FISH: Understanding Your Transparency Tube Measurements

Part of the "First Investigation of Stream Health" activity. Transparency tubes are popular for use in stream monitoring programs.
FISH: Understanding Your Transparency Tube Measurements - Articles


Tansparency Tube

Transparency tubes can be made or purchased.

What is a Transparency Tube?

A transparency tube is a relatively new piece of equipment that has become popular for use in volunteer stream monitoring programs and for educational activities. They came into use in the mid-90's as a way to apply the principles of measuring water clarity using a secchi disk to shallow bodies of water where a secchi disk in unusable. Secchi disks have been used since the 1800's. 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, like shown in the illustration. Secchi disks are lowered into lakes and ponds, tied to a rope, until the point where they are just barely visible. The distance along the rope is then measured to the water's surface, and a 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. The transparency tube, as shown in the illustration above, 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 though the water to identify the depth at which a smaller secchi disk, printed in the bottom of the tube, can be seen. Some transparency tubes have drain hoses at the bottom to lower the water column; others need to be carefully poured out in small amounts.

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 important for a number of reasons. Plants growing in the water need sunlight in order to complete photosynthesis. Photosynthesis is how plants make their own food and it also has the fortunate byproduct of producing oxygen which is released into the water, a key to increasing dissolved oxygen levels for aquatic animals to use for respiration. The water's transparency is also important 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 measured by determining the height of a column of water that is needed to completely obscure a beam of light. Technological advancements over time have changed exactly 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 levels of water clarity 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 chart given here.

<6.4>24021.7 to 24.13544.6 to 47.013
6.4 to 7.024024.2 to 26.73047.1 to 49.512
7.1 to 8.218528.8 to 29.22749.6 to 52.111
8.3 to 9.515029.3 to 31.82452.2 to 54.610
9.6 to 10.812031.9 to 34.32154.7 to 579
10.9 to 12.010034.4 to 36.81957 to 608
12.1 to 14.09036.9 to 39.41760 to 707
14.1 to 16.56539.5 ti 41.91570 to 856
16.6 to 19.14042.0 to 44.514>85<5
19.2 to 21.640

A few key measurements to consider as benchmarks for water clarity include:

  • >10 NTU = Fish and other aquatic wildlife begin to demonstrate signs of stress.
  • 1-5 NTU = EPA Drinking Water Standards (depending on the type of filtration system in place)
  • >5 NTU = not recommended for recreational use

Tips for Using a Transparency Tube

  • Follow the instructions that accompany your purchased transparency tube or the plans you used for making 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 that occurred in the days prior to your measurement. Also record the time of day. This information will help you identify why you might be seeing big differences between measurements.