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Equipment and supplies commonly used for field measurement of turbidity are listed in table 6.7-1. Before field use of water-quality instruments, become familiar with the manufacturer's instructions for calibration, operation, and maintenance. Test field instruments before use.

Table 6.7-1

Turbidity instruments. Three basic types of instrumentation are used to measure turbidity: turbidimeters (nephelometers), spectrophotometers, and multiparameter instruments with submersible sondes that can accommodate a turbidity sensor (commonly referred to as a turbidity probe). Choice of turbidity instrument depends on site characteristics and intended use of the data in addition to instrument specifications, performance, and reliability.2

arrow 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

arrow 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 probes (submersible sensors) are available for multiparameter instruments with pH, temperature, conductivity, and other sensors; this is convenient for monitoring turbidity along with other field measurements. For ground-water studies, multiparameter instruments are available with sondes that can be used in 2-in. diameter wells.

Field spectrophotometers can be convenient for qualitative turbidity measurements if additional sample properties will be measured spectrophotometrically.

USEPA--Approved Specs

Selected turbidity instruments were tested by the Hydrologic Instrumentation Facility. Referring to table 6.7-2, field tests of the Hach DR 2000TM indicated consistently higher FTU values compared with NTU values measured with the Analite 152TM, Hach 2100PTM, Hydrolab DataSonde 3TM, and YSI 3800TM. Not available at the time of testing were either the Hydrolab H20TM or YSI 6000TM multiparameter instruments with turbidity probe or the Analite 156TM. Refer to Hydrologic Instrumentation Facility (1994) for test details.

Table 6.7-2

The method used for sample handling falls into three general categories, as dictated by instrument capabilities: (1) manual (discrete) sample, using a cuvette-based instrument, with sample decanted into a sample cell (cuvette); (2) pumped sample, in which a sample is pumped through a "flowthrough cell," which is a turbidity-sensor-containing cuvette that is an internal part of the instrument; and (3) direct determination, by positioning a turbidity probe either in situ or into a flowthrough chamber that receives pumped sample (see NFM 6.0).

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.

arrow Obtain a debubbler from the instrument manufacturer, or construct one as shown on figure 6.7-2 in section 6.7.3.

arrow Probe-based instruments are available with a wiper mechanism that clears bubbles from the optical surface of the submersible sensor (probe).

Instruments with gas-sweep capacity. Condensation must be removed or reduced throughout turbidity determination. Some flowthrough-cell instruments have the capacity to continuously sweep the sample compartment with dry gas, reducing condensation on the sample cell; otherwise, condensation is to be removed manually every few minutes.


Check manufacturer's instructions for instrument maintenance, cleaning, and storage. Test equipment before each field trip and record all repairs in the instrument log book. Manufacturer's instructions and the log book should accompany the instrument at all times.

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.

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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.

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