Transfer Standards 

For Calibration of 

Air Monitoring 

Analyzers for Ozone 

Technical Assistance 

Document

 

October 2013

EPA‐454/B‐13‐004

October 2013

Transfer Standards 

For The Calibration of Ambient Air Monitoring Analyzers 

For Ozone 

Technical Assistance Document 

U.S. Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Air Quality Assessment Division 

Research Triangle Park, North Carolina 27711 
Ozone Transfer Standard Guidance Document 10/2013 

1

DISCLAIMER 

This document has been reviewed by the U.S. Environmental Protection Agency, and approved 

for publication. Mention of trade names or commercial products does not constitute endorsement or 

recommendation for use. 
Ozone Transfer Standard Guidance Document 10/2013 

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FOREWORD 

This Technical Assistance Document defines, specifies, and formalizes the verification of ozone transfer 

standards for calibrating ambient ozone analyzers. The procedures and guidance in this document provide 

monitoring agencies additional flexibility and specific benefits in designing and implementing an effective quality 

assurance program for their ambient ozone monitoring. This document replaces the 1979 document by the same title 

(EPA-600/4-79-056). Ultraviolet (UV) photometry has replaced most of the methods described in the 1979 

document. This document removes methods no longer in use and updates definitions and procedures as needed. 

The original analytical procedure prescribed by the U.S. Environmental Protection Agency (EPA) (under 

Title 40, Code of Federal Regulations (CFR), Part 50, Appendix D) for certifying local primary O3 concentrations 

had been a wet chemical technique based on spectrophotometric analysis of iodine generated by O3 in neutral 

buffered potassium iodide (NBKI) and referenced to an arsenious oxide primary standard. EPA has amended these 

regulations by replacing the NBKI technique with a better technique based on absorption of UV radiation and 

referenced to the well-established absorption coefficient of O3 at a wavelength of 254 nm (EPA 1979). 

The UV technique requires a stable O3 generator, a UV photometer, and a source of clean, dry, pollutantfree air. A flowing (dynamic) system is set up in which clean air is passed through the O3 generator at a constant 

flow rate and discharged into a multiport manifold. The O3 concentration in the manifold is assayed by the 

photometer and is available for calibration of O3 analyzers or verification of transfer standards. To verify some 

types of transfer standards, some modifications to this system may be required (see Section 3). After the air flow 

rate is adjusted, the O3 generator is adjusted to provide the approximate O3 concentration desired. The UV 

photometer is then used to measure the UV absorption of the generated concentration at a wavelength of 254 nm. 

This transmittance measurement, together with the well-established absorption coefficient of O3 at that wavelength 

and various instrument parameters, is used to calculate the O3 concentration by means of the Beer-Lambert 

absorption law. The accuracy of the photometer is critically important to this technique; however, certain 

commercial and laboratory photometers have been shown to be adequate to the task. 

In 2013, EPA identified some language in the 2010 document that needed revision. The 2010 document 

included the following sentences in Section 3 and in Appendix B: 

“ Only transfer standards that have met the requirements established in 40 CFR Part 50 Appendix D and are either 

an approved Federal Reference Method (FRM) or Federal Equivalent (FEM) should be used in ambient air 

monitoring and as such, should meet the qualification requirements described in this Appendix” 

Since the guidance allow the transfer standard to be generation device and a analyzer, just a analyzer, or 

just a generation device, there will be cases where the transfer standards will not be FRM/FEM devices. Therefore, 

this wording in Section 3 and Appendix B was removed, and additional minor edits were made to this version 
Ozone Transfer Standard Guidance Document 10/2013 

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PREFACE 

Ultraviolet (UV) photometry is currently the most accepted technique for assaying ozone calibration 

atmospheres in the sub-ppm concentration range to obtain primary ozone standards. Accordingly, the U.S. 

Environmental Protection Agency has adopted UV photometry as the prescribed procedure for the calibration of 

reference methods to measure ozone in the atmosphere. 

The ozone calibration procedure specifically allows the use of transfer standards for calibrating ambient 

ozone monitors. Such transfer standards, however, must be suitably referenced to a UV standard of higher authority 

and traceability. 

The U.S. Environmental Protection Agency believes that the use of ozone transfer standards can provide 

monitoring agencies with a number of worthwhile advantages. However, their use entails a considerable degree of 

technical expertise in selecting, qualifying, verifying, using, and maintaining the transfer standards. This document 

offers users technical guidance in these areas. 

For the experienced user, this document will help to formalize and standardize the qualification, 

verification, and use of transfer standards. For the less experienced user, it should serve as a learning aid and an 

information source. 

The accurate, long-term measurement of ozone concentrations in ambient air is not an easy task. Error 

processes are relentless; the infiltration and magnitude of errors must constantly be held to a minimum to realize 

quality data. Hopefully, attention to the information and guidance given in this document will help toward that end. 

Comments or criticism from readers and users are welcome; any changes resulting from such comments 

will also be included in any subsequent revision or supplement. 
Ozone Transfer Standard Guidance Document 10/2013 

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ABSTRACT 

On February 8, 1979 (Federal Register, 44:8221-8233), the U.S. Environmental Protection Agency 

amended Appendix D of Title 40, Code of Federal Regulations (CFR), Part 50, to prescribe a calibration procedure 

for the calibration of reference methods for measuring ozone in the atmosphere. The procedure is based on the use 

of ultraviolet (UV) photometry as the authoritative standard for ozone and allows the use of transfer standards for 

the calibration of ambient ozone monitors, provided such transfer standards are adequately referenced to a UV ozone 

standard of higher authority (level) and traceability. 

This document is intended as a reference aid to help users select ozone transfer standards and reference 

them to a higher level UV standard. It first describes the theory of traceability, defines ozone transfer standards and 

then discusses their purpose and role in calibrating ambient ozone analyzers. The various advantages and 

disadvantages of ozone transfer standards are pointed out to help users determine whether to use a transfer standard 

or the UV procedure directly. Different types of ozone transfer standards are described, including analytical 

instruments and ozone generation devices. 

The major part of the document is devoted to the procedures necessary to establish the authority of ozone 

transfer standards: qualification, verification, and periodic reverification. Qualification consists of demonstrating 

that a candidate transfer standard is sufficiently stable (repeatable) to be useful as a transfer standard. Repeatability 

is necessary over a range of variables such as temperature, line voltage, barometric pressure, elapsed time, operator 

adjustments, or other conditions, any of which may be encountered during use of the transfer standard. Tests and 

possible compensation techniques for several such common variables are provided. Detailed verification procedures 

are also provided together with the quantitative verification acceptance criteria. Finally, the periodic procedure and 

specifications necessary to maintain continuous verification of the transfer standard are given. 

For convenience, the UV primary ozone standard procedure from 40 CFR Part 50 is reproduced in 

Appendix A. Other appendices give more specific guidance for the qualification and verification of several common 

and practical types of transfer standards like ozone generators and ozone analyzers. 

This document replaces the 1979 document by the same title (EPA-600/4-79-056). Ultraviolet (UV) 

photometry has replaced most of the methods described in the 1979 document. This document removes methods no 

longer in use and updates definitions and procedures as needed. 
Ozone Transfer Standard Guidance Document 10/2013 

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CONTENTS 

Foreword ...................................................................................................................................................................... v 

Preface ....................................................................................................................................................................... vi 

Abstract ...................................................................................................................................................................... vii 

Figures/Tables ............................................................................................................................................................ ix 

Acknowledgments ....................................................................................................................................................... x 

1. TRACEABILTY AND THE USE OF TRANSFER STANDARDS ..................................................... 1-1 

The Family of Level 1 Standards- Standard Reference Photometers ............................................ 1-2 

The Family of Transfer Standards ............................................................................................... 1-3 

How Transfer Standards Are Used ............................................................................................... 1-4 

Advantages of Ozone Transfer Standards .................................................................................... 1-5 

Disadvantages of Ozone Transfer Standards ................................................................................ 1-6 

2. TYPES OF TRANSFER STANDARDS FOR OZONE ........................................................................ 2-1 

Analytical Instruments ................................................................................................................. 2-1 

Generation Devices ...................................................................................................................... 2-3 

Recommendations on the Use of Specific Types of transfer Standards ....................................... 2-4 

3. SUMMARY SPECIFICATIONS OF OZONE TRANSFER STANDARDS ........................................ 3-1 

Summary Specifications .............................................................................................................. 3-1 

Comparing Level 3 and Greater Standards to a Level 2 Ozone Standard .................................. 3-4 

4. VERIFICATION AND RE-VERIFICATION PROCESS .................................................................... 4-1 

Procedure for Initial Verification ................................................................................................. 4-1 

Procedure for Reverification ....................................................................................................... 4-4 

Appendices ..................................................................................................................................................................... 

A. Ultraviolet Photometric Procedure for primary Ozone Standards ........................................................ A-1

B. Qualification Process ............................................................................................................................ B-1 

C. Verification of an Ozone Generator as a Transfer Standard ................................................................. C-1 

D. Verification of an Ozone Analyzer as a Transfer Standard .................................................................. D-1 

E. Glossary ................................................................................................................................................ E-1 
Ozone Transfer Standard Guidance Document 10/2013 

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FIGURES 

Number Page

1.1 Ambient air ozone traceability scheme ............................................................................................................ 1-1 

1.2 Family of Level-1 SRPs .................................................................................................................................... 1-2 

3.1 Typical sample comparison set-up of an O3 authoritative standard to a assay type transfer standard .............. 3-4 

3.2 Typical sample comparison set-up of an O3 authoritative standard to an generation type transfer standard .... 3-5 

4.1 Example of a comparison of a Level 3 transfer standard to a Level 2 standard ............................................... 4-2 

4.2 Example of a preliminary calibration relationship for an adjustable O3 device ............................................... 4-3 

4.3 Example of a transfer standard verification relationship ................................................................................. 4-4 

4.4 Example of a chart showing reverification slope data for a transfer standard ................................................. 4-5 

4.5 Example of a chart showing reverification variability of slope and intercept for a transfer standard .............. 4-5 

A-1 Schematic diagram of a typical UV photometric calibration system .............................................................. A-9 

A-2 Schematic diagram of a typical UV photometric calibration system (Option 1) .......................................... A-10 

B.1 Example of temperature qualification test results showing up dependence on temperature ........................... B-4 

B.2 Example of a temperature dependence quantitatively defined as a correction factor ..................................... B-4 

B.3 Example of a defined barometric pressure dependence .................................................................................. B-6 

TABLES 

Number Page

3-1 Summary Specifications for the Family of Ozone Standards .......................................................................... 3-6 

4-1 6-Day initial Ozone Verification ...................................................................................................................... 4-2 
Ozone Transfer Standard Guidance Document 10/2013 

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ACKNOWLEDGMENTS 

This Guidance was the product of the combined efforts of the EPA Office of Air Quality Planning and 

Standards, the Office of Research and Development, the EPA Regional Offices, and the State, Tribal and 

Local monitoring organizations. The development and review of the material found in this document was 

accomplished through the activities of the Ozone Traceability Workgroup. The following individuals are 

acknowledged for their contributions. 

State, Tribal and Local Organizations

Anna Kelley, Brian Spreadborough, Ceresa Stewart, David Malorin, Don Gourley, Donald Hammond, 

Donovan Rafferty, Jeff Wasson, Lynn Geter, Melinda Ronca-Battista, Michael Tolocka, Paul Nichols, 

Rachael Townsend, Randy Dillard, Rayna Broadway, Robert Russell, Stan Paulsel, Stephanie McCarthy, 

Yousaf Hameed 

EPA Regions

 Region 

 1 Chris St. Germane 

 4 Greg Noah, Mike Crowe, Louie Pounds 

 5 Scott Hamilton 

 6 John Lay 

 7 Thien Bui, James Regehr 

 8 Michael Copeland, Joe Delwiche 

 9 Mathew Plate 

EPA Office of Research and Development – Air Pollution Prevention and Control Division 

Paul Groff, Scott Moore 

EPA Office of Chemical Safety and Pollution Prevention-Chemical Control Division 

Brian Lee 

Office of Air Quality Planning and Standards-Air Quality Assessment Division

Lewis Weinstock , Mark Shanis, Dennis Mikel, Dennis Crumpler 
Ozone Transfer Standard Guidance Document 10/2013 

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SECTION 1 

TRACEABILITY AND THE USE OF TRANSFER STANDARDS 

In ambient air monitoring 

applications, precise ozone 

concentrations called standards are 

required for the calibration of 

ozone analyzers. Gaseous ozone 

standards cannot be stored for any 

practical length of time due to the 

reactivity and instability of the gas. 

Therefore, ozone concentrations 

must be generated and “verified” 

on site. When the monitor to be 

calibrated is located at a remote 

monitoring site, it is necessary to 

use a transfer standard that is 

traceable to a more authoritative 

standard. Traceability is the 

“property of a measurement result

whereby the result can be related to 

a stated reference through a 

documented unbroken chain of 

calibrations, each contributing to 

the measurement uncertainty” 1

(ISO). 

A claim of traceability requires three elements: 

1. a declaration of the source of traceability (e.g., NIST ), 

2. a full description of the traceability chain from the source to the measurement of interest, and 

3. an uncertainty claim with supporting data. The responsibility for providing support for an uncertainty claim 

rests with the entity making the claim (i.e., the provider), but the responsibility for assessing the validity of 

such a claim rests with the consumer. 

Figure 1.1 represents the scheme that will be employed to ensure that the use of ozone transfer standards 

are applied in a manner that will ensure a specified level of measurement uncertainty and traceability. 

Measurement uncertainty describes a region about an observed value of a physical quantity which is likely to 

enclose the true value of that quantity. Measurement uncertainty is related with both the systematic and random 

error of a measurement, and depends on both the bias and precision of the measurement instrument. At each 

measurement phase (e.g., levels in Fig. 1.1) errors can occur, that in most cases, are additive. The goal of this 

guidance is to: 

                                                           

1 International Standards Organization (ISO)- International Vocabulary of Basic Terms in Metrology
Ozone Transfer Standard Guidance Document 10/2013 

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 identify the acceptable amount of measurement uncertainty in ozone transfer standards, and 

 develop a method to characterize measurement uncertainty through the transfer standard verification 

process in order to allow the use of transfer standards to an acceptable number of levels. 

This process is considered performance based. Monitoring organizations that keep measurements well 

within acceptance limits may be able to extend traceability to more levels (if needed). Transfer standards with 

considerably more variability may not be able to extend traceability to additional levels. As a general rule of thumb, 

monitoring organizations should always seek to minimize the use of additional levels of transfer standards. 

