file version of the plot above.
Hear the sound of the protons precessing in the Earth's field
For maximum signal strength the Magnum needs to be tuned to resonate at the frequency corresponding to the local magnetic field. To get an estimate of the strength of your local magnetic field you first of all need to know your latitude and longitude. The best way to do this is with a GPS. If you do not have a GPS then the following links may help.
Once you have your latitude and longitude, you can use a geomagnetic field model to calculate your approximate magnetic field. There are several places on the web that will do this calculation for you. NASA's National Space Science Data Center has an online form that you can fill in with your lat and long and it will calculate an approximate field.
An alternative to the above proceedure is to find a geomagnetic observatory near you and get some recent field measurement data from them. The U.S. Geological Survey operates several observatories. On the map showing the location of these observatories, click on the one nearest you and look at their recent measurement data. A map of observatories world wide is also available on the Intermagnet website.
With an estimate of your local magnetic field you can calculate the proton precession frequency using the calculator below. This is the frequency that the Magnum amplifier must be tuned to.
Converts from nanoTesla to frequency or vice versa. Click an arrow button to convert.
For signal processing software and application notes see our signal processing page.
Last modified Sept 26, 2005.
For more information contact:
Richard Hollos
richard[AT]exstrom DOT com
(303) 678-1487
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