INDUSTRIAL HYGIENE -INDUSTRIAL HYGIENE - PRINCIPLES AND PRINCIPLES AND

INSTRUMENTATION FOR INSTRUMENTATION FOR CALIBRATING AIR SAMPLING CALIBRATING AIR SAMPLING

EQUIPMENTEQUIPMENT

UNIVERSITY OF HOUSTON - CLEAR LAKEUNIVERSITY OF HOUSTON - CLEAR LAKE

PURPOSEPURPOSE

The accuracy and precision of any air The accuracy and precision of any air sampling procedure can be only as good sampling procedure can be only as good as the sampling and analytical error(s) as the sampling and analytical error(s) that are associated with the sampling that are associated with the sampling method. The difference between the air method. The difference between the air concentration reported for an air concentration reported for an air contaminant (on the basis of a meter contaminant (on the basis of a meter reading or lab analysis) and the true reading or lab analysis) and the true concentration at that time and place concentration at that time and place represents the overall error of the represents the overall error of the measurement. measurement.

ERRORS ERRORS The overall error may be due to a number of The overall error may be due to a number of

smaller component errors rather than a single smaller component errors rather than a single cause. To minimize the overall error, it may be cause. To minimize the overall error, it may be necessary to analyze each of the potential necessary to analyze each of the potential components and then concentrate efforts on components and then concentrate efforts on reducing the largest component error. reducing the largest component error.

Largest portion of sampling error is frequently due Largest portion of sampling error is frequently due to flow rate of air and therefore, underestimation/ to flow rate of air and therefore, underestimation/ overestimation of the total volume of air passed overestimation of the total volume of air passed through the sampler. To define exposure, the through the sampler. To define exposure, the quantity of the contaminant per unit volume of air quantity of the contaminant per unit volume of air must be accurately measured.must be accurately measured.

CALIBRATION DEFINITIONCALIBRATION DEFINITION““Set of operations which establishes, under Set of operations which establishes, under specified conditions, the relationship specified conditions, the relationship between values indicated by a measuring between values indicated by a measuring instrument or measuring system, and the instrument or measuring system, and the corresponding standard or known values corresponding standard or known values derived from the standard”. [ANSI]derived from the standard”. [ANSI]

OROR: Calibration process is a comparison of : Calibration process is a comparison of one instrument’s response with that of a one instrument’s response with that of a reference instrument of known response reference instrument of known response and accuracy. and accuracy.

CALIBRATION PROCEDURESCALIBRATION PROCEDURES

Potentially developed by manufacturers to Potentially developed by manufacturers to address the need for reliable valid sampling address the need for reliable valid sampling equipment. equipment.

Primary or secondary calibration equipment Primary or secondary calibration equipment is traceable to NIST following procedures by is traceable to NIST following procedures by ANSI, ASTM, ISA, IEC, ISO, CEN, etc. ANSI, ASTM, ISA, IEC, ISO, CEN, etc.

ISO provides procedural guidance and fills ISO provides procedural guidance and fills the need for standards outside of the the need for standards outside of the electrical and electronic disciplines. electrical and electronic disciplines.

TYPES OF CALIBRATORSTYPES OF CALIBRATORS

Differentiated by type of measurement: Differentiated by type of measurement: Volume meters – displacement bottles; Volume meters – displacement bottles;

spirometers; wet test/dry gas meters; spirometers; wet test/dry gas meters; Flow rate meters – variable-head (e.g. Flow rate meters – variable-head (e.g.

orifice meters) and variable-area meters orifice meters) and variable-area meters (e.g. rotameters); and,(e.g. rotameters); and,

Velocity meters – type of flow rate meters; Velocity meters – type of flow rate meters; respond by measuring velocity at a respond by measuring velocity at a particular point of the airflow cross-section particular point of the airflow cross-section (e.g. mass flow meters and pitot tubes). (e.g. mass flow meters and pitot tubes).

FLOW RATE CALIBRATORSFLOW RATE CALIBRATORS

Classification system is based on the Classification system is based on the accuracy and ability to directly measure the accuracy and ability to directly measure the internal dimensions of the calibrator.internal dimensions of the calibrator.

Primary standards Primary standards Intermediate standards Intermediate standards Secondary standards. Secondary standards.

Intermediate standards are now included in Intermediate standards are now included in the secondary standards category.the secondary standards category.

PRIMARY STANDARDSPRIMARY STANDARDSDevices for which measuring volume can be Devices for which measuring volume can be accurately determined by measurement of accurately determined by measurement of internal dimensions alone.internal dimensions alone.