A transfer standard is defined as a transportable device or apparatus which, together with associated 

operational procedures, is capable of accurately reproducing pollutant concentration standards or of producing 

accurate assays of pollutant concentrations which are quantitatively related to a higher level and more authoritative 

standard. The transfer standard’s purpose is to transfer the authority of a Level 1 pollutant standard to a remote point 

where it is used to verify or calibrate an air monitoring analyzer. 

Transfer standards may be used for many different purposes. In this document, however, the discussion of 

transfer standards for ozone (O3) applies to the family of standards that are used beyond standard reference 

photometers or Level 1 standards. EPA is attempting to reduce the number of common terms that were used in the 

past such as: primary standard, local primary standard, transfer standards and working standards that sometimes 

have been the cause of confusion. This document will identify the family of standard reference photometers (SRPs) 

as Level 1 standards. Beyond the SRPs, all standards will be considered transfer standards and will be numbered 

(starting with 2) based on its “distance in the traceability chain” from a verification against a Level 1 standard. 

THE FAMILY OF LEVEL 1 STANDARDS- STANDARD REFERENCE PHOTOMETERS (SRPs) 

Figure 1.2 represents the family of Level 1 standard 

reference photometers. Ideally, only one Level 1 (often termed 

reference standard) exists for each entity to be measured. The 

International Bureau of Weights and Measures (BIPM)2

 maintains an 

SRP (#27) as the World's ozone reference standard. All EPA SRPs 

are now traceable to SRP #27. Each member state of the Convention 

of the Meter3

 has one laboratory designated to provide traceability to 

its country. NIST represents that laboratory for the United States. 

Every 2 years, NIST compares the designated U.S. National ozone 

reference standard (SRP #2) to SRP #27 indirectly using SRP #0. 

For ozone, NIST has developed a number of identical SRPs 

                                                           

2 http://www.bipm.org/en/scientific/chem/gas_metrology/ozone_comparisons.html   3 http://www.speedylook.com/Convention_of_the_Meter.html  
Ozone Transfer Standard Guidance Document 10/2013 

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(numbered sequentially in order of date assembled) that it distributes to other countries and organizations as well as 

EPA. Within EPA, the EPA Office of Research and Development Metrology laboratory maintains EPA SRP #1 and 

#7. Each year it sends both SRPs to NIST for verification. The verification is deemed acceptable when the 

relationship of SRP #7 or #1 vs. SRP #2 is 1.00 + 0.01 (slope) and an intercept of 0.0 + 1 nmol/mol (ppbv). This 

relates to a bias of 1%. Upon acceptable verification, SRP # 7 is then sent to the EPA Regions to verify the SRPs 

operated in the Region and used for the ambient air program. Acceptance limits for this verification are basically 

the same as that for #7 and #1. At some frequency throughout the year, SRP#7 is compared to SRP #1 to ensure it 

maintains an acceptable level of data comparability. Based on this process, all transfer standards are traceable to 

BIPM #27. 

THE FAMILY OF TRANSFER STANDARDS 

The primary functions of a transfer standard are to duplicate and distribute concentration standards to 

places where comparability to a Level 1 standard is required. The limited number and expense of SRPs makes them 

impractical for use as transfer standards. Depending on the size and complexity of monitoring organizations, 

various levels of transfer standards are needed. Small monitoring organizations may be able to provide Level 2 

transfer standards to each monitoring site. Large monitoring organizations may bring one Level 2 standard for 

verification against an SRP and then station this standard at a lab location. In previous guidance documents this 

Level 2 standard was called the primary standard or the local primary standard because it acted as the reference 

standard for the monitoring organization. The monitoring organization would then bring one or more Level 3 

transfer standards to the Level 2 standard for verification. These Level 3 standards then are compared against the 

Level 4 standards that are used at the site for routine QC (span checks and one point QC checks) and for calibrating 

the ozone analyzer. In other cases, where QC standards are not at the monitoring sites, the Level 3 transfer 

standards may be used to perform the routine bi-weekly QC checks. There are other permutations that may occur 

with the implementation of these transfer standards. 

With each additional level, the number of standards available is multiplied. Each standard is traceable 

through a chain of “higher” standards to the Level 1 standard. However, as Figure 1.1 indicates, each lower 

standard in the chain must assume a somewhat greater uncertainty and less authority than the preceding standard. At 

some point the uncertainty may be beyond acceptable limits. 

Level 2 Standards— 

 

As mentioned above, depending on the size of monitoring organizations Level -2 standards can be a 

monitoring organization’s stationary reference standard or can travel to monitoring sites for verifying and calibrating 

monitoring instruments. In either case, the Level-2 standard is distinguished as the standard that is transported to a 

regional SRP for comparison once a year. The SRP comparison is accomplished by comparing a minimum of 7 

replicate values of a zero point and a minimum of 6 upscale audit concentrations points. This 7 replicate 6 upscale 
Ozone Transfer Standard Guidance Document 10/2013 

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concentration test is considered a cycle. At a minimum, the SRP comparison will consist of three cycles 

(approximately 1 hour per cycle). For an acceptable comparison, the average slopes of the 7 replicates for each audit 

point must be within + 3% for all three cycles and + 3ppb at the 0 intercept. There is a separate standard operating 

procedure (SOP)4

 for this comparison so it is not discussed in any detail in this guidance document. Table 3-1, 

which summarizes the transfer standard acceptance criteria, includes this comparison as a “verification/reverification 

to the SRP” and should not be misconstrued as an “initial 6x6 verification”. A new Level 2 standard or one that was 

adjusted or repaired would also be expected to undergo an initial 6x6 verification with an SRP. Using a Level 2 

Standard to perform a 6X6 verification of another Level 2 Standard is discouraged. More details of the 6x6 

verification are discussed in Section 4. 

HOW TRANSFER STANDARDS ARE USED 

In use, a transfer standard is first precisely related to a standard at least 1 level above by careful 

comparison. Then it is transported to the site of the analyzer to be calibrated and used to calibrate the analyzer (if 

necessary). For some types of transfer standards used in air monitoring, and particularly for O3 transfer standards, it 

is highly desirable to recompare the transfer standard to the standard one level above following its return from field 

use; this recomparison serves as a supplemental check on reliability. 

While this concept is relatively simple, the actual use of transfer standards for O3 is not so simple. Because 

of the nature of O3, transfer standards must be capable of accurately reproducing standard concentrations in a 

flowing system. Ozone transfer standards are complex systems consisting of devices or equipment that generate or 

assay O3 concentrations. Consequently, their verification and use must be in accordance with prescribed procedures 

that are specialized to each specific type of transfer standard. Section 2 describes several different types of O3

transfer standards and provides general information on the use of each. 

Due to the complexity of O3 transfer standards, there is always some uncertainty in their reliability. 

Therefore, a major part of the use of an O3 transfer standard is the need to qualify it, i.e., to determine and prove that 

it has adequate stability (repeatability) and reliability under conditions of use. General procedures for determining 

repeatability prior to use, and for assuring reasonable reliability during use are presented in Appendix B. 

ADVANTAGES OF OZONE TRANSFER STANDARDS 

As noted earlier, since the use of a Level 1 standard is not feasible for field use, the availability of O3

transfer standards provides an alternative: the Level 2 transfer standard can be used at a fixed location to certify one 

                                                           

4 http://www.epa.gov/ttn/amtic/srpqa.html. Not posted as of this draft since SOP is being updated.  
Ozone Transfer Standard Guidance Document 10/2013 

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or more O3 transfer standards which can then be transported to the various field sites and used to calibrate the O3

analyzers. Some possible advantages from using transfer standards are described below. 

Singularity - By using transfer standards, all O3 analyzer calibrations in a network can be related to a single UV 

photometer. All measurements in the network are then directly related to a single common standard (Level 2), 

which can be verified by intercomparison with other UV standards (see below) more easily and more frequently than 

multiple UV photometers could be. Concern about variations or discrepancies among multiple UV standards is then 

eliminated. 

Level 2 Standard Uncompromised - The use of transfer standards of Level 3 and greater allows the Level 2 

standard equipment and procedures to be used at a fixed laboratory location where the conditions of use can be 

carefully controlled. Neither the equipment nor procedures need to be compromised for field use, and there is no 

risk of damage to sensitive equipment during transport. Under the controlled conditions and fixed location, 

variability in the generated authoritative standard will be reduced, providing better accuracy and uniformity among 

all O3 analyzers in the network. If doubt arises in the quality of the Level 3 standard due to rough treatment in the 

field, it can be brought back in for reverification to the Level 2 standard. However, as stated earlier, some smaller 

monitoring organizations with a small number of ozone sites may have the opportunity to use only Level 2 transfer 

standards and maintain there traceability at some acceptable frequency with a Level 1 standard. 

Economy- Transfer standards (level 3 or greater) may be less expensive than the equipment required for the Level 2 

standard . The variety of types of transfer standards offer an agency flexibility in selecting transfer standards based 

on its available budget, equipment, and expertise. Spare or otherwise idle O3 analyzers may possibly be converted 

to transfer standards at little cost, or equipment previously used for O3 calibrations may be qualified for continued 

use. Small agencies may even be able to avoid the cost of Level 2 standard equipment by obtaining access to Level 

2 standards through Regional Offices, State agencies, or other cooperating agencies. There is also the possibility of 

purchasing periodic transfer standard verification services from commercial laboratories. 

Practicality and Convenience -Transfer standards (level 3 or greater) are generally more rugged and portable than 

Level 2 standard equipment. They can be designed to be more adaptable to a variety of applications and to be 

insensitive to various field or transportation conditions, i.e., they can be optimized for field use. They may be easier 

to use, require less operator training, and be less subject to operator error during use. Multiple transfer standards can 

be used simultaneously at different locations. Also, transfer standards are often more flexible or adaptable to 

calibration of various types or models of O3 analyzers. 

Intercomparison - Transfer standards can be used conveniently to intercompare among various Tribal, State and 

Local, and Federal agencies to assure accuracy and confidence. 
Ozone Transfer Standard Guidance Document 10/2013 

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DISADVANTAGES OF OZONE TRANSFER STANDARDS 

The use of transfer standards is not without some disadvantages. Since a choice is available between using 

or not using transfer standards, the disadvantages must be weighed carefully against the advantages to determine 

whether their use is appropriate or cost-effective. Some of the more important disadvantages are described below. 

Qualification -Before a device or procedure can be used as a transfer standard, it must be tested and shown to have 

adequate performance and reliability. It must be repeatable over reasonable periods of time and over the range of 

conditions encountered during field use and during transport. Qualification of a transfer standard may require a 

series of initial tests to determine reliability. This problem is addressed in more detail in Appendix B. 

Verification of Accuracy/Reliability -Most transfer standards for O3 involve complex apparatus or procedures or 

both. As a result, there is always some possibility of error, malfunction, drift, or some other cause for loss of 

repeatability. In addition, transfer standards have no authority until they are related or verified to a standard at least 

one level above it by critical comparison. Moreover, they must be reverified periodically to retain their verification. 

See Section 3, Table 3-1, for the recommended time frames. The time and effort necessary to verify/reverify 

depends on the type or nature of the transfer standard, and may vary considerably from one type to another. 
Ozone Transfer Standard Guidance Document 10/2013 

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SECTION 2 

TYPES OF TRANSFER STANDARDS FOR OZONE 

Several different types of devices or techniques can be considered for use as transfer standards for 

O3. They can be loosely grouped into two general categories: ozone analyzers/photometer5

 instruments and 

generation devices. Within these categories are a variety of techniques and devices that may differ in 

precision, reliability, portability, economy, and convenience. No single technique or device is necessarily 

best for all situations. An agency should select an appropriate type of transfer standard based on a 

complete evaluation of its situation with respect to available funds, available personnel and expertise, 

equipment on hand, location and distance to field sites, modes of transportation used, number and 

calibration frequency of analyzers to be calibrated or spanned, etc. 

A discussion of the two categories follows, together with examples of some techniques and 

devices which are currently available and have been used as transfer standards for O3. Any device or 

technique to be used must first qualify as an acceptable O3 transfer standard by demonstrating adequate 

repeatability (see Appendix B). Then, the transfer standard must be verified by relating it to a standard of 

higher authority. Sections 3 and 4 provide detailed information on the actual tests and procedures used to 

qualify, verify, and establish the reliability and accuracy of O3 transfer standards. Some of the tests are 

dependent on the level of standard. 

OZONE ANALYZERS 

Transfer standards which fall into this category employ an instrumental technique to assay stable, 

flowing O3 concentrations. The analyzer must be capable of measuring O3 concentrations adequately over 

the concentration range of interest, using a well-defined chemical or physical property of O3. Almost any 

commercial O3 analyzer designed for ambient air monitoring, or any other O3 analyzer that can measure O3

concentrations suitably in the appropriate concentration range can be used as a transfer standard. 

The 1979 version of this document (EPA-600/4-79-056) described two types of analytical 

instruments: UV analyzers and chemiluminescence analyzers. Due to the ease of use of UV analyzers, 

chemiluminescence analyzers are not in use and will not be discussed in this document. 

As with all O3 transfer standards, it must be related to a Level 1 standard once it has been 

qualified. This process is accomplished by simply allowing the transfer standard to sample a portion of the 

higher level standard O3 concentration obtained by the prescribed UV technique (see Section 3). Most 

ozone analyzers provide a measurement of O3 over a range of concentrations. The entire analytical range 

                                                           

5 Various documents and guidance may use the term photometer and analyzer synonymously. This

document will use the term analyzer throughout its sections.
Ozone Transfer Standard Guidance Document 10/2013 

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can be verified by comparing the response of the instrument to a series of different O3 concentrations over 

the range. (See “Verification” in Section 4.) The verification relationship between the transfer standard 

and the higher level O3 standard is then expressed mathematically and by a continuous plot of average 

instrument response versus higher level standard O3 concentration, as shown in Figure 4.3. 

When an analyzer is used as a transfer standard to calibrate an O3 monitor at a field site, some 

additional means must be provided to generate the O3 concentrations: usually, a UV O3 generator, an air 

pump, and an ambient air scrubber to provide clean zero air. The O3 generator must produce very stable 

concentrations of O3 (preferably less than ± 2% change per hour). 

There are many UV analyzers in the ambient concentration range commercially available and 

many contain not only analyzers but also a self-contained O3 generator that can serve as a transfer standard. 

