Proposal for $30 Turbidimeter

James Berg CEE 454 18 December 2004 $30 Turbidity Sensor

Purpose: Determine if a turbidity sensor that has a minimum resolution of at least 10 NTU can be made for around $30. This is the first step in developing a turbidity sensor that would be affordable to small rural villages and is accurate enough to be useful in judging the appropriate amount of chlorine to be added to water systems, about 1 NTU.

Method: It turned out to be relatively easy to make the turbidity sensor, all that was needed was a light tight box, a laser, and a light sensor. The basic design was construct a light box large enough to hold a 2 liter, the sample container, a pen laser and a 90° sensor. I made the light box out of black foam core which was effective but there turned out to have a lot of light leakage around the joints, which decreased the sensors ability to detect scattered light. I was able to increase the sensitivity of the system by adding a light well on the opposite side of the bottle from the laser. The light sensor also presented a bit of a problem, simply because I thought that photodiodes varied resistance with light, while in fact a photodiode is an extremely sensitive photo voltaic cell that generates its own voltage.

Analysis: As the table shows the whole system was put Item Cost together for less then $30, though this number lacks one of the most important components, the voltmeter. I expect that a Foam Core $2.00 voltmeter, either digital or analogue will cost at least $10, putting the cost right at $30. The results were very Photodiode $4.00 encouraging, the sensor very clearly picked up more reflected light from dirty samples then from clean ones. There are some 2ft Wire $1.80 anomalies, such as the lower voltage readings for the 3 and 44 NTU samples then the 0 and 40 NTU samples respectively. I Laser $12.00 don’t know exactly how to explain these other then to say that the measurements were not taken under the most ideal Total $19.80 circumstances. People were walking around the lab which caused spikes in the sensor reading and while I did my best to keep samples well mixed, the dirt did begin to settle out rather quickly. Also there are only five data points, many more experiments need to be done under more controlled circumstances and with a better light box.

Improvements: The most important thing to do in the future is to preferably find and if necessary make a voltmeter that is sensitive enough to give the unite a 1 NTU resolution. I fear that this may prove to be the most difficult part of the whole project. The sensor can also be easily improved by making the box out of something more light tight and more durable, probably plywood. An improvement could also be made in the sample container, a 2 Liter bottle was chosen because it would add to the sensitivity but a 2 Liter bottle takes a long time to fill through such a little hole. It may in fact be the best available solution but it is worth looking into other options.