Proton Precession Magnetometer (PPM)

This device is a highly accurate magnetometer which can be used to determine the exact absolute amount of the earth's magnetic field down to one Nanotesla. After a long development time and the building of several prototypes, this system reaches the specifications of commercial devices.

AM502FG PPM Steuergerät
AM PPM Sensor	AM502GL GPS Option

Features

This magnetometer is constructed without any electromechanical components and it can be used for the monitoring of the earth's magnetic field in a stationary and continuous way (for example to predict polar lights or to measure magnetic variances caused by solar flares) as well as for mobile usage (to search ferromagnetic material etc. in the ground). These are the basic features:

• Measurement range: 30000 to 60000nT
• Up to 1 measurement per second
• 0.1nT resolution, 50ppm absolute accuracy
• Completely transistorised, no relays, no moving parts
• Completely automatic tuning of the resonance-circuit and of the band-pass filter
• Optimized for 12V lead-acid batteries (11-15V, 2A max., 0.8A avg.)
• Short circuit and reverse-battery protection
• Non-volatile storage of 32768 measurement samples (including time/date and coordinates with GPS-option)
• Data logging output to a serial interface
• Buzzer for threshold alarms and for audible key-clicks
• Simple usage and display reading (differential bar graph)
• Designed for continuous 24x7 operation
• Sealed sensor, no need to open or fill it
• Robuste aluminium case (Made in Germany), case IP65, connectors IP40 or IP65 (optional)
• Dimensions of the control unit: 171x54x130mm (HxWxD)
• Dimensions of the sensor: Length: 315mm (without connector), diameter: 75mm, weight: 2.2kg

Motivation

The initial reason to develop this device was the wish to uncover some of the secrets of the earth's magnetic field. Especially because it is so impossible for the human senses to feel it. Sun storms, electrical currents, magnetic objects - Everything has an influence of the field that is always present around us. Even in former times seamen had sorrows because of stories about magnetic mountains that will pull out the nails out of their wooden ships. Enough reasons to approach this topic with a surprising measurement method that receives signals from the world inside of atoms.

The development from the first prototype (upper right) to the semi-professional unit was more the sportive enthusiasm, the wish to make things perfect, and of course the other amateurs who encouraged and supported me throughout the project. For these reasons there is now even a User Guide available. Well up to now in German only. There is a short Youtube Film too, it gives an impression about the basic usage of the instrument. The manual and the video refers not to the actual version. There is some English documentation too, please contact me if you are interested.

I wrote these pages here on the web to find people with similar interests. Maybe there are very fascinating usecases for such instruments, maybe there are new ideas for scientific projects or proposals for the improvement of the instrument?

Overview

The magnetometer is using the principle of nuclear magnetic resonance, similar to the medical nuclear magnetic resonance imaging systems but with much lower field strengths. With a generated magnetic field the spins of many nuclei will be aligned in the same direction. After switching of this "polarization field" the protons will precess like little gyros in the remaining earth's magnetic field. The precession frequency depends directly on the absolute earth magnetic field and can be picked up by an induction coil. The following figure shows the basic internal structure of the system:

The sensor (1) consists out of two coils wound in the opposite sense. This cancels out external magnetic disturbances of higher frequencies. A fluid like alcohol or parrafine is located in the core of the coils. Water could be used too but it is not an ideal fluid because it can freeze under the usual operation conditions of a magnetometer. The first purpose of the coils is the parallel alignment of the spin axis of the protons. To achieve this the coils will be switched to the power supply via the switches (3) and via the current limiter (4). A nearly lossless reverse-polarity protection (5) ensures that the unit cannot be easily damaged. The polarizing phase takes about 1 to 5 seconds, longer times are not of much use because at usual temperature not much more spins will be aligned after that. The following picture shows the sensor coils:

After the polarizing phase the current through the coils will be interrupted by (3). Due to the nature of an inductance a high voltage will be induced which is then limited by the voltage limiter (2) until the coil is electrically "discharged". Until the charge is completely zero, the decay will be damped by a resistor that is located in component (2) too.

After the polarization and discharge many spins are aligned, precessing in the earth's magnetic field. At typical field strengths the frequency lies in the range of 1.5 to 2.5kHz. The fascinating thing is that, although there are no metal things, just a non-magnetic fluid, a voltage will be induced in the coils (1). This is then connected by an analog switch (7) to to variable capacitor (8), which is in fact a capacitor bank, that can be configured by analog switches. This forms a resonance circuit, just like the receiver of a simple radio, which amplifies the signal approximately x10 which finally gives a few microvolts that are amplified by a low noise preamplifier (7).

After the pre-amplification (9) by 1000 or 2000 the noise is still dominant due to the very low signal amplitude. To reduce this, the signal is filtered by a programmable band-pass (10), with a band-width of less than 100Hz and an amplification factor of 20 to 50. The processed signal is then digitized by a comparator (11) and sent to an input of the CPU (12). Within a timeframe of about 500ms all zero-crossings will be detected. The timestamps will then be used to calculate the exact frequency. With an earth magnetic field of 47500nT, as it is the case in south Germany for example, the resulting frequency will be about 2022.35Hz

More details about the functional details of a proton magnetometer can be found on the Internet, for example in the related article on Wikipedia.