They are fairly simple to set up and operate, and are more tolerant of intermittent operation. Although not 

designed specifically for portability, the UV analyzers may be transported readily and relatively safely if 

given careful handling; the UV optical systems in these transfer standards, however, must not, be subjected 

to rough handling. The warm-up and stabilization time of the UV analyzers is normally less than an hour. 

They are likely to be sensitive to temperature and barometric pressure changes and may require corrections 

for those parameters. Line voltage sensitivity should also be checked carefully. 

It is debatable whether or not the clean air and O3 generator components should be considered a 

part of the transfer standard. Ideally, an O3 transfer standard should be self-contained such that it can 

completely reproduce O3 standards. To be self-contained, the O3 generation components must be an 

integral part of the transfer standard. An added advantage of this concept is that the O3 generation 

components can be inspected, tested, and serviced whenever the analytical instrument is recertified. On the 

other hand, the authority of the transfer standard is clearly contained in the verification of the analyzer. 

The O3 generation components could thus be considered incidental to the use of the transfer standard. 

Some advantages may therefore be obtained by equipping each field site with its own O3 generation system 

– which might also double as a zero-and-span system – and transporting only the analyzer from site to site. 

When used as a transfer standard, a UV analyzer may appear to function very much the same as 

the Level 1 standard (SRP) in the O3 procedure described in Appendix A. It is therefore easy to confuse the 

two. The distinction between the two is important and should be clearly understood. When a Level 1 

standard is used to generate O3 concentrations, it must meet all of the specifications prescribed in Appendix 

A, and the concentration measurements are referenced to the absorption coefficient of O3. In contrast, an 

O3 analyzer used as a transfer standard can only certify an O3 concentration by previous comparison to a 

standard of higher authority, reference to the O3 absorption coefficient is used only indirectly, although it 

may serve as an internal check of the transfer standard’s reliability (see Section 4). If an analyzer is used as 
Ozone Transfer Standard Guidance Document 10/2013 

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a transfer standard it should be designated as a federal equivalent method. These designations can be found 

on the AMTIC Website6

GENERATION DEVICES 

Transfer standards in this category are simply devices that generate accurate O3 concentrations 

without having any capability to assay the generated concentration output. The accuracy of these devices 

depends entirely on their inherent generation stability and reproducibility under changing conditions of use. 

The most common example of a generation device is the UV (photolytic) O3 generator. Other types of O3

generators capable of generating reproducible O3 concentrations in the appropriate range may also be 

suitable. 

Ozone generator transfer standards consist of an air pump, a device to provide zero air, a flow 

control system, a means to generate O3 within the flowing zero air stream, and an output manifold at which 

the O3 concentration standard is available for calibration or spanning of field O3 analyzers. The pump and 

air scrubber components could be considered ancillary to the O3 generator system, but ideally these 

components should be included as integral to the system, since the quality of the zero air and the flow 

regulation are important to the accuracy of the generation system. If possible, the generation system 

should be completely self-contained. 

Most generation devices have the ability to adjust the O3 output concentration over a considerable 

range for convenient calibration of field O3 analyzers. This adjustment normally has a digital readout, dial 

or scale associated with it that can be related to a higher authority O3 standard by a relationship as shown in 

Figure 4.2. In some generation systems, the output concentration is varied by changing the flow rate, 

which produces a curved (nonlinear) relationship to the higher authority O3 standard unless the 

concentration is plotted versus the reciprocal of the flowrate. Fixed or discretely adjustable O3 generators 

produce only one or a few fixed O3 concentrations that may have to be diluted at the field location to 

calibrate an O3 analyzer. 

In concept, it would seem that generation devices are well suited as transfer standards for O3. 

They are often of relatively simple design, easy to use, moderate in cost, fairly rugged, commercially 

available, and relatively immune to operator error. In practice, the devices fall somewhat short of the ideal. 

As noted above, the devices may have no assay capability and depend entirely on their own inherent 

stability. Consequently, their sensitivity to changing conditions from verification lab to field site and their 

stability with time must be checked carefully and frequently. Mechanical and electrical integrity and flow 

                                                           

6 http://www.epa.gov/ttn/amtic/criteria.html  
Ozone Transfer Standard Guidance Document 10/2013 

2-4 

stability are also important. Since most generation devices are sensitive to pressure changes (altitude) and 

often to temperature, corrections for these variables may be necessary. A warm-up period is normally 

required before the generation device has a stable, repeatable output. Read the manufacturers 

recommendation and consider local conditions (temperature during transport to monitoring site) on the 

amount of time necessary for warm-up. With some devices, a restabilization period may be required after 

each adjustment of the output concentration. 

Generation devices are likely to have lamps, glassware, and various electronic components that 

require reasonably careful handling. Those devices that require dilution of a fixed O3 concentration must 

have suitable apparatus and flowmeters to effect accurate dilution, putting an extra burden of accuracy on 

the operator. Since the generation devices provide their own O3 output concentrations, a slightly modified 

procedure is necessary to certify them against a higher authority O3 standard. 

RECOMMENDATIONS ON USE OF SPECIFIC TYPES OF TRANSFER STANDARD DEVICES 

Due to the advances in ozone monitoring technology, many of the newer transfer standards 

include both generators and analyzers. Therefore, it is strongly suggested that: 

 Level-2 standards and/or standards used in the verification of other transfer standards include both 

a generation device and a analyzer. 

 Level-3 standards be, at a minimum, an analyzer. The level 3 standard can be a analyzer and 

generator but should not be just a generator. 

 Level-4 and greater can be an ozone analyzer or an ozone generation device. 
Ozone Transfer Standard Guidance Document 10/2013 

3-1 

SECTION 3 

SUMMARY SPECIFICATIONS FOR OZONE TRANSFER STANDARDS 

As noted in previous chapters, the primary purpose of an O3 transfer standard is to transfer the accuracy of an 

O3 concentration standard of higher authority from one place and time to another. Because a transfer standard has 

initially (by definition) no authority of its own, its authority must first be established. The essence of establishing the 

authority of the transfer standard is to establish a high probability or confidence that O3 concentration standards 

obtained by means of the transfer standard, under a variety of operational conditions, are very nearly as accurate as the 

authoritative O3 standard. This confidence is established first by determining that the transfer standard has adequate 

reproducibility to qualify it as a useful transfer standard, then by verifying the transfer standard by relating it to a 

higher level UV standard, and finally by periodically reverifying its accuracy and stability. 

This section will first provide a summary description of the specifications for the ozone transfer standards and 

then provide details on the qualification, verification and re-verification steps. Table 3-1 at the end of this section 

provide a summary of the specifications. 

SUMMARY SPECIFICATIONS 

1. Apparatus -An O3 transfer standard should include all basic equipment, materials, and supplies (but not 

necessarily incidental items) required to carry out its function. 

2. Documentation -The following comprehensive documentation of an O3 transfer standard is required: 

2.1. A complete listing and description of all equipment, materials, and supplies necessary or incidental to the use 

of the transfer standard; 

2.2. A complete and detailed operational procedure for using the transfer standard, including all operational steps, 

specifications, quality control checks, etc.; 

2.3. Test data, rationale, evidence, and other information indicating that the transfer standard meets the 

qualification requirements given below; 

2.4. The current verification relationship information (slope and intercept) as described in step 4.6 and applicable 

to current use of the transfer standard, together with any corrections or restrictions in the operating 

conditions (temperature, line voltage, barometric pressure, etc.); and 

2.5. A logbook including a complete chronological record of all verification and reverification data, as described 

under “Verification” and “Reverification” above, as well as all O3 analyzer calibrations carried out with the 

transfer standard. 
Ozone Transfer Standard Guidance Document 10/2013 

3-2 





6

i 1

i I

6

1 I

3. Qualification ––(Details in Appendix B) An O3 transfer standard must meet the general requirements for 

qualification; the transfer standard output should not vary by more than ± 4% or ± 4 ppb (whichever is greater) 

from its indicated value over a stated range of any of the conditions to which it might be sensitive. Qualification 

was an important step in the initial 1979 guidance since few UV transfer standards were available and monitoring 

organizations may have been developing these devices internally. It is important that as monitoring organizations 

plan purchases of new generation transfer standards they consult with the manufacturers on the processes used to 

qualify the instruments. However, qualifying new instruments can help ensure the instruments will perform under 

various field conditions that might not get observed during a traditional certification processes described in 

Section 4 of this document. 

4. Verification- –(Details in Section 4) Prior to use, an O3 transfer standard must be verified by establishing a 

quantitative verification relationship between the transfer standard and the higher level O3 concentrations 

obtained by the UV calibration procedure as specified in Appendix D of 40 CFR, Part 50 (EPA 1979). The 

verification procedure follows: 

4.1 The verification relationship shall consist of the average of 6 individual comparisons of the transfer standard 

to the authoritative UV O3 standard system. Each comparison must be carried out on a different day. 

4.2 Each comparison shall consist of at least 6 comparison points at concentrations evenly spaced over the 

concentration range of the transfer standard, including 0 and 90% (± 5%) of the upper range limit. For the 6 

or more comparison points of each comparison, compute the slope and intercept by the least squares linear 

regression of the transfer standard output (either a generated O3 concentration or a concentration assay) and 

the UV authoritative O3 standard. 

4.3 For the 6 comparisons, compute the average slope ( m ): 

 (Eq. 1) 

and the average intercept (Ī): 

 (Eq. 2) 

 where mi and Ii are the individual slopes and intercepts, respectively, of each comparison regression. 

4.4 Compute the relative standard deviation of the 6 slopes, (sm): 

 ( ) %

6

1 ( ) 5

100 1 6

1

2

6

1

2





 



      i

i

i

m mi m

m

s (Eq. 3) 

 and the standard deviation SI defined in Equation 4 below for the 6 intercepts: 





6

i 1

mi 6

1

m
Ozone Transfer Standard Guidance Document 10/2013 

3-3 

 

 



 



      

6

1

2

6

1

2 ( ) 6

1 ( ) 5

1

i

i

i

I i s I I (Eq. 4) 

4.5 The value of Sm must be ≤ 3.7%, and SI must be ≤ 1.5 (ppb). 

4.6 If the transfer standard meets the specifications of 4.5, compute the verification relationship as: 

 Std. O3 conc. = 

m

1

 (Indicated O3 conc. – Ī) (Eq. 5) 

5 Reverification5

-–(Details in Section 4) To maintain continuous verification, an O3 transfer standard must be 

reverified as follows. In general, O3 transfer standards should be reverified at the beginning and end of the ozone 

season or at least every 6 month. 

5.1 At the time of reverification, carry out a comparison of the transfer standard to the UV primary standard as 

prescribed in step 4.2. 

5.2 The slope of the new comparison must be within the interval m  0.05m . 

5.3 If the transfer standard meets the specification in 5.2, compute a new m and a new I as prescribed in step 

4.3 using the 6 most recent comparisons (running averages). 

5.4 Compute a new sm and SI as prescribed in step 4.4 using the 6 most recent comparisons. 

5.5 If the new sm and SI meet the specifications given in 4.5 compute a new verification relationship as prescribed 

in step 4.6 using the updated m and I. 

5.6 If the transfer standard fails any of the reverification specifications, it loses its verification. Reverification 

then requires repeating all the initial verification steps (steps 4.1 – 4.6). 

NOTE- This document does not cover the procedures and requirements of the annual Level-2 standard 

comparison to the EPA SRPs6

. Table 3-1 lists acceptance criteria for the comparison. 

                                                           

5 Reverifications are normally considered for Level 3 and 4 standards since a level 2 is usually considered a monitoring 

organizations “bench standard” while levels 3 and 4 standards are transported to the field.

6 http://www.epa.gov/ttn/amtic/srpqa.html. Not posted as of this draft since SOP is being updated.
Ozone Transfer Standard Guidance Document 10/2013 

3-4 

COMPARING LEVEL 3 AND GREATER TRANSFER STANDARDS TO A LEVEL 2 OZONE STANDARD 

Basic to the qualification and verification of any O3 transfer standard is the need to compare the output (either 

a concentration assay or an O3 concentration) of the transfer standard to an O3 standard of higher authority (i.e., the 

reference standard), so that relationships such as shown in Figures 4.2 and 4.3 can be determined. Exactly how such a 

comparison is carried out depends on whether the transfer standard is of the assay-type or the O3-generation type. 

Assay-Type Transfer Standards

For transfer standards which provide an assay of an externally generated O3 concentration, the transfer 

standard is simply connected to the output manifold shown in Figures 1 and 2 of Appendix A. Make sure that the UV 

calibration apparatus can supply sufficient flow for both reference and transfer standard photometers. The output of 

the transfer standard is an indicated concentration (Fig 3.1), which can be compared directly to the primary (reference) 

standard concentration obtained from the UV calibration system. 

Ozone-Generation Type Transfer Standards

In comparing a generation-type transfer standard to a UV primary O3 standard, it obviously cannot be simply 

connected to the output manifold shown in Figures 1 and 2 of Appendix A; some alternate procedure is necessary. 

Described below is a procedure that may be used to compare a generation-type transfer standard to a UV primary O3

standard. 
Ozone Transfer Standard Guidance Document 10/2013 

3-5 

Other procedures are not necessarily precluded, but due concern for accuracy should be exercised. In 

designing or selecting a transfer standard, preference should be given to transfer standard configurations that allow 

direct comparison with the photometer, as described in the procedure below. Thus, a transfer standard should have an 

air supply capable of providing sufficient zero air for the photometer reference cycle without adversely affecting the 

generated O3 atmospheres. 

Procedure 1: Assay by Ultraviolet – 

The UV procedure for obtaining primary O3 standards (described in Section 2 and Appendix A) is basically 

an analytical technique using a UV photometer. The photometer (reference standard) can be used to assay the output 

concentration of a generation-type transfer standard. To do this, the photometer must be disconnected from its own O3

generation and output system and connected to the transfer standard output (see Figure 3.2). Care must be exercised to 

disturb the UV photometer as little as possible from its normal configuration, and to ensure that the output flow of the 

transfer standard exceeds the flow demand of the UV photometer. 

A significant problem arises with this procedure, however. In order to accurately measure IO for the 

transmittance (I/ IO) measurement, the UV photometer must be able to sample zero air from the same source as that 

used for the generation of the O3 concentrations (see Appendix A). If the zero air supply of the transfer standard is 

capable of providing sufficient additional zero air for the photometer, it may be tapped and connected to the two-way 
Ozone Transfer Standard Guidance Document 10/2013 

3-6 

valve as shown in Figure 3.2. Care must be exercised in the process to ensure that the transfer standard is not 

adversely affected. 

