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Water Turbidity Tube
Designed by Mark Little
 
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Water Turbidity Tube Design

The objective of the water turbidity tube is to allow the analysis of the characteristics of water at different levels of turbidity so that an automated turbidity meter can be designed, constructed and calibrated.

The higher the turbidity, the shorter the tube can be as the column of water that will obscure the bottom of the tube will be smaller. In this case, the accuracy of the reading will relate to the ability to read smaller and smaller graduations on the tube.

The lower the turbidity, the long the tube will need to be as column of water will need to be longer before the base of the tube is obscured.

ISO 7027 is a turbidity standard that originated in the brewing industry and is commonly used in Europe. The method has been accepted for regulatory reporting in water since the 1980s.

Since ISO 7027 defines the upper NTU measurement limit, without dilution of the sample, as 40 and the accepted NTU limit for potable water is five (5), this defines the range of lengths that may need to be measured. Using the formula that relates NTU to water depth, the turbidity tube must be at least 84 cm long to measure an NTU of 5.

The difference in depth between a turbidity reading of 39 and 40 is approximately 3.6 mm. Since the scale is logarithmic, the difference between 5 and 6 NTU is about 86.0 mm. As a result, the easiest way to scale the turbidity tube is to attach a tape measure to the side of the tube and the calculate the NTU from the depth of the water that obscures the target on the base of the tube.

Turbidity Tube Construction
The minimum length of the tube needs to be 84 cm, but a standard length of 20 mm OD (outside diameter) clear acrylic tube is 100 cm, so, for simplicity, this turbidity tube will also be 100 cm long, which gives it a minimum turbidity measurement capability down to 3.8.

For the tube to work correctly, the end must be sealed such that it can withstand the pressure of 100 cm of water without leaking or failing,
The ideal end cap is one where a target can be placed in the end cap without the target being shaded by the construction of the end cap. In this case, the round tubing insert meets those requirements.
To put a target in the end cap create a waterproof target the use of the turbidity tube will be looking for. Make it just a bit smaller than the internal space in the round tubing insert. Fill the space in the round tubing insert so that the target will sit at the top of the insert and fix it in place with waterproof adhesive. 
Once the target is firmly fixed in place and the adhesive has cured, put waterproof adhesive in the rings around the end cap and around the rime, then fully push the end cap into the end of the clear pipe.
Once the end cap is in place, clean off any excessive adhesive that may have spread out around the end cap and let the adhesive fully cure.
Although it was impossible to get the camera to focus correctly, the image to the left is an indication of what you can expect to see though the tube to the target.

The last task to to take a measuring tape or similar and glue it to the outside of the tube, making sure that the start of the scale likes up with the location of the target in the round tubing insert at the base of the tube. Once this is completed, the turbidity tube is ready for use.
This formula can be re-arranged to calculate the Turbidity from the depth in cm.

To use the tube, water is poured into the tube until the target at the bottom can be seen. A table relating depth and turbidity can be used to produce a look-up table.
These equations are not exact and have been extracted from a series of measurements on a turbidity meter, compared to physical observations using a turbidity tube.
The objective of building this turbidity tube is not to produce a sensor that can be used in a measurement campaign, rather, it has been build so that the design of an electronic turbidity meter has some sort of standard against which the reading from the meter during development can be compared.