Transcript What is metrology? - American Industrial Hygiene Association

Calibration of
Industrial Hygiene Instruments
David Silver, CIH
Industrial Hygiene Issues
• Accurate & repeatable measurements.
• Analytical results and confidence limits.
• Uncover the mystery of annual
calibrations.
• Field calibrations vs. annual calibrations.
Successful Outcomes
• Confident that instruments are performing
as they should.
• Results are accurate and repeatable.
• The analysis holds up to litigation.
• Accurate data provides a mean to
establish effectiveness of controls –
– Ventilation
– Work practices
Presentation Outline
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Calibration & metrology defined.
Primary Standards.
Uncertainty.
How industrial hygiene instruments are
calibrated.
Metrology Defined
Metrology establishes the international
standards for measurement used by all
countries in the world in both science and
industry
Examples: distance, time, mass,
temperature, voltage, values of physical and
chemical constants
Significance of Metrology
• Measurement & calibration procedures are
essential for quality control.
• Quality – minimize uncertainty in
measurements.
• Quality control system –
Direct reading instrument, sampling.
Measurement or analysis.
Results – variability.
Quality Systems
• Say what you do, do what you say.
– Standard operating procedures (SOPs)
– Calibration Procedures
– Work instructions
• International Standards Organization (ISO)
• ANSI Z540
Calibration Procedures
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Performance requirements – specs
Measurement standards – accuracy std
Preliminary operations – intrinsic safety
Calibration process – tolerances
Calibration results- documentation
Closing operation – labeling
Storage & handling – to ensure accuracy
Time Line
• Ancient Measurement – need to
standardize weights, weapons
• 732 A.D. – King of Kent – standard acre
• 1585 – Decimal system in Europe
• 1824 – George IV – Weights & Measures
Act
• 1958 – All countries agree on length and
mass
Measurement Philosophy
• Standardization is paramount.
• True value of a dimension.
– Speed of light, electron mass.
• Absolute units are a foundation for
standardization.
• Primary laboratories provide the standards
that are closest to the true value. Has the
least uncertainty.
Absolute Values
• Electric constant
• Magnetic constant
• Speed of light in a vacuum
Etc..
Clear Communication of Data
• Scientific Data in units understandable to
all in the scientific community.
• Allows for greater understanding,
compliance with occupational, safety and
health laws.
SI: The International System of
Units
Seven base units:
Lots of derived units:
Length: meter (m)
Area: m2
Mass: kilogram (kg)
Speed: m/s
Force: 1 Newton = 1 kg·m/s2
Time: second (s)
Electric current: ampere (A)
Voltage: 1 volt = 1 m2·kg/s3·A
Frequency: 1 hertz = 1/s
Thermodynamic temperature: Kelvin (K)
Power: 1 watt = 1 kg·m2/s3
Amount of substance: mole (mol)
Electric Charge: 1 C = 1 A·s
Luminous intensity: candela (cd)
Standards Accuracy
• More accurate methods to measure a unit
than intuitive common methods.
• Example – 1 kilogram
– Subjective – hold in hand & guess weight.
– Pan or spring balance – more accurate.
– Watt-balance – even more accurate.
– Avogadro’s number - # of atoms in a kilogram,
count them (not possible).
Clocks: Atomic time
One part per quadrillion accuracy!!!
Accurate frequency gives accurate
distance and time.
Artifact vs. quantum standards:
A metal bar:
1889-1960
The modern meter:
The meter is the length of the path
traveled by light in vacuum during
a time interval of 1/299,792,458 of
a second
The modern kilogram
The SI
kilogram
drifts!
Mass - possible replacements
Goal: 10 parts per billion accuracy
Avogadro’s number
6.0221415 × 1023
Watt-balance
Temperature: Kelvin, Celsius, and
Fahrenheit
21 C
70 F
294 K
0C
32 F
273.15 K
Water freezes
-196 C
-321 F
77 K
Air liquefies
4.2 K
Helium liquefies
-269 C
-452 F
-273.15 C -459.67 F
0K
Room temperature
Absolute zero
The Kelvin: the SI unit
The Kelvin, unit of thermodynamic temperature,
is the fraction 1/273.16 of the thermodynamic
temperature of the triple point of water.
(0.006 atm)
Primary Laboratories
Most technologically advanced
countries.
From Article I, section 8 of the U.S
Constitution:
“The Congress shall have Power To…
…fix the Standard of Weights and
Measures;”
Traceability
• Unbroken chain of comparison to national
standard.
• Measure uncertainty for each step in the
calibration chain.
• Documentation of procedures and results
for each step in the chain.
• Competence of each lab performing
calibrations.
Traceability
• Reference to SI units (National Primary
Laboratory).
• Re-calibration at appropriate intervals to
ensure accuracy of test instruments.
Calibration Standards
• National standard provides the basis for
fixing a value.
• Primary standard – highest metrological
standard (NIST).
• Secondary – based on comparisons to
primary.
• Reference – standard at a location
(metrology labs with NIST calibrated stds).
Calibration Standards
• Working standard – a standard not
reserved as a reference standard but
intended to verify test equipment.
• Transfer standard – the same as a
reference standard and transfers a
measurement parameter from one
organization to another for traceability
purposes.
Equipment Specifications
• Tolerance – a design feature that
defines the limits of a quality
characteristic.
• Specification – defines the
expected performance limits of a
large group of identical test units.
Uncertainty
• Goal – minimize measurement uncertainty.
• Measurement validity depends on random
distributions, fixed models, fixed variation
and fixed distribution curves.
• Central tendency.