Table 3-1 Summary Specifications for the Family of Ozone Standards. 

Requirement Frequency Acceptance Criteria Information /Action 

Regional Standard Reference Photometer (SRP) (Level 1 Standard) 

Verification 1/year Regression slope = 1.00 + 0.01 

and intercept <+ 1 ppb 

Usually at a Regional Office 

and compared against the 

traveling EPA SRP 

Ozone Level 2 Transfer Standard 

Qualification Upon receipt of transfer 

standard 

+4% or +4 ppb 

(whichever greater) 

Transfer Standard Doc 

EPA-454/B-10-001 App B 

 Verification (6x6) After qualification and 

upon 

receipt/adjustment/repair 

RSD of six slopes 3.7% 

Std. Dev. of 6 intercepts 1.5 

Transfer Standard Doc 

EPA-454/B-10-001 Section 

4.1 

Verification/ 

reverification to SRP 

Min- 6 upscale points 7 

replicates 

After qualification and 

upon 

receipt/adjustment/repair 

1/year 

Each individual 

point difference < + 3% 

Level 2 standard usually 

transported to EPA 

Region’s SRP for 

comparison 

 (if recertified via a 

transfer standard)1/

1/year Regression slopes = 1.00 + 0.03 

and two intercepts are 0 + 3 ppb 

Ozone Transfer Standards Levels 3 and Greater 

 Qualification Upon receipt of transfer 

standard 

+ 4% or +4 ppb 

(whichever greater) 

Transfer Standard Doc 

EPA EPA-454/B-10-001 

App B 

 Verification (6x6) After qualification and 

upon 

receipt/adjustment/repair 

RSD of six slopes  3.7% 

Std. Dev. of 6 intercepts 1.5 

Transfer Standard Doc 

EPA-454/B-10-001 

Section 4.1 

Reverification to Level 2 

standard if transfer 

standard includes an 

analyzer (photometer) 

Beginning and end of O3 

season or 1/6 months 

whichever less 

New slope = + 0.05 of previous 

and RSD of six slopes 3.7% 

Std. Dev. of 6 intercepts 1.5 

Transfer Standard Doc 

EPA-454/B-10-001 

Section 4.2 

Reverification to Level 2 

standard if transfer 

standard is only a 

generator 2/

Beginning, each quarter 

and end of O3 season 

New slope = + 0.05 of previous 

and RSD of six slopes 3.7% 

Std. Dev. of 6 intercepts 1.5 

Transfer Standard Doc 

EPA-454/B-10-001 

Section 4.2 

1/ Some monitoring organizations have implemented a practice where they send a transfer standard for verification 

against the Regional SRP rather than the “laboratory bench standard” or the standard the monitoring organization may 

consider its standard of highest authority. Once the transfer standard is verified against the SRP it’s transported back to 

the monitoring organization where it is immediately compared to the laboratory bench standard. If one reviews Figure 

1.1 it becomes evident that using this process in essence makes what the monitoring organization considers its 

standard of highest authority a level 3 transfer standard. Although it is realized that the practice has been used to 

“protect” the laboratory bench standard from damage during transport to the Regional SRP, the practice of verifying 

the monitoring organizations standard of highest authority via a transfer standard is discouraged. 

2/ Since this section includes level 4 transfer standards that may be just generators, more frequent reverification (i.e., 

quarterly) may be necessary on these standards to ensure stability. 
Ozone Transfer Standard Guidance Document 10/2013 

4-1 

SECTION 4 

VERIFICATION & REVERIFICATION PROCESS 

The accuracy of a transfer standard is established by (1) relating the output to an O3 standard of higher 

authority (level) and (2) demonstrating that the repeatability of the transfer standard is within the limits specified in

Section 3. 

After a transfer standard has been shown to meet the qualification requirements discussed earlier in this 

document, the transfer standard must be certified before it can be used. The prescribed formal verification 

procedure and specifications are set forth in Section 3, but are explained in more detail below. Refer to Section 3 

while reading the explanations; the verification procedure step numbers correspond. 

4.1 PROCEDURE FOR INITIAL (6x6) VERIFICATION 

4.1.1. Verification requires the averaging of 6 comparisons between the transfer standard and a higher level UV 

O3 standard. Each comparison must cover the full range of O3 concentrations and is to be carried out on a 

different day to a primary standard. 

4.1.2. Each comparison must consist of 6 or more individual comparison points, including 0 and 90% (± 5%) of 

the upper range limit of the transfer standard. The other points must be approximately evenly spaced between 

these points. For each comparison, the slope and intercept is computed by a least squares linear regression. The 

result should be similar to the example shown in Figure 4.1. Table 4-1 provides the data set that went into the 

points derived in Figure 4.1. Most analyzer-type transfer standards will be linear and the linear regression can 

be calculated directly. However, for non-linear transfer standards or generation-type transfer standards where 

the output is related to a control setting or an adjustable parameter, a preliminary calibration relationship such as 

shown in Figure 4.2 is required. Note that the curve shown in Figure 4.2 may have a considerable zero offset 

and may be nonlinear. This preliminary calibration should also include any necessary correction formulas for 

defined variables determined during qualification. A smooth curve fitting the points in Figure 4.2 should be 

drawn or calculated. There are no specific requirements on the form, number of points, linearity, or frequency 

of repetition for this preliminary calibration. However, excessive inaccuracy in this relationship will show up as 

variability in the verification comparison and may cause failure of the verification specifications. During the 

verification comparisons, the preliminary calibration relationship (see Figure 4.2) is used to obtain the indicated 

O3 concentration used in the linear regression calculations of Figure 4.1. (Note that Figure 4.1 should be linear 

even though Figure 4.2 is nonlinear.) 

4.1.3. When 6 comparisons as shown in Figure 4.1 have been completed, compute the average slope ( m ) from 

the 6 individual slopes (mi), and the average intercept (Ī) from the 6 individual intercepts, (Ii). 
Ozone Transfer Standard Guidance Document 10/2013 

4-2 

4.1.4. Compute the relative standard 

deviation (sm) of the 6 slopes (mi) and 

the standard deviation (sI) for the 6 

intercepts (Ii) defined by equations 3 

and 4 in Section 3. 

4.1.5. Compare sm to the 3.7% 

specification, and compare sI to the 1.5 

(ppb) specification. In the example in 

Table 4-1, both sm (0.34%) and sI (0.57 

ppb) were acceptable. If either of these 

specifications is exceeded, it indicates 

that the transfer standard has too much 

variability and corrective action must be 

taken to reduce the variability before 

the transfer standard may be certified. 

(Excessive variability in the UV 

authoritative standard is possible, 

although it is much less likely.) 

4.1.6. If the specifications are met, the 

verification relationship for the transfer 

standard consists of the average slope 

( m ) and the average intercept ( I) and 

can be plotted as shown in Figure 4.3. 

Note that the qualification restrictions 

must also be included with the 

verification relationship and should be 

shown on a plot of the verification 

relationship as illustrated in Figure 4.3. 

When the transfer standard is 

subsequently used, the standard O3

concentration is calculated from 

Equation 5. 
Ozone Transfer Standard Guidance Document 10/2013 

4-3 

Transfer Standards Having a Defined Dependence on Some Variable

Verification of transfer standards 

having a defined dependence on a variable 

such as barometric pressure or temperature is 

complicated somewhat by the need to take the 

variable into account. During verification, the 

variable must be measured accurately, ideally 

with the same measuring instrument that will 

be used during subsequent utilization of the 

transfer standard. A correction for effect of 

the variable must then be included, via the 

preliminary calibration, during the verification 

comparisons. The final verification 

relationship must also clearly identify the 

applicable calibration and correction associated with the transfer standard. 

If the effect of the variable is limited to the measurement of gaseous flow rates, the appropriate corrections 

should be applied to the flowrates during the verification comparisons, and an ordinary comparison similar to 

Figures 4.1 or 4.2 will result. Where a simple linear dependence on a variable exists, a suitable reference level 

should be defined (e.g., 1.01 kPa (760 mm Hg) for barometric pressure); then the appropriate mathematical 

correction (formula) to correct the output from the reference level to any other level within a reasonable range 

should be specified. An inverse correction is applied to the transfer standard output during establishment of the 

preliminary calibration (see Figure 4.2) to normalize the output to the reference level. The preliminary calibration 

relationship is then plotted with the normalized data. 

Other approaches can also be used. For example, if the linearity of the transfer standard is not affected by 

the variable, then the verification relationship can include a “correction factor” relationship as illustrated in Figures 

B.2 and B.3 of Appendix B. Such a correction factor relationship can be determined either by changing the variable 

during the preliminary calibrations or by calculation based on data obtained during qualification tests. 

Use 

After verification, when the transfer standard is used to reproduce O3 standards, the verification relationship 

(such as illustrated in Figure 4.3) is used to determine the certified O3 concentration from the concentration 

indicated by assay or variable setting. In using transfer standards, it is good practice to try to minimize any change 

in variables even though the transfer standard may be insensitive to them. 
Ozone Transfer Standard Guidance Document 10/2013 

4-4 

This insensitivity is 

particularly true of transfer standards 

where there are numerous variables 

that may not have been included in 

qualification tests. Special effort 

should be made to use the same 

standards, reagents, apparatus, 

technique, etc., to the greatest possible 

extent during verification and use. Of 

course, for any transfer standard which 

has a defined relationship to some 

variable, that variable must be 

accurately measured and the output of 

the transfer standard must be corrected 

accordingly. 

4.2 PROCEDURE FOR REVERIFICATION 

While the principle accuracy of a transfer standard is established during verification, the confidence in that 

accuracy is maintained by continual reverification to demonstrate stability. The objective is to show, to the greatest 

extent possible, that the transfer standard did not change significantly between verification and use. However, if the 

UV authoritative standard is located in a laboratory, the reverification process may always take place under nearly 

identical conditions (temperature, line voltage, barometric pressure, etc.). Therefore, occasional repetition of the 

qualification tests discussed in Appendix B may be an important and indispensable supplement to reverification. 

Procedure

A verified transfer standard of Level 3 and greater must be reverified at the beginning and end of the ozone 

season or at least every six months whichever is less. A transfer standard which loses its verification may cause the 

loss of ambient O3 measurements made with ambient monitors that were calibrated with the transfer standard. 

Consequently, a more frequent reverification schedule will reduce the magnitude and risk of any such loss. If a 

transfer standard in only a generation device (no photometer) it is strongly suggested that more frequent 

reverifications (i.e., quarterly) occur. More frequent reverification may also provide better accuracy, particularly for 

transfer standards that show slow but steady change (drift) over long periods of time. 
Ozone Transfer Standard Guidance Document 10/2013 

4-5 

4.2.1 The first step in the reverification procedure is to carry out one comparison (not a complete 6x6) of 6 

or more individual comparison points, including 0 and 90% (± 5%) of the upper range limit of the transfer 

standard to an authoritative standard as specified in step 4.1.2 (see also Figure 4.1). 

4.2.2 To maintain verification, the linear regression slope of the new comparison (m) must be within ± 5% 

of the average slope of the current verification relationship ( m ) (i.e., the average slope of the last 6 comparisons). 

Thus, m must be within the interval 0.95 m  m  1.05 m . A convenient way to monitor the performance of a 

transfer standard is to plot each new slope as shown in Figure 4.4. 

4.2.3 If the new slope is within the ± 5% specification, then a new average slope (m ) and a new average 

intercept ( I) are calculated using the new comparison and the 5 most recent previous comparisons. Thus m and I

are running or moving averages always based on the 6 most recent comparisons. The new m and the new ± 5% 

limits can also be plotted on the chart shown in Figure 4.4. 

4.2.4 New values for the relative 

standard deviation of the slopes (sm) and the 

quantity (sI) are calculated based on the new 

comparison and the 5 most recent previous 

comparisons. The formulas are equation 3 and 

4 in Section 3. These parameters can also be 

monitored with a chart format similar to 

Figure 4.5. 

4.2.5 The new sm and sI must again 

meet the respective 3.7% and 1.5 SD 

specifications given in step 4.1.5. If all 

specifications are met, then a new verification 

relationship (based on the updated m and I) 

is established according to step 4.1.6 and 

illustrated by Figure 4.3. 

4.2.6 If a certified transfer standard 

fails to meet one of the reverification 

specifications, it loses its verification. 

Reverification then requires 6 new 

comparisons according to the entire 

verification procedure starting at step 4.1. This failure could be due to a malfunction, which obviously should be 
Ozone Transfer Standard Guidance Document 10/2013 

4-6 

corrected before repeating the verification procedure. If a transfer standard has been repaired or serviced in a way 

which could affect its output, the complete verification procedure must also be repeated. Another possible cause for 

failure of the reverification specifications might be a change in the preliminary calibration (Figure 4.2), which 

should then be re-established before the verification is repeated. 

Cross Checks

Frequently, analyzer -type transfer standard is used with an O3 generator for which a preliminary 

calibration such as shown in Figure 4.2 is available (even though the O3 generator is not certified as a transfer 

standard). In such a case, any large discrepancy between the two could serve as a warning that the transfer standard 

may need reverification. 

