The last two weeks have been a complete blur of sanding, gluing & soldering, with simultaneous assembly of five next generation of flow meters, and six new dry cave drip sensors. With the leftover beta unit I used for the long bookshelf power drain test, and the new 5 Bar pressure sensing unit, we will have 13 deploy-able Cave Pearls with us this trip.
Laying all this out, I realized that while our clothing for would easily fit into a single carry on, I could not remember the last time my wife and I traveled without two 49.99 pound suitcases of equipment. I make detailed component identification sheets for the TSA inspectors, who always return the favor by putting their own little pieces of paper in our luggage. So far it has worked out OK.
Flow meter updates:
Since the last deployment, I have located better sensors for the flow meters and tweaked quite a number of things in the physical build. The new accelerometer is the BMA180, the only 14bit 1g accelerometer I could find on the market. The venerable DS18B20 temperature sensor has been replaced by the Sparkfun TMP102, in my quest for completely interchangeable I2C sensors. Several new epoxies will also be tested in this build, including a very expensive Arctic Alumina thermally conductive epoxy to see if it will improve the temperature sensor response.
I have built two special “Rosetta Stone” units for this deployment. One has the Ms5803-05, TMP102 & Ds18B20 temperature sensors, while the other has three accelerometers in it. Data from these units will give me a head-to-head comparison of the sensor performance, and allow me to unite the data sets that we are generating with each successive build. The multi-accelerometer unit will sport the BMA180, BMA250 and the ADXL345. I think the 180 will come out on top, but there is always the chance that the increased sensitivity of that acclerometer will contribute more noise than accuracy to the overall performance.
The anchoring system now uses modular 50 cm long connecting rods, so we can hang the pendulums at different heights in the cave passage simply by adding or removing sections. (and they are much easier to transport in the luggage) Replacing the stainless steel rim bolts with nylon removed about 80 grams of ballast mass, so I have embedded a ring of copper inside the upper clam-shell. Hopefully this will improve the accuracy of the data from the compass sensor.
Drip Sensor Progress:
Developed from a minimalist three component design, these are the first data loggers I have constructed where the mcu board possesses an always-on voltage regulator. Using only 3 AA batteries to power systems already sandbagged by the relatively high quiescent current of the MIC5205, really forced me to look for other ways to conserve power. Thanks to at tip from one 0f the advanced users at the Arduino Playground I discovered that the heartbeat LED pips are still quite visible when I use a whopping 10k limit resistor. This brings the 20mA LED currents well below 1mA, and if I sleep the processor while the LED is on, we save another 5mA for the pip duration. This little tweak will become part of my standard build from now on. Wherever possible I have replaced delay statements with brief MCU sleeps, and I am only reading Vcc once before the SD card writing process, since that is the only time the information becomes important. I originally conceived these units around the Sparkfun Pro Mini, but early bench tests are indicating that the Rocket Scream Ultra could be the board of choice for future Cave Pearl data loggers.