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Introduction
Magnetic susceptibility corresponds to the ability of a material to be magnetized in an external field. Bulk (volume) susceptibility χvol can be directly related to the relative permeability (μr ) of a material: χvol = μr - 1 and where μr is the ratio of permeability of material/permeability of vacuum.
Figure 1. Magnetic susceptibility of common rocks (from Bartington User Manual)
Instrument settings
Instrument units
The instrument is pre-set to display the susceptibility value directly in either SI or CGS units.
Numerical conversion from SI to CGS units (Table1) is accomplished by dividing the SI value by 4π, i.e. χCG = χSI/4π. The MS2 meter performs this function internally but by using the constant 0.4π to keep the numbers in a similar range of magnitude.
Table 1. Mass and volume magnetic susceptibility in SI and CGS units (from Bartington User Manual)
Instrument range selection
The instrument is pre-set to display the susceptibility value in either the 1.0 or 0.1 range. Table 2 shows the exponent value of the least significant digit (i.e. the last digit on the LCD) for each range. The units in Table 2 is for volume susceptibility.
Table 2. Exponent values for least significant digit in 1.0 and 0.1 ranges (from Bartington User Manual)
On the 0.1 range a decimal point will be displayed, effectively moving all the digits on the LCD to the left when compared with the 1.0 range. It should also be noted that one measurement in the 0.1 range will be an average of multiple readings taken over a period of 10 seconds.
MS2 Meter
The MS2 meter has front panel controls (Figure 2) and a rear panel (Figure 3).
Front panel (Figure 2):
- ‘Measure’ push button, labelled ‘M’: permits sample readings to be taken.
- Toggle switch: performs the same function as the push button but permits continuous measurements.
- ‘Zero’ push button, labelled ‘Z’: permits ‘air’ readings to be taken. By performing a measurement to ‘air’ this control re-sets the instrument and brings subsequent measurements within the range of the display.
- On/Off Switch: controls the internal battery supply and also permits the selection of either SI or CGS units.
- Range multiplier switch: allows selection of either x1 or x0.1 sensitivity range. In the second case the result is shown to the first place of decimal and a 10-fold increase in measurement time provides additional noise filtering. The switch also activates the battery indicator.
Figure 2. MS2 meter, front panel.
Rear panel (Figure 3):
- Power supply connector: the power adaptor plug should be inserted here.
- RS-232 serial interface: an MS2 RS-232 cable can be inserted here and connected to a computer to allow control and communication using Bartsoft software.
- Three-way rotary Switch: allows the user to select one of three RS-232 communication settings options.
- Charging light: this LED will light up when power is connected and the MS2 battery is being charged.
Figure 3. MS2 meter, rear panel.
Taking measurements
Connecting the sensor
Connect the sensor to be used to the front panel socket of the MS2 meter with the supplied 50Ω TNC-TNC cable, and switch on by selecting either SI or CGS units (Figure 4).
Figure 4. Connecting a sensor to the MS2 meter
Calibration Check
A calibration check core is provided. The serial number of the MS2C sensor appears on the calibration core supplied. Our calibration check core is S/N 675, with K = 1451 x 10-5 SI at 22degC (Figure 5). The value for the core, when used with the specified diameter sensor,
is printed around the middle of the core.
The stability of the sensor over time has been shown to exceed that of any core material which
might be used routinely to check the calibration. Therefore, the core should be used only to
identify when some catastrophic calibration error has occurred. If the sensor is within its factory
set calibration then the value obtained should be within 5% of the value printed on the core.
Figure 5. Calibration check core available on the JR and how to set the calibration core in the sensor loop
Measuring a sample
Set the meter to the 1.0 range for the initial measurement of any sample, to establish the
approximate result, before switching to the more sensitive 0.1 range. If the value of a sample is
greater than 1000 then the most significant digit will not be seen if measured on the 0.1 range,
leading to an apparent gross error in the result.
Irrespective of the type of sensor being used, measurements are accomplished using the
following procedure.
1. Take an ‘air’ reading by pressing the ‘Z’ push button with the sample to be measured away
from the influence of the sensor. The display will appear blank and a colon will appear to
show the instrument is ‘busy’. The completion of a Z cycle will be announced by a bleep and
the display will show all zeros.
2. Place the sample within the influence of the sensor and press the ‘M’ button. The ‘busy’ colon
will appear and the display will show the previous reading until updated at the completion of
the current measurement period. This is announced with a beep. At this time serial data will
be transmitted.
3. If either push button is enabled at the completion of a cycle then a repeat measurement will
be taken.
4. To measure weakly magnetic material, select the more sensitive x0.1 range and compensate
for any thermally induced drift by making a series of measurements:
a) Zero to air by pressing ‘Z’ button = R0 ( = 0)
b) Measure samples = R1 , R2 ....
c) Measure ‘air’ = RFinal
The mean of an ‘air’ measurement before and after the sample is subtracted from the nth
sample measurement Rn:
Corrected value Rk = Rn - n x (RFinal- R0)/N
where N is the total number of measurements after the initial zero (including RFinal)
In the case that only one sample measurement is taken: Rk = R1 - (RFinal- 0)/2
When taking manual measurements, attempt to keep the time between measurements as
consistent as possible. If recording values by hand then the previous value can be read whilst
the current measurement is taking place, thus saving time and improving precision.
The drift correction can be done automatically using Multisus or Bartsoft where the time of
each measurement is recorded, and the is drift applied linearly as a function of time.
Correcting the magnetic susceptibility value
estimating true values of susceptibility (cvol ) for narrow strata