Reverification Tests

Normally, the characteristics of a transfer standard would not be expected to change profoundly or 

suddenly. For example, a transfer standard that is not initially sensitive to line voltage changes is not likely to 

become so after a period of use. Malfunctions are a major exception: malfunctions in line voltage regulation or 

temperature regulation, or other variable control components can easily render a transfer standard sensitive to a 

variable at any time. Furthermore, the malfunction may not be obvious to the operator and could go undetected for 

some time. Even a reverification may not disclose such a malfunction. In consequence, some of the qualification 

tests described earlier should be repeated on some periodic basis. Such tests may be more cursory than the original 

tests, but are nevertheless important. Other techniques include warning lights or operation indicators on components 

that are critical to regulatory functions and that might otherwise provide no indication of malfunction. A user should 

always be somewhat skeptical that a transfer standard is operating properly. 
Ozone Transfer Standard Guidance Document 10/2013 

A-1 

APPENDIX A 

ULTRAVIOLET PHOTOMETRIC PROCEDURE FOR PRIMARY OZONE STANDARDS1

NOTE: No changes have been made to this Appendix since it reflects what is currently in 40 CFR 

Part 50 Appendix D 

CALIBRATION PROCEDURE 

1. Principle. The calibration procedure is based on the photometric assay of ozone (O3) 

concentrations in a dynamic flow system. The concentration of O3 in an absorption cell is determined from 

a measurement of the amount of 254 nm light absorbed by the sample. This determination requires 

knowledge of (1) the absorption coefficient () of O3 at 254 nm, (2) the optical path length (  ) through 

the sample, (3) the transmittance of the sample at a wavelength of 254 nm, and (4) the temperature (T) and 

pressure (P) of the sample. The transmittance is defined as the ratio I/IO , where I is the intensity of light 

which passes through the cell and is sensed by the detector when the cell contains an O3 sample and IO is 

the intensity of light which passes through the cell and is sensed by the detector when the cell contains zero 

air. It is assumed that all conditions of the system, except for the contents of the absorption cell, are 

identical during measurement of I and IO. The quantities defined above are related by the Beer-Lambert 

absorption law, 

αC

0

e

I

I Transmittance    

 (eq. 1) 

where: 

 = absorption coefficient of O3 at 254 nm = 308 ± 4 atm-1 cm-1 at 0°C and 760 torr. (1, 2, 3, 4, 5, 6, 7)

C = O3 concentration in atmospheres 

l = optical path length in cm 

In practice, a stable O3 generator is used to produce O3 concentrations over the required range. 

Each O3 concentration is determined from the measurement of the transmittance (I/IO) of the sample at 254 

nm with a photometer of path length and calculated from the equation, 

                                                           

1

Extracted from the Code of Federal Regulations, Title 40, Part 50, Appendix D, as amended February 8, 

1979 (Federal Register, 44:8221-8233). 
Ozone Transfer Standard Guidance Document 10/2013 

A-2 

















  

0 I

I ln 1

c(atm)  (2a) 

or, 

















  

0

6

I

I ln 10

c(ppm)  (2b) 

The calculated O3 concentrations must be corrected for O3 losses which may occur in the photometer and 

for the temperature and pressure of the sample. 

2. Applicability. This procedure is applicable to the calibration of ambient air O3 analyzers, either 

directly or by means of a transfer standard certified by this procedure. Transfer standards must meet the 

requirements and specifications set forth in Reference 8. 

3. Apparatus. A complete UV calibration system consists of an ozone generator, an output port or 

manifold, a photometer, an appropriate source of zero air, and other components as necessary. The 

configuration must provide a stable ozone concentration at the system output and allow the photometer to 

accurately assay the output concentration to the precision specified for the photometer (3.1). Figure 1 

shows a commonly used configuration and serves to illustrate the calibration procedure which follows. 

Other configurations may require appropriate variations in the procedural steps. All connections between 

components in the calibration system downstream of the O3 generator should be of glass, Teflon, or other 

relatively inert material. Additional information regarding the assembly of a UV photometric calibration 

apparatus is given in Reference 9. For verification of transfer standards which provide their own source of 

O3, the transfer standard may replace the O3 generator and possibly other components shown in Figure 1; 

see Reference 8 for guidance. 

3.1 UV photometer. The photometer consists of a low-pressure mercury discharge lamp, 

(optional) collimation optics, an absorption cell, a detector, and signal-processing electronics, as illustrated 

in Figure 1. It must be capable of measuring the transmittance, I/IO, at a wavelength of 254 nm with 

sufficient precision such that the standard deviation of the concentration measurements does not exceed the 

greater of 0.005 ppm or 3% of the concentration. Because the low-pressure mercury lamp radiates at 

several wavelengths, the photometer must incorporate suitable means to assure that no O3 is generated in 

the cell by the lamp, and that at least 99.5% of the radiation sensed by the detector is 254 nm radiation. 

(This can be readily achieved by prudent selection of optical filter and detector response characteristics.) 

The length of the light path through the absorption cell must be known with an accuracy of at least 99.5%. 

In addition, the cell and associated plumbing must be designed to minimize loss of O3 from contact with 

cell walls and gas handling components. See Reference 9 for additional information. 
Ozone Transfer Standard Guidance Document 10/2013 

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3.2 Air flow controllers. Devices capable of regulating air flows as necessary to meet the output 

stability and photometer precision requirements. 

3.3 Ozone generator. Device capable of generating stable levels of O3 over the required 

concentration range. 

3.4 Output manifold. The output manifold should be constructed of glass, Teflon, or other 

relatively inert material, and should be of sufficient diameter to insure a negligible pressure drop at the 

photometer connection and other output ports. The system must have a vent designed to insure 

atmospheric pressure in the manifold and to prevent ambient air from entering the manifold. 

3.5 Two-way valve. Manual or automatic valve, or other means to switch the photometer flow 

between zero air and the O3 concentration. 

3.6 Temperature indicator. Accurate to ±1°C. 

3.7 Barometer or pressure indicator. Accurate to ±2 torr. 

4. Reagents. 

4.1 Zero air. The zero air must be free of contaminants which would cause a detectable response 

from the O3 analyzer, and it should be free of NO, C2H4, and other species which react with O3. A 

procedure for generating suitable zero air is given in Reference 9. As shown in Figure 1, the zero air 

supplied to the photometer cell for the IO reference measurement must be a derived from the same source as 

the zero air used for generation of the ozone concentration to be assayed (I measurement). When using the 

photometer to certify a transfer standard having its own source of ozone, see Reference 8 for guidance on 

meeting this requirement. 

5. Procedure. 

5.1 General operation. The calibration photometer must be dedicated exclusively to use as a 

calibration standard. It should always be used with clean, filtered calibration gases, and never used for 

ambient air sampling. Consideration should be given to locating the calibration photometer in a clean 

laboratory where it can be stationary, protected from physical shock, operated by a responsible analyst, and 

used as a common standard for all field calibrations via transfer standards. 
Ozone Transfer Standard Guidance Document 10/2013 

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5.2 Preparation. Proper operation of the photometer is of critical importance to the accuracy of 

this procedure. The following steps will help to verify proper operation. The steps are not necessarily 

required prior to each use of the photometer. Upon initial operation of the photometer, these steps should 

be carried out frequently, with all quantitative results or indications recorded in a chronological record 

either in tabular form or plotted on a graphical chart. As the performance and stability record of the 

photometer is established, the frequency of these steps may be reduced consistent with the documented 

stability of the photometer. 

5.2.1 Instruction manual: Carry out all set-up and adjustment procedures or checks as 

described in the operation or instruction manual associated with the photometer. 

5.2.2 System check: Check the photometer system for integrity, leaks, cleanliness, proper 

flowrates, etc. Service or replace filters and zero air scrubbers or other consumable materials, as necessary. 

5.2.3 Linearity: Verify that the photometer manufacturer has adequately established that the 

linearity error of the photometer is less than 3%, or test the linearity by dilution as follows: Generate and 

assay an O3 concentration near the upper range limit of the system (0.5 or 1.0 ppm), then accurately dilute 

that concentration with zero air and reassay it. Repeat at several different dilution ratios. Compare the 

assay of the original concentration with the assay of the diluted concentration divided by the dilution ratio, 

as follows: 

100%

A

A A / R

E

1

1 2

            

 (3) 

where: E = linearity error, percent 

 A1 = assay of the original concentration 

 A2 = assay of the diluted concentration 

 R = dilution ratio = flow of original concentration divided by the total flow 

The linearity error must be less than 5%. Since the accuracy of the measured flowrates will affect 

the linearity error as measured this way, the test is not necessarily conclusive. Additional information on 

verifying linearity is contained in Reference 9. 

5.2.4 Intercomparison: When possible, the photometer should be occasionally 

intercompared, either directly or via transfer standards, with calibration photometers used by other agencies 

or laboratories. 
Ozone Transfer Standard Guidance Document 10/2013 

A-5 

5.2.5 Ozone losses: Some portion of the O3 may be lost upon contact with the photometer 

cell walls and gas handling components. The magnitude of this loss must be determined and used to 

correct the calculated O3 concentration. This loss must not exceed 5%. Some guidelines for quantitatively 

determining this loss are discussed in Reference 9. 

5.3 Assay of O3 concentrations. 

5.3.1 Allow the photometer system to warm-up and stabilize. 

5.3.2 Verify that the flowrate through the photometer absorption cell, Fp, allows the cell to be 

flushed in a reasonably short period of time (2 liter/min is a typical flow). The precision of the 

measurements is inversely related to the time required for flushing, since the photometer drift error 

increases with time. 

5.3.3 Insure that the flowrate into the output manifold is at least 1 liter/min greater than the 

total flowrate required by the photometer and any other flow demand connected to the manifold. 

5.3.4 Insure that the flowrate of zero air, FZ, is at least 1 liter/min greater than the flowrate 

required by the photometer. 

5.3.5 With zero air flowing in the output manifold, actuate the two-way valve to allow the 

photometer to sample first the manifold zero air, then FZ. The two photometer readings must be equal (I = 

Io). 

 NOTE: In some commercially available photometers, the operation of the two-way valve and 

various other operations in section 5.3 may be carried out automatically by the photometer. 

5.3.6 Adjust the O3 generator to produce an O3 concentration as needed. 

5.3.7 Actuate the two-way valve to allow the photometer to sample zero air until the 

absorption cell is thoroughly flushed and record the stable measured value of Io. 

5.3.8 Actuate the two-way valve to allow the photometer to sample the ozone concentration 

until the absorption cell is thoroughly flushed and record the stable measured value of I. 
Ozone Transfer Standard Guidance Document 10/2013 

A-6 

5.3.9 Record the temperature and pressure of the sample in the photometer absorption cell. 

(See Reference 9 for guidance.) 

5.3.10 Calculate the O3 concentration from equation 4. An average of several 

determinations will provide better precision.

) L

10 )( P760 )( 273T )( II ln 1 [O ] (

6

0

3 OUT

   

 (4) 

where: [O3]OUT = O3 concentration, ppm 

 = absorption coefficient of O3 at 254 nm = 308 atm-1 cm-1 at 0°C and 760 torr 

l = optical path length, cm 

 T = sample temperature, K 

 P = sample pressure, torr 

 L = correction factor for O3 losses from 5.2.5 = (1-fraction O3 lost) 

NOTE: Some commercial photometers may automatically evaluate all or part of equation 4. It is 

the operator’s responsibility to verify that all of the information required for equation 4 is obtained, either 

automatically by the photometer or manually. For “automatic” photometers which evaluate the first term 

of equation 4 based on a linear approximation, a manual correction may be required, particularly at higher 

O3 levels. See the photometer instruction manual and Reference 9 for guidance. 

5.3.11 Obtain additional O3 concentration standards as necessary by repeating steps 5.3.6 to 

5.3.10 or by Option 1. 

5.4 Verification of transfer standards. A transfer standard is certified by relating the output of the 

transfer standard to one or more ozone standards as determined according to section 5.3. The exact 

procedure varies depending on the nature and design of the transfer standard. Consult Reference 8 for 

guidance. 

5.5 Calibration of ozone analyzers. Ozone analyzers are calibrated as follows, using ozone 

standards obtained directly according to section 5.3 or by means of a certified transfer standard. 

5.5.1 Allow sufficient time for the O3 analyzer and the photometer or transfer standard to 

warm-up and stabilize. 
Ozone Transfer Standard Guidance Document 10/2013 

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5.5.2 Allow the O3 analyzer to sample zero air until a stable response is obtained and adjust 

the O3 analyzer’s zero control. Offsetting the analyzer’s zero adjustment to + 5% of scale is recommended 

to facilitate observing negative zero drift. Record the stable zero air response as “Z”. 

5.5.3 Generate an O3 concentration standard of approximately 80% of the desired upper 

range limit (URL) of the O3 analyzer. Allow the O3 analyzer to sample this O3 concentration standard until 

a stable response is obtained. 

5.5.4 Adjust the O3 analyzer’s span control to obtain a convenient recorder response as 

indicated below: 

100 Z

URL

[O ] recorder response (%scale) 3 OUT  



 



   (5) 

where: URL = upper range limit of the O3 analyzer, ppm 

 Z = recorder response with zero air, % scale 

Record the O3 concentration and the corresponding analyzer response. If substantial adjustment of 

the span control is necessary, recheck the zero and span adjustments by repeating steps 5.5.2 to 5.5.4. 

5.5.5 Generate several other O3 concentration standards (at least 5 others are recommended) 

over the scale range of the O3 analyzer by adjusting the O3 source or by Option 1. For each O3

concentration standard, record the O3 concentration and the corresponding analyzer response. 

5.5.6 Plot the O3 analyzer responses versus the corresponding O3 concentrations and draw 

the O3 analyzer’s calibration curve or calculate the appropriate response factor. 

5.5.7 Option 1: The various O3 concentrations required in steps 5.3.11 and 5.5.5 may be 

obtained by dilution of the O3 concentration generated in steps 5.3.6 and 5.5.3. With this option, accurate 

flow measurements are required. The dynamic calibration system may be modified as shown in Figure 2 to 

allow for dilution air to be metered in downstream of the O3 generator. A mixing chamber between the O3

generator and the output manifold is also required. The flowrate through the O3 generator (FO) and the 

dilution air flowrate (FD) are measured with a reliable flow or volume standard traceable to NBS. Each O3

concentration generated by dilution is calculated from: 
Ozone Transfer Standard Guidance Document 10/2013 

A-8 

















  

0 D

0

3 OUT 3 OUT F F

F [O ] [O ] (6) 

where: [O3]'OUT = diluted O3 concentration, ppm 

 FO = flowrate through the O3 generator, liter/min 

FD = diluent air flowrate, liter/min 

REFERENCES FOR APPENDIX A 

1. E.C.Y. Inn and Y. Tanaka, “Absorption Coefficient of Ozone in the Ultraviolet and Visible Regions”. 

J. Opt. Soc. Am., 43,870 (1953). 

2. A.G. Hearn, “Absorption of Ozone in the Ultraviolet and Visible Regions of the Spectrum”, Proc. 

Phys. Soc. (London), 78,932 (1961). 

3. W.B. DeMore and O. Raper, “Hartley Band Extinction Coefficients of Ozone in the Gas Phase and in 

Liquid Nitrogen, Carbon Monoxide, and Argon”, J. Phys. Chem., 68,412 (1964). 

4. M. Griggs, “Absorption Coefficients of Ozone in the Ultraviolet and Visible Regions”, J. Chem. Phys., 

49,857 (1968). 