• Linear and non-linear interpolation.
Step 1: Determine the
uncertainty contributors
• Each element in the chain of calibration.
• Example – soap film calibrator.
– Dimensional volume.
– Timer.
– Operator start stop timer at bubble mark.
– Variable flow in air mover.
– Drag on soap bubble.
Step 2: Determine Contribution.
• Dimensional error – Type B buret is 6
ml/1000ml = 0.6%.
• Timer = +/1 one minute per year =
negligible.
• Stop Start operator = +/- 0.5 seconds x 2 =
1 second. 10% for 10 second run.
• Variable flow in air mover = 0.1 lpm for 5
lpm pump = 2%.
95% Uncertainty
• Combined standard deviation = sq.rt. (0.62
+ 102 + 22) = 10.21
• Uncertainty 95% = k * s =
• 2 * 10.21 = 20.42 %
• By using an electro-optical sensor we
reduce the 10 % operator error.
Measurement Methods
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Direct
Differential
Indirect
Ratio
Reciprocity
Substitution
Transfer
Direct
• Direct – Measurement
that is in direct contact
with the measurand
and provides a value
representative of the
measurand as read
from an indicating
device.
• Example – measuring
electrode resistance of
a moisture meter.
Differential
• Differential – A measurement made by
comparing an unknown measurand with a
standard.
• Example – comparing reading from a heat
stress monitor and compare to a NIST
traceable thermometer.
Indirect
• Indirect – a measurement made of a nontargeted measurand that is used to
determine the value of the targeted
measurand.
• Example – measuring the time a piston
traverses a cylindrical volume in a piston
prover and calculating flow.
Reciprocity
• Reciprocity – makes use
of a transfer function
relationship in comparing
two or more
measurement devices
subject to the same
measurand.
• Example – determination
of microphone sensitivity
via the response of
another microphone.
Substitution
• Substitution – using a known standard to
establish a measurand value after the
known standard is removed and the test
unit is inserted to determine the test unit
response.
• Example – measuring weight using a
single pan scale.
Transfer
• Transfer – an
intermediate device
used for conveying a
known measurand
value to an unknown
test device.
• Example – generating
a known volume of gas
to a test gas meter.
Industrial Hygiene
Measurements
• Flow – bell prover, flow test stand, flow
calibrator.
• Frequency – time bases, frequency
standards.
• Humidity – environmental chamber, salts.
• Luminance – calibrated light source.
• Temperature – chamber, triple point of
water.
Flow Calibration
Soap bubble meter.
Pump is attached to the top of a
volumetric glass tube containing
a small amount of liquid soap.
While the air flow causes the
soap film to move from one
volume mark to another, the
travel time is measured with a
stopwatch. The flow rate can
then be directly calculated using
the travel time and the known
tube volume.
• ±2% per reading volumetric
calibrations.
Flow Calibration
• High-speed,
hands-free
measurements.
• 3 Cells
• ±1% per
reading
volumetric
calibrations.
Calibration of Flow Calibrators
• Brooks Vol-U-Meter
• Precision bore
borosilicate glass
cylinder combined with
photo-electric switches.
• Mercury O-ring piston
seal is virtually
frictionless. Accuracy =
0.2% of indicated
volume.
Calibration of Velocity Meters
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Wind Tunnels
Laminar Flow
Comparative
Referent
velocity
pressure
Calibration of Heat Stress
Monitors
• Chamber –
cold/hot
• NIST traceable
Instrulab
platinum
resistance
thermometer
Platinum Resistance
Thermometer
• Platinum RTD sensor, 100
ohms.
• Instrument + sensor
accuracy up to ±0.08ºC.
• Resolution up to 0.01ºC.
• Wide range: -60º to
+300ºC, -76ºF to +572ºF.
• Self-check calibration.
• Traceable to NIST.
Calibration of Sound Level
Meters & Noise Dosimeters
• ANSI
Standards.
• Accuracy of dB
measurements,
response time
and frequency.
• Anechoic
Chamber
Acoustic Laboratory
• Sound level meters, noise dosimeters,
microphones, octave filters and microphones.
• Frequency response calibration of microphones
using electrostatic and acoustical method
• Sensitivity calibration of microphones using the
insert voltage method.
• Sound level meter calibration in ANSI 1.4
• Test of fractional octave filters.
Calibration of Mass
Concentration Meters
• Arizona Road
Dust Standard.
• Laminar flow
chamber.
• Comparative
Standard –
R&P 1400A
R&P 1400a
• TSI 3400 Fluidized Aerosol Generator
maintains Arizona road dust
concentrations in laminar flow chamber.
• Particle Mass is proportional to frequency
of tapered element.
• Highly precise and accurate.
• Mass calibration is NIST traceable.
Calibration of Optical Particle
Counters
• ASTM Standard
• Spherical Latex
Particles
• Aerosol Generator
• Mini-Chamber
• Classifier.
• Bi-polar ion
generator.
• Referent CNC /
OPC.
Polymer Particle Standards
• Duke Scientific's
standards contains a
Certificate of
Calibration and
Traceability to NIST
which includes a
description of the
calibration method
and its uncertainty,
and a table of
chemical and physical
properties.
Calibration of Gas Meters
• “Canned Gas” –
most common.
• Permeation gas
– advantages:
– Long shelf life.
– Physical
principals.
– Repeatable.
Permeation Tubes
• Permeation devices
provide a stable
concentration of a
specific trace chemical,
including those with low
vapor pressures.
Calibration gas
generators, used with
their respective
permeation devices,
generate known
concentrations of
various gases and liquid
vapors.