Bartington - Model Grad-01-1000L - Vertical Component Magnetic Gradiometer
The Gradiometer contains two directionally sensitive fluxgate elements spaced 1m apart on a very stable beam together with the necessary electronics to provide an analog output representing the magnetic gradient along the main axis. It is housed in a rugged, lightweight 38mm diameter protective tube which is fully sealed for operation under wet conditions.
The unit operates from a 12V unregulated power supply, has a bandwidth of d.c. to 10Hz and a range which can be switched between ±100nT/m (±1mGauss/m) and ±1000nT/m (±10mGauss/m). The analog output is ±4V full scale. Beyond four volts, a logarithmic compensation extends the range to 3 and 30ìT/m. The interface allows directional and offset errors of the sensing elements to be nulled electronically. The influence of the background field can thereby be eliminated and only anomalies in the field will be recorded. This compensation also applies where the gradiometer is rigidly mounted near a magnetic structure. The enclosure is clearly labelled with a direction arrow to facilitate orientation during set-up, see figure 3.
- Sensor element spacing : 1m
- Gradient range : +IOO11T/111 or ±1000nT/m full scale (3|.iT or 30jiT compressed)
- Output : +4V ftill scale, output impedance lkQ (±5V compressed)
- Accuracy : +1%
- Maximum ambient field : ±100|iT
- Noise : lOOpTpk-pk max.
- Differential Drift : <0.02nT/°C (warm up time 2-3 minutes)
- Bandwidth * : d.c. to 10Hz min. with -12dB/octave roll off
- Power supply : 12V nominal (9.5 -18.5V) unregulated, polarity protected
- Power supply current : 58mA
- Pull-up current (pins 5-11) : 0.5mA when held low
- Connector : 12 pole Tajimi R04-R12M
- Environmental : IP65
- Operating temperature : -20°C to +70°C
- Size : 38mm diameter x 1052mm in lenath
- Weight : 0.82kg (1.81b)
The unit requires a power supply of 12V unregulated. The input is protected against reversed polarity. The analog output of ±4V full scale is referenced to power ground within the gradiometer. A separate signal ground connection permits the use of a differential input data logger for good noise immunity. The normal scale factor is ±100nT full scale but a high scale of ±1000nT can be selected by pulling input /HR to ground at any time. The analog output is active irrespective of the status of the other digital control lines. Enquiries are welcomed for suitable data loggers and power supplies.
The gradiometer output represents the difference between the outputs of the two sensors. The resolution of any fluxgate gradiometer is limited by small errors in offset, gain and angular alignment between these sensors. These errors appear in response to changes in the sensor orientation. The Grad-01-1000L has a digital interface to allow the user to minimise these errors. The errors are classified as follows:
Offset Error - O
This is the departure from zero output regardless of the orientation of the gradiometer.
Vertical Error – V
This error alternates in magnitude when the long axis of the gradiometer is alternately inverted and non-inverted. This error increases in significance as the inclination of the terrestrial field increases, that is, towards the poles.
North/South Error – N
This error is due to misalignment of the sensors in the direction of the arrow and therefore is discovered by pointing the arrow alternately north and south.
East/West Error – E
This error is due to misalignment of the sensors at right angles to the direction of the arrow and is discovered by pointing the arrow east and west.
Compensation for the above errors is set using six CMOS/TTL inputs. The most recent settings are stored internally even with the power disconnected. They may be revised at any time whilst in use. The digital lines are active in the low state and are fitted with internal pull-up resistors. All lines are heavily protected against electrical damage and false operation. The lines are inhibited for a time of 2 seconds following power up. The lines operate as follows:
Device Select /DS
This line must be held low to select the gradiometer which is to be adjusted and held low during adjustment. Settings are stored when this line returns high but only if the /INC line is stable and high. A delay of 20mS must be allowed for the /DS line to stabilise after each level change.
Parameter Address Lines A, B, C
Three lines are used to address the relevant control within the gradiometer, as shown below.
This line determines the polarity of the desired correction (increase or decrease).
Increment - /INC
Each time this line goes low the selected compensation setting is incremented one step to remove the error under investigation. A delay of 1mS must be allowed for the /INC line to stabilise after each level change.