USDA Forest Service
 

Pacific Southwest Research Station

 
Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Water & Watersheds

Instrumentation

A Campbell CR10X data logger platform.
The Turbidity Threshold Sampling program is executed from the Campbell CR10X data logger platform.

Mention of product names is not an endorsement by the USDA Forest Service

Data Logger and Sampling Logic

A programmable data logger is required to make the required sampling decisions. For remote locations, it is important that the data logger has low power requirements in order to preserve the battery's capacity. The Turbidity Threshold Sampling (TTS) program only requires input information about stage and turbidity to decide what actions to take.

Wake-up intervals are either set at 10-minutes for small, flashy watersheds, or at 15-minute intervals for larger basins. At the beginning of each wake-up interval, the OBS-3 turbidity probe, under control of the program logic, collects 60 measurements in 30 seconds.

Next, the raw turbidity values are sorted and the median value is determined. We have found that these two operations effectively reduce outlier values. In the case of the DTS-12, the sampling frequency and period, and reported statistics, are controlled by the sensor's onboard processor.

The program next collects 150 stage readings in three seconds from a pressure transducer and computes the mean stage. The mean stage is then compared against the minimum operating stage to determine if the turbidity probe and sampler intake are adequately submerged (stage is above "baseflow") to allow sampling. If the program logic determines that a sample is required, based on the rules discussed above, it activates an automatic water sampler to collect one sample. Other instruments, such as tipping bucket rain gages and water temperature probes, may be connected to the data logger to provide additional information.

Finally, all pertinent records are written to data logger memory. The TTS logic, discussed above, has been developed for Campbell data loggers.

Turbidity Probe
Looking upward into the trees. In the center is a pole with an assembly of passive samplers used for monitoring air quality.A passive sampler labeled with each of the air pollutant monitors labeled: ozone, nitric acid vapor, ammonia, and nitrogen oxide.
Left: OBS-3 backscatter nephelometer; Right: DTS-12 digital backscatter nephelometer with wiper

The OBS-3 turbidity probe, manufactured by D&A Instrument Company, is a backscatter nephelometer that emits infrared radiation (IR) into the water column. The distance the IR penetrates the water depends on the probe's optical configuration and the amount and type of sediment in suspension. 

The penetration, or volume sampled, decreases with increasing concentration of material. The scattered IR returned to the sensor's detector is a function of particle size and shape and the number of particles in suspension.

Comparisons made with different turbidimeters should be viewed with some skepticism due to inconsistencies in light sources, calibrations, and the sampled volume.

Periodic calibration of the turbidity sensor in formazin standards is required to compensate for instrument drift and scratched optical surfaces. Sensors with a small viewing area (1 cm or less) reduce the chance that large debris will be viewed by the optics and allow for shallow deployment. Small viewing areas often do not provide adequate sampling volume and may produce noisy data. Large viewing areas (7 to 25 cm) have the opposite characteristics. A viewing area of 4 to 7 cm is a good choice.

Turbidity Probe Housing
DTS-12 turbidity probe housing.
The DTS-12 turbidity probe housing protects the sensor and reduces fouling.

The turbidity probe housing reduces contamination from organics by shedding debris. The housing, if properly designed, can reduce hydrodynamic noise caused by turbulence and the entrainment of air or re-suspension of sediment close to the sensor. The housing also protects the sensor from direct impacts by large submerged organic debris.

Sampling Boom
DTS-12 turbidity probe housing.
Upper Jacoby Cr. boom in stormflow (housing, sensor, and intake submerged).

The boom positions the turbidity probe and sampler intake at the appropriate position and depth in the stream. Since the boom is articulated, large floating organic debris can, on impact, lift the vertical arm of the boom to the surface and pass underneath.

Increasing water velocity and depth pushes the vertical boom arm downstream, raising the turbidity sensor higher in the water column. A counterweight prevents the boom from rising to the water surface.

The highest probability of contamination by organics, and resulting loss of data, occurs during flood stages when organic material is recruited from flood plains.

A bank-, cable-, or bridge-mounted retrievable boom is desirable for all but the smallest streams to allow debris removal during high flows.

The depth of the turbidity probe can be adjusted as needed to position the probe above the zone of bedload transport and below the water surface. Changing the depth of the turbidity probe can change the ratio of coarse and fine particles sampled by both the turbidity probe and sampler intake.

Pressure Transducer
A Campbell CR10X data logger platform.
The Druck 1830 pressure transducer measures stage.

The pressure transducer measures the head, or water pressure, at the sensor. The pressure transducer is mounted below the lowest expected water stage. A vent tube inside the cable, open to the atmosphere where the cable terminates, compensates for changes in barometric pressure. The pressure transducer is calibrated before installation by submerging the sensor to known depths and recording the voltage signal.

The data logger uses this relationship to convert the sensor's voltage readings to depth. It is possible, with the proper placement and orientation of the pressure transducer housing, coupled with averaging of multiple readings, to eliminate the need to dampen wave pressure with a stilling well.

Automatic Water Sampler
Looking upward into the trees. In the center is a pole with an assembly of passive samplers used for monitoring air quality.A passive sampler labeled with each of the air pollutant monitors labeled: ozone, nitric acid vapor, ammonia, and nitrogen oxide.
Side and top views of the ISCO 3700 automatic water sampler.

Samples for laboratory analysis are collected by an automatic pumping sampler. The intake tubing runs from the sampler's pump to within close proximity of the turbidity probe on the boom. Both the intake tubing and cable for the turbidity probe are routed inside the boom to provide protection. 

In some situations, locating the sampler intake and turbidity probe in different stream locations can increase the variability between the two measurements if the transported sediment is not adequately mixed.

An ISCO pumping sampler is capable of collecting 24 samples under control of the TTS program. Sample volumes are set to approximately 350ml, or about 1/3 of available bottle volume. When the TTS program determines that all the rules have been met for collecting a threshold sample, the data logger triggers the sampler to collect one sample. The sampler's distributor arm then advances to the next empty bottle position and waits until the next signal from the data logger.

Additional samples, via the TTS program, may be collected under control of field personnel to match depth-integrated manual samples or to increase the frequency of sampling under certain conditions. The bottles containing samples are removed for laboratory analysis at the same time that the data is transferred from the data logger.

In situations where the transported sediment is predominantly coarse (>0.5mm), and the required lift (head-height of the sampler above the stream) is more than approximately 10 feet, the line speed of the water sediment mixture in the intake tubing may be inadequate to capture a representative sample of large sediment particles. The particles' momentum may be too great for the sampler to reverse, or their settling rate may be too great, permitting them to fall out of suspension before reaching the sample bottle.

A Campbell CR10X data logger platform.
South Lake Tahoe station with solar panel and cell antenna
Remote Sites

Sites that have difficult access benefit from solar panels and telecommunications. For sites with adequate solar access, a small solar panel can provide all the necessary power to keep the battery fully charged.

Telecommunications (land-line phone, cellular phone, radio RF, or satellite uplinks) allow for inspection of the data and reduce the number of site visits.

Last Modified: Aug 29, 2016 11:24:30 AM