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DRAFT 

EPA Traceability Protocol 

 for 

Qualification and Certification of Evaporative HCl Gas Standards 

and 

 Humidification of HCl Gas Standards from Cylinders

Prepared by: 

Office of Research and Development 

National Risk Management Research Laboratory 
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1.0 INTRODUCTION 

For regulatory purposes, hydrochloric acid (HCl) calibration gas standards of known 

concentration and known uncertainty are needed to quality-assure data recorded by HCl 

continuous emission monitoring systems (CEMS) and measurements made with instrumental 

HCl reference test methods. This EPA traceability protocol reflects the current state of 

development of procedures for quantifying hydrochloric acid (HCl) concentrations from 

gas/vapor blending devices designed to produce HCl calibration standards. 

This traceability protocol provides procedures for: (1) establishing the NIST traceability and 

quantitative performance (i.e., “calibration”) of key components of HCl gas/vapor blending 

devices; (2) determining the uncertainty contributed by each of these components; (3) calculating 

certified output concentrations and expanded, combined uncertainty values for the gas standards 

produced by the devices, and (4) verifying the output concentrations of the evaporative through 

comparison to a NIST traceable gaseous HCl reference standard. This protocol assumes a 

minimum “target” expanded, combined uncertainty value of 5% for HCl gas/vapor blending 

devices (see Section 4.7) unless otherwise specified in the applicable regulations. 

Demonstrating that a particular HCl gas/vapor blending device meets the requirements of this 

protocol establishes that the device output is traceable to NIST standards by means currently 

acceptable to EPA. 

Section 2, “Overview,” discusses the different approaches to delivering HCl gas standards, 

explains the certification process for gas/vapor blending devices, and describes the two types of 

blending devices that are used for HCl gas standards. Section 3, “Qualification,” contains 

provisions designed to demonstrate the adequacy of a particular model prior to performing the 

actual certification tests. Section 4, “Certification,” explains specific procedures for 

quantitatively determining certified output concentrations from each type of device and for 

calculating and reporting the expanded, combined uncertainties of the HCl output concentrations. 

Section 5, “Verification,” shows how the certified device output is verified based on 

measurements of the actual output concentration. Section 6, “Quality Assurance,” details 

procedures for periodic data quality assessments, which provide assurance of continued 

acceptable HCl gas/vapor blending device performance. 

The technical specifications and requirements for HCl calibration gases will have to be evaluated 

for each regulatory application as new HCl emission standards and their associated monitoring 

and testing requirements are developed. These issues may be addressed during formal 

rulemaking processes or by the regulatory agency responsible for implementing the standards, 

and are not addressed in this or other EPA traceability protocols. Therefore, for each regulatory 

application, the maximum acceptable uncertainty (if needed) and other implementation 

requirements and/or alternatives for devices should be specified by the responsible agency or 

organization. 
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2.0 OVERVIEW

There are essentially three ways to deliver HCl gas standards currently in use: (1) direct delivery 

of bottled dry gas of NIST-traceable HCl concentration, (2) a device that quantitatively blends a 

bottled dry gas with water vapor or humidified zero gas, and (3) an evaporative HClstandard, 

which produces gas standards of known concentration by vaporization of aqueous solutions of 

HCl and quantitatively blending the resultant vapor with a diluent carrier gas. The first option, 

delivery of a dry HCl gas standard, is outside the scope of this document. The second option was 

developed as a way to cope with the technical challenges of delivering dry HCl gases without 

losses and response lags. The third option delivers a humidified gas standard without the need of 

a NIST-traceable bottled gas. Any of these options is suitable for both CEMS and instrumental 

testing applications. 

The latter two options listed above are essentially variations of a gas/vapor blending device, and 

are described in more detail below. The specific design, configuration, and features of any HCl 

gas/vapor blending device are left to the judgment of the manufacturer and are not constrained 

by explicit design specifications or EPA requirements. In order for the output concentration of a 

device to be NIST-traceable, however, all of the inputs must be calibrated using NIST-traceable 

standards, and the uncertainties of these inputs must be combined to calculate the uncertainty of 

the resulting gas standard. 

