Understanding Transparency Tube Measurements
What is a Transparency Tube?
A transparency tube is a relatively new piece of equipment that has become popular for volunteer stream monitoring programs and educational activities. They came into use in the mid-90s to apply the principles of measuring water clarity using a Secchi disk to shallow bodies of water where a Secchi disk is unusable. Secchi disks have been used since the 1800s. They are round disks, usually about 8-10 inches in diameter, with a high-contrast black and white pattern printed on them. Typically, that pattern is in wedges, as shown in the illustration. Secchi disks are tied to a rope and lowered into lakes and ponds until they are barely visible. The distance along the rope is then measured to the water's surface, and that length is recorded as the water's transparency measurement.
Applying a Secchi disk to a shallow stream, or even a deep stream, is often not realistic. Either the water is not deep enough for the disk to become not visible, or the current carries the disc downstream and prevents a reading. As shown in the illustration above, the transparency tube is a clear (typically plastic) tube that can be filled with water from a stream (or any body of water) to create a vertical column of water. The user then looks down through the water to identify the depth at which a smaller Secchi disk, printed inside the bottom of the tube, can be seen. Some transparency tubes have drain hoses at the bottom to lower the water column; you must carefully pour out the water in small amounts from other styles of tubes.
What Do The Results Mean?
A Secchi disk measures water clarity in terms of transparency (the ability for light to penetrate the water.) The transparency of water is vital for several reasons. Plants growing in the water need sunlight to complete photosynthesis. Photosynthesis is how plants make their own food. It also has the fortunate byproduct of producing oxygen released into the water, a key to increasing dissolved oxygen levels for aquatic animals to use for respiration. The water's transparency is also essential to aquatic animals for navigation, finding food, and avoiding predators. In addition, decreased water transparency is an indicator of other issues that may be occurring, such as high sediment pollution and increased algae growth.
A more scientific measure of water clarity is to measure turbidity using a turbidity meter or test kit. Turbidity is calculated by determining the height of a column of water needed to obscure a beam of light completely. Technological advancements have changed precisely how this measurement is accomplished and in what units turbidity measurements are recorded. For the most part, modern measurements are made using a meter referred to as a nephelometer, and results are recorded in units called Nephelometric Turbidity Units (NTU).
When discussing the effects of varying water clarity levels on the environment, wildlife, and even humans, you will usually see clarity referred to in turbidity units (such as NTU or somewhat equivalent FTU or JTU measurements). Transparency measurements recorded using a transparency tube can be converted to NTU values using the conversion table given here.
| cm | NTU | cm | NTU | cm | NTU |
|---|---|---|---|---|---|
| <6.4 | >240 | 21.7 to 24.1 | 35 | 44.6 to 47.0 | 13 |
| 6.4 to 7.0 | 240 | 24.2 to 26.7 | 30 | 47.1 to 49.5 | 12 |
| 7.1 to 8.2 | 185 | 28.8 to 29.2 | 27 | 49.6 to 52.1 | 11 |
| 8.3 to 9.5 | 150 | 29.3 to 31.8 | 24 | 52.2 to 54.6 | 10 |
| 9.6 to 10.8 | 120 | 31.9 to 34.3 | 21 | 54.7 to 57 | 9 |
| 10.9 to 12.0 | 100 | 34.4 to 36.8 | 19 | 57 to 60 | 8 |
| 12.1 to 14.0 | 90 | 36.9 to 39.4 | 17 | 60 to 70 | 7 |
| 14.1 to 16.5 | 65 | 39.5 to 41.9 | 15 | 70 to 85 | 6 |
| 16.6 to 19.1 | 40 | 42.0 to 44.5 | 14 | >85 | <5 |
| 19.2 to 21.6 | 40 |
A few key measurements to consider as benchmarks for water clarity include:
- >10 NTU = Fish and other aquatic wildlife demonstrate signs of stress.
- >5 NTU = not recommended for recreational use
- 1-5 NTU = EPA Drinking Water Standards (depending on the type of filtration system in place)
Tips for Using a Transparency Tube
- Follow the instructions accompanying your purchased transparency tube or the plans you used to make your own tube.
- If you wade into the stream to collect water, collect upstream from where you have walked to avoid collecting sediment disturbed by your feet.
- Don't let the tip of the transparency tube touch the stream bottom and disturb sediment while filling.
- Attempt to take your measurements in indirect sunlight to keep comparisons as consistent as possible.
- Record current weather conditions, including cloud cover and any significant weather from the days before your measurement. Also, record the time of day. This information will help you identify why you might see substantial differences between measurements.
