1. A Low-cost, User-friendly, Smartphone-based
Measurement Technique for Dissolved Nutrients
J. Feng, B. Khakipoor, K. E. Siman, A. W. Smith, H. T. King
The University of Akron
Akron, OH, U.S.A.
April 2018

2. Outline Need for Accessible Water-quality Measurement Techniques
Design of an Accessible Water-quality Measurement System
Prototype of a Low-cost and User-friendly Spectrometer
Standard Operation Procedure and Calibration Results
Directions for Future Modification and Development

3. California Drought Global Fresh-water Shortage
Source: UNESCO World Water Assessment Programme
California Drought
Source: USGS "Earth's water distribution"
Fresh water is limited: 2.5% of water on earth is fresh water, 1.2 % of which is surface water
Demand for water is rapidly growing
Water shortage problem manifests in various ways all around the world

4. The Need for Water-quality Data
USD $ 300k +
Source: Woods Hole Oceanographic Institution
Regional water issue: Lake Erie HABs
Lack of water quality data (e.g., in the great lake area)
Current technology are expensive and inaccessible to ordinary citizens
Public concern towards environment provides opportunity for innovations
USD $ 3,000 +

5. Accessible by Design: the Open-source Way
SpecPhone
e.g., d = 1.0 μm, m = 1
color λ θ
Blue
445 nm
26.4o
Green
532 nm
32.1o
Red
650 nm
40.5o
Grating Equation:
d∙sinθm = m∙λ
(m = 0, ±1, ±2 …)

6. A Low-cost Spectrometer
small strip of diffraction grating
light-blocking cover
battery pack
solder-less connection
3D-printed sample holder
mirror
current-limiting resistor installed inside battery pack
bolt and spacer
Laser-cut Acrylic enclosure

7. A Low-cost Spectrometer
Designed to be easy-to-make and easy-to-use
Custom parts are made with 3D printer and laser cutter

8. Standard Operation Procedure
Add 2mL of nitrate sample to test tube
Add 4 drops of reagent_1
Shake rigorously for 60 s
Add 4 drops of reagent_2
Shake rigorously for 60 s
spectra
wait 10 m
put cuvette into spectrometer
water
sample

9. Commercially-available Test Kits
Nitrates
Phosphates
Nitrites
Ammonia pH
High-range pH

10. Calibration for Dissolved Nitrate
Color difference between solutions with difference nitrate concentrations is detectable by eye
Mixing of red and green is perceived as yellow / orange
Color change from light yellow to dark red corresponds to decrease of green
It is observed that concentration of nitrate is correlated with absorption of green light
0.0 ppm
1.0 ppm
2.0 ppm
5.0 ppm
10.0 ppm
water
1.0 ppm
2.0 ppm
5.0 ppm
10.0 ppm

11. Misalignment of Diffraction Grating
Data Extraction from Smartphone Photos
sample
reference
reference spectrum
sample spectrum
diffraction grating
slit
spectrum
Misalignment of Diffraction Grating
Edge of spectrum (along width) is defined by slit
Color bands in spectrum always align with grating
Spectrum images may be rotated to correct for grating misalignment

12. Quantification of Spectra in RGB Channels
Pixel Intensity Averaged across Width of each Spectrum
Pixel Value
Pixel Number
sample spectrum
reference spectrum
Random noise can be filtered out by averaging
Signal processing in each RGB channel should be performed separately

13. Correction for Exposure and White Balance
1.0 ppm
2.0 ppm
5.0 ppm
10.0 ppm

15. Correlation between Absorbance and Concentration
535 ± 5 nm
A: measured absorbance
L: path length
c: analyte concentration
−log10(I / I0) = A = ε∙L∙c
Beer-Lambert Law:
Sample spectrum are rescaled the same way that allows reference spectrum to be aligned
Light intensity are calculated as the integrals under the normalized intensity curves
A narrow band of 10 nm (in wavelength) is used to account for smearing of spectrum
Beer-Lambert law is observed from test data
log10IG = a∙c + b
a = −ε∙L
b = log10IG,0

16. Correlation between Absorbance and Concentration
535 ± 5 nm

17. Correlation between Absorbance and Concentration
500 nm – 570 nm
Bands of 10 nm (in wavelength) from 500 nm to 570 nm is checked for log-linear relationship:
505 ± 5 nm = 500 nm – 510 nm
515 ± 5 nm = 510 nm – 520 nm
525 ± 5 nm = 520 nm – 530 nm
535 ± 5 nm = 530 nm – 540 nm
545 ± 5 nm = 540 nm – 550 nm
555 ± 5 nm = 550 nm – 560 nm
565 ± 5 nm = 560 nm – 570 nm

18. Phosphate Tests
All three channels change intensity

19. Phosphate Tests R-channel Result

20. Future Modifications Mobile App & Website
Light Source: replace LED light plate with stronger LED to improve signal-to-noise ratio
Mobile App & Website
Cover: a snap-on cover will be implemented for pedagogical purpose
Recyclability: replace Acrylic with plywood as housing material to allow for recycling
Modularity: modular design to allow for easy repair/replace and upgrade
Extended Functionalities: measuring heavy metals and chlorophyll; soil measurement

21. Grating, light source and battery
Error Estimation
Grating, light source and battery
Mixing and operation
User and phone
manufacturing