Qualification tests are required for any blending device that is designed to produce NISTtraceable gas standards. For the purposes of this protocol, the tests that are routinely performed 

by the manufacturer will suffice, provided the test results demonstrate that the specific model has 

adequate stability and repeatability and can operate reliably over a stated range of conditions 

(e.g., line voltages, back pressures, ambient temperatures, delivery back pressures, and other 

applicable factors). These tests may either be performed on each individual device or on selected 

devices that are representative of the model. The manufacturer must provide the end user with a 

statement of disclosure that explains proper set-up and use of the device and documents the 

range of conditions over which reliable operation can be expected. 

This EPA Traceability Protocol specifies the procedures for establishing the NIST traceable 

quantitative output of HCl gas/vapor blending devices. These devices may be certified either: (a) 

at the manufacturer’s facility, prior to shipping, or (b) at the site where the device will be used 

(or at another convenient location). The manufacturer or equipment supplier performing the 

certifications must maintain documentation of all certifications. The test date, certified Hg 

concentrations, and uncertainties must be provided in writing to the user. All certification test 

records must be available for inspection by regulatory officials. 

The final step in the process of establishing the acceptability of a blending device as a calibration 

standard is to verify that the measured output concentration agrees with the calculated and 

certified concentration. This verification step is essentially a confirmation that everything 

feeding into the device is being delivered to the outlet undeterred and unaltered; it is a pass/fail 

test which does not result in any change to the certified output concentrations. 

 
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Following the initial certification of a blending device, it can be used in accordance with the 

manufacturer’s written instructions and operational specifications for regulatory applications in 

emission monitoring or testing. Periodic quality assurance (QA) checks and/or quality control 

(QC) procedures are required to verify that the device continues to provide accurate and reliable 

calibration standards. Recertification of a device is required periodically to maintain NIST 

traceability. Recertification may also become necessary when certain malfunctions, operating 

conditions, or other factors identified by the user or manufacturer occur. 

2.1 Moisture Blending with Bottled HCl Gas Standards 

The presence of moisture assists in transporting HCl gas standards through calibration lines and 

sample lines (i.e., umbilical) and through other system components upstream of the analyzer. As 

such, for using bottled HCl gas standards it may be advantageous, if not necessary, to introduce 

some moisture into the gas stream. There are several technologies for introducing moisture into a 

gas stream, and describing them is beyond the scope of this protocol. 

Since introducing any additional flow into a gas stream dilutes it, the output HCl concentration 

of any gas/vapor blending device must be calculated based on the measured inputs: the flow rates 

of any liquid and gas streams that are added, and flow rate and concentration of the original HCl 

gas standard. For purposes of certifying the output concentration of the blending device, the 

input measurements must be NIST-traceable. The HCl gas standard should come with its 

certificate of analysis, providing the certified concentration and uncertainty. Liquid feed rates are 

established by using a balance to gravimetrically calibrate the feed rate meter relative to NISTtraceable mass standards. The HCl gas standard flow rate, along with any carrier gas flow rate 

associated with the vapor delivery, is established by calibrating the gas flow rate meter(s) 

relative to a reference standard that has a NIST-traceable calibration (see Sections 4.2 and 4.7.1).

2.2 Evaporative HCl Standard

In an evaporative HCl standard, a liquid stream containing a known concentration of HCl is 

metered and then dynamically mixed with a known flow rate of a carrier gas. The liquid feed is 

nebulized into small droplets, evaporated in a heated chamber, and then diluted with the carrier 

gas. Some devices may include a second liquid feed stream to facilitate control of the moisture 

concentration of the generated calibration gas. This principal of operation assumes that all of the 

HCl in the feed solution is emitted from the evaporator without re-condensing or reacting 

chemically with any of the device components or other feed streams. The output HCl 

concentration is calculated from the measured inputs: the HCl solution concentration, the liquid 

feed rate(s), and the carrier gas flow rate (see Section 4.2). 

Certification of evaporative HCl standards requires NIST-traceability for the inputs listed above. 

The HCl concentration of the working liquid feed solution is established through the use of 

commercially available NIST-traceable HCl liquid standards and, if necessary, the quantitative 

dilution of these standards. Like the simpler blending devices described in Section 2.1 above, 

liquid feed rates are established by using a balance to gravimetrically calibrate the feed rate 

meter relative to NIST-traceable mass standards, and the carrier gas flow rate is established by 
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calibrating the gas flow rate meter relative to a primary laboratory standard that has a NISTtraceable calibration. 

