A Geomagnetic Monitoring Station is being added to the Observatory in Spring 2025 for monitoring changes in the earth's magnetic field and to supplement and build upon the Observatory's existing Aurora monitoring that use a Black/White AllSky Camera and new NorthCam Colour Camera.

Earlier notes regarding selection of SAM III System are covered in: Review - Potential Geomagnetic Monitoring Station (2025-01-24)

The following notes describe the Parts Assembly & Preliminary Testing Phase

Other notes  :
| Review | SAM III | Site Prep | Assembly Design | Parts Assembly | Testing | Final Assembly | Installation | Commissioning | Remedial Work |

System Burn-In (2025-04-02 - 2024-04-05)

A 72 hour 'burn-in' has been commenced on the workbench (aka dining room table) with sensors hooked up to SAM III Unit and PC. This is too check that everything is working reliably and there are no failures of any of the components (better to find out something now than after the fixture is buried !).   The Burn-In period will allow Bx, By, Bz readings to be followed in an attempt to understand the cause and effect of any deviations seen in the readings

Sensor sensitivity - 2025-04-05

A 72 hour 'burn-in' has been successfully completed.  System working throughout period, though not connected to the PC for the entire period.

The magnetograms show that the sensors are very sensitive to temperature, which is of no surprise given the relativey large temperature coefficient of the sensors (between -100 & -150nT/°C), the fact that sensors are not insulated and are subject to various daily changes to temperatures and to central heating cycles.  

The sensors are also demonstrated to be very sensitive to any small or larger movement of the sensor.

Other effects such as changes to ferro-magnetic environment caused by a car on the driveway (10m) or a new skip / moving wheelbarrows on the estate road (20m) are detectable but cause much lower deviations to magnetometer readings than temperature/precise position & orientation do.

During the 72 hour burn in there was geomagnetic changes (noted by the presence of viewable aurora and Scandinavian magnetometers deviations), but these effects (the signal) is much smaller than the other changes (especially temperature)









Noise due to i) thermal effects,  ii) movements of the magnetometer sensors,  iii) changes in large ferro-magnetic objects in front of the house will be largely eliminated by enclosure in magnetometer fixture assembly and by burial in the rear garden to a depth of at least 80cm below ground surface.

Review (2025-04-06)

Whitham Reeve has looked at the Magnetometers (2025-04-06) and beside the recommendation to continue the tests for longer to fully appreciate the cause and effect from different factors, he considers that the magnetogram traces look normal for the test conditions described and that there is nothing fundamentally holding me from final assembly of sensor fixture and its burial.

Extended Monitoring (2025-04-07)
Monitoring was continued the following day



Temperature Relationships (2025-04-08)

A 14 hour test was run during during which magnetometer sensor positioning was stable (no knocks or changes etc) and temperature monitoring running. The test occured through a time interval in which the central heating (thermostat in an adjacent room) is automatically turned down to 16°C at 21:30 UT, turned up to 20°C at 07:00 UT, and turned slightly down to 19°C at 08:30 UT,  producing a nearly 4°C change in ambient room temperature. 

Data was compiled using AstroMag (a custom application that read the SAM III data file and the DS18B20 temperature sensor) and then plotted in Excel.

The following chart shows the changes in Bx, By, Bz and Bh (blue, red, green & magenta lines) from the point that the run commenced, Temperature (black line), and the modelled magnetometer change based on temperature coefficients of 100 nT/°C and 150 nT/°C (dashed line).  



The Central Heating Controller (thermostat located in an adjacent room) is set to reduce thermostat threshold turns down to 16°C at 21:30 UT, turns up to 20°C at 07:00 UT, and turns slightly down to 19°C at 08:30 UT.  This effectively means that radiator heating is off during the chart period 22:00 to 07:00 and then on again at 07:00 to around 09:00 or so.  During this time there is around a nearly 4°C change in ambient room temperature.  Minimum outdoor temperature during the night was 1°C ( It is possible that there is some central heating between 06:30 and 07:00 dependant on the temperature in the adjacent room (c. +/- 16°C). It is also possible that the workbench room begins to stabilise in temperature at this time due to daytime solar radiation from outdoors)

The setup used during the test is shown in the following photo. The 3 magnetometer sensors are all located together (within 30cm of each other) at a distance of around 2.2m from the room's radiator and around 1.4m from the room's window. 



