ASTM D7512-09(2023)

Standard Guide for Monitoring of Suspended-Sediment Concentration in Open Channel Flow Using Optical Instrumentation

SIGNIFICANCE AND USE
5.1 This guide is general and intended as a planning guide. To satisfactorily monitor a specific site, an investigator must sometimes design specific installation structures or modify those given in this guide to meet the requirements of the site in question. Because of the dynamic nature of the sediment transport process, the extent to which characteristics such as mass concentration and particle-size distribution are accurately represented in the monitoring program depends on the type of equipment used and method of collection of the SSC samples used to calibrate the optical readings. Sediment concentration is highly variable in both time and space. Numerous samples must be collected and analyzed with proper equipment and standardized methods for the rating of the optical equipment at a particular site (see Guide D4411 and Practice D3977).  
5.2 All optical equipment have an upper limit for valid readings, beyond which the meter will not read properly, commonly referred to as “blacking out.” If upper range of SSC are expected to cause optical instrument black out, then some other means should be devised, such as automatic pumping samplers, to collect samples during this period. See Edwards and Glysson (1)3 and Glysson (2) for information on collection of suspended sediment samples using pumping samplers. It should be noted that other technologies, such as lasers and acoustic dopplers, are also being used to monitor SSC continuously.  
5.3 The user of this guide should realize that because different technologies and different models of the same technology of turbidity meters can produce significantly different outputs for the same environmental sample, only one manufacturer and model of the turbidity meter can be used to develop the relationship between the SSC and turbidity readings at a site. If a different manufacturer or a different model type of turbidity meter is used, a new relationship will need to be develop for the site.
SCOPE
1.1 This guide covers the equipment and basic procedures for installation, operation, and calibration of optical equipment as a surrogate for the continuous determination of suspended-sediment concentration (SSC) in open channel flow.  
1.2 This guide emphasizes general principles for the application of optical measurements to be used to estimate suspended-sediment concentration (SSC) in water. Only in a few instances are step-by-step instructions given. Continuous monitoring is a field-based operation, methods and equipment are usually modified to suit local conditions. The modification process depends upon the operator skill and judgment.  
1.3 This guide covers the use of the output from an optical instrument, such as turbidity and suspended-solids meters, to record data that can be correlated with suspended-sediment concentration. It does not cover the process of collecting data for continuous turbidity record, which would require additional calibration of the turbidity readings to the mean turbidity of the measurement cross section. For the purposes of this method it is assumed that the dependent variable will be mean cross-sectional suspended-sediment concentration data.  
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 14-Dec-2023
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: D19 - Water
• Drafting Committee: D19.07 - Sediments, Geomorphology, and Open-Channel Flow

Relations

• Replaces: ASTM D7512-09(2015) - Standard Guide for Monitoring of Suspended-Sediment Concentration in Open Channel Flow Using Optical Instrumentation
• Effective Date: 15-Dec-2023

Overview

ASTM D7512-09(2023), "Standard Guide for Monitoring of Suspended-Sediment Concentration in Open Channel Flow Using Optical Instrumentation," provides comprehensive guidance for the continuous determination of suspended-sediment concentration (SSC) in open channel waters utilizing optical devices. Developed by ASTM International, this standard emphasizes site-specific considerations, equipment selection, proper installation, calibration, operation, and maintenance practices required to ensure reliable SSC measurements with optical measurement systems such as turbidity and suspended-solids meters.

Continuous monitoring of sediment loads is vital for water resource management, environmental compliance, sediment transport studies, and infrastructure maintenance. The guide underlines the importance of tailoring monitoring systems and protocols to site conditions, and the need for careful calibration and validation of instrument outputs with physical SSC samples.

Key Topics

• Equipment & Installation:
  The standard discusses the use of various optical monitoring configurations, including flow-through systems, in-situ sensors, and self-contained recording sondes. Each has unique advantages and disadvantages concerning power requirements, maintenance, site accessibility, and vulnerability to environmental factors.

• Calibration & Maintenance:
  To achieve accurate readings, frequent calibration and cleaning of optical sensors are required. The guide provides protocols for field calibration checks, handling fouling (biological growth, sediment deposition), and addressing calibration drift over time. Only one model and manufacturer of turbidity meter should be used for site-specific calibration, as outputs may vary significantly between technologies or models.