2.3 Uncertainty

In addition to the mass balance calculations that establish the certified concentration(s) from HCl 

gas/vapor blending devices (as a function of input parameters), the certification must also include 

the expanded, combined uncertainty of the certified HCl concentration(s). Combined uncertainty 

is a function of (1) the uncertainties of the input parameters, as included in the mass balance, and 

(2) the mathematical sensitivity of the output concentration to changes in each input parameter. 

A detailed discussion of propagation of uncertainty is outside the scope of this protocol. 

Equations are provided in Section 4 for all of the expected combinations of gas flows, liquid 

flows, and gas or liquid concentrations of HCl. Expanded uncertainty is simply the combined 

uncertainty multiplied by a coverage factor (k=2) to represent the required confidence level. 

3.0 QUALIFICATION 

Manufacturers are required to evaluate and specify the range of operating conditions over which 

the device can operate reliably and deliver the certified concentration(s). Information about the 

limitations of the device must be provided to the end user by means of a “Manufacturers 

Disclosure” (see Section 3.2) defining the limits of the environmental conditions and operating 

parameters which must not be exceeded. Manufactures shall conduct qualification tests to 

demonstrate the adequacy of each gas/vapor blending device (as defined in Section 3.1) prior to 

undertaking actual certification tests. Documentation of the qualification test procedures and 

results must be available upon request for inspection by EPA and other regulatory officials. 

3.1 Definitions 

3.1.1 Blending Device Model – A specific configuration identified by the instrument system 

design, including: 

(a) The use of specific gas flow rate measurement/control devices, evaporation chamber, 

temperature controllers, temperature or pressure measurement and compensation devices, 

and other components; 

(b) The physical arrangement of principal components; 

(c) The specific electronics configuration and signal processing; and 

(d) The specific software/firmware version and data processing algorithms, as implemented 

by a particular manufacturer and subject to an identifiable quality assurance system. 
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Note: The following changes to a gas/vapor blending device do not constitute a model change. 

However, the manufacturer must document all such changes: 

• Minor changes to software, firmware, or data outputs that do not affect data processing 

algorithms or status outputs; 

• Software installed on external devices, (including external computer systems), and used for 

processing of the device output to generate reference values, operational information, or to 

activate alarms; and 

• Substitution or use of new measurement components, provided that the device manufacturer 

verifies that the performance of the new components is as good, or better, than the replaced 

components and documents supporting information for all such changes. 

3.1.2 Intrinsic Adjustment – An automatic and essential feature of a gas/vapor blending 

device that provides for the internal control of specific components or adjustment of the 

device output in a manner consistent with the manufacturer's design and its intended 

operation. Intrinsic adjustments are either non-elective or are configured according to 

factory recommended procedures; they are not subject to change by the user. 

3.1.3 External Adjustment – Either (1) a physical adjustment to a component of the gas/vapor 

blending device that affects its output or performance, or (2) an adjustment applied by the 

data acquisition system (for example, mathematical adjustment to compensate for drift) 

that is external to the device. External adjustments are made at the election of the end 

user but may be subject to various regulatory requirements. 

3.2 Manufacturer’s Disclosure 

As part of each gas/vapor blending device’s certification, a “Manufacturer’s Disclosure” is 

required. This disclosure must include documentation and other relevant information for the 

blending device model, in accordance with subsections 3.2.1 through 3.2.5, below. This 

information shall be provided to each end-user in the operation manual and/or other documents. 

3.2.1 Design Description 

The gas/vapor blender manufacturer shall provide a description of the blender design and 

operation in sufficient detail to afford users with a factual basis to assess the parameters that can 

reasonably be expected to affect performance. The description must identify the critical 

components, features for the control or compensation for changes in external influences, alarms 

or fault indicators, and necessary external devices or connections. 

3.2.2 Range(s) 

The manufacturer shall specify the nominal concentration range(s) (minimum and maximum 

HCl concentrations in units of ppm) over which the blending device model has been 
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demonstrated to meet all of the manufacturer’s specifications. The description of the range 

should explain the parameter values that must be changed or selected by the user to establish the 

effective operating range and/or HCl concentration values for a specific device in a particular 

measurement application. 

3.2.3 Operational and Environmental Conditions 

The manufacturer shall explicitly state the operational range or limitations within which the 

device model can be expected to operate reliably and meet all of the manufacturer’s 

specifications for: 

3.2.3.1 Line Voltage 

The acceptable line voltage limits over which the device can provide reliable HCl gas 

concentrations. 