The graph above shows a firm (inverse) relationship between Magnetometer Change and Temperature.

However it is unclear why

i) the Y sensor shows a significantly stronger relationship to temperature than that expected (based on the published -100 to -150 nT/°C coefficient range), and
ii) the X sensor shows a significantly weaker relationship to temperature than that expected.  

- Do the 3 sensors have a significantly greater range in temperature coefficient than the 'published' range
- Does the self-built circuitry associated with each sensor cause the wider variation ?

I'm a poor amateur at soldering (I'm too embarrassed to show the photos here!). Whilst the circuits all work, I've almost certainly used more solder than an expert would.  I purposely applied solder to both upper and low sides of the mini-boards (to minimise chance that any one solder join fails through the life-time of the Station).  The precise distances between the circuit and the sensor is slightly different between the 3 builds (e.g. 4.5 cm for X , 5.5cm for Y) and there are potentially other small points of difference on the 3 boards. Whilst the capacitors are all in the correct place in a circuit sense and the (longer) pin of the tantalum capacitor is in the correct "+" position on all the 3 boards, the height at which I have the tantalum capacitor mounted above the board varies (due to my poor skill level).  For the X Sensor Circuit it is mounted close to the board but in the Y sensor Circuit it is mounted around 6mm higher than for X.  Ideally I would have constructed all the circuits to be precisely identical but I was learning as I was going using a new (digital controlled) solder iron.    With tiny boards one really needs 4 hands (one to hold the soldering iron, one to hold the solder wire , one to hold board and  one to hold the electrical wire or component !) Q. Can any of this explain the difference in Temperature Coefficients ?

Earlier tests using each sensor with each circuit in turn and in each of the 3 orientations was inconclusive regarding the impact of the each circuit. This was due to the precise effect of sensor positioning and the large thermal effects). Those tests did show that each sensor needed a significantly different offset to bring its readings in line with those expected for my location.
 
In the final installation where thermal variations will be much smaller, differences in the temperature coefficient between the 3 sensors (including self-built circuit) will be much less important. However if there are strong differences in the sensitvity/responses of the 3 sensors to non-thermal magnetic changes that is of more concern.


The recorded magnetometer values during the same time period are shown in the chart below:



These compare with expected readings at the Observatory location of
X:   16467nT ,  Y:      -100 nT ,   Z:   47326 nT ,   H :   16468 nT

These include following offsets from raw (-82000 based values) :
X -833    Y +13584    Z+11815

Note : These offsets will be updated again once sensor fixture is installed & buried at final site.

Sensors Decisions (2025-04-09)

Based on the Sensor2's higher sensitivity to temperature changes (Y in above burn-in & monitoring tests) it has been decided to use it as the Z Axis Sensor.  This means it will lie deepest of the 3 sensors and can be expected to experience the least temperature fluctuations.

As a consequence Sensor3 (Z in above burn-in & monitoring test) will be used as the Y Axis Sensor

Sensor1 will continue as the Z Axis Sensor

The sensor cables have been re-coded with coloured electrical tape at both ends for identification

X Axis (Sensor1 / CableA)   - Blue
Y Axis (Sensor3 / CableC)  - Red
Z Axis (Sensor2 / CableB)  - Green

Began assembly but ran into problems with

i) a couple of wire to 3-pin terminal breaking (at circut board end and/or terminal end)  requiring resoldering,
ii) the 3 socket plastic terminal coming loose from SG-3+1 pins when trying to work the circuit board into place in the Side-Elbow Junction.
One of them was eventually reinserted  but in the other case
iii) the blue wire to the Y Axis Sensor broke again and the after reattaching to the terminal socket it was found that
iv) the socket could not be inserted far enough into the terminal so that
v) a reliable connection to the SG-3+ GND Pin couldn't be archieved

 May need to buy some new pipework for housing the Y axis Sensor (End Stop, Straight Junction and Connector & Side-Elbow Junction)

| Review | SAM III | Site Prep | Assembly Design | Parts Assembly | Testing | Final Assembly | Installation | Commissioning | Remedial Work |