Hardware Reference
In-Depth Information
solution and allowed to digest at 150 °C for 2 h. Concentration (mg COD/l) is determined by
measuring the increase in absorbance at 606 nm, the absorbance peak for the chromic ion, or
High-range COD digestion vials from Hach Company were used to digest samples pro-
duced during unrelated research activities conducted to develop precision and bias statements
imeter and a commercial Hach DR890 portable colorimeter, the open-source colorimeter res-
ults (2 mg COD/l) were well within the stated precision of 17 mg COD/l of the Hach method
As usual, the OSH alternative is considerably less costly than closed commercial versions.
The open-source colorimeter can be built for one-tenth of the price (approximately US$50) of
the least expensive, commercial COD-only instrument available and two orders of magnitude
less than the Hach DR890 that it was compared to in this evaluation. Both the hardware design
and software are now freely available online and under the creative commons CC-BY-SA li-
cense (see
Chapter 2
for more details) such that they can be modified and new designs de-
rived. To put this work in further perspective, for about the cost of a standard commercial
COD instrument, a research lab can purchase an open-source 3-D printer (or the parts for sev-
here. Thus, perhaps most importantly, the ease and low cost of this approach for developing
sensor-based tools enables increasingly sophisticated tools to be used in low-funded develop-
ing world laboratories, helping to disseminate open-source appropriate technology for sus-
such as the colorimeter described here, provides public, nonprofit and nongovernmental insti-
tutions, schools and amateur scientists the tools necessary to conduct real science, while driv-
ing down the costs of research tools at our most prestigious corporate, government, and aca-
6.4.2 Open-source pH meter
The measure of pH is a measure of the hydrogen ion concentration—or a method to measure
the acidity or alkalinity of a solution. By definition, aqueous solutions at 25 °C with a pH less
than seven are acidic, while those with a pH greater than seven are basic or alkaline. A pH
level of 7.0 at 25 °C is defined as “neutral” because the concentration of H
3
O
+
equals the con-
centration of OH
−
in pure water (H
2
O). A
pH meter
is an electronic device used for measur-
ing the pH of a liquid. A typical pH meter consists of a measuring probe (a glass electrode,
which is a type of ion-selective electrode made of a doped glass membrane that is sensitive to
a specific ion) connected to an electronic meter that measures and displays the pH reading.
pH measurements are important in a long list of applications including environmental scien-
ce (e.g. environmental monitoring of soils, water of rivers, lakes, and rain), medicine, biology,
chemistry, agriculture, aquaculture, forestry, food science, oceanography, civil engineering
(e.g. monitoring of sewage treatment tanks), chemical engineering, nutrition, water treatment
and water purification, and many other applications. It is a useful and fundamental property
needed in dozens of ields.
The open-source pH meter discussed in this section was developed by Carlos Neves, a
pH meter using the Arduino microcontroller detailed in
Chapter 4
and is thus compatible with
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