There are many ways to build a turbidity tube. On this page, we will look at the key components of a tube and then discuss possible substitutions for these different components.
This section is very broad to allow flexibility in design and material use.
A detailed set of instructions for a specific turbidity tube design can be found here: The Turbidity Tube_ Simple and Accurate Measurement of Turbidity.
How a Turbidity Tube Works
The turbidity tube uses the correlation between visibility and turbidity to approximate a turbidity level. A marker is placed at the bottom of the turbidity tube until it can no longer be seen from above due to the “cloudiness” of the water. This height from which the marker can no longer be seen correlates to a known turbidity value. Although this correlation is less accurate than what would be obtained from other methods, it is almost certainly accurate enough for most applications in the developing world. Generally, the cost savings of using a turbidity tube outweigh this loss of accuracy.
A turbidity tube is made up of four key components (see pic):
- A Clear Tube
- A Tube Cap
- A Viewing Disc
- A Measuring Device
(1) Clear Tube
The clear tube will hold the water sample being tested. The tube must be clear to allow for maximum light reflectance off of the marker being viewed.
Even a light colored or white plastic tube will not let in enough light for the tube to work properly.
A clear plastic tube will provide the most durability and reduce the chances of damage during transport, but a glass tube can be used if handled with caution.
- Possible Clear Tube Materials: Fluorescent light sleeve, graduated cylinder, etc.
(2) Tube Cap
The tube cap prevents the water sample from leaving the clear tube.
A seal to the end of the tube can be used, but a removable tube cap is preferred for cleaning of the tube and view disk. Make sure that whatever cap is used prevents leakage (a good seal is more important than removability).
The size of your cap will depend on the size of your tube.
- Possible Tube Caps: Rubber stopper, PVC pipe cap, Gatorade lid with rubber washer, chair leg end cap, etc.
(3) Viewing Disc
The viewing disk will be submerged in the water sample. A clear pattern must be visible on the disk as well.
Generally, it is best to use a white background that is colored with a black checker pattern like the one shown in Figure 2 (this is the pattern typically found on a Secchi disk 4 as well).
The contrast makes the viewing disk very clear, which improves the accuracy of the reading. A white plastic disk patterned with black permanent marker works extremely well.
The disk should be sized to fit inside the plastic tube. If necessary, the disk can be made of a porous material such as wood or cardboard, but it must be sealed by lamination or with varnish.
- Possible Viewing Discs: Yogurt container lid cut into a circle, white poker chip, etc.
- Possible Marking Device: Black permanent marker, black paint, etc.
(4) Measuring Device
The level of the water at the point of non-visibility needs to be measured.
This can be done in two ways.
- The water level can be directly measured from the viewing disc to the top of the water, and a chart can be used to find the turbidity level that corresponds to the measurement.
- A better way is to mark the turbidity tube with the corresponding turbidity levels before testing begins so that no conversion is necessary.
Your choice will depend on the availability of materials and the construction of your tube (for example, if the removal and reinsertion of your tube cap changes the height of your viewing disk, the marking will no longer be correct.)
- Possible Measuring Device: Ruler, tape measure, etc.
As stated earlier, these instructions are very broad to encourage adaptations in the design. A set of specific instructions can be found on page 9 (Procedure for Turbidity Tube Construction).
There are many ways to build a turbidity tube.
A detailed set of instructions for a specific turbidity tube design can be found in pdf The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field.
Turbidity Conversion Chart
The Turbidity Conversion Chart converts the measured value in cm to Nephelometric Turbidity Units (NTUs).
This is the official unit of measurement to quantify how much light is scattered due to the suspended sediments.
You can convert your turbidity tube reading from a height to NTUs using the chart below.
|Distance from bottom of tube (cm)||NTU's|
|< 6.25||> 240|
|6.25 to 7||240|
|7 to 8||185|
|8 to 9.5||150|
|9.5 to 10.5||120|
|10.5 to 12||100|
|12 to 13.75||90|
|13.75 to 16.25||65|
|16.25 to 18.75||50|
|18.75 to 21.25||40|
|21.25 to 23.75||35|
|23.75 to 26.25||30|
|26.25 to 28.75||27|
|28.75 to 31.25||24|
|31.25 to 33.75||21|
|33.75 to 36.25||19|
|36.25 to 38.75||17|
|38.75 to 41.25||15|
|41.25 to 43.75||14|
|43.75 to 46.25||13|
|46.25 to 48.75||12|
|48.75 to 51.25||11|
|51.25 to 53.75||10|
|53.75 to 57.5||9|
|57.5 to 60||8|
|Over the top||6|
Design from Elizabeth Myre & Ryan Shaw
The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field
Michigan Technological University