• Data Quality & Collection:
  SSC in open channel flows is highly variable spatially and temporally. The standard recommends collecting multiple calibration samples representing a range of flow conditions to create robust correlations between optical readings and physical SSC measurements. Proper record-keeping and field notes are essential for traceability and data interpretation.

• Site Selection:
  Selecting an appropriate monitoring location is critical. Ideal sites are hydrologically stable, easily accessible (especially during high flow events), and allow for representative sampling. Considerations include distance from tributaries, susceptibility to backwater effects, power availability, and physical security of equipment.

• Data Processing & Corrections:
  The standard outlines methods for applying corrections to raw optical data to account for fouling, drift, and instrument error. This includes protocols for documenting corrections, handling rapid environmental changes, and maintaining calibration records.

Applications

• Water Quality Monitoring:
  Supports regulatory compliance and resource management by providing real-time or near real-time SSC data, enabling the detection of pollution events, and helping to meet national or regional sediment control standards.

• Hydrologic and Sediment Transport Studies:
  Enables researchers and engineers to better understand sediment dynamics, assess morphological changes in rivers, and inform sediment management strategies.

• Infrastructure Design & Maintenance:
  Continuous SSC data support the design and integrity assessment of dams, bridges, and irrigation systems, by quantifying sediment loads that can affect capacity and operational reliability.

• Flood and Erosion Control:
  Monitoring sediment concentrations can be critical for predicting and mitigating flood risks and understanding erosion patterns in watersheds.

Related Standards

• ASTM D4411: Guide for Sampling Fluvial Sediment in Motion
• ASTM D3977: Test Methods for Determining Sediment Concentration in Water Samples
• ASTM D7315: Test Method for Determination of Turbidity Above Turbidity Unit (TU) in Static Mode

Additional guidance on equipment, sensor maintenance, and calibration solutions is referenced throughout the standard. This ASTM guide aligns with internationally recognized principles for environmental monitoring and standardization, as endorsed by the WTO Technical Barriers to Trade (TBT) Committee.

