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

• Calibration & metrology defined.

• Primary Standards.

• Uncertainty.

• How industrial hygiene instruments are calibrated.

Metrology DefinedMetrology 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

• 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

Length: meter (m)

Mass: kilogram (kg)

Time: second (s)

Electric current: ampere (A)

Thermodynamic temperature: Kelvin (K)

Amount of substance: mole (mol)

Luminous intensity: candela (cd)

Seven base units: Lots of derived units:

Area: m2

Speed: m/s

Force: 1 Newton = 1 kg·m/s2

Voltage: 1 volt = 1 m2·kg/s3·A

Frequency: 1 hertz = 1/s

Power: 1 watt = 1 kg·m2/s3

Electric Charge: 1 C = 1 A·s

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 timeOne part per quadrillion accuracy!!!

Accurate frequency gives accurate distance and time.

Artifact vs. quantum standards:

A metal bar:1889-1960 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 meter:

The modern kilogram

The SI kilogram drifts!

Mass - possible replacements

Watt-balance

Avogadro’s number6.0221415 × 1023

Goal: 10 parts per billion accuracy

Temperature: Kelvin, Celsius, and Fahrenheit

294 K70 F21 C

273.15 K32 F0 C

77 K-321 F-196 C

4.2 K-452 F-269 C

0 K-459.67 F-273.15 C

Water freezes

Air liquefies

Helium liquefies

Room temperature

Absolute zero

The Kelvin: the SI unitThe 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

“The Congress shall have Power To……fix the Standard of Weights and Measures;”

From Article I, section 8 of the U.S Constitution:

Most technologically advanced countries.

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

• 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 non-targeted 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

• 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.