In general it is important to know that proton magnetometers can only be used outside of buildings, away from power lines, fences, cars etc. because they are disturbing the magnetic field, causing fast fluctuations and/or field gradients that will disturb the precession of the protons so that the induced signal will fade away too fast to be measured. In addition the sensor should be placed 1m or more above ground to reduce the disturbances caused by the small electric currents flowing in the soil below.

Measurement Results

The following graph (please click to enlarge) shows the variations of the earth's magnetic field at the 15th and 16th July 2012 close to Ludwigsburg in Germany (red) and in Braunschweig, Germany (blue). The Technische Universitaet Braunschweig operates a magnetic laboratory and observatory, providing their measurement data on their Internet pages which can be found here.

The width of the red trace is noise, caused by magnetic interferences from power lines and trains. The spikes are caused by vehicles, passing by at the sensor in around 10m. Here a magnified example for a vehicle that comes and parks in a distance of about 15m:

The noise floor in urban regions with houses is still around +-2 to 3nT:

In regions far from human homes even more reduced fluctuations can be observed. The distance to buildings is now about 3km, the way to the next street about 1.5km. The red data shows the measurements taken by the magnetometer described here while the blue data comes from the professional Overhauser-magnetometer mounted in the observatory station of the Technische Universitaet Braunschweig:

Here are the fluctuations of the device, recorded in a forest about half a kilometres far from buildings, with maximum measurement frequency (1Hz). It can be assumed that this is noise is to the most part the true noise level of the instrument. The effective value of the noise floor in this example is about 0.25nT RMS.

Finally the presentation of some measurement results will be closed by the "voice" of protons. It is fascinating each time to hear this strange echo coming out of a liquid that doesn't seem to have magnetic properties. Here is the signal in the WAV-format (165kB) which was acquired in an urban region with buildings around and power lines in the ground. The spectral analysis shows harmonics of 50 and 100Hz up in the range of kHz. They are so strong that the cancellation of precisely wound coils does not work completely. Therefore power lines are always a problem, especially when using proton magnetometers in mobile surveys.

A magnetometer software tool (PPMTOOL) has been developed to create so called contour-maps, visualizing colored maps of the magnetic field with the help of the recorded GPS coordinates. In the following example the disturbances due to the (mostly remanent) magnetism of a water pipeline can be seen. The data has been acquired by using this magnetometer. With KML it is possible to use such maps as Ground-Overlay in Google. These can be even viewed interactively.

This example shows that it is possible to achieve the resolution and accuracy of professional devices by amateurs in their circuit cellars.

Magnetometer Software Tool

Measurement data recorded during mobile usage can be downloaded to a Windows-PC. Data with GPS coordinates can be quickly displayed as interpolated colormap.

The tool is simple and the main purpose is to download data. The values can be saved in CSV-format for further analysis. This allows to use them in other programs (for example Surfer) too.

For the pure readout of actually measured values no special software is required. The values are send as ASCII strings via the RS232 interface. The tool is required just for the download of saved data records.

Early Versions

During the different phases of the development one of the early versions was the AM502C in a very small housing. Unfortunately the small case allowed connectors on the rear panel only so that the portable usage in a shoulder-bag was not really possible. The toggle-switch was also not protected against dust and humidity:

The version on the following picture (AM502FL) had solid industrial connectors on the front panel but the case was not classified for dust/humidity and it had a plastic frame that was not so robust compared to the full aluminium case of the latest variant. However, the toggle-switch was already replaced by a membrane type:

Further interesting Documents

Here are some links for documents that are worth to read.

• Applications Manual for Portable Magnetometers (S. Breiner). From my point of view this is some kind of a standard document for the mobile use of high-sensitive magnetometers for searching objects.
• Practical Guidelines for building a Magnetometer by Hobbyists (Part 1: Introduction to Magnetometer Technology) (W. Bayot). This PDF contains a lot of information for those who are interested in the detailed design and working principles of proton-magnetometers. It also contains a link to an Excel-spreadsheet for calculating the parameters of sensor coils for proton magnetometers.
  (archived copy of the pdf)
  (add on: KoehlerMag.pdf)
• Practical Guidelines for building a Magnetometer by Hobbyists (Part 2: Practical Building Guidelines) (W. Bayot). Everyone who thinks about building a proton magnetometer should have read this PDF.
  (archived copy of the pdf)
• Guide for magnetic Measurements and Observatory Practice (JERZY JANKOWSKI and CHRISTIAN SUCKSDORFF) Very interesting for those who are not so much interested on the mobile usage of magnetometers but more in the scientific observation of the earth's magnetic field.

For a general introduction it is recommended to read the related articles about nuclear magnetic resonance (NMR) and proton magnetometers on Wikipedia.

Who else builds Proton Magnetometers?

Here is an overview (not complete for sure) of some developers and producers of proton magnetometers.

Professional High-End Manufacturers

Semi-Professional Amateur Instruments

Educational Projects