3.2.3.2 Gas Supply 

The minimum and maximum supply pressure of air, nitrogen, or dry HCl gas standard, the 

allowable variation in supply pressure, the required gas flow rate, and the quality of carrier or 

HCl gas required. 

3.2.3.3 Enclosure Temperature Operating Range 

The range of ambient temperatures over which the thermal stability of the device has been tested, 

and can deliver reliable HCl gas concentrations. 

3.2.3.4 Back Pressure 

The range of back pressures over which the device has been tested, and can deliver reliable HCl 

gas concentrations. 

3.2.4 Operation, Maintenance, and QA 

The manufacturer shall provide written procedures for installation, start-up, operation and 

maintenance, and quality assurance (QA) of the HCl gas/vapor blending device. The 

manufacturer shall identify those activities and/or QA check/maintenance intervals, or other 

factors, that may need to be adjusted based on site-specific conditions or other application 

factors. The manufacturer shall also identify the specific conditions and factors that require 

recertification of the device, including, as applicable, known malfunctions or failures, exposure 
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to unacceptable operating conditions, component replacements, component recalibrations, or 

other conditions and/or factors. 

3.3 Qualification Tests 

Manufacturers shall perform and pass a series of standardized qualification tests for each HCl 

gas/vapor blending device model. These tests will assess the repeatability of the delivery 

concentration, and evaluate the effect of changes in line voltage, supply pressure, ambient 

temperature, and delivery back pressure. The tests will require an HCl detector or measurement 

system capable of accepting, either under positive pressure or by pulling a sample at its own rate, 

the output gas from the HCl gas/vapor blending device. The HCl detector need not be precisely 

calibrated with NIST-traceable gases, but must be capable of drift-free operation and generate a 

continuous, linear signal proportional to the HCl concentration. 

3.3.1 Repeatability Test 

Prior to all other qualification tests, the repeatability of the gas/vapor blending device must be 

assessed over the range of possible delivery concentrations and operating conditions. Table 1 

shows an example list of test conditions, representing an evaporative HCl standard feeding a 

0.00555 M solution of HCl. 

Table 1: Repeatability Test Conditions 

Concentration, ppm Liquid Flow, mg/min Gas Flow, ml/min 

1.2 (minimum) 100 (minimum) 10000 (maximum)

18 (maximum) 350 (maximum) 2000 (minimum)

4.2 (mid-range) 350 (maximum) 10000 (maximum)

4.2 (mid-range) 210 (mid-range) 6000 (mid-range)

To conduct the repeatability test, both the gas/vapor blending device and the HCl measurement 

system must be operational and stabilized in a controlled environment (e.g., no recent 

temperature changes or power interruptions). 

(a) Use the blending device’s user interface to set one of the test conditions and activate. 

Verify that the device is calculating and displaying the expected concentration. 

(b) Continue feeding until the HCl measurement response stabilizes and an average response 

is recorded. Proceed to the next test condition, and repeat until a response is recorded for 

each condition. 

(c) After the first set of responses is recorded, repeat from the beginning to record a second 

set of responses. For each matched pair of responses ( 1

x ,

2

x ), calculate a relative percent 

difference. 
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( ) 100%2 1 2

1 2

×

+

−

=

x x

x x

RPD 

(d) If the relative percent difference is greater than 2% for any of the test conditions, take 

corrective action and conduct a new repeatability test for the full set of conditions. 

3.3.2 Line Voltage Test 

Since HCl measurement equipment often operates in an industrial environment which can be 

prone to electricity voltage sags and surges, the operation of gas/vapor blending devices must be 

verified over a range of supply voltages. These tests may be conducted under controlled 

laboratory conditions using a simple variable transformer (e.g., Variac®) to adjust the line 

voltage up and down from its nominal value. Like the repeatability test, the output concentration 

of the gas/vapor blending device will be measured with a suitable detector during the test. The 

high and low voltage test conditions may be set based on the manufacturer’s disclosure, but must 

include the range shown in Table 2. If the device is designed to be switchable between the two 

nominal voltages shown, this test must be conducted at each setting. 

