IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 56, NO. 3, JUNE 2007 907 A New Design of Low-Cost Four-Beam Turbidimeter by Using Optical Fibers Antonio García, Miguel A. Pérez, Gustavo Jacinto Grillo Ortega, and Javier Tejerina Dizy Abstract—Turbidity is one of most important water-quality parameters; turbidity measurement and removal is as important in water for human use as for industrial-process water. This paper presents a new low cost, small size, and good accuracy turbidime- ter based upon the use of optical fibers. The design takes advantage of using optical fibers for light transmission, and consequently, waterproof protection becomes unnecessary, remote measurement is feasible, and high signal-to-noise ratio is obtained, decreasing overall uncertainty. Index Terms—Fiber optics, industrial sensors, IR sensors, supervision, turbidity, water quality. I. I NTRODUCTION N OWADAYS, environmental-pollution measurement and removal, particularly on water and the atmosphere, is one of most important research fields due to the fact that the reduction is essential for any kind of life survival [4]. Water-contamination determination is extremely important, and several subjects must be taken into account [8]: 1) biological characteristics, referring to live-organism pres- ence like viruses or bacteria; 2) chemical characteristics, such as minerals in suspension; 3) physical characteristics, like color, smell, or turbidity. Turbidity is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample. It is caused by suspended impurities that interfere with the clarity of water [1]. Turbidity can produce health concerns because impurities provide protection for bacteria by reducing their exposure to disinfectants. Turbidity can also introduce problems for me- chanical systems in water, causing progressive damage and, perhaps, final destruction. Several methods and systems are available to calculate tur- bidity on a water sample, but the most precise is the neph- elometer or turbidimeter that uses an incident-light source on the sample and catches both the light transmitted and light scat- tered. The ratio between these measurements provides values that allow us to calculate turbidity in nephelometer turbidity Manuscript received June 15, 2005; revised September 7, 2006. A. García, M. A. Pérez, and G. J. Grillo Ortega are with the Department of Electrical, Electronic, Computer, and System Engineering, Oviedo University, 33204 Gijón, Spain (e-mail: antonio@ate.uniovi.es; Miguel@ate.uniovi.es; ggrillo@ate.uniove.es). J. T. Dizy is with the Desarrollo y Mantenimiento de Electrónica S.L., Fernández Balsera, 46 Avilés, Spain (e-mail: jdizy@deyman.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2007.894222 units (NTUs). Turbidimeter is essentially constituted by one or more light emitters, a sample cell, and one or more light receivers. Most used light emitters are tungsten filament lamps and infrared LED [3]. A photo detector will be chosen accord- ing to the kind of light source: For tungsten-filament lamps, PhotoMultiplier Tubes (PMTs) are used, or photodiodes are used when the emitter is an LED. This paper presents a new system that can be used in online measurements; this design takes advantage of using optical fibers as a light-transmission path; so, several benefits are obtained. 1) Waterproof protection becomes unnecessary because no electrical parts are under water. A consequence is that we get a small and low-cost sensor. 2) The use of fibers allows us long-distance measurement without interference or attenuation risks. Therefore, this kind of system will be extremely interesting for remote measurement in a heavy Electro-Magnetic Interference (EMI) environment. 3) The overall design is valid for any configuration topology and for any light source and kind of detector. In the next sections, an optical-fiber turbidimeter design is presented, including block diagrams, design criteria, and experimental results. II. DESIGN CONSIDERATIONS OF TURBIDIMETER There are several turbidimeter topologies, but only three are approved by the Environmental Protection Agency (EPA) [1] and the International Organization for Standardization [2]: 1) simple-beam design; 2) ratio-algorithm design; 3) four-modulated-beam design. The simple-beam design is the most simple and less precise design. It does not have a good response with colored samples, and it is mainly used for low turbidity ranges. Results are more precise in the ratio-algorithm design, owing to the use of more detectors at different angles, which allows us partial cancellation of errors due to wavelength absorption in colored samples. The-four-modulated beams (see Fig. 1) design is the most precise of all the designs. It uses an algorithm similar to ratio design, where turbidity is calculated from two independent measurements, and all errors, due to water, color, and attenu- ation, are cancelled, providing precise measurement in a wide turbidity range. The four-beam operation is quite simple: Each emitter is ON in alternate periods of 0.5 s (see phases 1 and 2 in Fig. 1). 0018-9456/$25.00 © 2007 IEEE