The disk can be all white and vary in size from 7.5"-10" diameter. The diameter is not as important as using the same disk every time and doing it at relatively the same time of day and similar lighting conditions i.e. with sun shine. The distance or depth that the disk can be seen from above can vary with size and color of the disk.
You can easily make a homemade secchi disk (see below). I have used various types of white plastic lids. Ideally it should be divided into quadrants alternating black & white, although this is not absolutely necessary. An all white disk can also work okay to monitor or check the water clarity as long as the same disk is always used for each measurement. You ideally want to monitor the changes in water clarity of your pond.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=157758&page=1 MATERIALS: � 20 cm diameter circle of acrylic disk 3/8� or 1/2� in thickness (aluminum or steel may be substituted, but wood is not recommended) � 15 cm circle or square of 1/8� galvanized steel (used to weight the disk) � hand drill � eye bolt � 5/16� x 2� � two flat 5/16� washers � one locking 5/16� washer � two 5/16� nuts � flat black rust resistant spray paint � flat white rust resistant spray paint � masking tape � nylon rope (cut long enough to be the depth of your lake deep spot. Avoid cotton rope because it stretches)
INSTRUCTIONS: 1. Divide the 20 cm acrylic disk into quadrants using masking tape. Spray paint alternating quadrants black and white, so that you have a disk that is similar to that pictured below. Let the paint dry. Apply a second coat of paint if necessary. 2. Drill a hole of 3/8� through the center of the acrylic disk and the galvanized steel disk. 3. Assemble disks with eyebolt (5/16� in diameter). Use flat washers between disk and nut, and between steel plate and locking washer. Use 5/16� nuts at the top of the eyebolt and to bolt the steel plate on the underside of the acrylic disk. 4. Attach a brass chain or rope calibrated by 0.5 meter increments to the secchi-disk to use in the lake. (Note: If rope is used, avoid using cotton rope or clothesline since it stretches when it is wet. Use non-stretching white plastic coated wire-core clothesline. Make sure to bend and straighten the line before you buy it to make sure that it will lie straight. Calibrate the rope at 0.5 meter increments using permanent pen, or by tying knots at each 0.5 meter interval.)
Nephelometric Turbidity Unit (NTU) is an old standard method. Here is a link to some examples of water color and how they relate to NTU. http://www.water.ncsu.edu/watershedss/info/turbid.html Turbidity. Several methods are used to measure turbidity. All of them involve measuring the amount of some type of light that can pass through a standard width of water.
A Formazin Nephelometric Unit (FNU) is similar to a Nephelometric Turbidity Unit (NTU) in that both measure scattered light at 90 degrees from the incident light beam, but the FNU is measured with an infrared light source according to the ISO 7027 method whereas the NTU is measured with a white light according to EPA method 180.1.
Background. From Wikipedia. http://en.wikipedia.org/wiki/Turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. The propensity of particles to scatter a light beam focused on them is now considered a more meaningful measure of turbidity in water. Turbidity measured this way uses an instrument called a nephelometer with the detector set up to the side of the light beam. More light reaches the detector if there are lots of small particles scattering the source beam than if there are few. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). To some extent, how much light reflects for a given amount of particulates is dependent upon properties of the particles like their shape, color, and reflectivity. For this reason (and the reason that heavier particles settle quickly and do not contribute to a turbidity reading), a correlation between turbidity and total suspended solids (TSS) is somewhat unique for each location or situation.
Turbidity in lakes, reservoirs, channels, and the ocean can be measured using a Secchi disk. This black and white disk is lowered into the water until it can no longer be seen; the depth (Secchi depth) is then recorded as a measure of the transparency of the water (inversely related to turbidity). The Secchi disk has the advantages of integrating turbidity over depth (where variable turbidity layers are present), being quick and easy to use, and inexpensive. It can provide a rough indication of the depth of the euphotic zone with a 3-fold division of the Secchi depth, however this cannot be used in shallow waters where the disk can still be seen on the bottom.
An additional device, which may help measuring turbidity in shallow waters is the turbidity tube. The turbidity tube condenses water in a graded tube which allows determination of turbidity based on a contrast disk in its bottom, being analogous to the Secchi disk.
Last edited by Bill Cody; 09/14/2202:20 PM.
aka Pond Doctor & Dr. Perca Read Pond Boss Magazine - America's Journal of Pond Management