5. K.H. Becker, U. Schurath, and H. Seitz, “Ozone Olefin Reactions in the Gas Phase. 1. Rate Constants 

and Activation Energies”, Int’l. J. Chem. Kinetics, VI,725 (1974). 

6. M.A.A. Clyne and J.A. Coxom. “Kinetic Studies of Oxy-Halogen Radical Systems”, Proc. Roy. Soc., 

A3O3,207 (1968). 

7. J.W. Simons, R.J. Paur, H.A. Webster, and E.J. Bair, “Ozone Ultraviolet Photolysis. VI. The 

Ultraviolet Spectrum”, J. Chem. Phys., 59, 1203 (1973). 

8. “Transfer standards for Calibration of Ambient Air Monitoring Analyzers for Ozone”, EPA 

publication available from EPA, Department E (MD-77), Research Triangle Park, North Carolina 

27711. 

9. “Technical Assistance Document for the Calibration of Ambient Ozone Monitors”, EPA publication 

available from EPA, Department E (MD-77), Research Triangle Park, North Carolina 27711. 
Ozone Transfer Standard Guidance Document 10/2013 

A-9 

Figure 1. Schematic diagram of a typical UV photometric calibration system. 
Ozone Transfer Standard Guidance Document 10/2013 

A-10 

Figure 2. Schematic diagram of a typical UV photometric calibration system (Option 1). 
Ozone Transfer Standard Guidance Document 10/2013 

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APPENDIX B 

QUALIFICATION PROCESS 

The first step in establishing the authority of a candidate transfer standard is to prove that it qualifies for use 

as a transfer standard. In other words, can the output (either an actual O3 concentration or a concentration assay, 

depending on the type) of the candidate transfer standard be trusted under the changing conditions of use that might 

be encountered in field use. Qualifying new instruments can help ensure the instruments will perform under various 

field conditions that might not get observed during a traditional verification processes described in Section 4 of this 

document. 

The primary requirement of a transfer standard is repeatability – repeatability under the stress of variable 

conditions that may change between verification and use. A candidate transfer standard is qualified by proving that 

it is repeatable over an appropriate range for each variable likely to change between the time and place of 

verification and the time and place of use. According to the specifications in Section 3, the repeatability must be 

within ± 4% or ± 4 ppb, whichever is greater, for each condition or variable that may change between the point of 

verification and the point of use. 

Selecting the conditions that are likely to vary and that may affect the repeatability of the device or 

procedure is largely a matter of intelligent, informed, judgment. To a large extent, the variables will depend on the 

nature of the device or procedure; for some candidate transfer standards, the variables to be considered may be quite 

numerous. It is the user’s responsibility to determine all of the conditions to be considered in the demonstration of 

repeatability before a candidate transfer standard can be considered qualified for use as a transfer standard. 

Common conditions likely to affect a wide variety of types of transfer standards include such items as ambient 

temperature, line voltage, barometric pressure, elapsed time, physical shock, etc. These variables are discussed 

individually later in this section. Conditions not likely to affect the transfer standard can usually be eliminated from 

consideration. The user must, however, be constantly alert for the unusual situation where an unexpected condition 

may significantly affect the repeatability of a transfer standard. 

Note that a transfer standard does not necessarily need to be constant with respect to these variables, only 

repeatable or predictable. While it is certainly desirable that a device or procedure be insensitive to any given 

variable, it may still qualify as a transfer standard if it is repeatable. For example, it may be difficult to find or 

design a generation-type transfer standard device that is insensitive to barometric pressure. However, if it is 

repeatable with respect to barometric pressure, the relationship can be quantitatively defined by a curve or table. At 

the time of use, the local barometric pressure must be measured and the curve or table used to “correct” the transfer 

standard’s indicated output. This technique is acceptable for one or perhaps two variables. But beyond two 
Ozone Transfer Standard Guidance Document 10/2013 

B-2 

variables, the difficulties of determining and specifying the relationship to the variables may become impractical. 

Fortunately, sensitivity to most variables can be reasonably controlled. 

Demonstration of repeatability for a candidate transfer standard normally requires testing for each condition 

that could or may affect it. Typical tests for common conditions are discussed below. Again, intelligent judgment is 

required to determine what conditions to test and the extent of testing required to qualify the device or procedure. 

For commercially available transfer standard devices, some or all of the testing may be carried out by the 

manufacturer, thereby reducing the burden on the user. In some cases it may be possible to judiciously substitute 

design rationale for actual testing. For example, a device whose power supply is designed to be highly regulated 

electronically may not require specific line voltage tests. However, such situations should be viewed with 

considerable skepticism. 

The preceding discussion brings up the further question of whether candidate transfer standards must be 

tested individually or whether they can be qualified by type, model, or agency. The units of commercially produced 

transfer standard devices are designed and manufactured to be identical and should therefore have very similar 

characteristics. The manufacturer could carry out the necessary qualification tests on representative samples, 

sparing the user the burden of testing each unit or the cost of paying the manufacturer to test each unit individually. 

Under this concept, it would certainly be appropriate to require the manufacturer to guarantee that each unit meets 

appropriate performance specifications. However, the user should assume a skeptical attitude, in view of 

manufacturing tolerances and possible defective components, and carry out at least some minimal tests to verify that 

each unit is acceptable. 

In the case of unique devices assembled by users, testing for all pertinent conditions which could or might 

affect the device are normally required. 

QUALIFICATION TESTS 

Some of the more common conditions likely to be encountered or to change while using transfer standards 

and that may often affect the repeatability of the device or procedure are discussed below. Also discussed are ways 

or approaches to test for sensitivity to the condition. As noted previously, the exact conditions or variables that must 

be considered depend on the specific nature of the device or procedure. The user (or manufacturer, etc.) should 

determine the conditions for each case on an intelligent judgmental basis derived from a complete understanding of 

the operation of the device or procedure and supported by appropriate rationale. Specific recommendations for the 

common O3 generators, and O3 analyzer transfer standards are provided in the appropriate Appendices. 
Ozone Transfer Standard Guidance Document 10/2013 

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Once the conditions to be considered have been determined, the objective of the qualification tests is either 

a or b: 

a) to demonstrate that the candidate transfer standard’s output is not affected by more than ± 4% or ± 4 

ppb (whichever is greater) by the condition over a range likely to be encountered during use of the 

device or procedure; 

b) to demonstrate that the candidate transfer standard’s output is repeatable within ± 4% or ± 4 ppb 

(whichever is greater) as the variable is changed over a range likely to be encountered during use, and 

to quantify the relationship between the output and the variable. 

Temperature

Changes in ambient temperature are likely to occur from place to place and from one time to another. 

Temperature changes are very likely to affect almost all types of transfer standards unless appropriate means are 

used to avoid adverse effects. Temperature affects transfer standards in many ways: changes in the action of 

components, changes in chemical reactions or rates of reaction, volume changes of gases, electronic drift, variable 

warm-up time, etc. The most important effects may well be (1) changes in the output of generation devices, (2) 

changes in the sensitivity of O3 analyzer-type systems, and (3) changes in the volume of air flows which must be 

measured accurately. 

Temperature effects can be minimized in several ways. Since shelters for ambient air monitoring are 

normally maintained at about 20 – 30o

 C (with some flexibility for fluctuations) all transfer standards should be 

proven to be repeatable within this range. Transfer standard devices may be made insensitive to temperature 

changes by design, such as thermostatic regulation of sensitive components or of the entire device, or by temperature 

compensation. 

Temperature effects on air flow measurement can be minimized by the use of mass flowmeters, which do 

not measure volume, or by the regulation of gas temperatures. In another approach, ordinary ideal-gas-law 

corrections could be made manually to adjust to measured volumetric flowrates. However, when using orifice 

control or measurement devices such as critical orifices and rotameters, be sure to use an appropriate correction 

formula. 

Testing a candidate transfer standard for sensitivity to temperature is facilitated by the use of a controlled 

temperature chamber. However, successful temperature tests can be carried out in many ordinary laboratories where 

the temperature can be manually controlled by adjusting thermostats, blocking air vents or outlets, opening doors or 

windows, or using supplemental heaters or air conditioners. A reasonable temperature range would be 20 to 30°C 

(68 to 86°F). Broader temperature ranges could be used if appropriate. 
Ozone Transfer Standard Guidance Document 10/2013 

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The candidate transfer standard is tested by comparing its output to a stable concentration reference. This 

reference should ideally be a UV analyzer system at least one level above the level of the transfer standard. It 

would be best to locate the reference outside of the variable temperature test area. The candidate transfer standard 

should be tested at a minimum of 3 

different points over the temperature 

range, including the extremes, and at a 

minimum of 3 different 

concentrations. Be sure to allow 

sufficient time for the device or any 

instruments or equipment associated 

with the transfer standard to 

equilibrate (min time period or until 

reading stabilized) each time the 

temperature is changed. The test 

results should be plotted in a fashion 

similar to the example shown in Figure 

B.1. 

If the candidate transfer 

standard has a significant temperature 

dependence, additional test points at 

various concentrations and 

temperatures should be taken to define 

the relationship between output and 

temperature accurately. Furthermore, 

if the candidate turns out to have a 

dependence on more than one 

condition or variable, tests must be 

carried out over the range of both 

variables simultaneously to determine 

any interdependence between the two 

variables. Once the test data are 

acquired, they should be analyzed to determine if some general formula or curve can be derived (either analytically 

or empirically) to predict the correct O3 concentration at any temperature in the range (see Figure B.2). The 

correction formula or curve must be accurate within ± 4% or ± 4 ppb, whichever is greater. If two or more variables 
Ozone Transfer Standard Guidance Document 10/2013 

B-5 

are involved, a family of curves may be required; unless the relationship is rather simple, this situation may prove 

impractical in actual use. 

Line Voltage

Line voltage is very likely to vary from place to place and from one time to another. Good electrical or 

electronic design of the transfer standard should avoid sensitivity to line voltage variations, but poorly designed 

equipment can easily be affected. In addition, line voltage sensitivity may appear only as long-time thermal drift, a 

rather subtle effect. 

Aside from adequate design, line voltage effects can be minimized by the addition of an outboard line 

voltage regulator. However, such devices may distort the line voltage waveform, thereby adversely affecting some 

types of equipment. If such regulators are used, it is important that the same regulator is used during both 

verification and use of the transfer standard. Restriction of the transfer standard to a line voltage range in which the 

effects are insignificant is another alternative, but that would require monitoring the voltage during use and may 

preclude use at some sites. 

Testing for line voltage sensitivity can be carried out along the same lines as described for temperature 

testing. The line voltage can be varied by means of a variable voltage transformer (“Variac”) and measured by an 

accurate ac voltmeter. Do not use electronic “dimmer” controls which operate on a delayed-conduction principle, as 

such devices cause drastic waveform distortion. 

A line voltage range of 105 to 125 volts should adequately cover the vast majority of line voltages available 

in the U.S. If the transfer standard is used when powered by a small power generator, it should be checked for 

frequency dependence. 

Barometric Pressure/Altitude

Since O3 concentrations are gaseous in nature, all transfer standards will probably have some basic or 

inherent sensitivity to change in barometric pressure. Unfortunately, it is rather difficult to minimize barometric 

pressure effects by design. Air pressures can be regulated mechanically against an absolute reference, but most such 

schemes are not practical when working with O3 concentrations because of restrictions to inert materials such as 

glass or Teflon. At a constant altitude, normal day-to-day variation in barometric pressure is only a few percent. If 

the use of the transfer standard can be restricted to altitudes within a hundred meters of the verification altitude, it 

may be acceptable to neglect the barometric effect entirely. However, if the use of a transfer standard is necessary at 

altitudes significantly different than the calibration altitude, then pressure effects cannot generally be ignored. 
Ozone Transfer Standard Guidance Document 10/2013 

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Although not readily preventable, pressure effects are likely to be repeatable. As a result, barometric 

pressure may be the variable most likely to be handled by the defined-relationship approach discussed previously in 

connection with temperature effects. The 

technique is very similar to the technique 

used to determine a temperature 

relationship; hopefully, a unique quantitative 

relationship will result, such as that 

illustrated in Figure B.3. Remember that in 

any work with O3 concentrations at altitudes 

significantly above sea level, the 

concentration units must be clearly 

understood. The volume ratio concentration 

units must be clearly understood. The 

volume ratio concentration units (ppm, ppb, 

etc.) are independent of pressure, while 

density units such as μg/m3

 are related to pressure. (However, the μg/m3 unit defined and used by EPA is 

“corrected” to 1.01 kPa (760 mm Hg) and 25°C and is therefore related to ppm by a constant.) 

Testing with respect to barometric pressure may be difficult. The use of a variable pressure chamber is the 

best approach, but few laboratories have access to such facilities. It is conceivable that various pressures could be 

obtained in a manifold setup, but construction of such an apparatus is difficult and of questionable validity. The use 

of a mobile laboratory vehicle which can be driven to various altitudes to conduct tests may offer the most feasible 

solution. Some types of transfer standards may not require pressure tests because their pressure sensitivity is well 

known. Some analyzer-type devices are clearly related directly to gas density, where a simple ideal-gas-law 

correction can be applied. Pressure tests are not needed for these types. For commercially-produced devices, the 

manufacturer would be expected to carry out the necessary qualification tests and to offer the devices as typeapproved, at least with respect to pressure effects. 

As a final note of encouragement, automatic compensation for barometric pressure is rapidly becoming 

economically feasible for some types of O3 transfer standards by the incorporation of microprocessor technology. 

At least two manufacturers have used this approach in commercially available instruments. 

Elapsed Time

As the elapsed time between verification and use increases, the confidence in the repeatability decreases. 

As a result, periodic reverification is needed. Some types of O3 generation devices have a definite loss of output 

(decay) with time. This decay is usually associated with use-time or on-time rather than total elapsed time. Since 
Ozone Transfer Standard Guidance Document 10/2013 

B-7 

the decay rate tends to be quantifiable, it can be accommodated with the defined-relationship mechanism discussed 

in connection with temperature effects: the transfer standard is equipped with an hours meter or another time 

measuring device and a series of tests over a sufficient time period can then be used to determine the decay rate. 

During use, a correction to the output is applied based on the number of hours of on-time since the last verification. 

Another approach is to recertify such a transfer standard often enough so that the error due to decay never 

exceeds the ± 4% or ± 4 ppb specification. 

