Calibration, Certification, and Verification of Standards

This page last reviewed April 29, 2010

The Air Resources Board's Standards Laboratory conducts tests of instrument performance and assays of pollutant gas mixtures composition and concentration. Many of the procedures used were developed at the Air Resources board using Standard Reference Materials (SRMs) from the National Institute of Standards and Technology (NIST).  There are different types of evaluations of comparability. 

The Standards Laboratory performs verifications of ozone and flow rate primary standards, calibrations and certifications of ozone and flow rate transfer standards, and certification of compressed gas cylinders.  Instruments sent to the Standards Laboratory for certification must be in good working order.  They must also be accompanied by an owners manual.  Standards Laboratory staff will perform no maintenance.  If an instrument fails certification or verification it will be returned to the owner for repair.

If an instrument has not been previously certified by the Standards Laboratory, please request approval from Robert Russel prior to sending the instrument to the Standards Laboratory for certification.  In addition, any instrument that has power requirements other than conventional North American single phase, 120V/60Hz, must be clearly marked as such on both the case and power cord.  Standards Laboratory staff will accept no responsibility for damage to an instrument that is not clearly marked as having special power requirements.

Ozone

Verification of an ozone primary standard consists of one ok comparison against a primary standard reference photometer (SRP).  For the verification to be valid, the linear regression must have a correlation coefficient of 0.9999 or greater, the slope must be within 3 percent of the expected value, and the intercept must be less than 3 parts per billion (ppb) ozone.  The comparison's slope and intercept should not be used to correct the instrument readings.  They should only be used to verify the operation of the instrument. The U.S. EPA established the verification criteria in the user's guide entitled "Standard Reference Photometer for Verification and Certification of Ozone Standards." 

Certification of ozone transfer standards requires six OK comparisons against the SRP.  Each comparison must have a correlation coefficient of 0.9999 or greater, each slope must be within 5 percent of the expected value, and each intercept must be less than 3 ppb ozone.  A certification is valid if the six most recent comparisons have a Relative Standard Deviation (RSD, Equation 1) less than 1.5 percent for the slope and a Full Scale Relative Standard Deviation (FRSD, Equation 2) less than 0.5 percent for the intercept.  For recertifications, the current comparison's slope must be within 1 percent of the most recent comparison's slope, otherwise, another comparison must be performed to verify the change.  Additional comparisons are not required if the six most recent comparisons meet the certification requirements.  The certified slope and intercept is the average of each for the six comparisons and should be used by the client to correct or adjust the instrument's displayed ozone concentration. 

The Standards Laboratory established the certification criteria to be more stringent than those required by the U.S. EPA in the Technical Assistance Document (TAD), "Transfer Standards for Calibration of Air Monitoring Analyzers for Ozone."  This ensures increased transfer standard repeatability, allowing for less frequent recertifications. Recertifications are performed once per calendar quarter, as opposed to twice per quarter as called for in the TAD. 

Equation 1. Relative Standard Deviation (RSD) = (Population St Dev / Mean) x 100

Equation 2. Full Scale Standard Deviation (FRSD) = (Population St Dev / Full Scale) x 100

Calibration of an ozone analyzer consists of one multi-point comparison against the SRP. The comparison must have a correlation coefficient of 0.9999 or greater.  The derived slope and intercept is used by the client to correct or adjust the instrument's displayed ozone concentration. 

Flow Rate

Low-Volume Flows (.005 to 50 lpm)

Verifications consist of one multi-point comparison against one of two primary flow calibrators: Molbox/MolblocA or Molbox/MolblocB Flow Calibrator. For a verification to be valid, the linear regression of the comparison must have a correlation coefficient of 0.9999 or greater, the slope must be within 3 percent of the expected value, and the intercept must be less than 1 percent (full scale) from the calibrator's intercept.  The comparison's slope and intercept should not be used to correct the meters reading.  They should only be used to verify the instrument is operating properly.  The Standards Laboratory established the verification criteria to be more stringent than those required by the U.S. EPA in the Code of Federal Regulations (CFR) . 

Certifications require four consecutive comparisons against a primary flow calibrator.  It is preferred to alternate primary flow calibrators for each comparison.  Each comparison must have a linear regression with a correlation coefficient of 0.9999 or greater. A certification or recertification is valid if the four most recent comparisons have a RSD less than 1 percent for the slope and a FRSD less than 1 percent for the intercept.  For recertifications, the current comparison's slope must be within 1 percent of the most recent comparison's slope, otherwise, another comparison must be performed to verify the change.  Additional comparisons are not required if the four most recent comparisons meet the certification requirements.  The certified slope and intercept is the average of each for the four comparisons and should be used by the client to correct or adjust the instrument's displayed flow rate.  The Standards Laboratory has established the certification criteria   to be more stringent than those required by the U.S. EPA in the CFR. 

A calibration consists of one comparison against a primary flow calibrator.  The comparison must have a linear regression with a correlation coefficient of 0.999 or greater.  The derived slope and intercept should be used by the client to correct or adjust the instrument's displayed flow rate. 

The type of flow rate device dictates which analysis method is required.  Typically, bubble meters and piston-type flow devices undergo verifications; mass flow meters and controllers undergo certifications; and rotometers and Vol-O-Flow meters undergo calibrations.  Mass flow meters and controllers can also undergo calibrations, depending on their intended use. 

High-Volume Flows (566 to 2,360 lpm)

Certifications of high-volume flows are performed with a Rootsmeter certified every two years by the original manufacturer.  A certification or recertification is valid if the two most recent comparisons have a RSD less than 0.7 percent for the slope and intercept.  In order for each comparison to be valid, all the points in the assay must be within 2 percent of the regression line.  The certified slope and intercept is the average of each for the two comparisons.  A slope and intercept is provided to determine both the Actual Flow (Qa) and Standard Flow (Qstd).  The Standards Laboratory established the certification criteria to be more stringent than those required by the U.S. EPA in "Certification of an Orifice Transfer Standard," U.S. EPA Quality Assurance Handbook for Air Pollution Measurement Systems, Volume II, Section 2.11.2.2.1. 

Compressed Standards Gases

Standards contained in gas cylinders are assayed with method MLD 5722 using calibration standards traceable to NIST. A standards certified concentration is the average of the three most recent assays.

Certification services for instruments and gas standards are provided at no charge (apart from shipping costs) to California's Air Districts.

-  The certification validity for ozone, high-volume orifice, and ambient compressed gas standards are set by the U.S EPA.  The certification validity for low-volume flow and source compressed gas standards are set by the client's procedures and methods.

STANDARDS LABORATORY CERTIFICATION SCHEDULE

STANDARD OPERATING PROCEDURES

For further information about the Standards Laboratory, please contact:

Robert Russell

Publications and Downloadable files

Essential Standards used in California for the Measurement of Toxics in Ambient Air, Ozone Precursors from Automotive Emissions and Cleaner Burning Gasoline (MS Powerpoint 10,516 KB), (Acrobat 661 KB),
Donald Hammond.  Pittsburgh Conference in New Orleans, Impact of NIST Chemical Measurement and Standards Programs: Customer Views, March 4-9, 2001.

Automated Method for Establishing National Institute of Standards and Technology Traceable Certifications of Secondary and Tertiary Compressed Gas Standards, Gerhard H. Achtelik, Kitty Howard, Elbert V. Lawrence. Technical paper presented at the Air & Waste Management Association 86th Annual Meeting & Exhibition, Denver, Colorado, June 13-18, 1993.