Optical sensor for water quality
STS have worked in collaboration with many institutions over its 25+ years many of whom
have developed a technically great idea but cannot move it easily from the drawing board
or test lab to a marketable product.
One such development was the use of fluorescence to detect a chemical that could be used as
a surrogate for BOD (biological oxygen demand) a universally accepted indicator of organic pollution
in freshwater. In this instance the University of Birmingham (UK) had developed a technique to measure an amino acid called Tryptophan which had a very strong correlation with high BODs derived from fecal organic matter. The measurement system however relied on a laptop to process the data and a box containing a sensor and light source - not suitable for resale!
STS sought a grant to take this prototype unit and develop it to a robust portable field instrument that would stand up to the rigors of field work whilst providing accurate and rapid assessment of water quality samples.
The basis of the initial research work and early development were sound but STS investigated potential improvements to the system which would both enhance sensitivity and functionality. The original unit used a spectrometer to make the measurement which although very sensitive is also very expensive. The method developed by STS replaced this with a packaged Photomultiplier which was considerably less costly - although still the major expense in the build costs. To determine the best way to utilise the PMT and achieve best performance STS turned to one of its sub contractors who has years of experience in optical design and processing of the resulting signals.
The first major finding was that we could replace the halogen lamps used with new specialised UV LEDs - although expensive they offered much greater reliability, a longer life span and a much smaller package size. These were therefore incorporated into the design and the optical arrangement agreed. Due to the size of the LED it was possible to use the best possible arrangement of light source to detector for fluorescence work that is at 90 degrees to each other without the use of mirrors or lenses.
Of particular importance to the design was the production cost, as this device was designed to have only one specific
measurement then it had to be competitively priced against other products that offer a wider range of measurements. Simplicity was therefore key in ensuring that the project was delivered in a cost conscious manner. Conversely as the unit was so specific the measurement had to be accurate and having designed out costly mirrors and lenses meant that the UV led and Photomultiplier detector had to be high quality- and therefore more expensive.
The resultant design proved to have the sensitivity level required, was robust and durable for field operation and satisfied the financial constraints set. The technology has subsequently been used in production of water quality measurement equipment.