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Chapter A6. Field Measurements

National Field Manual for the Collection of Water-Quality Data (TWRI Book 9)

Conversion Factors, Selected Terms and Symbols, Chemical Symbols and Formulas, and Abbreviations

See Errata for Chapter A6 for changes to this section.

Conversion Factors

Multiply By To obtain
millimeter (mm)0.03937inch (in.)
centimeter (cm)0.3937inch
square centimeter (cm2)0.155square inch (in2)
meter (m)3.281foot
micrometer (µm)3.3 x 10-6foot (ft)
kilometer (km)0.6214mile
square meter (m2)10.7639square foot (ft2)
square kilometer (km2)0.3861square mile
cubic meter per second (m3/s)35.31cubic foot per second (ft3/s)
liter (L)0.264gallon (gal)
milliliter (mL)0.0338ounce, fluid

2.64 x 10-4gallon
gram (g)0.03527ounce, avoirdupois
milligram (mg)3.527 x 10-5ounce, avoirdupois
microgram (µg)3.52 x 10-8ounce, avoirdupois
nanogram (ng)3.52 x 10-11ounce, avoirdupois

Temperature: Water and air temperature are given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by use of the following equation: °F = 1.8 (°C) + 32

Use the following equation to convert temperature from degrees Celsius to degrees Kelvin (K):
0°C = 273.150 K

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Selected Terms and Symbols

*The definitions identified with an asterisk were adopted by the Water Resources Division of the U.S. Geological Survey in Water Resources Division Memorandum 91.09.

*accuracy: The extent to which the measured value of a quantity agrees with the accepted value for that quantity.

approximately: ~

approximately equal to:sign

*bias: Systematic error that is manifested as a consistent positive or negative deviation from the known or true value. It differs from random error, which shows no such deviation.

dissolved constituent: A solute in a water sample--often operationally defined by the method and media properties used to separate the aqueous solutes from the particulate or colloidal phase.

District: A water-data-collecting organizational unit of the USGS located in any of the states or territories of the United States of America.

filtered sample: A sample passed through a commercial filter membrane of identified media, diameter, and pore size.

Formazin turbidity unit (FTU): (See Nephelometric turbidity unit).

gpm: gallons per minute

greater than: >

greater than or equal to: sign

less than: <

less than or equal to:

method detection limit (MDL): The minimum concentration of a substance that can be identified, measured, and reported with 99-percent confidence that the analyte concentration is greater than zero; determined from analysis of a sample in a given matrix containing analyte.

micrometer (µm): The millionth part of the meter; the pore diameter of filter membranes is given in micrometer units.

micromoles per liter (µmoles/L): A solution having a concentration of one millionth of a mole of a substance per liter solution (micromolar solution). A mole of substance is its atomic or molecular weight in grams.

µS/cm: microsiemens per centimeter at 25 degrees Celsius.

milliequivalents per liter (meq/L) or microequivalents per liter (µeq/L): One equivalent per liter is equal to one thousand milligram-equivalents per one thousand milliliters (meq/mL). Chemical analyses of solutes in a sample are expressed in unit concentrations that are chemically equivalent in terms of atomic or molecular weight and electrical charge.

milligrams per liter (mg/L) or micrograms per liter (µg/L): Milligrams per liter is a unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million.

millivolt (mV): A unit of electromotive force equal to one thousandth of a volt.

min: minute

minimum reporting level (MRL): The smallest measured concentration of a constituent that may be reliably reported using a given analytical method. In many cases, the MRL is used when documentation for the method detection limit is not available.

molal (m): Moles per kilogram (1,000 grams) of solvent.

molar (M): Concentration in moles per liter of solution, where one mole of a substance is its formula weight expressed in grams.

nanometer (nm): The millionth part of a millimeter.

Nephelometric turbidity unit (NTU): A measure of turbidity in a water sample, roughly equivalent to Formazin turbidity unit (FTU) and Jackson turbidity unit (JTU).

normality (N): The number of equivalents of acid, base, or redox-active species per liter (equivalents/L) of solution. Examples: a solution that is 0.01 F in HCl is 0.01 N in H+. A solution that is 0.01 F in H2SO4 is 0.02 N in acid. Formality (F) is the number of atomic (formula) weights per 1,000 grams of solution.

plus or minus: ±

*precision: The degree of similarity among independent measurements of the same quantity, without reference to the known or true value. It often is presented as the inverse of the standard deviation.

