Water Resources--Office of Water Quality
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|If measuring turbidity for regulatory or compliance purposes, the only method approved by the USEPA employs Method 180.1 (STORET NO. 00076) (USEPA, 1979).3|
|For nonregulatory monitoring purposes, either a
submersible sensor that measures turbidity using a near-infrared light
source or a spectrophotometer in absorbance mode may be used.|
Turbidity-free water. Turbidity-free water is used for preparation of turbidity standards and is prepared by filtering either sample water or deionized water (DIW) through a 0.2-mm or smaller pore-sized membrane. Turbidity-free water is recommended instead of unfiltered DIW for preparation of standards.
Turbidity standards. USEPA (1979) guidelines recommend monthly preparation of the stock turbidity suspension for the calibration standard, and daily preparation of the standard turbidity suspension at the dilutions needed (see Section 6.7.2). Formazin stock solution is available commercially.
Debubbler/degassing system. Bubbles in the sample will give false turbidity readings. A debubbler or degassing system is required if sample contains effervescing gases. The equipment plumbing must be set up to maintain a constant head, resulting in constant velocity through the turbidimeter's flowthrough cell. When using a turbidity probe within a flowthrough chamber, it might be necessary to direct debubbled water through the chamber.
|Obtain a debubbler from the instrument manufacturer, or construct one as shown on figure 6.7-2 in section 6.7.3.|
|Probe-based instruments are available with a wiper mechanism that clears bubbles from the optical surface of the submersible sensor (probe).|
Turbidity instruments. Protect instruments from extreme temperatures. Shield the instrument LED display panel from direct sunlight. If a bench-top turbidimeter gets wet, allow it to dry thoroughly before the next use (field turbidimeters are constructed to withstand moisture). Check and replace batteries routinely.
Sample cells (cuvettes). Handle and store sample cells in a manner to prevent dirt, scratches, or other damage. Follow instrument manufacturer instructions for the maintenance of sample cells. Keep sample cells scrupulously clean, inside and out. After each use, (1) wash with nonphosphate laboratory detergent, (2) rinse repeatedly with deionized water until all detergent residue is removed, and (3) allow cells to air dry in a dust-free environment.
Submersible turbidity probe. Exercise care that optical surfaces of probes are not scratched during cleaning, operation, or storage. Scratched or damaged probes must be replaced. Keep optical surfaces free of all foreign material by wiping with moist lens-cleaning paper or cloth.
Standard solutions. Discard turbidity standards with elapsed expiration dates. Protect turbidity standards from extreme temperatures. Never pour used standard or a portion of unused standard back into its original (stock) container.
2Turbidity instruments are being developed and improved by several companies; investigate instrument performance and reliability before making an equipment selection. 3The USEPA also approves the GLI-2 method turbidity instrument system (a microprocessor-based turbidity system using a pulsed-light, four-beam sensor); the GLI-2 provides stable and reproducible turbidity readings to 0.5 NTU but it is not a portable instrument.
Maintainer: Office of Water Quality
Webversion by: Genevieve Comfort
Last Modified: 22JUNE98 ghc