Figure 1: Completed magnetometer circuit in use
Setting up
To set the instrument up it should be constructed as shown, and the sensor placed in the intended location.
So that it may be calibrated, the sensor should be placed inside a coil wound on a former that is large enough to hold the sensor, and at least three times as long. The entire assembly, including the temperature sensor should then be placed in a suitable container to provide thermal mass, and insulation, eg. dry sand contained in a polystyrene box. It should be stable within the box.
The calibration coil will have no effect unless voltage is applied to it.
The temperature sensor should be in intimate contact with the FGM-3, and also mechanically stable. If the sensor is located inside a coil former, the ends of the former should be 'stopped' with a polystyrene bung to maintain the internal temperature.
The sensor head assembly should be aligned East / West, and by making very small horizontal adjustments, it will be possible to obtain approximately 500 Hz output from the mixer. It should now be left to stabilise for several hours.
If desired, rather than moving the assembly to obtain the offset, it can be obtained by passing a small current through the coil to set up a small local offset, or by placing small magnets close to the assembly, but if this is done, they must be rigidly fixed in place once adjusted.
Calibration
The calibration process is used to determine the change in magnetic field required to cause the output frequency of the sensor to change by 1 Hz, and to then measure the change in output voltage that occurs as a result of 1 Hz change.
Sensor
The sensor can be calibrated to establish the change in frequency due to external magnetic fields, and you should also calibrate it to determine the change due to temperature.
Both figures are available for a family of sensors, and can be used, but for better accuracy, the change due to temperature can be determined by observing the change in output frequency against the change due to temperature.
Field / period calibration
It is possible to achieve a reasonably good relative calibration with simple equipment. The process is described in detail in the Application Notes for the FGM-3.
The current applied to the calibration coil should be varied, and the output of the LM2917 should be observed to obtain the nT/mV calibration for the instrument. Typically, this figure will be approximately 1.6nT/mV.
Temperature / period calibration
The relationship between temperature and output period is essentially linear. An accurate calibration may be achieved by monitoring the device over an extended period, and then using the copy facility to paste the temperature output, and the output from the f/V into a spreadsheet program such as Microsoft Excel.
By adding a ‘trend line’, Excel will display the formula in the form y = mx+c, where:
'c' is an arbitrary offset based on the starting temperature of the experiment, typically approximately 100
'm' will be the effect of temperature (x), and will be approximately 2.6
This formula may be used to calculate a compensated value for the variation in the local field.
F/V converter
Whilst it is possible to calibrate the frequency to voltage converter independently, the calibration should be derived from the process of calibrating the sensor described earlier.
Carrying out the experiment
The instrument should be allowed to stabilise in situ before being set up as it will take several hours for all the components to reach a stable operating temperature.
It should be observed during this period to ensure that there are no unexpected external influences affecting the sensor. The instrument is more than capable of detecting small vehicles, e.g. a Ford Ka at over 100 foot, so will be influenced by traffic.
The process of setting the system up need only be done once, the calculations required are stored with results, and may be recalled from the file as required, and can be refined if necessary.
Note, the instrument gathers data relative to the time that a recording run is started, so the current values for output and temperature are effectively zeroed at start time.
Sampling
The sampling interval should be selected to reflect the desired information, for example, if a warning of a solar event had been received, the interval might be set to a relatively short period, eg. 1 minute or less. For more general observation of the near field, the sampling rate can be set to a larger interval.
Absolute values
If the absolute value of the local field is known, it may be added using the calculated parameters.
There are a number of stations in the world that make the current value of the near field available including SAMNET in the UK. As well as providing an instant value, the SAMNET website provides a means of checking one's results against an established monitoring station.