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** Water Monitoring Equipment & Supply (a division of Lawrence Enterprises Inc.) manufactures products upon requests of researcher, scientist and field study personnel. These individuals or groups set the protocol and specifications to the equipment itself. We simply manufacture it to their specifications. Questions regarding the protocol in utilizing this piece of equipment should be addressed to your specific project supervisor or coordinator
WHAT IS TRANSPARENCY?
Pure water naturally reduces the intensity of light as the light travels farther through the water, with higher attenuation at the longer light wavelengths. The lower energy, reddish light suffers the greater natural attenuation. Dissolved chemicals, colloids, and suspended particles in water cause further attenuation by absorbing and reflecting/scattering of the incident light beam. Transparency is the ability of water to transmit light. Any incident light attenuated, reflected/scattered, or absorbed decreases the transparency of the water; thus the "dirtier" the water, the lower the transparency.
HOW IS TRANSPARENCY MEASURED?
Two basic methods are used; one employing incident light and the other depending upon reflected light. The incident light reading is accomplished by an electrical devise that sends a light beam in a single direction, and compares the amount of light emitted to the amount of light transmitted (i.e. received by a photo cell.) If the water contains a high level of contaminants in the form of dissolved chemical, colloids, and suspended solids then less of the incident beam will reach the receiver, because most of the light will be reflected/scattered or absorbed by the foreign materials, thus a low transparency reading.
The other method, which is well known to lake monitors, is the Secchi disk reading. This method relies upon the reflected light of the sun from a disk lowered through the water column. Typically the disk is lowered until it disappears and this depth is averaged with the depth at which it reappears as the disk is raised. This produces a transparency reading.
Another method for quantifying water "cloudiness" is turbidity. Turbidity is measured most commonly with a nephelometer. This device sends an incident light beam in a single direction, and senses the light which is scattered /reflected at a right angle to the incident beam. Particles in the water scatter the light: the more particles; the more scattered the light; and so the higher the turbidity reading. Very clean water scatters very little light, and so has a low turbidity reading. The unit of measurement for a nephelometer is the nephelometric turbidity unit (NTU).
It should be noted that transparency is measured either in line with the incident light beam or at an acute angle as with the Secchi disk. A nephelometer measures at a right angle to the incident light. Because the optical measurement methods are different, the readings of transparency and a nephelometer can not be directly related. There are no tables that convert NTUs to Transparency for all waters. Monitors can relate NTUs and transparency for a particular water body only by comparing nephelometer readings with transparence readings for a number of samples, throughout the different seasons of the year. And then generate a custom conversion table.
Under budget restraints and eagerness to get the job done some have used a hammer to drive a screw. Just because a scale of NTUs is on the side of a tube does not make it a nephelometer nor if the tube is calibrated with formazin standards is it a nephelometer.
WHAT ARE THE APPROPRIATE APPLICATIONS OF A TRANSPARENCY TUBE?
Transparency tubes are most frequently used to document the clarity of running water. Rather than using a series of adjectives (good, better, best,) a numeric scale in centimeters is affixed to the tube. Thus the monitor is able to document numerically the transparency results. This follows the typical Secchi disk reading for lake transparency. From these results, correlations can be made to other data collected from the watershed. Watershed data may include: rainfall, soil types, per cent of different land use, delinquent misuse of the land and water (poor development practices, over grazing, improper discharges, etc.).
Other resources for protocols and information concerning the Transparency/Turbidity Tube:
|The Volunteer Monitor Newsletter||Vol 16, Num 1-Winter 2004|
|Vol 14, Num 2-Summer 2002|
This method corresponds to the method used in viewing the Secchi disk in the lake environment, only with a much smaller disk.
Several groups have developed a correlation between Secchi disk readings and NTU's for a specific watershed or biogeo-region. But no known universal conversion factor is available.
** Water Monitoring Equipment & Supply (a division of Lawrence Enterprises Inc.) manufactures products upon requests of researcher, scientist and field study personnel. These individuals or groups set the protocol and specifications to the equipment itself. We simply manufacture it to their specifications. Questions regarding the protocol in utilizing this equipment should be addressed to your specific project supervisor or coordinator.