Variability

The adequacy of the relationship between a transfer standard and a authoritative reference O3 standard is 

dependent on the variability of the transfer standard. Variability reduces confidence in transfer standards. A high 

degree of variability may be cause for disqualifying a device or procedure for use as a transfer standard, or for 

selecting one with lower variability. Although the verification procedure in Section 4 includes a test for variability, 

more extensive tests for variability may be necessary to qualify a transfer standard because the verification test is for 

variability in the slope of the verification relationship and not for individual point variability. Furthermore, 

variability may be due to changes in conditions not encountered during verification. 

Many different types of transfer standards may have excessive variability for a variety of reasons. 

Qualification variability testing is perhaps most needed to test for the effect of a variety of non-specific or nonquantitative variables that cannot be tested individually. Whenever increased variability can be assigned to a 

specific cause, corrective actions or restrictions can be and should be applied to reduce the variability. 

The variability test should be carried out on a single-point basis. A series of at least 6 single-point 

comparisons should be made between the candidate transfer standard and a UV reference at each of at least two 

fixed concentrations – one low concentration (less than 0.1 ppm) and one high concentration (over 80% of the upper 

range limit). These comparisons should be made over a variety of conditions and situations and over a number of 

days. For each concentration, verify that all O3 concentration measurements determined by the UV reference 

standard are very nearly equal. Then calculate the average of the 6 (or more) concentrations indicated by the 

transfer standard, using the following equation: 





n

i 1

i y

n

1 Ave

Where: n = number of comparisons 

 yi = O3 concentration indicated by the transfer standard 
Ozone Transfer Standard Guidance Document 10/2013 

B-8 

Determine the difference between each concentration indicated by the transfer standard and the average 

concentration (yi − Ave). Each difference must be less than ± 5% of the average (for concentrations over 0.1 ppm) 

or less than ± 5 ppb (for concentrations less than 0.1 ppm). 

For this test, the acceptable limits are ± 5% or ± 5 ppb rather than ± 4% or ± 4 ppb, because the test is for 

general variability, which may derive from a number of non-identifiable causes. Under these circumstances slightly 

wider limits than those allowed for the other qualification tests are acceptable. 

One technique that can reduce variability and improve accuracy is repetition and averaging. For example, 

the variability of assay procedures can be reduced by assaying each concentration several times and averaging the 

results. Of course, if this technique is used, it becomes a necessary part of the transfer standard procedure and must 

be carried out each time the transfer standard is used and certified. 

Relocation

A transfer standard obviously needs to maintain repeatability after being moved and possibly encountering 

mechanical shocks, jolts, and stress. Any electrical or thermal stress incident to turning the device or equipment on 

and off frequently is also of concern, as is consideration of orientation or set-up factors. 

Tests for these conditions, while perhaps not particularly quantitative, should include actually moving the 

candidate device or equipment to different locations and comparing the output each time it is returned. Tests could 

also include mild shock or drop tests, or tests for any set-up factors which can be specifically identified, e.g., 

physical orientation, removal of covers, any set-up variations. Any cause-and-effect-relationship discovered should 

be investigated completely. The tests may be conveniently combined or included with those discussed previously 

for variability. 

Operator Adjustments

Those transfer standard devices whose output is to be related to an operator adjustment (such as an 

adjustable O3 generator) should be tested for repeatability with respect to the adjustment. Mechanical adjustments 

might need to be tested for play, backlash, hysteresis, slippage, and resolution. Other types of adjustments may 

require tests for analogous aspects. If possible, specific tests should be used. For example, approaching a given 

setting from both above and below the setting might be appropriate for testing play or hysteresis. If specific tests 

cannot be designed, then simple repeatability tests at several different settings should be carried out. 
Ozone Transfer Standard Guidance Document 10/2013 

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Malfunctions

The usefulness of a transfer standard is dependent on the degree of confidence that can be put on its ability 

to reproduce O3 standards. While any device is subject to occasional malfunctions, frequent malfunctions would 

certainly compromise the purpose of a transfer standard. Of particular concern are non-obvious type malfunctions 

that can cause a significant error of which the operator is unaware. While no specific tests for malfunctions are 

normally used, the tests described above for the other conditions need to be repeated periodically to check for nonobvious malfunctions. After a malfunction has been corrected, the transfer standard must be recertified. 

Other Conditions

Any other condition that might affect a candidate device or procedure or that might cause change between 

the point of verification and the point of use should be tested. 
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APPENDIX C 

VERIFICATION OF AN OZONE GENERATOR AS A TRANSFER STANDARD 

Following the specifications in Section 3 and the guidance in Section 4, this appendix attempts to 

provide more specific instructions for verifying an O3 generator as a transfer standard. 

This appendix assumes that the O3 generator is of the common UV lamp type. The generator has a 

means to adjust the O3 concentration over a convenient range without changing the flowrate, but the device has 

no means to assay the output O3 concentration. 

PRELIMINARY REQUIREMENTS 

The first requirement is a source of zero air. A means of reasonable flow regulation for the zero air is 

also needed, since the concentration of O3 generated by the UV lamp varies with flow. Some commercial O3

generation systems may include some or all of the required zero air source and flow components. Otherwise, the 

zero air subsystem must be provided by the user. Scrubbed ambient air is preferable to cylinder zero air as the 

latter may vary in oxygen content or impurity level from one cylinder to another. Decide whether or not the zero 

air subsystem is to be an integral part of the transfer standard. If not, the flow regulation and flow monitoring 

components at least should be integral. Be aware that differences in zero air from one zero air system to another 

may affect the repeatability of the O3 generator. 

The O3 generator also needs an output manifold which meets the requirements specified in Appendix A. 

The manifold may be as simple as a tee where one of the legs serves as a vent. 

Access to a UV calibration system as described in Appendix A is required for verification of the 

transfer standard and is also recommended for the qualification tests. Review the O3 procedure for comparing 

the output of an O3-generation-type transfer standard to a UV authoritative reference O3 standard (see Section 3). 

Review any operation information or instructions provided by the manufacturer of the O3 generator to become 

familiar with its operation. 

Review the documentation requirements specified in Section 3 and complete item 2.1. 

QUALIFICATION 

The next step is to qualify the transfer standard by demonstrating that it is repeatable to within the 

qualification specifications given in Section 3 (± 4% or 4 ppb, whichever is greater). The variables likely to 
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affect an O3 generator are the same as those discussed generally below. Refer to Appendix B for additional 

guidance on each of the qualification tests. 

Before starting qualification tests, first prepare a tentative preliminary calibration relationship as shown 

in Figure 4.2, where the O3 output concentration is related to the O3 adjustment setting. Although you may want 

to prepare a more complete preliminary calibration relationship after qualification, this tentative relationship is 

necessary to carry out the qualification tests. Prepare the relationship as shown in Figure 4.2 by plotting the 

transfer standard’s output concentration as measured by the UV reference system at various O3 settings. Note 

the temperature, barometric pressure, line voltage, and other pertinent conditions. During the qualification tests, 

use this preliminary relationship to determine each “indicated output” from the transfer standard for a given 

setting of the concentration adjustment (i.e., sleeve setting or current setting). 

QUALIFICATION TESTS 

Temperature

If possible, select an O3 generator which has temperature regulation, preferably one which has a 

temperature indicator or other warning device to indicate whether the temperature regulator is working properly. 

Keep in mind that such systems will require a warm-up period before the temperature stabilizes. 

Select a temperature range over which the O3 generator is to be qualified. Temperatures from 20 to 

30°C (68 to 86°F) might be appropriate, or possibly 15 to 30°C (59 to 86°F). To show how the indicated output 

of the generator varies as the temperature changes, test the generator over this temperature range at several O3

concentrations as suggested by Figure B.1. Be sure (1) that the proper temperature and pressure corrections are 

made to the UV standard, (2) that the O3 generator is allowed to equilibrate each time the temperature is 

changed, and (3) that the O3 setting is repeated precisely for each different temperature. If the manufacturer has 

tested the O3 generator (or one like it), only enough tests are needed to show that the generator is operating 

properly and meets the specifications. 

If the O3 generator’s actual output concentration does not vary more than ± 4% over the entire 

temperature range, then it is qualified over that temperature range. If it does not meet those specifications, the 

following options are available: 

a) determine if the generator has a malfunction or inadequacy, attempt to correct it, and then retest it; 

b) reduce the temperature range to a range over which the generator does meet the specifications (this may 

inconveniently restrict the subsequent use of the generator); or 

c) attempt to determine, either analytically or empirically, the temperature-output relationship such as 

illustrated in Figure B.2. When this relationship is used to calculate a “corrected” indicated output, the 
Ozone Transfer Standard Guidance Document 10/2013 

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O3 generator should meet the specifications. If so, this correction formula becomes a necessary and 

integral part of the transfer standard and must be included with the preliminary calibration relationship 

(see Figure 4.2) and ultimately with the verification relationship (see Figure 4.3). 

Line Voltage

Line voltage tests are conducted in a fashion analogous to the temperature tests, substituting line 

voltage variation for temperature variation. A range of 105 to 125 volts should be appropriate. A well designed 

O3 generator should show little or no sensitivity to line voltages over this range. If the generator does not meet 

the specifications, you have the same three options as with temperature, plus a fourth option of adding an 

external voltage regulator. 

Barometric Pressure/Altitude

Virtually all UV O3 generators are sensitive to pressure changes unless they are specifically designed to 

compensate for pressure effects. If use of the O3 generator can be restricted to altitudes within a range of about 

100 meters (300 feet), the range of pressure variations is only about 2 or 3% (about 1% for altitude variations 

and about 1 to 2% for normal barometric pressure changes). Under these conditions an uncompensated O3

generator operating at ambient pressure can be expected to meet the qualification specifications. Pressure 

changes could then be ignored and no tests would be necessary. 

Where a larger altitude range is needed (or for applications requiring more accuracy), the O3 generator’s 

sensitivity to pressure changes must be compensated, either by design or by defining the pressure-output 

relationship and developing a correction formula (see Figure B.3). 

Testing for pressure sensitivity is most practically carried out by moving both the O3 generator and the 

UV standard system to various altitudes. In doing this, be sure (1) that the proper temperature and pressure 

corrections are made to the UV standard, (2) that the O3 setting is repeated precisely for each new pressure, and 

(3) that other variables (temperature, line voltage, etc.) are controlled so that they do not affect the generator. 

From the test data, determine an appropriate correction formula and include it with the preliminary calibration 

relationship and then ultimately with the verification relationship. 

Elapsed Time

The output of a UV O3 generator is likely to decrease somewhat with usage time. Comparisons with a 

UV reference over a period of time are necessary to determine the rate of decay. Then, either the transfer 
Ozone Transfer Standard Guidance Document 10/2013 

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standard can be recertified often enough so that it stays within specifications between verifications, or a 

correction factor based on elapsed time can be determined and used with the preliminary calibration relationship. 

Variability

General variability (variability other than that associated with a specific known variable) is not likely to 

be a problem with an O3 generator unless it is very poorly designed. If any of the other qualification tests show 

variability not strongly correlated with a specific variable, then a test for general variability would be needed. If 

the generator meets the specifications for the other tests, no specific test for general variability is required. 

Relocation

Relocation tests help to establish the ruggedness of the O3 generator. During the course of the elapsedtime or other tests, move the O3 generator about as it might be moved during subsequent use to see if any 

malfunctions, variability, or other dependability problems are observed that might make use of the generator 

questionable or inconvenient. 

Operator Adjustments

Test the O3 generator for repeatability of O3 concentration setting. Check each of several settings 

repeatedly, approaching sometimes from a higher setting and sometimes from a lower setting. Intersperse the 

various settings. Note if a stabilization period is required for each new setting, or if any other observable 

peculiarities are evident. A well-designed O3 generator is not likely to fail this test, but the information obtained 

will help to achieve better precision. 

Malfunctions

There is no special test for malfunctions. During other tests, be observant for any malfunctions that 

occur or other characteristic weaknesses in the generator that could cause unreliability. Understand the design 

and operation of the generator so that you can be alert to non-obvious types of malfunctions such as failure of 

temperature, line voltage, flow or other regulation mechanisms. 

Other Conditions

The tests described above should cover the performance variables for most common UV-type O3

generators. However, new types of generators or unusual generator designs may require additional special tests. 
Ozone Transfer Standard Guidance Document 10/2013 

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VERIFICATION 

Before conducting the verification tests, decide whether a new preliminary calibration relationship (see 

Figure 4.2) should be prepared. 

A new preliminary calibration relationship is advisable if: 

a) the transfer standard needs one or more correction formulas for defined-relationship variables; 

b) the original relationship was rough, inaccurate, or incomplete; 

c) the original relationship indicates output concentrations more than 30% different than the UV standard; 

or 

d) the qualification tests provided other information to suggest that a new relationship should be prepared. 

If a new preliminary calibration relationship is needed, prepare it carefully and accurately, including 

enough points to define it precisely over the entire operating range. Ozone generators that are non-linear or have 

appreciable variability need additional comparisons to define the relationship precisely. If any correction 

formulas are needed, clearly specify the conditions at the time the preliminary calibration relationship is 

established, and be sure that the specified correction formula is accurate. If necessary, various qualification tests 

should be repeated to verify that the corrections are accurate. Note or clarify any special operating instructions, 

operating restrictions, limits, or other pertinent information. 

When the preliminary calibration relationship has been prepared, proceed to verify the transfer standard 

as specified in Section 4. It is very important to complete the documentation requirements specified in Section 

3. 

USE 

In using the O3 generator as a transfer standard, review the guidance in Section 2. Reverify the transfer 

standard as required by Section 4. Consider occasional cross checks of the transfer standard to other O3

standards, and occasionally repeat the qualification tests to be sure the transfer standard is maintaining adequate 

reliability. 
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APPENDIX D 

VERIFICATION OF AN OZONE ANALYZER AS A TRANSFER STANDARD 

Following the specifications in Section 3 and the guidance in Section 4, this appendix attempts to provide 

more specific instructions for verifying an O3 analyzer as a transfer standard. 

The analyzer does not have to be a UV-type analyzer; any type of O3 analyzer may be considered for use as 

a transfer standard. However, UV-type analyzers may be more readily portable or have other practical advantages 

over other types of O3 analyzers. 

An analyzer used as a transfer standard should receive special treatment consistent with its authoritative 

status; careful handling and storage, frequent maintenance and service and operation by a fully competent operator. 

In particular, the analyzer should not be used for ambient monitoring between uses as a transfer standard, as dust or 

dirt buildup in the cell and other operational degradation may occur. Where it is necessary to use an analyzer that 

has had previous service as an ambient monitor, the analyzer should be thoroughly cleaned and reconditioned prior 

to verification as a transfer standard. 