*quality assurance (QA): All those planned or systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements for quality.

*quality control (QC): The operational techniques and the activities used to fulfill requirements of quality.

recommended (recommend, recommended, recommendation): Pertains to USGS protocols and indicates that USGS Office of Water Quality policy recognizes that one or several alternatives to a given procedure or equipment selection are acceptable on the basis of research and (or) consensus. Specific data-quality requirements, study objectives, or other constraints may affect the choice of recommended equipment or procedures. The recommended equipment or procedures selected must be documented and can be based on referenced research and good field judgment. Departure from or modifications to recommended procedures must be quality assured and documented.

required (require, required, requirements): Pertains to USGS protocols and indicates that USGS Office of Water Quality policy has been established on the basis of research and (or) consensus of the technical staff and reviewed by water-quality specialists and selected District personnel. Departure from or modifications to the stipulated requirements that might be necessary to accomplishing specific data-quality requirements or study objectives must be quality assured and documented.

specific electrical conductance (conductivity) (SC): Conductivity of water is expressed in microsiemens per centimeter at 25 degrees Celsius (µS/cm). This unit is equivalent to micromhos per centimeter at 25 degrees Celsius (µmho/cm), formerly used by the U.S. Geological Survey.

unfiltered sample: Sometimes referred to as a wholewater or raw sample--pertains to a water sample collected for subsequent chemical or physical analysis without undergoing a phase-separation procedure.

Chemical Symbols and Formulas

CaCO3calcium carbonate
CO2carbon dioxide
CO32-carbonate ion
H+hydrogen ion
H2SO4sulfuric acid
HClhydrochloric acid or hydrogen chloride
HCO3-bicarbonate ion
HNO3nitric acid
K4Fe(CN)6·3H20potassium ferrocyanide
K3Fe(CN)6potassium ferricyanide
KClpotassium chloride
Na2CO3sodium carbonate
NaOHsodium hydroxide
OH-hydroxide ion


ALK alkalinity
ANC acid neutralizing capacity
ASTM American Society for Testing and Materials
ATC automatic temperature compensator
AU Attenuation Unit (see Turbidity, NFM 6.7)
BNC bayonet nut connector
BU Backscatter Unit
DIW deionized water
DO dissolved oxygen
EDI equal-discharge increment
EDTA ethylene diaminetetracetic acid
emf electromotive force
EWI equal-width increment
FAU Formazin Attenuation Unit (see Turbidity, NFM 6.7)
FBU Formazin Backscatter Unit (See Turbidity, NFM 6.7)
FNMU Formazin Nephelometric Multibeam Unit (See Turbidity, NFM 6.7)
FNRU Formazin Nephelometric Ratio Unit (see Turbidity, NFM 6.7)
FNU Formazin Nephelometric Unit (see Turbidity, NFM 6.7)
FTU Formazin turbidity unit
HIF Hydrologic Instrumentation Facility
IPT inflection-point titration
ISO International Standards Organization
LS land surface
MI multiparameter instruments
MP measuring point (for water level measurements)
MSDS Material Safety Data Sheets
N normal
NASQAN National Stream Quality Accounting Network
NAWQA National Water-Quality Assessment Program
NFM National Field Manual for the Collection of Water-Quality Data
NIST National Institute of Standards and Technology
NTMU Nephelometric Turbidity Multibeam Unit
NTRU Nephelometric Turbidity Ratio Unit
NWIS National Water Information System of the USGS
NWQL National Water Quality Laboratory of the USGS
OWQ Office of Water Quality of the USGS
PAO phenylarsine oxide
PTFE polytetrafluoroethylene polymer (a variety of TeflonŠ)
QA quality assurance
QC quality control
QW quality of water, or water quality
QWDATA Water-Quality Data Processing Routine (part of the USGS NWIS system)
redox reduction-oxidation potential (also referred to as oxidation-reduction potential, or ORP)
SC specific electrical conductance (conductivity)
STORET Storage and Retrieval (USEPA Water-Quality Data Management System)
T temperature
TBY turbidity
TC to contain
TD to deliver
TWRI Techniques of Water-Resources Investigations
USEPA U.S. Environmental Protection Agency
USGS U.S. Geological Survey
WRD Water Resources Division of the U.S. Geological Survey
YSI Yellow Springs Instrument Company

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Last Modified: 27Sept06

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