Table 2: Line Voltage Test Conditions 

Nominal Line Voltage Highest Line Voltage Lowest Line Voltage 

120 VAC 126 VAC 110 VAC

240 VAC 252 VAC 220 VAC

The line voltage test may be conducted at the same settings that were used for the repeatability 

test, or a subset thereof, but must include conditions at the high and low limits of each flow 

measurement (gas and liquid) that is incorporated into the device. 

(a) Plug the device into the variable transformer set to 100% (no change from line voltage), 

turn on and allow to stabilize. 

(b) Use the blending device’s user interface to set one of the test conditions and activate. 

Verify that the device is calculating and displaying the expected concentration. 

(c) Continue feeding until the HCl measurement response stabilizes and an average response 

is recorded for this nominal voltage. 

(d) Slowly adjust the variable transformer to the lowest voltage corresponding to the nominal 

voltage in Table 2. Allow at least 5 minutes for any shifted response to stabilize, and 

record an average response for this voltage. 

(e) Slowly adjust the variable transformer to the highest voltage corresponding to the 

nominal voltage in Table 2. Allow at least 5 minutes for any shifted response to stabilize, 

and record an average response for this voltage. 
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(f) Using the highest and lowest response averages ( highest x and lowest x , from among the three 

recorded responses), calculate a relative percent difference. 

( ) 100%

2

×

+

−

=

highest lowest

highest lowest

x x

x x

RPD 

(g) Proceed to the next operating condition on the gas/vapor blending device, allow the 

response to stabilize, and cycle through the voltage settings in any order, making sure to 

allow at least 5 minutes for responses to shift before recording the response. Calculate a 

RPD for this condition. 

(h) Once all operating conditions have been tested, review the RPD values. If the value is 

greater than 3% for any of the operating conditions, take corrective action and conduct a 

new line voltage test. 

3.3.3 Supply Pressure Test 

The supply pressure test may be conducted under controlled laboratory conditions using a 

pressure regulator, assuming the supply pressure to that regulator is at least as high as the 

maximum pressure provided on the manufacturer’s disclosure. Like the other tests, the output 

concentration of the gas/vapor blending device will be measured with a suitable detector during 

this test. The high and low pressure test conditions must be set based on the manufacturer’s 

disclosure, if a range is given. If the manufacturer documentation provides specific warnings 

against overpressure, the high pressure may be set as much as 10% below the maximum. If the 

manufacturer only provides a nominal value for input pressure with no range, use pressures of 

10% above and below the nominal value for the high and low pressure settings. Table 3 shows 

some examples of manufacturer pressure specifications, and acceptable ranges for this pressure 

supply test. 

Table 3: Example Ranges for Pressure Supply Test 

Manufacturer Spec. High Supply Pressure Low Supply Pressure 

2-6 bar 6 bar 2 bar

50-150 psig 135 psig 50 psig

30 psig 33 psig 27 psig 

The pressure supply test may be conducted at the same settings that were used for the 

repeatability test, or a subset thereof, but must include conditions at the high and low limits of 

each flow measurement (gas and liquid) that is incorporated into the device. 

(a) Connect the device to an air supply or bottled gas that can deliver at least the high supply 

pressure that will be used for this test. Ensure that the supply includes a regulator that can 

bring that pressure down to the low supply pressure, and that there is a gauge to read the 

pressure supplied to the gas/vapor blending device. Set the pressure regulator for the low 

supply pressure. 
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(b) Use the blending device’s user interface to set one of the test conditions and activate. 

Verify that the device is calculating and displaying the expected concentration. 

(c) Continue feeding until the HCl measurement response stabilizes and an average response 

is recorded for this supply pressure. 

(d) Slowly adjust the pressure regulator to the high supply pressure. Allow at least 5 minutes 

for any shifted response to stabilize, and record an average response for this supply 

pressure. 

(e) Using the response averages for the high and low pressure settings ( highP x ,

lowP x ), 

calculate a relative percent difference. 

( ) 100%

2

×

+

−

=

highP lowP

highP lowP

x x

x x

RPD 

(f) Proceed to the next operating condition on the gas/vapor blending device, allow the 

response to stabilize, and record responses at the high and low supply pressure settings, 

making sure to allow at least 5 minutes for responses to shift, if they do. 

(g) Once all operating conditions have been tested, review the RPD values. If the value is 

greater than 3% for any of the operating conditions, take corrective action and conduct a 

new supply pressure test. 

