1. UNIVERSITY OF HOUSTON - CLEAR LAKE SPRING 2015
INDUSTRIAL HYGIENE - PRINCIPLES AND INSTRUMENTATION FOR CALIBRATING AIR SAMPLING EQUIPMENT
UNIVERSITY OF HOUSTON - CLEAR LAKE
SPRING 2015

2. PURPOSE
The ACCURACY and PRECISION of any air sampling procedure can be only as good as the Sampling and Analytical Error(s) [SAE] associated with the referenced method. Difference between the air contaminant concentration reported (from meter reading or lab analysis) and the true concentration represents the OVERALL error of the measurement.

3. ERRORS
Overall error may be related to a number of smaller components rather than single cause. To minimize, analyze each potential component and concentrate efforts on reducing the largest component error. Largest portion of sampling error is frequently due to flow rate and therefore, underestimation/ overestimation of total air volume sampled. To define exposure, quantity of 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
Developed by manufacturers to address 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 certain disciplines (e.g. electrical and electronic).

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)
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 internal dimensions of the calibrator.
Primary standards
Intermediate standards
Secondary standards
Intermediate standards now included as part of 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. Included in secondary standards.

10. SECONDARY STANDARDS
Devices for general use that are calibrated against primary or intermediate standards. Typically more portable, rugged, and versatile than other devices and have accuracies of +/- 5% or better. The need for recalibration depends on 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 since potential contamination may exist during process. 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 sample validity may be affected; with a difference of less than 5%, the flow rate used for total volume calculations will be determined by professional judgment. Routine utilization of pre/post-average. Lower flow rate number – calculation errs for increased worker protection; for enforcement – larger flow rate used for calculation errs on side of employer.

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

14. HIERARCHY AND TRACEABILITY
Traditional concept of measurement traceability in U.S. focused on unbroken hierarchical pathway of measurements, that leads, ultimately, to a national standard. Examples – calibration curve; primary/secondary calibration standards; lab calibration/vendor with certificate, etc. Involves 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 calb standard used - Specification of calb environmental conditions when correction factors should be applied if standard or equipment were used under different environ conditions. Maintain records with NIST certification.

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 total volume (V) of gas passed through the meter during operation. The time period (t) of operation and the temperature and pressure of the gas moved through meter are also measured. The average flow rate (Q) is derived from Q = V divided by t.

18. SPIROMETERS
Spirometers include instruments that measure volume directly and also those that measure velocity or pressure differences with electronic conversion to volume. Not commercially available; found in lab and universities; function as primary standard calibrators and also training tools.

19. SPIROMETER

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

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

22. DISPLACEMENT BOTTLE

23. 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 variety of materials which directly impact meter cost, accuracy, and portability.

24. 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%

25. PISTON METERS

26. BUBBLE FLOW METER

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

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

29. VARIABLE HEAD METERS
- Venturi meters – devices that produce a pressure differential caused by restriction in airflow stream. - Critical flow orifice – widely used; 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 calibration curve.

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

31. ROTAMETER

32. 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. Calibration with primary standard; generate calibration curve with temp & pressure.

33. 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. Flow meters with a sufficiently low flow resistance usually bulky and/or expensive. A common metering element is the by-pass rotameter which measures small fraction of the total flow that is proportional to total flow.

34. VELOCITY METERS
Because flow profile is rarely uniform, the measured velocity invariably differs from the average velocity. Shape of the flow profile usually changes with flow rate, and the ratio of point-to-average velocity also changes. - Mass flow meters - Thermo-anemometers - Pitot tubes - Other velocity meters.

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

36. THERMO-ANEMOMETERS
Anemometer is used to measure velocity. Heated element anemometer uses flowing air that cools sensor in proportion to air velocity; essentially non-directional with single element probes that measure airspeed, but not direction. Usually have reference elements to provide output used to compensate or correct errors due to temperature variations.

37. PITOT TUBES
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 in a plane that is eight diameters from opening. The difference between the static and impact pressure is velocity pressure. Several serious limitations in most sampling flow calibrations.

38. CALIBRATION PROGRAM
Each element of sampling system should be calibrated accurately prior to initial field use. Protocols established for periodic re-calibration since performance changes with accumulation of direct corrosion, leaks, and mis-alignment due to vibration/shocks. Frequency should initially be high until experience is accumulated to show that safe reductions can be employed.

39. CALIBRATION NEED AND FREQUENCY
- Instrument characteristics – sensitivity and experience with stability under similar use - Equipment use – rough handling, moving, heavy usage, and changing environments necessitate frequent calibration - Various users – multiple persons, and different skills and experience. Document nature and frequency of calibrations and meet legal and scientific requirements. [e.g. EPA, NIOSH, OSHA]. Formalized calibration audits, and consideration of systematic framework for documented procedures.

40. CALIBRATION PROGRAM
Effective program performed: - with use of documented and controlled procedures by competent individual; - in a repeatable manner; and, - under controlled conditions. Repeated unambiguously and meet defined traceability requirements. Safety assurance, calibrations should have an effective quality system with references.