The accuracy of this type of meter The accuracy of this type of meter

is +/- 1% or better. is +/- 1% or better.

INTERMEDIATE STANDARDSINTERMEDIATE STANDARDSDevices that are more versatile than primary Devices that are more versatile than primary standards, but for which physical standards, but for which physical dimensions cannot be easily measured. dimensions cannot be easily measured. Intermediate standards are calibrated Intermediate standards are calibrated against primary standards under controlled against primary standards under controlled laboratory conditions. laboratory conditions.

The accuracy of this category of device is The accuracy of this category of device is usually +/- 2% or better.usually +/- 2% or better.

Most of this group included in secondary Most of this group included in secondary standards. standards.

SECONDAY STANDARDSSECONDAY STANDARDSDevices for general use that are calibrated Devices for general use that are calibrated against primary or intermediate standards. against primary or intermediate standards. Typically more portable, rugged, and Typically more portable, rugged, and versatile than devices in the other two versatile than devices in the other two categories. These devices generally have categories. These devices generally have accuracies of +/- 5% or better.accuracies of +/- 5% or better.

The need for recalibration depends on the The need for recalibration depends on the amount of handling, frequency of use, and amount of handling, frequency of use, and type of operational environment. type of operational environment.

ExampleExample: rotameters – every 3 months: rotameters – every 3 months

CALIBRATIONCALIBRATIONCalibration of air sampling pumps is Calibration of air sampling pumps is performed before and after each sampling performed before and after each sampling event with media “in-line”, as the media event with media “in-line”, as the media provides resistance affecting the flow rate. provides resistance affecting the flow rate.

Individual sample media is not used for Individual sample media is not used for calibration and sampling as the individual calibration and sampling as the individual media may become contaminated during media may become contaminated during calibration from potential airborne exposure. calibration from potential airborne exposure.

OSHA requires performance within 5% OSHA requires performance within 5% based on calculation of SAE. based on calculation of SAE.

CALIBRATION RECORDSCALIBRATION RECORDSPre/Post-calibration records maintained. Pre/Post-calibration records maintained. When difference of more than 5%, the When difference of more than 5%, the validity of the samples may be affected; with validity of the samples may be affected; with a difference of less than 5%, the flow rate a difference of less than 5%, the flow rate used for total volume calculations will be used for total volume calculations will be determined by professional judgment. For determined by professional judgment. For most instances, pre/post-average is utilized. most instances, pre/post-average is utilized.

Lower flow rate number – calculation errs for Lower flow rate number – calculation errs for increased worker protection; for increased worker protection; for enforcement – larger flow rate used for enforcement – larger flow rate used for calculation errs on the side of the employer. calculation errs on the side of the employer.

CERTIFICATIONCERTIFICATIONFor air sampling equipment, may be done For air sampling equipment, may be done by NIOSH, Mine Safety and Health by NIOSH, Mine Safety and Health Administration (MSHA), or by a contract Administration (MSHA), or by a contract testing laboratory such at Underwriters testing laboratory such at Underwriters Laboratories (UL), or by third party testing Laboratories (UL), or by third party testing such as Safety Equipment Institute (SEI). such as Safety Equipment Institute (SEI).

These organizations may certify the These organizations may certify the equipment or oversee part or all of the equipment or oversee part or all of the assembly process. assembly process.

HIERARCHY AND TRACEABILITYHIERARCHY AND TRACEABILITY

The traditional concept of measurement The traditional concept of measurement traceability in the U.S. has focused on an traceability in the U.S. has focused on an unbroken hierarchical pathway of unbroken hierarchical pathway of measurements, that leads, ultimately, to a measurements, that leads, ultimately, to a national standard. national standard. ExampleExample – calibration curve; – calibration curve; primary/secondary calibration standards; primary/secondary calibration standards; lab calibration/vendor with certificate, etc.lab calibration/vendor with certificate, etc.Involved some form of traceability back to Involved some form of traceability back to an acceptable reference standard of known an acceptable reference standard of known accuracy. [i.e. NIST]accuracy. [i.e. NIST]

NIST AUDIT TRAILNIST AUDIT TRAIL-- The assigned value The assigned value

- - A stated uncertaintyA stated uncertainty

- - Identification of the standard used in the Identification of the standard used in the calibration calibration

- The specifications of any environmental - The specifications of any environmental conditions of the calibration when conditions of the calibration when correction correction factors should be applied if the factors should be applied if the standard standard or equipment were to be used or equipment were to be used under under different environmental conditions. different environmental conditions.