PRELIMINARY REQUIREMENTS 

First, a stable source of O3 to be assayed by the analyzer/transfer standard, and an attendant source of zero 

air for the O3 generator are required. Some analyzers have internal or associated O3 generators that may be used; 

otherwise, an O3 generator/zero air system must be provided by the user. In addition, some means of reasonable 

flow regulation for the zero air is usually needed, since the concentration of O3 generated by most O3 generators 

varies with flow. Decide whether or not the O3 generation/zero air system is to be an integral part of the transfer 

standard. If not, arrangements must be made for an adequate O3 generator/zero air supply at each site where the 

transfer standard will be used. 

The O3 generator will also need an output manifold meeting the requirements specified in paragraph 3.4 of 

Appendix A. The manifold may be as simple as a tee where one of the legs serves as a vent. 

Access to a UV calibration system as described in Appendix A is required for verification of the transfer 

standard and is also recommended for the qualification tests. Comparing the output (indicated concentration) of the 

O3 analyzer to the UV authoritative reference O3 standard is easy, as the analyzer is simply connected to the output 

manifold of the UV authoritative O3 standard. 
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Review any operation information or instructions provided by the manufacturer of the O3 analyzer to 

become familiar with its operation. Review the documentation requirements specified Section 3 and complete item 

2.1. 

QUALIFICATION 

The next step is to qualify the standard by demonstrating that it is repeatable to within the specifications 

given in Section 3 (± 4% or ± 4 ppb, whichever is greater). The variables likely to affect an O3 analyzer are 

normally the same as those discussed generally in Appendix B and more specifically below. Refer to Appendix B 

for additional guidance on each of the qualification tests. 

Generally, a preliminary calibration relationship (as shown in Figure 4.2) is not necessary for O3 analyzers, 

as most analyzers are linear and provide a direct output indication of concentration. The zero and span of the 

analyzer should be adjusted for approximate calibration (with respect to the UV standard) over the desired 

concentration range. The final “calibration” of the analyzer is the verification relationship. 

An output-indicating device such as a chart recorder or digital meter is a helpful accessory. But, if such an 

output indicator is used, it should be permanently associated with the transfer standard analyzer and employed 

during qualification, verification, and use of the analyzer as a transfer standard. 

QUALIFICATION TESTS 

Temperature

If possible, select an O3 analyzer that has good temperature regulation (or compensation), preferably one 

with a temperature indicator or other warning device to indicate whether the temperature regulator is working 

properly. Keep in mind that temperature regulation systems require a warm-up period before the temperature 

stabilizes. 

Select a temperature range over which the O3 analyzer is to be qualified. Temperatures of 20 to 30°C (68 to 

86°F) might be appropriate, or possibly 15 to 30°C (59 to 86°F). Test the O3 analyzer over this temperature range at 

several O3 concentrations as suggested by Figure B.1 to depict how the indicated output of the analyzer varies as the 

temperature changes. Be sure (1) that the proper temperature and pressure corrections are made to the UV standard, 

(2) that the O3 analyzer is allowed to equilibrate each time the temperature is changed, and (3) that the O3 analyzer’s 

span is not adjusted between each different temperature (adjustment of other parameters to nominal values is 

permitted). If the manufacturer has tested the O3 analyzer (or one like it), only enough tests are needed to show that 

the analyzer is operating properly and meets the specifications. 
Ozone Transfer Standard Guidance Document 10/2013 

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If the O3 analyzer’s indicated concentration does not vary more than ± 4% or ± 4 ppb over the entire 

temperature range, then it is qualified over that temperature range. If it does not meet those specifications, the 

following options are available: 

a) determine that the analyzer has a malfunction or inadequacy, attempt to correct it, and then retest it; 

b) reduce the temperature range to a range over which the analyzer does meet the specifications (this may 

inconveniently restrict the subsequent use of the analyzer); or 

c) attempt to determine, either analytically or empirically, the temperature-output relationship such as 

illustrated in Figure B.2. When this relationship is used to calculate a “corrected” indicated output reading, 

the analyzer should meet the specifications. If so, this correction formula becomes a necessary and integral 

part of the transfer standard and must be included with the ultimate verification relationship (see Figure 

4.3). 

Line Voltage

Line voltage tests are conducted in a fashion analogous to the temperature tests, substituting line voltage 

variation for temperature variation. A range of 105 to 125 volts should be appropriate. A well-designed O3 analyzer 

should show little or no sensitivity to line voltage changes over this range. If the analyzer does not meet the 

specifications, the same three options as with temperature are available, plus a fourth option of adding an external 

voltage regulator. 

Barometric Pressure/Altitude

An O3 analyzer is likely to be sensitive to pressure changes unless it is specifically designed to compensate 

for pressure effects. If the use of the transfer standard can be restricted to altitudes within a range of about 100 

meters (300 feet), the range of pressure variations is only about 2 or 3% (about 1% for altitude variations and about 

1 to 2% for normal barometric pressure changes). Under these conditions an uncompensated O3 analyzer which 

operates at ambient pressure could be expected to meet the qualification specifications. However, the analyzer 

should be tested over a normal range of barometric pressure to be sure. Test the analyzer on various days when the 

barometric pressure is different and plot the results as shown in Figure B.1. 

Where a larger altitude range is needed (or for applications requiring more accuracy) the O3 analyzer’s 

sensitivity to pressure changes may have to be compensated, either by design or by defining the pressure-output 

relationship and developing a correction formula (see Figure B.3). 
Ozone Transfer Standard Guidance Document 10/2013 

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Testing for pressure sensitivity is most practically carried out by moving both the O3 analyzer and the UV 

standard system to various altitudes. In doing this, be sure (1) that the proper temperature and pressure corrections 

are made to the UV standard, (2) that the O3 analyzer is not adjusted between each different pressure, and (3) that 

other variables (temperature, line voltage, etc.) are controlled so that they do not affect the analyzer. From the test 

data, determine an appropriate correction formula and include it with the verification relationship. 

Elapsed Time

Ozone analyzers operated exclusively as transfer standards are not likely to change much with elapsed 

time. The specified reverification frequency should be sufficient to compensate for any long-term response changes 

in the analyzer. 

Variability

General variability (variability other than that associated with a specific known variable) is not likely to be 

a problem with an O3 analyzer unless it is very poorly designed. If any of the other qualification tests show 

variability not strongly correlated with a specific variable, then a test for general variability would be needed. 

Otherwise, if the analyzer meets the specifications for the other tests, no specific test for general variability is 

required. 

Relocation

Relocation tests help to establish the ruggedness of the O3 analyzer. During the course of the barometric 

pressure or other tests, move the transfer standard about as it might be moved during subsequent use to see if any 

malfunctions, variability, or other problems are observed that might make use of the analyzer questionable or 

inconvenient. Make sure that the zero, span, and other adjustments can be locked so they don’t change when the 

analyzer is moved. 

Operator Adjustments

Test the analyzer for repeatability with respect to any operator settings such as flow or gas pressure. Note 

if a stabilization period is required after an adjustment. Note any other observable peculiarities. A well-designed O3

analyzer is not likely to fail this test, but the information obtained will help to achieve better precision. 
Ozone Transfer Standard Guidance Document 10/2013 

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Malfunctions

There is no special test for malfunctions. During other tests, be observant for any malfunctions that occur 

or other characteristic weaknesses in the analyzer that could cause unreliability. Understand the design and 

operation of the analyzer so that you can be alert to non-obvious types of malfunctions such as failure of 

temperature, line voltage, flow or other regulation mechanisms. 

Other Conditions

The tests described above should cover the performance variables for most common types of O3 analyzers. 

However, new types of analyzers or unusual analyzer designs may require additional special tests. 

VERIFICATION 

Before conducting the verification tests, make any zero, span, or other adjustments to the analyzer as 

necessary so that the analyzer readings are close to the O3 concentrations obtained from the UV reference standard. 

When the adjustments are complete, lock the adjustments, record their values, and proceed to verify the 

transfer standard as specified in Section 4. If any correction formulas are needed, use them during the verification 

procedure and clearly specify the conditions at the time of the verification relationship. Be sure that the specified 

correction formulas are accurate. Note any special operating instructions, operating restrictions, limits, or other 

pertinent information. It is very important to complete the documentation requirements specified in Section 4. 

USE 

In using the O3 analyzer as a transfer standard, review the guidance under “Use” in Section 4. Make any 

zero, flow, or other adjustments except span adjustment to the values recorded at the time of verification. Reverify 

the transfer standard as required by Section 4. Consider occasional cross checks of the transfer standard to other O3

standards, and occasionally repeat the qualification tests to be sure the transfer standard is maintaining adequate 

reliability. 
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APPENDIX E 

GLOSSARY 

Calibration 

the comparison of a measurement standard, instrument, or item with a standard or instrument of higher accuracy to 

detect and quantify inaccuracies and to report or eliminate those inaccuracies by adjustment [1] 

set of operations that establish, under specified conditions, the relationship between values of quantities indicated by 

a measuring instrument or measuring system and the corresponding values realized by standards [2] 

Certification 

refers to the qualification processes used to determine if a product has passed performance and quality assurance 

tests stipulated in regulations and nationally accredited test standards, or that it complies with a set of minimum 

performance criteria designed to ensure acceptability for the products intended use. (see conformity assessment) 

the procedure by which written assurance is given that a product or service conforms to a standard or specification 

[3] 

Notes 

Certification verifies that a particular product meets a given level of quality or safety, providing the user with 

explicit or implicit information about the characteristics and/or performance of the product. [3]

Comparability 

property of measurement results enabling them to be compared because they are metrologically traceable to the 

same stated metrological reference 

Conformity assessment 

is defined in ISO/IEC(1) Guide 2: 1996 as: "any activity concerned with determining directly or indirectly that 

relevant requirements are fulfilled." Conformity assessment procedures provide a means of ensuring that the 

products, services, or systems produced or operated have the required characteristics, and that these characteristics 

are consistent from product to product, service to service, or system to system. Conformity assessment includes: 

sampling and testing; inspection; certification (of both products and personnel); and quality and environmental 

system assessment and registration 

Standard Reference Photometer (SRP) 

Stationary photometers maintained and operated by EPA. They are referenced directly to the NIST primary 

photometer. Due to the longer UV paths, the SRPs are more accurate and stable than other photometric devices. 

Measurement uncertainty 

describes a region about an observed value of a physical quantity which is likely to enclose the true value of that 

quantity 

Qualification 

process demonstrating that a transfer standard is sufficiently stable (repeatable ) to be used as a transfer standard. 

For ozone transfer standards acceptance is + 4% or + 4 ppb whichever is greater, for each condition or variable that 

may change between point of certification and point of use 
Ozone Transfer Standard Guidance Document 10/2013 

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Repeatability [2] 

property of a measuring system to provide closely similar indications for replicated measurements of the same 

quantity under repeatability conditions 

Standard [2] 

embodiment of the definition of a given quantity, with stated value and measurement uncertainty, used as a 

reference 

national standard [2] 

measurement standard designated as a national stated metrological reference 

primary standard [2] 

measurement standard whose quantity value and measurement uncertainty are established without relation to 

another measurement standard for a quantity of the same kind 

NOTES 

1. This definition implies that the establishment of a primary measurement standard has to refer to the 

definition of any unit concerned, particularly an SI unit 

2. The first measurement standard of a calibration hierarchy is always a primary measurement standard. 

reference standard [2] 

standard, generally having the highest metrological quality available at a given location or in a given 

organization, from which measurements made there are derived 

secondary standard [2] 

measurement standard whose quantity value and measurement uncertainty are assigned through calibration 

against, or comparison with, a primary measurement standard for a quantity of the same kind 

NOTES 

1. The relation may be obtained directly between the primary measurement standard and the secondary 

measurement standard, or involve an intermediate measuring system 

2. calibrated by the primary standard and assigning a measurement result to the secondary standard 

3. A measurement standard having its quantity value assigned by a ratio primary measurement procedure 

is a secondary measurement standard. 

transfer standard 

transportable device or apparatus which, together with associated operational procedures, is capable of 

accurately reproducing pollutant concentration standards or of producing accurate assays of pollutant 

concentrations which are quantitatively related to an authoritative standard

transfer device [2] 

measurement device used as an intermediary to compare measurement standards
Ozone Transfer Standard Guidance Document 10/2013 

E-3 

Traceability 

the “property of a measurement result whereby the result can be related to a stated reference through a documented 

unbroken chain of calibrations, each contributing to the measurement uncertainty (ISO). A claim of traceability 

requires three elements: 

 

1. a declaration of the source of traceability (e.g., NIST ), 

2. a full description of the traceability chain from the source to the measurement of interest, and 

3. an uncertainty claim with supporting data. The responsibility for providing support for an uncertainty claim 

rests with the entity making the claim (i.e., the provider), but the responsibility for assessing the validity of 

such a claim rests with the consumer. 

NOTES 

1. the unbroken chain of comparisons is called a traceability chain [2]. 

2. Institutes should maintain as direct a path as possible between their laboratory standards and NIST 

Traceability chain [2] 

chain of alternating measuring systems with associated measurement procedures and measurement standards, from a 

measurement result to a stated metrological reference 

NOTE 

A metrological traceability chain is defined through a calibration hierarchy from the measurement result to the stated 

metrological reference 

Verification [1] [2] 

confirmation by examination and provision of objective evidence that specified requirements have been fulfilled. 

NOTES 

1. comparison made without calibration 

2. Verification should not be confused with calibration of a measuring system, or vice versa. 

Verification of the accuracy of a transfer standard is established by (1) relating the output to a O3 standard of higher 

authority (level) and (2) demonstrating that the repeatability of the transfer standard is within the limits 

Validation [1] [2] 

Confirmation through examination of a given item and provision of objective evidence that it fulfills the 

requirements for a stated intended use 

References 

[1]American National Standard Quality Systems for Environmental Data and Technology Programs ANSI /ASQ E4 

http://www.asq.org/ 

[2] ISO Publications, International vocabulary of basic and general terms in metrology, Revision of 1993 edition 

The abbreviation of this title is VIM http://www.iso.org/iso/pressrelease.htm?refid=Ref1106 

 [3] National Institute of Standards and technology, The ABC's Of Certification Activities In The United States 

NBSIR 88-3821 July 1988 http://ts.nist.gov/standards/information/cerprime.cfm 
United States 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Air Quality Assessment Division 

Research Triangle Park, NC 

Publication N