A turbidity tube is a clear tube 50-60 centimeters long and 3-4 centimeters wide, sealed at the bottom with a clear disc, with a thick black cross on white paper beneath it, and you read the depth of water at which the cross just disappears. Over 30 centimeters is good, 10-30 marginal and under 10 bad, so the tube must be longer than 30 centimeters, which a single drink bottle is not.
Part of Community Water Testing
A turbidity tube is a simple depth-based clarity tester: a transparent tube filled with water until a black marking viewed from above is no longer visible, with the water depth at that point read from a scale on the side.
The Tube Must Be Long Enough To Show The Good Result
The carrier’s tube is 50-60 cm long and 3-4 cm in diameter, with a thick black cross or circle drawn on white paper under it. It suggests that a clear water bottle with the bottom cut off works. Two cautions follow from the geometry.
First, an open-bottomed tube leaks when lifted from its paper, and a bottle with its base cut off leaves the neck and cap in the way of the view. The bottom must be a clear, sealed window. Second, a single drink bottle is rarely much taller than 30 cm (measure yours). The “good” band begins above 30 cm, so a bottle can hardly show a good reading: it fills and the mark is still visible. Use a tube or two bottles joined, so the working height exceeds the band you are measuring.
What You Need
- A clear plastic or glass tube (a pipe, or two clear drink bottles with their ends cut off and joined with tape and sealant), 50-60 cm long, 3-4 cm diameter
- A clear bottom disc, glass or clear plastic, sealed on with waterproof glue or sealant
- A black cross painted or drawn thick on white paper, fixed under the disc or on the underside of the base
- A ruler or scale marked on the side every 5 cm
- A permanent marker or scratches for the scale
Method: Build, Mark, Read
Step 1: Cut the tube square and smooth the edges.
Step 2: Seal the clear bottom disc and test it full of water for a day. It must not leak.
Step 3: Draw the cross. Thick black lines on white; make one master drawing and copy it, so every tube’s mark is the same. A different mark gives different readings.
Step 4: Mark the scale on the side. Measure the depth from the inside of the bottom up. Test with a ruler.
Step 5: Read. Stand in shade. Look straight down through the water. Fill slowly until the cross just disappears. Then pour a little out until it reappears, and read the depth at that point. Repeat three times and average.
Step 6: Record the depth, the place, the date and the weather.
Step 7: Wash the tube between sources.
Worked Example: Volumes And A Reading
- Water in a full 3 cm tube of 60 cm: π × (0.015 m)² × 0.6 m = 0.000424 m³ = 424 ml.
- For a 4 cm tube: π × (0.02 m)² × 0.6 m = 0.000754 m³ = 754 ml.
Take a sample of spring water after rain. The cross vanishes at 28.5 cm; pouring out 1 cm brings it back at 27.5 cm. The reading is 28 cm, in the marginal 10-30 cm band: investigate. A second sample from a stream reads 6 cm: bad, do not distribute without treatment. A third fills the whole tube to 55 cm with the cross still visible: record “over 55 cm (tube full)”, which is good; you cannot say how much better than the 30 cm line.
Limits
The tube gives a relative measure, not a reading in standard turbidity units (NTU). Without a standard to calibrate against, a cm figure from your tube cannot be compared with a figure quoted from someone else’s tube. The bands here are the carrier’s screening bands. A reading over 30 cm does not mean the water is clear enough to chlorinate or to skip filtration (What Turbidity Means).
Common Mistakes
- A leaking bottom. Test before use.
- A tube shorter than the “good” band. It can never show good.
- Reading in sunlight. Glare hides the cross.
- Changing the cross design. The readings are then no longer comparable.
- Letting the sample stand. Particles settle; read at once.
Safety
Cut edges and sample water
Glass tubes break; use plastic where possible, and smooth every cut edge. Do not drink test samples. A sample from a suspect source can carry germs: wash hands after handling it, and do not wash the tube in a clean-water container.
What’s Next
- Learn next: What Turbidity Means — what a reading tells you and what it does not
- Combine with: Test Log — recording readings so trends show