General setup
The general measurement scheme is exactly as described in the earlier Fluxgate Magnetometer tests document. The FGM-3h magnetometer requires a stablized 5 volt supply, and gives a stream of TTL-compatible pulses, whose period is approximately proportional to the magnetic field. Corrections for slight non-linearity of this relationship is discussed in the earlier document. For zero field (i.e. with the magnetometer aligned exactly along the geomagnetic E-W axis) the pulse frequency is approximately 60 kHz. The responsivity was measured and plotted in the earlier test document, but is around 1 Hz change in frequency for a 1 nT change in magnetic field.
For the earlier measurements, the output of the magnetomete was fed to a frequency counter (an Optoelectronics model 3000) with an RS232 interface, so that results could be logged to a computer (a dedicated 286 laptop). The counter was set to a gate period of 10 seconds, so the frequency of around 60 kHz could be measured to about 0.1 Hz precision. For the measurements described here, a circuit using a Basic Stamp II was used. The schematic (which is more complicated than it needs to be) of the Basic Stamp II data acquisition system is available here. Some "tricks" were necessary to get the best out of the Stamp circuit - including making it monitor its own temperature, with a thermistor, and to apply corrections accordingly. One day, I'll write those details down in another web document. Throughout the measurements described here, the Stamp circuit and the original Optoelectronics counter were both collecting data simultaneously, logging data into independent laptop computers. Although data from the Stamp circuit were used for the analysis, both sets of data were regularly compared, and were found to be almost indistinguishable from each other.
The FGM-3h magnetometer used in these measurements is fairly well isolated. It is placed inside a vaccum (thermos) flask, which contains capsules of water to give thermal intertia. The screw-top of the thermos flask is replaced by a 2-inch styrofoam plug, of which about 1 inch pushes tightly into the flask. This gives better thermal isolation than the original screw top, and conveniently allows wires to the magnetometer, and to a temperature sensor probe, to enter the flask. The temperature probe is a thermistor, whose resistance is measured by the Stamp II microcontroller. The calibration from thermistor resistance to temperature is performed by the 286 computer. Long, thin wires were used to the temperature and magnetometer sensors, to reduce heat conduction to and from the outside world. This vacuum flask is inside a styrofoam picnic container, which is packed with freezer gel bags, to give more thermal inertia. This styrofoam container is placed inside a second styrofoam container, made of 2-inch thick styrofoam sheets. Inner and outer surfaces of the styrofoam boxes are coated with aluminum foil to reduce radiation transference of heat. The outer box is placed in a fairly secluded corner of my living room, against 2 outer stone walls of the house. All this thermal isolation quite effectively damps out the daily temperature variations that would otherwise badly affect the magnetometer readings. However, the inside temperature of the vacuum flask does change slowly, following the mean daily temperature, but with a time lag of a couple of days.
Magnetometer and temperature readings inside the flask are recorded
by the Stamp II microcontroller every 10 seconds. The 286 computer calibrates
and averages both magnetic and temperature data, recording the averages
approximately every 2 minutes. The measurements described here are from
the first 50 days of the year 2000.
Below I show plots of my measured magnetic field, averaged in 24-hour blocks,
for the first 50 days of the year 2000. I show matching plots of the
temperature measured at the magnetometer; the correlation between changes
of temperature and apparent changes in magnetic field is astoundingly
good. The apparent change in field is entirely an instrumental effect,
coming either from the magnetometer itself, or from the associated
electronics. Because the apparent field change and temperature track
each other so well, I believe that electronics outside my thermal
enclosure cannot be responsible. More work is needed to locate the cause
of this drift.
I have used the measured temperature changes to correct the measured
magnetic field, assuming an empirical factor of 46 nT/deg F. Figure 3
shows the corrected values. Figure 4 shows measurements over the
same period from the USGS Tucson Magnetic Observatory.
Although the steady drift in the USGS data (+0.5 nT/day) is smaller
than my own (-2.3 nT/day), the residual rms noise in these daily
averages, after subtracting the linear drift, is nearly identical in my
own measurements to those from the Tucson Magnetic Observatory - i.e.
14 vs. 15.3 nT.
I've compared long term, day-to-day drifts in measurements from my
Speake FGM-3h magnetometer with variations in temperature at the
magnetometer. There is extremely good correlation. A simple correction,
subtracting [(T-72)*46] nT from the data, makes a substantial improvement.
There is a slow residual drift of -2.3 nT/day superposed on the residual.
I don't know what causes the steady drift, but apart from that the
residuals probably result from temperature variations of the electronics
I use to collect the data.
Compared to the USGS Tucson Magnetic Observatory measurements, I'm not
doing too badly. Their measurements show a much smaller drift of only
+0.5 nT/day, but the rms noise of deviations from the steady drift
is about the same magnitude (14 vs. 15.3 nT) on the USGS and my own data.
Finally, note that these measurements only show long term, day-to-day
drifts and noise.
Other measurements show that the short term noise of my FGM-3 h measurements,
over a period of a few hours, is less than about 0.5 nT. Fortunately,
most magnetic events of interest last from minutes to an hour or so,
so the residual long term drift is of little consequence.
Measured Magnetometer drifts
Figure 1
The plot below shows the temperature measured at the magnetometer,
for the first 50 days of the year 2000. The temperature was measured
every 10 seconds, but has been averaged over 24-hour blocks,
0h UTC to 2400h UTC for each of the 50 days.
Figure 2
The plot below shows the magnetic field measured with the FGM-3h
magnetometer. The data have been averaged into 24-hour blocks.
Figure 3
The plot below shows the corrected magnetic field, after subtracting
a correction for temperature drifts. Changes in temperature were
scaled by 46 nT/deg F to derive the correction.
The straight line is a least squares fit to the data, and has a slope of
-2.3 nT/day. The rms deviation of the data from that straight line is 15.3 nT.
Figure 4
This plot (below) shows data over the same period
taken from the USGS Tucson Magnetic Observatory.
The data are available with 1-minute sampling, but have been averaged
into 24-hour blocks to compare with my own measurements.
The straight line is fitted to the data, and has a slope of
+0.5 nT/day. The rms deviation from that straight line is 14 nT.
Note that, from standard models, the expect drift in the "Y" component
at my location (Tucson, AZ) and at this time of year is only
about -0.02 nT/day.
Conclusions
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Fluxgate magnetometer tests: Calibration of the FGM-3h magnetometer
Darrel Emerson top page