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Implementing ASTM D7512-09(2023) helps ensure accurate, defensible, and actionable data for the assessment and management of sediment in open channels using optical instrumentation. For organizations seeking to improve water quality monitoring, support research, or comply with environmental regulations, this standard provides essential guidance tailored to real-world field conditions.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7512 − 09 (Reapproved 2023)
Standard Guide for
Monitoring of Suspended-Sediment Concentration in Open
Channel Flow Using Optical Instrumentation
This standard is issued under the fixed designation D7512; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This guide covers the equipment and basic procedures 2.1 ASTM Standards:
for installation, operation, and calibration of optical equipment D3977 Test Methods for Determining Sediment Concentra-
as a surrogate for the continuous determination of suspended- tion in Water Samples
sediment concentration (SSC) in open channel flow. D4411 Guide for Sampling Fluvial Sediment in Motion
D7315 Test Method for Determination of Turbidity Above 1
1.2 This guide emphasizes general principles for the appli-
Turbidity Unit (TU) in Static Mode
cation of optical measurements to be used to estimate
suspended-sediment concentration (SSC) in water. Only in a
3. Terminology
few instances are step-by-step instructions given. Continuous
3.1 Definitions:
monitoring is a field-based operation, methods and equipment
3.1.1 calibration drift, n—the error that is the result of drift
are usually modified to suit local conditions. The modification
in the sensor reading from the last time the sensor was
process depends upon the operator skill and judgment.
calibrated, and is determined by the difference between
1.3 This guide covers the use of the output from an optical
cleaned-sensor readings in calibration standards and the true,
instrument, such as turbidity and suspended-solids meters, to
temperature-compensated value of the calibration standards.
record data that can be correlated with suspended-sediment
3.1.2 continuous, adj—refers to a time series of unit values
concentration. It does not cover the process of collecting data
that are close enough in time to simulate a continuous record.
for continuous turbidity record, which would require additional
3.1.2.1 Discussion—Generally, in studies of open-channel
calibration of the turbidity readings to the mean turbidity of the
flow, 15-min intervals are used and are adequate to estimated
measurement cross section. For the purposes of this method it
continuous record. However, the time interval maybe as little
is assumed that the dependent variable will be mean cross-
as a minute or as great as an hour.
sectional suspended-sediment concentration data.
3.1.3 fouling, n—the error that can result from a variety of
1.4 The values stated in SI units are to be regarded as
sources (such as biological growth on the sonde and covering
standard. No other units of measurement are included in this
of the probe with sediment), and is determined by the differ-
standard.
ence between sensor measurements in the environment before
1.5 This standard does not purport to address all of the
and after the sensors are cleaned.
safety concerns, if any, associated with its use. It is the
3.1.4 sonde, n—part of the monitoring equipment that con-
responsibility of the user of this standard to establish appro-
tains the measurement sensors.
priate safety, health, and environmental practices and deter-
3.1.4.1 Discussion—A sonde may be either a single param-
mine the applicability of regulatory limitations prior to use.
eter sensor or a combination of different sensors of different
1.6 This international standard was developed in accor-
parameters.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
4. Summary of Guide
Development of International Standards, Guides and Recom-
4.1 This guide covers the equipment and basic procedures
mendations issued by the World Trade Organization Technical
for installation, operation, and calibration of optical equipment
Barriers to Trade (TBT) Committee.
as a surrogate for the continuous or near continuous determi-
nation of SSC in open channel flow.
This guide is under the jurisdiction of ASTM Committee D19 on Water and is
the direct responsibility of Subcommittee D19.07 on Sediments, Geomorphology,
and Open-Channel Flow. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Dec. 15, 2023. Published January 2024. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2009. Last previous edition approved in 2015 as D7512 – 09 (2015). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/D7512-09R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7512 − 09 (2023)
4.2 This guide emphasizes general principles for one par- 6.2.1 The Alliance for Coastal Technologies (ACT) did an
ticular application of optical measurements in water. This in-situ evaluation of different nephelometer technologist. Users
guide covers the use of nephelometers and backscatter type of this guide may be interested in the results of this study which
turbidity meters to record data that can be correlated with SSC. can be found online.
6.3 Before selecting the type of meter to be used, the
5. Significance and Use
operator needs to review the site requirements in order to
ensure selection of the proper instrumentation. Things to
5.1 This guide is general and intended as a planning guide.
consider, but are not limited to, are: the instruments ability to
To satisfactorily monitor a specific site, an investigator must
survive the study site environment, the degree of fouling that
sometimes design specific installation structures or modify
may take place, and the range of readings likely to be
those given in this guide to meet the requirements of the site in
encountered at the site.
question. Because of the dynamic nature of the sediment
transport process, the extent to which characteristics such as
