Requirements for Obtaining High Accuracy with Proton Magnetometers Part 1

Definition of Absolute Accuracy

Absolute accuracy of a measurement is the difference between measured and true values. Obviously, nobody knows the true value, so we end up defining the limits we know the true value must be within.

Presently the limits of accuracy of measurement of the magnetic field of the Earth can be pushed to better than 1ppm. In a field of, say, 50,000nT, this is better than 0.05nT.

However, there are numerous difficulties and conditions that must be fulfilled to obtain that kind of absolute accuracy. Parameters that are involved include:

• Gyromagnetic constant’s accuracy
• Time reference stability and accuracy
• Method of measurement of the acoustic precession frequency in continuous and pulsed systems
• Signal-to-noise ratio of the precession signal
• Sensor cleanliness
• Presence of the AC magnetic fields

In the Earth’s magnetic field (0.2-0.6G) protons produce precession frequency between 850Hz and 2.5 kHz. Spectral line widths of liquid sensors are anywhere from few nT to tens of nT. Spectral line widths (or decay time in pulsed systems) depend on temperature, gradient over the sensor and of course the tуре of liquid used. All liquids have a chemical shift due to local magnetic fields produced by the atoms of the particular molecule. Span of all chemical shifts is 10ppm. Gyromagnetic constant is usually given for water at 25°C. Water has about 3.5 pрm chemical shift related to tetramethyl silane the accepted reference for high resolution NMR.

Methanol, that we use in our sensors is about l ppm away from water. Kerosene, widely used liquid in proton precession magnetometers is а mixture of chemicals and its chemical shift is not known. Kerosene spectral line is very wide; about 15 nТ (decay time is about 0.5 sec dependent on temperature).

Stay tuned for part 2 of this article, “Gyromagnetic Constant”