Water turbidity sensing using a smartphone† I. Hussain, a K. Ahamad b and P. Nath * a This paper demonstrates a rapid, cost-effective and field-portable smartphone based turbidimeter that measures turbidity of water samples collected from different natural water resources and in drinking water. The working of the designed sensor is based on a Mie-scattering principle where suspended micro (m-) particles in water medium scatter a strong light signal along the normal direction of the incoming light signal, which can be detected by an infra-red (IR) proximity sensor embedded in the smartphone. Two freely available android applications were used to measure the irradiance of the scattered flux and analyse the turbidity of the medium. With the designed sensor, water turbidity variation as low as 0.1 NTU can be measured accurately in the turbidity value ranging from 0 to 400 NTU. The sensor responses for these ranges of turbid media are found to be linear. A high repeatability in the sensor characteristics is also been observed. The optics design involved for the development of the proposed smartphone turbidimeter is simple and is robust in operation. The designed sensing technique could emerge as a truly portable, user-friendly and inexpensive turbidity sensing tool that would be useful for different in-field applications. Introduction The presence of organic and inorganic m-particles in water bodies affects the lives of aquatic living organisms. 1 Also, the presence of such m-particles in drinking water above the threshold level may cause serious health hazards to our society. 2 Thus, accurate monitoring of such m-particles in water medium bears a great relevance as far as water quality monitoring of natural water resources and drinking water is concerned. Turbidity is a measure of water clarity which means how much the suspended m-particle in water medium affect the passage of light through it. Highly turbid water causes strong scattering of the incident light signal which in turn, may lead to reduction in photosynthesis process of aquatic ora. This may affect the production of dissolved oxygen in aquatic medium. 3 Further, a highly turbid water or water having high levels of suspended solids absorbs more sunlight and may cause rise in temperature of the water bodies. This may subsequently affects the aquatic lives of some living organisms. 4 Highly turbid water medium is a favorable condition for growth of waterborne pathogens. 5 All suspended inorganic, organic and m-organisms in water causes light scattering which can be estimated by commercially avail- able turbidimeter such as 2100Q portable turbidimeter from Hach Inc. 6 Two of the common approaches for monitoring of water turbidity are Jackson candle method and Secchi disk. 7 Both of these techniques depend on the observers' perception thus, turbidity readings may vary from observer to observer. In recent years several groups have demonstrated the usability of optical bers for monitoring of turbidity in different liquid medium. 8,9 Very recently, single photon counting based tech- nique 10 has been proposed to monitor turbidity of liquid medium. Using this technique water turbidity as low as 0.1 NTU could be measured accurately. Another optical approach that measures turbidity in liquid medium is based on light scat- tering principle. 11 Nephelometric 90  light scattering measure- ment is considered to be the standard method for turbidity measurement in nephelometric turbidity unit (NTU). 12 This approach is based on intensity comparison of the scattered light beam from a given water sample under investigation to the laboratory prepared standard reference turbid medium such as formazin standard suspension. 13 The technique is primarily based on Mie-scattering principle where quasi-collimated light beam from an IR source is allowed to incident on a turbid medium and the scattered ux from this medium is monitored at right angle to the direction of the incident beam. For overall estimation of suspended particle concentration where size of the particles may vary from less than 2 mm to 2000 mm, Mie- scattering principle is valid. Hence, for monitoring of overall quality of water, nephelometric technique is being widely used. With the improved hardware and soware, smart built-in sensors and freely downloadable applications that enable easy data analysis and interpretation make smartphones a primary choice for many researchers across the globe to develop it as alternative sensing tools for different physical, chemical and biological sensing applications. 14–19 High megapixel imaging sensor of the smartphone nds its applications in imaging and a Applied Photonics and Nanophotnics Laboratory, Department of Physics, Tezpur University, Napaam 784028, India. E-mail: pnath@tezu.ernet.in b Department of Civil Engineering, Tezpur University, Napaam 784028, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02483a Cite this: RSC Adv. , 2016, 6, 22374 Received 27th January 2016 Accepted 13th February 2016 DOI: 10.1039/c6ra02483a www.rsc.org/advances 22374 | RSC Adv. , 2016, 6, 22374–22382 This journal is © The Royal Society of Chemistry 2016 RSC Advances PAPER