6.4 If a flow-through or in-situ monitoring device is used, a
mass concentration and particle-size distribution are accurately
recording system must be installed. The recording system must
represented in the monitoring program depends on the type of
have enough storage capacity to store all data recorded
equipment used and method of collection of the SSC samples
between site service visits. See manufacturer’s advice on
used to calibrate the optical readings. Sediment concentration
which recording devices will work best for the type of monitor
is highly variable in both time and space. Numerous samples
being used.
must be collected and analyzed with proper equipment and
6.5 Remote access and near real-time transmission of data
standardized methods for the rating of the optical equipment at
from the site to the office can be very important in meeting the
a particular site (see Guide D4411 and Practice D3977).
objectives of the monitoring station. Remote access and the
5.2 All optical equipment have an upper limit for valid near real-time transmittal of the recorded data take other
equipment (such as a data collection platform (DCP), and
readings, beyond which the meter will not read properly,
commonly referred to as “blacking out.” If upper range of SSC transmittal antenna) in addition to the optical sensor. A DCP
performs the same fundamental function as a basic data
are expected to cause optical instrument black out, then some
other means should be devised, such as automatic pumping recorder (BDR). They both collect data from attached sensors
on a timed interval and store the results. The difference is the
samplers, to collect samples during this period. See Edwards
and Glysson (1) and Glysson (2) for information on collection BDR retains the data until it is retrieved manually, while the
of suspended sediment samples using pumping samplers. It DCP has the ability to transmit the collected data to another
should be noted that other technologies, such as lasers and location. Since data is transferred elsewhere for storage shortly
acoustic dopplers, are also being used to monitor SSC continu- after collection, a DCP may have less memory than a BDR.
ously. The data may be transmitted via telephone modem, line-of-site
radio link or satellite. It is beyond the scope of this guide to
5.3 The user of this guide should realize that because
discuss how to instrument a site for remote data transmission
different technologies and different models of the same tech-
and no single reference on how to do this is available for
nology of turbidity meters can produce significantly different
reference here. The user is encouraged to visit the U.S.
outputs for the same environmental sample, only one manu-
Geological Survey’s Hydrologic Instrumentation Facility web-
facturer and model of the turbidity meter can be used to
site for information on equipment needs and used by the
develop the relationship between the SSC and turbidity read-
USGS.
ings at a site. If a different manufacturer or a different model
6.6 The installation of an automatic pumping sampler,
type of turbidity meter is used, a new relationship will need to
especially in remote areas, will allow samples to be collected
be develop for the site.
that can be used to relate the optical reading to the suspended-
sediment concentration in the stream and also address the
6. Apparatus
blackout periods discussed in 5.2. Detailed information con-
6.1 In general, three types of configurations of installations
cerning the installation and operation of pumping samplers is
of monitors can be used: (1) the flow-through monitoring
beyond the scope of this guide. See Edwards and Glysson (1)
system, (2) the in-situ monitoring system, and (3) the self
and Glysson (2) for more information on the use of automatic
contained, combined sensor and recording system.
pumping samplers.
6.2 Optical instruments such as photoelectric nephelometer
7. Site Selection
(best used for lower levels of SSC) and backscatter sensors
(best used for higher levels of SSC) provide the basis for this
7.1 The procedure for establishing a sampling location
method. For more information concerning the advantages and
should emphasize the quest for a stream-data site. A stream-
disadvantages of each, see Test Method D7315. As they
data site is as a cross section displaying relatively stable
become available, other sensors may be used.
hydrologic characteristics and uniform depths over a wide
3 4
The boldface numbers in parentheses refer to a list of references at the end of Available from http://www.act-us.info/evaluation.php.
this standard. Available from http://water.usgs.gov/hif.
D7512 − 09 (2023)
range of stream discharges, from which representative sedi- given stream conditions at one stage would produce the same
ment data can be obtained and related to a stage-discharge sampling point location relative to the flow conditions at a
rating and optical readings from the site. This is an idealized different stage. Therefore, generalized guidelines (modified
concept because the perfect site is rare at best. Therefore, it is from (1)) are outlined here and should be considered on a
necessary to note the limitations of the most suitable site case-by-case basis when selecting a sampling point.
available and build a program to minimize the disadvantages 8.1.1 Select a stable cross section of reasonably uniform
and maximize the advantages. depth and width to maximize the stability of the relationship
between sediment concentration at a point and the mean
7.2 Sites that may be affected by backwater conditions
sediment concentration in the cross section. This guideline is of
should be avoided whenever possible. Backwater condition is
primary importance in the decision to use an optical meter in a
a transient condition that occurs when water is backed up or
given situation; if a reasonably stable relation between the
retarded in a stream by tides or from inflow from another
sample-point reading and mean cross-section concentration
stream. Backwater affects both stage-discharge and velocity-