Maintain records with certification to NIST. Maintain records with certification to NIST.

PRIMARY STANDARDSPRIMARY STANDARDS

-- Spirometers and Meter Provers Spirometers and Meter Provers

- - Displacement BottleDisplacement Bottle

- - Frictionless Piston Meters Frictionless Piston Meters

-- Soap-Film Pistons or Bubble Meters Soap-Film Pistons or Bubble Meters

-- Mercury-Sealed PistonsMercury-Sealed Pistons

-- Glass and Graphite Pistons Glass and Graphite Pistons

PRIMARY STANDARDSPRIMARY STANDARDSSpirometers and Meter ProversSpirometers and Meter Provers

Measure the total volume (V) of a gas that is Measure the total volume (V) of a gas that is passed through the meter during operation. passed through the meter during operation.

The time period (t) of operation and the The time period (t) of operation and the temperature and pressure of the gas passed temperature and pressure of the gas passed through the meter also are measured.through the meter also are measured.

The average flow rate (Q) is derived from The average flow rate (Q) is derived from

Q = V divided by t.Q = V divided by t.

SPIROMETERSSPIROMETERSSpirometers include instruments that Spirometers include instruments that measure volume directly as well as those measure volume directly as well as those that measure velocity or pressure that measure velocity or pressure differences and convert these indicators differences and convert these indicators through the use of electronics to a volume through the use of electronics to a volume reading. No longer commercially available. reading. No longer commercially available. Found in commercial laboratory and Found in commercial laboratory and university settings and function as primary university settings and function as primary standard calibrators and training tools. standard calibrators and training tools. Calibrated at factory using a volume syringe.Calibrated at factory using a volume syringe.

METER PROVERSMETER PROVERSProven tank capacity used to check the Proven tank capacity used to check the volumetric accuracy of a gas or liquid that is volumetric accuracy of a gas or liquid that is delivered by a positive-displacement meter. delivered by a positive-displacement meter. Primary volume standards similar to Primary volume standards similar to spirometers, but bell provers are designed spirometers, but bell provers are designed to specifically function as primary volume to specifically function as primary volume calibrators. Employ a low vapor pressure oil calibrators. Employ a low vapor pressure oil seal instead of water and an internal bell or seal instead of water and an internal bell or tank to reduce the overall volume of the tank to reduce the overall volume of the liquid used to make the seal.liquid used to make the seal.

DISPLACEMENT BOTTLEDISPLACEMENT BOTTLEProver bottle is a volume and flow rate Prover bottle is a volume and flow rate calibrator that operates similarly to the bell calibrator that operates similarly to the bell prover, except that it measures displaced prover, except that it measures displaced water instead of displaced gas. Bottle water instead of displaced gas. Bottle usually has a valve at bottom that allows usually has a valve at bottom that allows water to be drained which draws air into water to be drained which draws air into bottle in response to lowered pressure. Air bottle in response to lowered pressure. Air volume drawn in is equal to the change in volume drawn in is equal to the change in water level multiplied by the cross-section at water level multiplied by the cross-section at the water surface. Or collect water and the water surface. Or collect water and measure time to displace a set volume, etc..measure time to displace a set volume, etc..

PISTON METERSPISTON METERSFrictionless piston meters are cylindrical air Frictionless piston meters are cylindrical air displacement meters that use nearly displacement meters that use nearly frictionless pistons to measure flow rates as frictionless pistons to measure flow rates as primary flow calibrators. Pistons form gas-primary flow calibrators. Pistons form gas-tight seals of negligible weight and friction tight seals of negligible weight and friction and made from a variety of materials which and made from a variety of materials which directly impact the meter cost, accuracy, directly impact the meter cost, accuracy, and portability.and portability.

TYPES OF PISTON METERSTYPES OF PISTON METERS-- Bubble meter – vacuum source, pump, Bubble meter – vacuum source, pump, connected to graduated tube with use of connected to graduated tube with use of soap/water solution; flow rate (volume soap/water solution; flow rate (volume displacement per unit time) by measuring displacement per unit time) by measuring time for bubble to pass a known volume; time for bubble to pass a known volume; accurate +/- 1%.accurate +/- 1%.