3.3.4 Thermal Stability Test 

The thermal stability test requires an environmental chamber or enclosure large enough to 

contain all components of the gas/vapor blending device, with penetrations for electrical and 

tubing connections. This chamber must be able to maintain temperatures at the high and low 

extremes listed on the manufacturer’s disclosure. Similar to other tests, the output concentration 

of the gas/vapor blending device will be measured with a suitable detector or measurement 

system located in a climate controlled laboratory space or control room outside of the 

environmental chamber. For this test, because it takes longer to perform, the detector may need 

to be calibrated or adjusted for drift before each measurement. 

The high and low temperature test conditions must be set based on the manufacturer’s disclosure, 

if a range is given. If no range is specified, set the high temperature to 40°C, and the low 

temperature to 5°C. The thermal stability test may be conducted at the same settings that were 

used for the repeatability test, or a subset thereof, but must include conditions at the high and low 

limits of each flow measurement (gas and liquid) that is incorporated into the device. Ideally, 

these test conditions will be pre-programmed into the device, or activated remotely, in order to 

minimize intrusions into the environmental chamber during the test. 

(a) Place the device into the environmental chamber, routing the HCl gas output and all 

power and signal wires outside the chamber as necessary. Set the chamber to a 
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representative ambient temperature (e.g., 20-25°C). If the chamber has humidity controls, 

set them to a dewpoint temperature below the low temperature condition. 

(b) Use the blending device’s user interface to set one of the test conditions and activate. 

Verify that the device is calculating and displaying the expected concentration. 

(c) Continue feeding until the HCl measurement response stabilizes and an average response 

is recorded on the HCl detector. Without disturbing the environmental chamber settings, 

switch the blending device to the next test condition, allow the reading to stabilize for at 

least 5 minutes, and repeat until a response is recorded for each test condition. 

(d) Change the environmental chamber controls to the low temperature setting (i.e., 5°C or 

the low end of the manufacture-specified range, whichever is lower). Allow at least 4 

hours for the device to equilibrate at this temperature. 

(e) Perform any necessary calibrations or drift adjustments to the HCl detector to assure that 

its responses will be comparable to the earlier measurements. Set the gas/vapor blending 

device to each of the test conditions, as before, and record the detector responses to each 

test condition. 

(f) Change the environmental chamber controls to the high temperature setting (i.e., 40°C or 

the high end of the manufacture-specified range, whichever is higher). Allow at least 4 

hours for the device to equilibrate at this temperature. 

(g) Perform any necessary calibrations or drift adjustments to the HCl detector to assure that 

its responses will be comparable to the earlier measurements. Set the gas/vapor blending 

device to each of the test conditions, as before, and record the detector responses to each 

test condition. 

(h) Process the detector response data. For each gas/vapor blending device operating 

condition, identify the lowest and highest response average ( highest x and lowest x , from among 

the three recorded response averages), and calculate a relative percent difference. 

( ) 100%

2

×

+

−

=

highest lowest

highest lowest

x x

x x

RPD 

(i) If the relative percent difference is greater than 3% for any of the test conditions, take 

corrective action and conduct a new temperature stability test. 

3.3.5 Delivery Back Pressure Test 

The delivery back pressure test may require some modification of the tubing connection between 

the gas/vapor blending device and the HCl measurement system. By inserting one or more valves 

or back pressure regulators into the detector and/or vent lines, create a system where the back 

pressure at the HCl gas/vapor blending device outlet may be varied and maintained 
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independently of the device operating conditions (e.g., outlet flow rate). As always, any portion 

of the connections leading to the detector must be designed to not alter the delivered 

concentration of HCl. 

Unless the manufacturer’s disclosure specifies otherwise, the low back pressure test condition 

will be ambient pressure, or whatever the minimum back pressure is that will deliver adequate 

sample to the detector. The high back pressure test condition should be specified in the 

manufacturer’s disclosure; if not, the high back pressure test condition will be 28 kPa. If the 

manufacturer documentation provides specific warnings against too much back pressure, the 

high back pressure test condition may be set as much as 10% below the manufacturer-specified 

maximum. The delivery back pressure test may be conducted at the same HCl gas/vapor 

blending device settings that were used for the repeatability test, or a subset thereof, but must 

include conditions at the high and low limits of each flow measurement (gas and liquid) that is 

incorporated into the device. 

