INDUSTRIAL HYGIENE - PRINCIPLES AND INSTRUMENTATION FOR CALIBRATING AIR SAMPLING EQUIPMENT

1. INDUSTRIAL HYGIENE -PRINCIPLES AND INSTRUMENTATION FOR CALIBRATING AIR SAMPLING EQUIPMENT UNIVERSITY OF HOUSTON - CLEAR LAKE

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

3. ERRORS The overall error may be due to a number of smaller component errors rather than a single cause. To minimize the overall error, it may be necessary to analyze each of the potential components and then concentrate efforts on reducing the largest component error. Largest portion of sampling error is frequently due to flow rate of air and therefore, underestimation/ overestimation of the total volume of air passed through the sampler. To define exposure, the quantity of the contaminant per unit volume of air must be accurately measured.

4. CALIBRATION DEFINITION “Set of operations which establishes, under specified conditions, the relationship between values indicated by a measuring instrument or measuring system, and the corresponding standard or known values derived from the standard”. [ANSI] OR: Calibration process is a comparison of one instrument’s response with that of a reference instrument of known response and accuracy.

5. CALIBRATION PROCEDURES Potentially developed by manufacturers to address the need for reliable valid sampling equipment. Primary or secondary calibration equipment is traceable to NIST following procedures by ANSI, ASTM, ISA, IEC, ISO, CEN, etc. ISO provides procedural guidance and fills the need for standards outside of the electrical and electronic disciplines.

6. TYPES OF CALIBRATORS Differentiated by type of measurement: • Volume meters – displacement bottles; spirometers; wet test/dry gas meters; • Flow rate meters – variable-head (e.g. orifice meters) and variable-area meters (e.g. rotameters); and, • Velocity meters – type of flow rate meters; respond by measuring velocity at a particular point of the airflow cross-section (e.g. mass flow meters and pitot tubes).

7. FLOW RATE CALIBRATORS Classification system is based on the accuracy and ability to directly measure the internal dimensions of the calibrator. • Primary standards • Intermediate standards • Secondary standards. Intermediate standards are now included in the secondary standards category.

8. PRIMARY STANDARDS Devices for which measuring volume can be accurately determined by measurement of internal dimensions alone. The accuracy of this type of meter is +/- 1% or better.

9. INTERMEDIATE STANDARDS Devices that are more versatile than primary standards, but for which physical dimensions cannot be easily measured. Intermediate standards are calibrated against primary standards under controlled laboratory conditions. The accuracy of this category of device is usually +/- 2% or better. Most of this group included in secondary standards.

10. SECONDAY STANDARDS Devices for general use that are calibrated against primary or intermediate standards. Typically more portable, rugged, and versatile than devices in the other two categories. These devices generally have accuracies of +/- 5% or better. The need for recalibration depends on the amount of handling, frequency of use, and type of operational environment. Example: rotameters – every 3 months

11. CALIBRATION Calibration of air sampling pumps is performed before and after each sampling event with media “in-line”, as the media provides resistance affecting the flow rate. Individual sample media is not used for calibration and sampling as the individual media may become contaminated during calibration from potential airborne exposure. OSHA requires performance within 5% based on calculation of SAE.

12. CALIBRATION RECORDS Pre/Post-calibration records maintained. When difference of more than 5%, the validity of the samples may be affected; with a difference of less than 5%, the flow rate used for total volume calculations will be determined by professional judgment. For most instances, pre/post-average is utilized. Lower flow rate number – calculation errs for increased worker protection; for enforcement – larger flow rate used for calculation errs on the side of the employer.

13. CERTIFICATION For air sampling equipment, may be done by NIOSH, Mine Safety and Health Administration (MSHA), or by a contract testing laboratory such at Underwriters Laboratories (UL), or by third party testing such as Safety Equipment Institute (SEI). These organizations may certify the equipment or oversee part or all of the assembly process.

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

15. NIST AUDIT TRAIL - The assigned value - A stated uncertainty - Identification of the standard used in the calibration - The specifications of any environmental conditions of the calibration when correction factors should be applied if the standard or equipment were to be used under different environmental conditions. Maintain records with certification to NIST.

16. PRIMARY STANDARDS - Spirometers and Meter Provers - Displacement Bottle - Frictionless Piston Meters - Soap-Film Pistons or Bubble Meters - Mercury-Sealed Pistons - Glass and Graphite Pistons

17. PRIMARY STANDARDS Spirometers and Meter Provers Measure the total volume (V) of a gas that is passed through the meter during operation. The time period (t) of operation and the temperature and pressure of the gas passed through the meter also are measured. The average flow rate (Q) is derived from Q = V divided by t.

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

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

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

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

22. TYPES OF PISTON METERS - Bubble meter – vacuum source, pump, connected to graduated tube with use of soap/water solution; flow rate (volume displacement per unit time) by measuring time for bubble to pass a known volume; accurate +/- 1%. Also electronically determined volumes; need annual calibration. - Mercury-sealed pistons – lab use only - Glass and graphite pistons – 1-2%

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

24. SECONDARY STANDARDS - Variable-head meters - fixed restriction because the differential pressure varies with flow. - Variable-area meters – a constant pressure differential is maintained by varying the cross-sectional area of the meter; e.g. rotameters. AS FLOW RATE METERS Flow rate meters operate on the principle of the conservation of energy.

25. VARIABLE HEAD METERS - Venturi meters – devices that produce a differential in pressure caused by a restriction in the airflow stream. - Critical flow orifice – widely used with nozzles; only one flow possible and pressure differential is HIGH; can clog and erode over time and require regular examination/calibration against other reference meters as part of a program; involves use of a calibration curve.

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

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

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

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

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

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

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

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

34. CALIBRATION NEED AND FREQUENCY - Instrument characteristics – sensitivity and experience with stability under similar use - Instrument use – rough handling, moving, heavy usage, and changing environments necessitate frequent calibration - Instrument users – multiple users and users of various skill and experience Document nature and frequency of calibrations and check to meet legal as well as scientific requirements. [e.g. EPA, NIOSH, OSHA]. Formalized calibration audits, and consideration of systematic framework for calibration procedures.

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