coming soon: a cheap, single-board kit for measuring physiological processes in plants
A Pretty-Good Philosophical Instrument board (“PI” for short), with all standard components installed by the user, is shown at left. As you can see, PI are square. With suitable attachments, a PI can measure:
* light
* air temperature
* humidity
* carbon dioxide
* hydrogen, ethanol vapor, methane, or carbon monoxide
* respiration
* photosynthesis
* transpiration
* biological nitrogen-fixation rate
* respiration cost of nitrogen fixation
* root-nodule physiology
* leaf nitrogen content
* optical density (e.g. of bacterial cultures)
The name “Pretty Good PI” honors the memory of Alex Pretti and Renee Good and acknowledges the importance of the Raspberry Pi Pico microcontroller to its functionality. “Philosophical Instrument” was an old name for things like microscopes and barometers, when natural science was a subfield of philosophy.
Is “Pretty Good” Good Enough?
The PI substitutes Python code on the Pico for expensive circuitry and relies on a host computer (typically a Raspberry Pi) for keyboard, display, and some computation. So a PI may be less accurate or less portable than much-more-expensive devices that make some subset of the same measurements. You can increase absolute accuracy by calibration (e.g., using a CO2 standard), but a PI will be most useful for relative comparisons and educational applications. It should not be used for medical diagnosis or other applications where limited accuracy could lead to health risks or economic loss.
For research on biological nitrogen fixation (the initial motivation for designing the PI), a PI measures production of hydrogen gas, a byproduct of nitrogen fixation. That approach will not work with most cyanobacteria (including nitrogen-fixing symbionts in some lichens) or with some root-nodule rhizobia, because they recycle hydrogen. For example, the graph at left shows that a few field isolates of rhizobia that apparently fixed large amounts of nitrogen (as inferred by soybean seed yield) released little or no hydrogen.
The sensor that measures hydrogen, methane, ethanol, or carbon monoxide can not distinguish among them, so a PI will only be useful in cases where the gas mixture is dominated by only one of them. Sensitivity to these gases depends on composition of the background gas (e.g., humidity and oxygen), so electrolytic generation of hydrogen for calibration in each gas mix is an included feature.
To measure the rate of photosynthesis, respiration, transpiration, or nitrogen fixation, the focal plant needs to be in some kind of gas-exchange cuvette, such as the Pouch-As-Cuvette-Kit (PACK) for roots, which doesn’t need to be completely leak-tight. Measuring transmission of red light by leaves (a proxy for leaf nitrogen) or by liquids (e.g., to estimate cell numbers in bacterial cultures) requires the Assay-Leaves-And-Measure-Optical_Density-Electronically (ALAMODE) attachment. Measuring root-nodule oxygen permeability and nodule-interior respiration rate requires the Put-Around-Nodule (PAN) attachment and a source of low-pressure oxygen and either nitrogen or argon. These gases are also required to measure the respiration cost of nitrogen fixation.
The PI board is designed to support additional functions, using components added by users who are competent to design such additions and to assess any risks to the original functions. On the other hand, you can save even more money by leaving off components you don’t need.
Pretty Good Philosophical Instruments 