cannot be attained by the following outlined steps, the meter
discharge relationships. Therefore, a given discharge may have
should not be installed and an alternate location considered.
varying stage and mean stream velocity and thus have varying
8.1.2 Consider only the part of the vertical that could be
sediment transport rates.
sampled using a standard U.S. depth- or point-integrating
7.3 A sediment-measuring site located downstream from the
suspended-sediment sampler, excluding the unsampled zone,
confluence of two streams may require extra sediment mea-
because data collected with a depth- or point-integrating
surements due to incomplete mixing of the flows from the
sampler will be used to calibrate the optical meter. See Guide
tributaries. Moving the sampling location far enough down-
D4411 and Edwards and Glysson (1) for information on the
stream to ensure adequate mixing of the tributary flows should
unsampled zone, proper procedures, and equipment to collect
be investigated.
samples for SSC analysis.
7.4 Because sediment samples must be obtained more 8.1.3 Determine, if possible, the depth of the point of mean
frequently during high flows, and it is imperative that a site be sediment concentration in each vertical for each size class of
selected where obtaining data during these times are feasible. particles finer than 0.250 mm, from a series of carefully
Particular attention should be given to the ease of access to the collected point integrated samples. See Edwards and Glysson
water-stage recorder and to a usable bridge or cable during (1) for information on the collection of point samples and Guy
(5) for procedures for determining particle size of sediment
high flows, many of which occur at night. Sites accessible only
by poorly maintained backroads or trails should be avoided. samples.
8.1.4 Determine, if possible, the mean depth of occurrence
7.5 The average monitoring site will consist of a shelter to
of the mean sediment concentration in each vertical for all
house and protect the equipment and some means for either
particles finer than 0.250 mm.
bringing water to the meters or conduits that will allow the
8.1.5 Use the mean depth of occurrence of the mean
meter to be placed in the flow zone of the channel. The shelter
sediment concentration in the cross section as a reference depth
should be located to minimize the distance between the stream
for placement of the sampling point.
and the meters, and at a high enough elevation to protect the
8.1.6 Adjust the depth location of the sampling point to
equipment during flooding events. Some instruments may need
avoid interference by dune migration (which may bury the
AC power and therefore the proximity to power lines is
probe) or contamination by bed material.
important.
8.1.7 Adjust the depth location of the sampling point to
7.6 For additional information and discussion on the selec-
ensure submergence at all times.
tion of a site for the collection of surface water and sediment
8.1.8 Locate the sampling point laterally in the flow at a
data see Edwards and Glysson (1), Wagner et al. (3), and Rantz
distance far enough from the bank to eliminate any possible
et al. (4).
bank effects such as inflow of overland flow for the banks and
localized increased SSC do to bank erosion.
8. Installation of Equipment
8.1.9 Place the sampling point in a zone of high velocity and
8.1 Placement of sensor in cross section of the stream: The
turbulence to improve sediment distribution by mixing and
primary consider when placing an optical meter, sensor, or
reduce possible deposition on or near the sampling point.
water intake (collectively referred in this section as “sampling
8.2 Flow Thru—The flow-through monitoring system has a
point.”) in the streamflow at a cross section is that only one
pump to convey water from the stream to a tank inside a shelter
point in the flow is being sampled. Therefore, to yield reliable
that contains the monitoring sensor or sonde (Fig. 1). Typical
and representative data, the sampling point should be placed at
pumps require 120-volt alternating current (AC) and deliver
the point where the concentration approximates the mean SSC
about 10 gallons of water per minute. If access to AC power is
for the cross section for the full range of flows. SSC data may
not a problem, then other site considerations become important
have to be collected from several verticals in the cross section
(Table 1); the advantages and disadvantages of the flow-
to help define where the mean SSC values is most likely to
through monitoring system must be compared to the data
accrue (see (1)). This is an idealistic concept and the mean
objectives.
cross-section concentration almost never exists at the same
point under varying streamflow conditions. It is even less likely 8.3 In-Situ—The sensors in the in-situ monitoring system
that specific guidelines for locating a sampling point under are placed at the measuring point in the stream cross section
D7512 − 09 (2023)
FIG. 1 Typical Flow-Through Water-Quality Monitoring Station (Modified from (3))
TABLE 1 Principle Advantages and Disadvantages of Flow-Through Monitoring Systems
Advantages Disadvantages
Unit can be coupled with chlorinators to reduce fouling. 120-volt AC power source is needed.
Sensor systems can be secured in vandal-proof shelters. Higher installation costs are incurred.
Calibration can be performed in the shelter. Pumps tend to clog in streams with algal fouling or high sediment loads.
Can help eliminate interferences caused by ambient light and bubbles. Electrical shock protection is required.
Pumps may be damaged by corrosive waters.
Pump maintenance is required.
Pumping may cause changes in water-sediment concentration and particle size
distribution.
(Fig. 2). Cables run from the sensors to the recording equip- requirements of the sensors and recording equipment. In-situ
ment that is housed in a shelter. The primary advantage of this water-quality monitoring systems can be installed at remote
