Summary

Build your own colorimeter! More details can (will eventually) be found athttp://www.appropedia.org/Open-source_colorimeter. Note that present firmware only reports absorbance and transmittance; a calibration protocol is forthcoming.

More and better to come...

The STLs will build a case for a colorimeter and the electronics BOM will yield a device designed for performing high range Chemical Oxygen Demand (COD) determination, measuring absorbance at 606 nm, an absorbance peak for the chromic ion. The cuvette holder is designed for Hach COD digestion vials.

The light source was specifically chosen for chromic ion determination; it can be altered to measure other ions (aquarist: think nitrates, ammonia and pH). We're working on other designs and useful applications and invite you to join us and share your designs.

Colorimeters are exceptionally handy items used for determining the concentration of dissolved species typically in aqueous solutions. The principles of operation are simple and form the backbone of several other analytic methods.
This is part of a larger project to reduce the cost of scientific
equipment using open-source hardware at Michigan Tech. Read a short
piece published by Science talking about it:
http://mtu.academia.edu/JoshuaPearce/Papers/1935580/Building_Research_Equipment_with_Free_Open-Source_Hardware. More open-source research tools:
http://www.thingiverse.com/jpearce/collections/open-source-scientific-tools.

Instructions

01.31.13 - If I'm understanding Git, I successfully uploaded the firmware. (I sure wish you linux folks would learn to speak colloquially). http://github.com/Chowderhead/colorimeter

BOM:
Arduino Uno
Adafruit LCD shield (www.adafruit.com/products/772)
LED having peak around 606nm (like: http://www.jameco.com/1/1/3120-lef3833-led-orange-620nm-t-1-3-4-1700mcd-1-7v-v-a12-irled.html)
A suitable resistor for the LED you choose
TSL230R light-to-frequency sensor
Proto board (like: http://www.radioshack.com/product/index.jsp?productId=2102845&znt_campaign=Category_CMS&znt_source=CAT&znt_medium=RSCOM&znt_content=CT2032230)
Coductors (Cat 5 cable works great)
Black ABS or PLA filament
12 M3 screws (just about any length; 10-12mm is good)
12 M3 nuts
20 M3 washers

Print the parts and clean them up so everything fits together nicely. Push M3 nuts into their appropriate slots at each corner of the case body - slots open to interior.
Cut the proto board down to size (about 27mm x 46mm) and drill holes to match those in the sides of the case.
Loosely attach the boards to the interior of the case with a couple screws each and push the cuvette holder into place (no cover) and mark the approximate locations where the sensor and LED must be placed on the boards to align with the windows in the cuvette holder.
Remove the boards from the case and solder the components to their respective boards at the points marked. Leave the LED leads a bit long so it can be moved to aim the beam through the hole.
Solder the conductors per the schematic. (The io pins can be soldered to directly on the LCD shield if you're careful, otherwise different means will be required, like not using the shield as a shield.)
Fit the boards back into the case, this time firmly.
Download and install the firmware on the Aurduino.
Fit the LCD shield and power the device (surplus wall wart of appropriate voltage or USB power will work).
Place the cuvette holder back into position (no cover) and use the menu system to select "Calibrate". The LED will illuminate for a few seconds - make sure that the majority of light passes as straight as possible through the cuvette holder windows and impinges upon the sensor. If the LED/sensor are high or low, reshape the cuvette windows with a small rat tail file or suitably sized drill bit.
After the LED is properly aimed, remove the cuvette holder and align and affix the cover to the case with four M3 screws and washers.
Push the cuvette holder through the opening in the cover and check that the lid fits nicely into recess.
Follow the appropriate protocol for calibration (yet to be built into the firmware - forthcoming).