(a) Connect the device, through the back pressure arrangement, to the HCl measurement 

system. Adjust the back pressure arrangement to the low back pressure setting. 

(b) Use the blending device’s user interface to set one of the test conditions and activate. 

Verify that the device is calculating and displaying the expected concentration. 

(c) Continue feeding until the HCl measurement response stabilizes and an average response 

is recorded for this delivery back pressure. 

(d) Slowly adjust the back pressure arrangement to the high setting. Allow at least 5 minutes 

for any shifted response to stabilize, and record an average response for this delivery 

back pressure. 

(e) Using the response averages for the high and low back pressure settings ( highBP x ,

lowBP x ), 

calculate a relative percent difference. 

( ) 100%

2

×

+

−

=

highBP lowBP

highBP lowBP

x x

x x

RPD 

(f) Proceed to the next operating condition on the gas/vapor blending device, allow the 

response to stabilize, and record responses at the high and low delivery back pressure 

settings, making sure to allow at least 5 minutes for responses to shift, if they do. 

(g) Once all operating conditions have been tested, review the RPD values. If the value is 

greater than 3% for any of the operating conditions, take corrective action and conduct a 

new delivery back pressure test. 

3.3.6 Other Tests 

The manufacturer’s disclosure may include operating ranges for other critical component 
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parameters (e.g., evaporator temperature for an evaporative calibration gas standard). 

Qualification tests for these parameters shall be structurally similar to the other tests in this 

section. The tests may be conducted at the same HCl gas/vapor blending device settings that 

were used for the repeatability test, or a subset thereof, but must include conditions at the high 

and low limits of each flow measurement (gas and liquid) that is incorporated into the device. 

4.0 CERTIFICATION 

4.1 Introduction 

For each candidate HCl gas/vapor blending device, this section provides procedures and 

calculations to determine certified reference values at specific operating conditions and to 

calculate the combined, expanded uncertainties of those reference values. Documentation that a 

particular HCl gas/vapor blending device has been evaluated according to these procedures, and 

has met the applicable acceptance criteria, satisfies the NIST traceability requirements for the 

device. 

4.2 Device Calculations 

HCl gas/vapor blending devices calculate and report the delivered concentration using simple 

algebraic functions. The inputs to these functions are flow measurements, concentrations, and 

conversion factors. The functions themselves will vary based on the type of device and what it 

measures. Table 4 shows four device designs, and the associated expressions to calculate outlet 

concentration in ppm. 

Table 4: HCl Gas/Vapor Blending Device Designs 

Bottled HCl Gas Humidifier Evaporative HCl Gas Standard 

Direct Inert Gas Carrier Single Liquid Water Addition 

w dry

dry dry

m

C

ν

ν

⋅ +

⋅

.001244 &

w dry add

dry dry

m

C

ν ν

ν

⋅ + +

⋅

.001244 & sol add

ws sol

m

M

ν

ν

⋅ +

⋅ ⋅

.001244 &

22.414

( ) w sol add

ws sol

m m

M

ν

ν

⋅ + +

⋅ ⋅

.001244 & &

22.414

Cdry Concentration of dry HCl gas, ppm 

ν dry Flow rate of dry HCl gas, liters/min 

Mws Working solution Molarity, mol/liter 

 ν sol Working solution flow, μl/mina

 msol & Working solution flow, mg/mina

mw

& Flow rate of liquid water, mg/min 

ν add Flow rate of inert carrier gas, liters/min 

. liters/mg Specific volume of water vapor at stan 001244 dard conditions (0°C, 1 atm) 

22 liters/mol Molar volume of ideal gas at standar .414 d conditions (0°C, 1 atm) 

 

a

 For both water and working solutions, a density of 1.000 g/mL can be assumed for purposes of calculating μl/min 

injected into the evaporator when the actual measurement is based on mass. See 4.4.1.4. 
- 15 - 

There are essentially three types of inputs to the concentration calculation: (1) HCl feedstock 

(gas or liquid) concentration, (2) gas flow rate(s), and (3) liquid feed rate(s). The feedstock 

concentration is provided by the supplier or the laboratory, and is provided as an input parameter 

to the blending device. The gas flow and liquid flow rates, however, must be measured in real 

time and used by the device to calculate the expected HCl concentration. In order to translate an 

expected concentration into a NIST-traceable concentration, the calculation inputs must be 

NIST-traceable, and the uncertainties must be propagated through all calculations to the final 

“certified” HCl concentration and uncertainty. These final calculations may be performed 

internally or externally to the device’s firmware or software. 

