It's been quite a while since our last blog update - I shamefully admit to have neglected my duties as project correspondent somewhat this last week. Everyone (else) has day jobs, which means that most of us are working in our separate dens most of the time, and only really come together once a week, so I haven't been keeping up to date with the rest of the team quite as much as I should have. What's worse, though, is having brought my vidcam to the second meeting in a row, only to get so wrapped up the discussions and following work that I entirely forgot to do the Noisebridge tour and team intro I was planning. I'll remedy in part tomorrow and take you all on a quick tour of my favourite (and admittedly, only) hackerspace. Right now, though, I want to introduce you to our two new team members, Rolf and Otute, tell you how we're coming along with the project, and show you some pictures of what the team's been up to.
Otute is also a chemistry-dude-by-day, and has taken on the non-trivial task of building a DIY pH probe. A pH sensor is composed of two electrodes, a measuring electrode, which are normally encased in a special type of glass that is permeable to hydrogen (H+) ions. Both electrodes are made of silver chloride AgCl and bathed in a potassium chloride solution. The measuring electrode builds up potential directly related to the concentation of H+, but the reference electrode is in a separate chamber, and thus not in electrical contact with the solution being measured, so it provides a baseline measurement for calibration. The selectively permeable glass is expensive and difficult to come by, though, so instead Otute is going to try to make a selectively permeable membrane by dissolving polyvinylchloride (PVC, a plastic) in tetrahydroflouride (THF, a solvent) and mixing it with tribenzylamine to form a matrix. Still waiting for reagents, though, so it'll be a little while yet before we see the results of his efforts. The recipe is taken from a recipe in M.J. Goldcamp et al. (2010) "Inexpensive and Disposable pH Electrodes". Journal of Chemical Education, Vol. 87, no. 11, pp. 1262-64, and the major challenge here is presumably going to be getting the membrane solution to set right in the tube.
Meanwhile, Rolf's been hacking away at an amplifier circuit for the pH sensor based on the pHduino project - I've no idea what Rolf does by day, but by night, he's obviously busy solving any and all issues with regards to circuitry and electronics components on this project. There will be much refining of the whole package after the challenge, but we're going with the viable functionality model for now, so unfortunately Rolf will not be getting his much-desired custom-made PCBs just yet - we're going with strip boards for now - although I highly doubt I'll be able to stop him from designing them. As a small compensation, he's been mandated by the group to get a regular pH probe to play with while he fine-tunes the amp circuit.
The strip board Sean and Rolf are building their circuits on
As I believe I've already mentioned, Rolf has also taken over the sensor part of the NIR probe from me (much to my relief), which is modeled on a commercial Optek probe and composed of a near-infrared LED emitting at 850nm and a phototransistor with equivalent reception. Both look exactly like regular LEDs to the untrained eye (i.e. mine), and will eventually be silicone-mounted in a pair of acrylic discs fused onto the end of a pair of acrylic tubes. The major challenge with this set-up is going to be firmly mounting these directly opposed to each other across an open gap, while water-proofing the leads + wires coming off both. All other components will be on the board end for now, so it'll be wires only, but the fact that the leads are at 180° could make it tricky nonetheless. I'll be buying/sourcing acrylic tubes of appropriate diameter and a tube of aquarium silicone (we're assuming it's food as well as fish-safe) tomorrow, so pictures of this soon to come.
The basic NIR sensor circuit
A rough sketch of the NIR probe design
On the oxygen front, Sean is still missing the catalyst for his dissolved oxygen sensor, but the circuitry is (almost) done - he was still waving a smoking soldering iron like mad hacker when I left Noisebridge at close to midnight, so I feel pretty sure we'll be hearing more from him soon. At this stage, the whole thing seems to be coming along pretty much according to plan - except for the darned catalyst - so we're not expecting any major difficulties. There's always the potential outcome that we fail miserably and it doesn't work at all, but Sean's a pretty sound dude, and I'm confident that he's got this sussed out.
Sean's dO probe circuitry
The schematics for the dO sensor circuit
Last you heard from us was an update from Charlie, who's been working on testing several different thermal sensor designs. His latest iteration of the thermistor (I believe this is a 10kΩ model?) has produced beautifully linear results within the desired range (15-40°C) and an estimated resolution of about 0.1°C, with an as-of-yet unknown absolute accuracy, likely within the ±0.2-0.5°C range. This is really encouraging (and frankly better than I had dared hope for), and since water-proofing thermistor leads + wires is really simple (don't worry, we'll make sure to show you how), we don't foresee much of a challenge with this part of the project - and that's critical, as temperature is a key parameter both in the biological processes we're hoping to measure, as well as in pH and dO calculations. Rolf also got the DTS (digital thermal sensor) to work, so we'll have two different thermometers at once - also useful, as time response may vary, and could potentially be important, at least in the dO measurements.
The Dallas 1-Wire DTS (black semi-circle in the lower left-hand corner of the board) and the Arduino read-out
On the software end, Marc now has both the Arduino + ethernet shield assembly successfully transmitting data to a Rails server, which type- and timestamps all data values, logs them to a database, and plots them on a basic graph. The next feature add-ins are going to be a Comet server which will give us real-time graphics, a pared-down version of the Arduino (pure board, no shield, just a USB cable to any old box), and a de-centralized version of the data logging app so we don't have to host a server for this project indefintely, but can instead enable people to do it themselves. We're all pretty excited about this end of the project, as the data visualization really is very central to the educational perspective. Seeing is believing!
Alright, folks, that's all for now - stay tuned for the hectic rush to get all the sensors finished before next week's meeting, and don't forget: be excellent to each other, dudes!