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==Dissolved oxygen (DO) probes== *[http://www.vernier.com/probes/do-bta.html DO-BTA Dissolved Oxygen Sensor] $209 - cheapest commercial product I could find. In this case, it seems relatively safe to assume that bulding one ourselves would be cheaper. ===Potential DIY designs and progress=== *'''Mod an automotive O2 sensor to make it a membrane electrode''' - New sensors for out of date cars are available on eBay for $10. Although these sensors typically operate at ~300C (won't work for us), they do have the required platinum, anodes, and teflon membrane. I'm thinking I can knock out the zirconium matrix and add a KCl electrolyte and see if we can get a reaction started at room temp (fingers crossed). **Progress thus far: Ordered 3 $6-$10 probes on ebay to futz with *'''Build an intensity or time based optode from scratch''' - Recently, people have been using a [http://www.sigmaaldrich.com/catalog/ProductDetail.do?lang=en&N4=85793|FLUKA&N5=SEARCH_CONCAT_PNO|BRAND_KEY&F=SPEC ruthenium complex] as a visual (fluorescent) indicator of oxygen concentration. This complex is excited by a blue LED, then its transmission is measured by a filtered photoresistor (more details [http://www.env.gov.nl.ca/env/waterres/rti/rtwq/07_14.pdf here in pdf]). There could be some serious tecnical hurdles to overcome on this one, but if it works, this would be a way better sensor in the long run - no calibration needed, all solid state (super low maintenance). The Ru molecule is expensive (~$70/mg), but could probably be used for quite a few electrodes. * A better optode might be based on Erythrosine B (FD&C Red No. 3) (details here: http://nathan.instras.com/documentDB/paper-429.pdf). A fairly complete list of dyes that would work is in this paper http://www.jbc.org/content/262/12/5476.full.pdf but erythrosine is probably the easiest/cheapest/most readily available of these. It even has better sensitivity (phosphorescence lifetime change with change in oxygen) and produces a stronger signal. The drawbacks are that it is somewhat non-linear and it does photodegrade (slowly). Erythrosine dispersed in any kind of oxygen permeable and optically clear medium would work, including in silicone or aerogel/xerogel. Clear silicone is probably easiest. Silicone caulk from the hardware store, preferably aquarium grade, may work (ie: would not react with the dye during curing). If not, then platinum cured silicone (LSR) such as Smooth On [http://www.smooth-on.com/Silicone-Rubber-an/c2_1115/index.html] will almost certainly work. A green LED (530nm) is ideal for inducing phosphorescence. The silicone/erythrosine sensor can be completely sealed in black silicone to prevent interference from ambient light, and allowing an unfiltered light sensor to be used. Exact probe dimensions are not critical: thin probes (1-2mm) would respond faster to changes in oxygen (< a minute), thicker probes (5-10mm, loaded with enough dye that they are almost opaque) would have a stronger signal (phosphorescence up to 0.5-1% of the brightness of the illuminating LED). ([[User:AI|AI]]) * Optode signal processing might have the highest sensitivity if it is based on phase detection: a sine-wave input signal at 10-100kHz (with dc offset) is fed to the illuminating LED, then the output from the photodiode is filtered to remove dc offset and high-frequency noise, and both the input and output are fed through zero-crossing detection (only from above to below zero). The duration of pulses during which the input signal is below zero but the processed output signal is above zero is a function of the phase angle, and directly related to the phosphorescence decay time. Lowpass filtering of those pulses will produce a DC voltage which directly corresponds to oxygen levels. This measurement should be very sensitive, and immune to most types of noise/interference. This does sound complicated, but it may be easier to get good results with this method than with gating+integration, because it relies a lot less on specific part tolerances. ([[User:AI|AI]])
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