Transcript WORKPLACE EXPOSURE ASSESSMENT AND FIELD …

UNIVERSITY OF HOUSTON - CLEAR LAKE
SPRING 2015
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.
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.
“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.
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).
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).
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.
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.
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.
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.
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.
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.
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.
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].
-
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.
-
Spirometers and Meter Provers
Displacement Bottle
Frictionless Piston Meters
Soap-Film Pistons or Bubble Meters
Mercury-Sealed Pistons
Glass and Graphite Pistons
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.
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.
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.
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..
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.
- 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%
- 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
- 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.
- 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.
- 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 13.15 insert!
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.
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.
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.
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.
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.
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.
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.
- 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.
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.