Proton mag question

[signman]

I've designed a proton precession magnetometer. I took it out for the first test this past Saturday. Using a .8 second polarization time, which gives me roughly a 1 second reading interval, the unit achieved readings all within 1.3nT of each other (from minimum to maximum reading). When I increased it to 1 second polarization time, it achieved readings that varied within .5nT of each other. Is this considered a success? or should I be getting something more repeatable? I'm trying to understand if this is acceptable for a proton mag or should I try to improve it?
Thanks,
Signman

[Krzysztof]

Hi Signman.
Congratulations, you have chosen an interesting hobby.
You have pretty good results.
You can always go further.
I'm interested in, as shown in the presence of your Mag noise and EMI.
As a further indication of change in one place.
Give the results of the next several measurements.
Show off your design.
Best regards Chris.

[Willy Bayot]

Hi,

It is indeed a good result for a first try.
Depending on where you make your tests even at a fixed location, you should expect some diurnal field variations of a few nT over periods of minutes.
Now, you should indeed check that your absolute B field values are indeed the real measurement of the local precession frequency and not any type of auto-oscillation which would also give you very good and very stable results.
You should try to put a ferrous object close to the sensor and see if the field drastically changes.

Anyway, it would be interesting to know more about your design.


Willy

[signman]

no oscillations

Thanks for the reply guys. Yes it is magging. After trying a couple of sensors I wound on my lathe along with different fluids, I had a friend with me begin walking near the sensor. He was wearing a handgun on his side. At about 15feet it began to lower the magnetic field value. Also changing the tuning (filters and resonance) affected the operation of the unit. I just didn't know if I should expect better consistancy of readings or if that was considered good. My design uses an FET based front end pre amp, digital bandpass filters, and dsp for frequency measurements.
Thanks,
Signman

[Willy Bayot]

congratulations.

What fluid are you using?
What are the specs of your coils?


The variations of your results over time without any explicit disturbances come from a mix of three potential effects:
1. The natural diurnal field variations
2. Human-generated electromagnetic activities in the surroundings generating varying underground currents
3. The internal lack of precision in your signal processing due to noise and spikes.

To make the difference between the three, you could feed your sensor with an AF generator through a secondary coil and make measurements without any polarization but with a variable level of injected noise. This should give you an evaluation of the variations due to your amplifying, filtering and signal processing in front of various noise levels.
If you find that your system gives you very constant frequencies in these conditions (i.e. less than 0.1nT), you can then be sure that the variations you have observed in practice in the field were natural or artificial variations.

Another method that I have used before is to use two synchronized PPM in parallel with their sensors put at a few meters of distance at a fixed location and without any movement around them. The variations of the difference between their corresponding readings is the evaluation of the precision of your systems. If that precision would be absolute, the two curves should be exactly parallel (constant value of the differences) in spite of their variations due to the natural or artificial field variations. The variations of the differences are only due to the system itself.

In your particular case, if you observed variations of only 0.5nT, you should congratulate yourself, it is a nice achievement.
Can I know how long you have worked on your project before getting this type of results?

Willy

[signman]

Mag

Hi,
I wound a coil inline with 2 counter wound coils of 2.5" each with 2 inches seperating them. The coil comes out to 45mH or so. I was trying charcoal starter when I got the .8 second polarize time to work. 1 second greatly increased the repeatability. I was surprised at the improvement for an extra .2 seconds. I've been working on this off and on for about 3 years. I worked on it in spurts.
Thanks,
Signman