configuration is that no power is needed to pump water (Table locations where AC power is not available, but the advantages
2). Direct current, 12-volt batteries easily meet the power and disadvantages of the in-situ monitoring system also must
be considered.
8.4 Self-Contained—The third water-quality monitoring
system is a combined sensor and recording sonde that is
self-contained, requires no external power, and reduces expo-
sure to vandalism. Power is supplied by conventional batteries
located in a sealed compartment, and sensor data are stored
within the sonde on nonvolatile, flash-memory, recording
devices (Fig. 3). The advantages and disadvantages of the
self-contained sensor and recording system must be considered
(Table 3).
8.5 Placement of Recorders in Relation to Sensor—
Recorders should be placed as close to the sensor as possible,
but at a high enough elevation to protect it from being
inundated by high water. The recorder should be placed in a
secure structure to protect it from the environment and from
possible vandalism.
9. Operation of Meters
9.1 Maintenance—The operation of a sediment monitoring
station is intended to produce the greatest amount of correct-
able field record that can be verified. The general operational
FIG. 2 Typical In-Situ Water-Quality Monitoring Station (Modified
from (3)) categories include maintenance of the station and equipment;
D7512 − 09 (2023)
TABLE 2 Principle Advantages and Disadvantages of In-Situ Monitoring Systems
Advantages Disadvantages
No power is needed to pump water. Sensors are susceptible to vandalism.
Remote locations are possible. The water flow cannot be treated to reduce fouling.
Smaller shelters can be used. In shallow bank installations, the proper location of sensors in the cross section
is difficult.
Pumping maintenance is not required. Servicing sensors during flooding can be difficult.
Freeze protection is provided to the sensors. Sensors are susceptible to debris at high flows and chemical and biological
fouling.
Electrical hazards are reduced. Shifting channels may cause movement of the equipment.
No pumping therefore no lift limit. Sensors may be susceptible to interferences for light and surface reflections or
bubbles.
stations equipped with telemetry, or some other form of real
time remote connection, can be recognized quickly.
9.1.1 In addition to the maintenance/servicing instructions
recommended by the manufacturer, the general maintenance
functions at a water-quality monitoring station include:
9.1.1.1 Daily review of sensor function for sites equipped
with telemetry;
9.1.1.2 Inspection of the site for signs of physical disrup-
tion;
9.1.1.3 Inspection of sensor(s) for fouling, corrosion, or
damage;
9.1.1.4 Battery (or power) check;
9.1.1.5 Time check;
9.1.1.6 Routine sensor cleaning and servicing;
9.1.1.7 Calibration (if needed); and
9.1.1.8 Downloading of data.
9.2 Sensor Inspection—The purposes of the sensor inspec-
tion are to provide an ending point for the interval of optical
meter record since the last service visit, a beginning point for
the next interval of record, and verification that the sensor is
working properly. This is accomplished by recording the initial
sensor readings, servicing the sensors, recording the cleaned
sensor readings, performing a calibration check of sensors by
FIG. 3 Typical Self-Contained Water-Quality Monitoring Sensor
using appropriate standards, and if the readings of the moni-
and Recording System (Modified from (3))
toring sensor are outside the range of acceptable differences
(see 9.3.2), recalibrating the sensor. A final environmental
sensor reading is required after the calibration check or after
recalibration. The difference between the initial sensor reading
inspection and recording of sensor readings; cleaning,
and the cleaned sensor reading is the sensor error as a result of
calibration, and troubleshooting of sensors and recording
fouling; the difference between the calibration-check reading
equipment; cross section measurements; and accurate record
and calibrated-sensor reading, if necessary, is a result of drift.
keeping. Optical sensors are prone to fouling. Maintenance
All information related to the sensor inspection must be
frequency is generally governed by the fouling rate of the
recorded on a field form or in a field notebook. Contact the
sensors, and this rate varies by sensor type, hydrologic
manufacturer if any unexpected condition with the monitoring
environment, and season. The use of wiper or shutter mecha-
sensor is detected.
nisms on modern optical instruments has decreased the fouling
9.2.1 The initial sensor readings (before cleaning) of the
problem significantly. For stations with critical data-quality
monitoring equipment are taken before removing the monitor
objectives, service intervals may be weekly or more often.
sonde for servicing. This initial sensor reading becomes the
Monitoring sites with nutrient-enriched waters and moderate to
ending point of the data record since the last servicing.
high temperatures may require service intervals as frequently
as every third day. In cases of severe environmental fouling or 9.2.2 Upon removal from the water, the monitoring sonde is
remote locations, the use of an observer for servicing the inspected for signs of chemical precipitates, stains, siltation, or
water-quality monitor should be considered. In addition to aquatic growth. Readings are then taken on standards appro-
fouling problems, physical disruptions (such as pump failure, priate for the optical meter being used and the readings
recording equipment malfunction, sedimentation, electrical recorded, and that represent the estimated range the meter had
disruption, debris, ice, or vandalism) also may require addi- read since the last visit. At a minimum, a high and low standard
tional site visits. The service needs of sediment monitoring must be checked. These observations are recorded in the field
D7512 − 09 (2023)
TABLE 3 Principle Advantages and Disadvantages of the Self-Contained Mon
...