Also electronically determined Also electronically determined volumes; need annual calibration.volumes; need annual calibration.-- Mercury-sealed pistons – lab use only Mercury-sealed pistons – lab use only -- Glass and graphite pistons – 1-2%Glass and graphite pistons – 1-2%

SECONDARY STANDARDSSECONDARY STANDARDS

-- Wet Test Meters – function primarily as Wet Test Meters – function primarily as a lab calibrating standard; also frequently a lab calibrating standard; also frequently used to meter the flow of other gases used to meter the flow of other gases directly; correct volumes to standard directly; correct volumes to standard conditions. conditions. - - Dry-Gas Meter – second most widely Dry-Gas Meter – second most widely used air flow calibration device; calibrated used air flow calibration device; calibrated against a primary standard; returned to the against a primary standard; returned to the manufacturer for annual calibration. manufacturer for annual calibration.

AS VOLUME METERSAS VOLUME METERS

SECONDARY STANDARDSSECONDARY STANDARDS

-- Variable-head meters - fixed restriction Variable-head meters - fixed restriction because the differential pressure varies because the differential pressure varies with flow.with flow.

- - Variable-area meters – a constant Variable-area meters – a constant pressure differential is maintained by pressure differential is maintained by varying the cross-sectional area of the varying the cross-sectional area of the meter; e.g. rotameters.meter; e.g. rotameters.AS FLOW RATE METERSAS FLOW RATE METERSFlow rate meters operate on the principle of Flow rate meters operate on the principle of the conservation of energy.the conservation of energy.

VARIABLE HEAD METERSVARIABLE HEAD METERS

-- Venturi meters – devices that produce Venturi meters – devices that produce a a differential in pressure caused by a differential in pressure caused by a

restriction in the airflow stream.restriction in the airflow stream.- - Critical flow orifice – widely used with Critical flow orifice – widely used with

nozzles; only one flow possible and nozzles; only one flow possible and pressure differential is HIGH; can clog pressure differential is HIGH; can clog and erode over time and require and erode over time and require

regular regular examination/calibration against examination/calibration against other other reference meters as part of a reference meters as part of a program; program; involves use of a calibration involves use of a calibration curve.curve.

VARIABLE AREA METERSVARIABLE AREA METERS

-- Rotameters – most popular field instruments Rotameters – most popular field instruments for flow rate measurements. for flow rate measurements.

e.g. precision – temperature/pressuree.g. precision – temperature/pressure-- Device consists of a float that is free to move Device consists of a float that is free to move up and down within a vertical tapered tube that up and down within a vertical tapered tube that is is larger at the top than the bottom. Floats are larger at the top than the bottom. Floats are conventionally read at the highest point of conventionally read at the highest point of maximum diameter unless otherwise indicated. maximum diameter unless otherwise indicated. - - A deviation of more than +/- 5% of the A deviation of more than +/- 5% of the calibration value [OSHA] is considered to be a calibration value [OSHA] is considered to be a significant shift. (individual calb or up to 25%).significant shift. (individual calb or up to 25%).-- Accuracy of readings - major limiting factor.Accuracy of readings - major limiting factor.

ROTAMETERSROTAMETERSPrecision rotameters have accurate Precision rotameters have accurate numerical scales and read correctly only at numerical scales and read correctly only at ambient pressure and temperature. ambient pressure and temperature. Restriction to inlet may produce significant Restriction to inlet may produce significant errors in readings – never placed between errors in readings – never placed between sampling media and the pump. Need sampling media and the pump. Need calibration with a primary standard; generate calibration with a primary standard; generate a calibration curve with temperature and a calibration curve with temperature and pressure conditions.pressure conditions.

BY PASS FLOW INDICATORSBY PASS FLOW INDICATORSMeters with Both Variable Head and Meters with Both Variable Head and Variable Area ElementsVariable Area Elements

In most high-volume samplers, the flow rate In most high-volume samplers, the flow rate is strongly dependent on the flow resistance, is strongly dependent on the flow resistance, and flow meters with a sufficiently low flow and flow meters with a sufficiently low flow resistance are usually too bulky or resistance are usually too bulky or expensive. A common metering element is expensive. A common metering element is the by-pass rotameter which measures only the by-pass rotameter which measures only a small fraction of the total flow that is a small fraction of the total flow that is proportional to the total flow. proportional to the total flow.

