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Journal notes, fluid testing.

GELLER Labs "Backyard Science"

Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project

The goal of this project is a low cost high performance proton magnetometer (a digital magnetometer) kit for amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. There is a regular daily (diurnal) variation in the Earth's magnetic field. During events related to solar activity, there can be sudden changes in the field (such as a sudden impulse) as well as large excursions in the field which can be more than ten times the regular diurnal variation caused by magnetic storms.

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(Note: We are measuring the decay constant of the NMR proton precession signal. We have been referring to this value as the NMR Tau2 value. The NMR Tau2 value is traditionally measured by NMR spin echo methods. Until we learn more about these fundamental measurements, we will more correctly refer to the "decay constant of proton precession signal" (the free induction decay signal or FID).)

April, 2011 - Afternote: These measurements were made for a fixed polarization time. With our improved understanding of NMR Tau 2 and Tau 2 as a function of temperature, we are working on improved methods for fluid comparison. These results might have been compromised in part by changes in temperature during the tests. Also, some amplitude differences can be attributed to polarization time and Tau 1 (related to the polarization portion of the cycle) and Tau 2 which is more directly related to the decay of the precession signal.

Wednesday, November 25, 2009

Liquids or fluids to use in the sensor of a proton precession magnetometer

Now that the PPM test stand - proof of principle magnetometer is providing more stable and repeatable shot-to-shot data, fluid type testing was repeated.

In past years, I thought one of the most difficult hurdles to achieving a working proton precession magnetometer was the purity of the fluid, typically distilled water as an introductory fluid.

In fact, it is almost difficult to find a fluid that does not have a low field NMR response.

In other words, once the small signal AC issues, such as input referred noise, noise bandwidth, and common mode filtering (both at the input and at the end of a remote cable) are solved, just about anything works in the low field "single point" NMR machine. Even regular tap water works.

I am thinking that concerns that typically lead amateur scientists towards volatile PPM fluids, high currents, large wire gauges, and high fluid volume can all be met by good front end electronics design (e.g. low input referred noise and a few hundred Hz effective noise bandwidth) and attention to common mode and RF filtering. The present test stand powers a single coil (of a counter-wound pair) of #24 wire at 1.2 A for 2 seconds and uses a relatively simple FET-Relay hybrid switching scheme (discussed above).

In repeating the fluid type testing, I tried a variety of household fluids that can be easily obtained by the amateur scientist. I am not recommending some of the harsher fluids, such as bleach or acetone. However, with low volume (125 ml), low temperature (under 50F), and low current (1.2A), who could resist a little bit of the Thomas Edison approach, try everything in sight!

Now, turning to today's results of fluid type testing, the summary of the results are reported as amplitude (volts, but a relative scale from an hp 3581A wave analyzer "Y" output) plotted against precession envelope fall time (read using the measure functions of a LeCroy LT344L scope). Raw amplified PPM signals were on the order of 0.6 V RMS. The most desirable points are the ones farthest out from the origin, roughly along (on either side of) a 45 degree line from the origin. Also, low temperature performance is needed for cold winter climates. The RainX and Prestone low temperature deicers continue to look like the best choice. Here is a pdf of the fluid type testing. Here is the raw data and results in Excel.

Scope screen prints were taken for each fluid type (typically of the last data point in each series of 10 points, which were later averaged with standard deviation noted on the excel sheet): Acetone, Clorox Bleach, Distilled water (Walgreens), Distilled water (home distiller), Tap water (single filtered, whole home carbon filter), Tap water (double filtered, additional under sink filter), Simple Green home cleaner, RainX, RainX (old bottle been sitting around for more than a year in the garage)

 

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