As I mentioned previously, the
SAM sensors were not suitably insulated/isolated from temperature variations.
2-3 degree C variations occurred overnight resulting in ~100 to 130 nT
fluctuations per degree C change in temperature. A more robust means has
been devised. The sensors will be placed into the 27lt cooler then filled
with dry sand (approx 40kg of sand) to act as a thermal mass. This 27lt
cooler will then be placed into a 62lt marine cooler with the void between the 2
filled with ~25lt of polystyrene beads (bean bag filler). The 62lt marine
cooler has very thick polyurethane filled walls and a properly sealed top.
The aim is simply to keep the temperature of the sensors within 0.1-0.3 degrees
over a 24-48 hr period based on external temperature variations of no more than
10 degrees C.
Figure 1: The SAM III electronics
The temperature sensor supplied
with the SAM kit was not reliable so I replaced it with a kit version using an
LM335 sensor. Velleman kits K8067 and K8055 provided the hardware and
software required for the job. The sensor array is wired for the 3 FGM-3
sensors and the K8067 thermometer sensor. The K8067 is rather picky about
it's input voltage so I chose to use a DC-DC converter on the 12v Powerpack to
produce exactly 12v (many power adapters output a great deal more than 12v.
Of the 4 I had, 3 output 14.8v + and only 1 was anywhere near 12v). To
reduce dependency on power packs I chose to power the K8067 from the SAM III 12v
out. All components were then sealed with Duct tape and Silicone (to
prevent any ingress of sand) before being tapper to the inside of the cooler to
ensure that the sensors orientation did not shift when being filled with sand.
Figure 2: Isolation and Insulation of the Sensors
The final step is to setup the software to output and plot the
data. I modified the code that came with the K8055 USB interface to
provide a variable output real time graph and log of the temperature data.
Calibrating the temperature sensor was an issue as the K8067 is very sensitive
to input voltage but given I am only interested in relative temperature, as long
as the temperature remained in the K8067's output band that's all that matters.
The SAM III output is picked up by an AutoFTP agent (FTP Watchdog in this case)
and uploaded periodically (every 5 minutes) to my web page. From there it
is picked up by the world network site and plotted alongside the other
monitoring sites.
Figure 3: Where does all that data go?
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