Journal notes, Measuring the Polarization Field with an Axial Hall Probe
GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The FDM MAGNETOMETER1 project is a low cost high performance proton magnetometer (a digital magnetometer) kit under development for universities and amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. Magnetic storms can cause large excursions in the field and are of concern to interests ranging from electrical power grids, radio communications, and satellite operations, to aurora watchers and amateur radio operators.
1 Filter Diagonalization Method "FDM" (harmonic inversion), see Jan 21 and Jan 23 entries, based on: Vladimir A. Mandelshtam, Howard S. Taylor, Harmonic inversion of time signals and its applications, Journal of Chemical Physics (1997), Volume 107, Issue 17, 1997, Pages 6756-6769
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Project Documentation, Links and References (very early stages)
Journal Notes:
Monday, January 17, 2011
Running for a while today on the LabView Windows distribution executable PDF, TXT.
Tuesday, January 18, 2011
Overnight: PDF, TXT Early evening, continued quiet PDF.
Wednesday, January 19, 2011
Beta Tester Carl Olsen prepared a detailed coil winding assembly note and a sensor stand assembly note, both of which can now be found on our Docs page under Coils and Coil Stand. Again, many thanks!
Thursday, January 20, 2011
Carl Olsen's beta test FDM PPM in Colorado, US, is now operational, PDF, TXT Congratulations! Note that in this example PDF, apodization (the exponential window) is turned off by setting the parameter Exp FV to 1.000.
Apodization is not needed for FDM (except to apply a CW calibration signal, since FDM only solves for a decaying waveform.) I have been running for some time now with Exp FV (final value) set to 0.002. I believe the trade off to be an emphasis in early data with a somewhat lower average figure of merit (FOM) versus better rejection of CW EMI/RFI intereference. With the other improvements of recent months, I will try to run with the window off (Exp FV 1.000) for a while and see how the system performs. Our local FID is relatively noisy, PDF, however this level of noise is not a problem, FDM is a very robust frequency estimator. sample spectra PDF, Log spectra PDF
Long term readers will recall that I have been trying from time to time over many months to characterize the affect on instrument performance of having a vehicle normally within about 33 feet of the counter-wound coil pair sensor. Also, recall, that a +18 nT offset on our displayed total F scalar field reflects what the reading would be (presumably a more accurate value for our locale) if the vehicle were not nearly always present.
This afternoon when the vehicle went out (effectively at infinite distance) it ocurred to me, that this would be a good time to do another good quick check for any adverse affects caused by the vehicle being normally close-in. This pdf shows the figure of merit (FOM), magnetic field, and FDM amplitude before, during the absence of the vehicle, and after its return. PDF Here is the corresponding instrument display (magnified in time and field over our regular display) PDF, TXT. There does not appear to be any significant correlation between any change in FDM FOM and/or FDM amplitude and the vehicle presence or absence. This lack of change seems counter intuitive to me, since I would have thought the increased (spacial) gradient related to the 18 nT depression in field (with the vehicle present) would have a detrimental affect on precession signal amplitude and/or quality of the free induction signal (FID) as reflected by the FOM numbers (related to a spacial field gradient induced across the sample volume, albeit a relatively small 125 mL sample bottle).
Friday, January 21, 2011
Overnight: PDF, TXT, sample spectra PDF, Log spectra PDF
Saturday, January 22, 2011
Overnight: PDF, TXT, somewhat later overnight view (very quiet overnight) shows the beginning of the normal diurnal cycle.
Sunday, January 23, 2011
To enjoy the warm indoors (-20 degress F (-29 C) tonight, I understand for some readers that is still relatively warm!), I am winding another coil for DC field measurements. This coil bobbin design (which I later reject, See Jan 25), using PVC parts left on hand, is a little different, with two outside couplers on a 3.5" length of 2" PVC pipe to give a 2" winding length. JPG This time I used the pipe to draw circles with a pencil on the 3M electrical tape covers and cut them with a sharp X-acto knife. It was more time consuming than using the hole saw, but gives a nice tight fit (so I don't lose end windings out the side at the bottom).
I realized yesterday that I could carefully take apart the transverse probe holder for my surplus Teslameter and temporarily mount the relatively small hall element at 90 degrees at the end of a wood holder to make an axial probe. It takes an axial probe to measure the field at the center of the solenoid. JPG (If it looks odd, this Telsameter still needs a new red plastic display filter to complete the refurbishing project). These extremely accurate and highly stable Group3 Digital Teslameters, have been widely used in "big science projects" such as at the national laboratories and sold/sell new for about $7,000 with a probe (since about the mid 80's or early 90's, so some surplus instruments might be older units). They can often be had today, with a probe, at the usual surplus forums such as eBay, for under $200. However, it is unlikely that a broken unit will be user repairable, so it's a gamble at best. Also, don't bother buying one without a probe unless you already have one, since the temperature compensated probes sell new for about $2,500 each! For readers at the big labs or in industry, you can still buy these fine instruments new (no affiliation) from GMW Associates in the US.
The precession signal (tau 2) is noticably shorter and the peak amplitude is down at -10 F, however, the system continues to find the fundamental frequency with no trouble (still on the way to -20 overnight) PDF. Here is a sample measurement with Exp FV set to 1, showing the "raw" FID. PDF.
Project Documentation, Links and References (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller at gellerlabs dot com
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