VELOCITY METERSVELOCITY METERS

Because the flow profile is rarely uniform Because the flow profile is rarely uniform across the channel, the measured velocity across the channel, the measured velocity invariably differs from the average velocity. invariably differs from the average velocity. The shape of the flow profile usually The shape of the flow profile usually changes with changes in flow rate, the ratio changes with changes in flow rate, the ratio of point-to-average velocity also changes. of point-to-average velocity also changes. -- Mass flow meters Mass flow meters - - Thermo-anemometers Thermo-anemometers -- Pitot tubesPitot tubes-- Other velocity meters.Other velocity meters.

VELOCITY METERSVELOCITY METERSA thermal meter measures mass airflow or A thermal meter measures mass airflow or gas flow rate with negligible pressure loss. gas flow rate with negligible pressure loss. Mass flow meters consist of a heating Mass flow meters consist of a heating element in a duct section between two element in a duct section between two points at which the temperature of the air points at which the temperature of the air stream or gas stream is measured. The stream or gas stream is measured. The temperature difference between the two temperature difference between the two points depends on the mass rate of flow and points depends on the mass rate of flow and the heat input.the heat input.

THERMO-ANEMOMETERSTHERMO-ANEMOMETERSAnemometer is any instrument used to Anemometer is any instrument used to measure velocity. Heated element measure velocity. Heated element anemometer uses flowing air that cools the anemometer uses flowing air that cools the sensor in proportion to the velocity of the air. sensor in proportion to the velocity of the air. Essentially non-directional with single Essentially non-directional with single element probes that measure the airspeed element probes that measure the airspeed but not its direction. Usually have reference but not its direction. Usually have reference elements that provides an output that can be elements that provides an output that can be used to compensate or correct errors due to used to compensate or correct errors due to temperature variations. temperature variations.

PITOT TUBESPITOT TUBESA standard pitot tube consists of an impact A standard pitot tube consists of an impact tube with an opening facing axially into the tube with an opening facing axially into the flow and a concentric static pressure tube flow and a concentric static pressure tube with eight holes spaced equally around it in with eight holes spaced equally around it in a plane that is eight diameters from the a plane that is eight diameters from the impact opening. The difference between the impact opening. The difference between the static and impact pressure is the velocity static and impact pressure is the velocity pressure. There are several serious pressure. There are several serious limitations to pitot tube measurements in limitations to pitot tube measurements in most sampling flow calibrations. most sampling flow calibrations.

CALIBRATION PROGRAMCALIBRATION PROGRAMEach element of the sampling system Each element of the sampling system should be calibrated accurately prior to initial should be calibrated accurately prior to initial field use. Protocols also should be field use. Protocols also should be established for periodic re-calibration since established for periodic re-calibration since performance changes with accumulation of performance changes with accumulation of direct, corrosion, leaks, and misalignment direct, corrosion, leaks, and misalignment due to vibration or shocks in handling. The due to vibration or shocks in handling. The frequency should initially be high until frequency should initially be high until experience is accumulated to show that safe experience is accumulated to show that safe reductions can be employed.reductions can be employed.

CALIBRATION NEED AND CALIBRATION NEED AND FREQUENCYFREQUENCY

-- Instrument characteristics – sensitivity and Instrument characteristics – sensitivity and experience with stability under similar useexperience with stability under similar use

- - Instrument use – rough handling, moving, Instrument use – rough handling, moving, heavy usage, and changing environments heavy usage, and changing environments necessitate frequent calibrationnecessitate frequent calibration

-- Instrument users – multiple users and users Instrument users – multiple users and users of of various skill and experiencevarious skill and experienceDocument nature and frequency of calibrations Document nature and frequency of calibrations and check to meet legal as well as scientific and check to meet legal as well as scientific requirements. [e.g. EPA, NIOSH, OSHA].requirements. [e.g. EPA, NIOSH, OSHA].Formalized calibration audits, and consideration of Formalized calibration audits, and consideration of systematic framework for calibration procedures. systematic framework for calibration procedures.

CALIBRATION PROGRAMCALIBRATION PROGRAMEffective program to be performed under a Effective program to be performed under a definite, documented, and controlled definite, documented, and controlled procedure by a competent individual; in a procedure by a competent individual; in a repeatable manner; and under controlled repeatable manner; and under controlled conditions. It must repeated unambiguously conditions. It must repeated unambiguously and meet defined traceability requirements. and meet defined traceability requirements. For safety assurance, the calibrations For safety assurance, the calibrations should have an effective quality system. should have an effective quality system. There should be demonstrated competence There should be demonstrated competence in activities that affect reliability, safety, and in activities that affect reliability, safety, and performance. [ISO 9000, ANSI, etc.]performance. [ISO 9000, ANSI, etc.]