4.3 Dry HCl Gas Concentration 

The NIST-traceability of dry HCl gas standards is beyond the scope of this protocol. For the 

purpose of certifying devices that use these gas standards as feedstock, it is assumed that NISTtraceable standards are commercially available, and that the certificate of analysis reports the 

certified concentration and an uncertainty. Unless the certificate states otherwise, the reported 

uncertainty will be assumed to have a coverage (K) factor of 2, so this number will be divided by 

2 to get the standard (K=1) uncertainty which will be used in subsequent calculations of 

combined uncertainty. 

4.4 Working Solutions for Evaporative HCl Standards 

Working HCl feed solutions may be directly provided by commercial manufacturers or prepared 

by the end user from commercially available standards. Commercial HCl standards that have 

certified concentrations with an uncertainty of ± 0.5% or better should be used. 

The commercial standard is guaranteed to be stable and accurate up to the expiration date, 

provided that the solution is kept tightly capped and stored under normal laboratory conditions. 

Note: Standards with accuracy specifications of ± 0.1 to 0.5% are available. For calculation 

purposes, the guaranteed accuracy is considered to be the expanded uncertainty of the 

commercial standard. (The assumed coverage factor is 2.0.) 

4.4.1 Working Solution Preparation

Direct use of a commercially prepared HClstandard solution is preferred. However, subsequent 

dilution of a commercially prepared HCl solution to prepare a working solution is permissible, 

when necessary. 

(a) Determine the target concentrations of the working solutions that are appropriate for the 

anticipated liquid feed rates, gas dilution flow rates, and other applicable factors that are 

necessary to generate each of the desired HCl gas concentrations. These target 

concentrations may be supplied by the device manufacturer. 
- 16 - 

(b) If necessary, prepare each target working solution by diluting the NIST traceable HCl 

solution to the target concentration using the procedure in Section 4.2.2.1, below, or a 

similar procedure. 

(c) Note: The preparation of working solutions described in Section 4.2.2.1 should be 

performed by qualified persons who are knowledgeable and experienced with the use of 

proper laboratory procedures and equipment. 

4.4.1.1 Gravimetric/Volumetric Preparation

This approach offers considerable flexibility while maintaining suitable working solution 

uncertainties and combines gravimetric and volumetric preparation techniques. An aliquot of the 

commercial stock solution is added to a tared volumetric flask, weighed using a balance, and 

then brought to volume. The required precision of the balance depends on the mass of the stock 

solution aliquot. For aliquots 5g and greater, either a 3-place or a 4-place balance may be used. 

For aliquots less than 5g, a 4 place balance is required. Stock solution aliquots less than 0.5 g are

not recommended. 

a) Prepare a 3- or 4-place balance capable of reading in the desired range by zeroing it and 

then weighing NIST-traceable weights in the range of measurement necessary to prepare 

each target working solution concentration. 

b) Tare an empty Class A volumetric flask (e.g., 100 mL to 1000 mL). 

c) Add the targeted mass of the standard HCL solution to the flask (e.g., a 10 mL aliquot 

will weigh approximately 10 g). Record the total weight. Subtract the tare weight of the 

flask from the total weight to determine the exact weight of the aliquot. 

d) Dilute with deionized (DI) water to the exact mark on the volumetric flask. 

4.4.1.2 Working Solution Concentration (mg/liter)

(a) Calculate the concentration of the working solution using Equation 1A or Equation 1B, 

depending on the units of the commercial standard solution: 

 

( )( )

( )( ) total std

std std

ws V

C

C w

ρ

= × ×

3

1 10 (Equation 1A) 

Where: Cws = HCl concentration of the working solution (mg/liter) 

Cstd = HCl concentration of the standard solution (g/liter) 

wstd = Weight of standard solution (g) 

Vtotal = Final total volume of working solution (mL) 

ρ std = Density of standard solution (g/ml) 

3

1×10 = Conversion factor (mg/g) 

 
- 17 - 

( )( )( )

( )( ) total std

std HCl std

ws V

N MW

C w

ρ

= × ×

3

1 10 (Equation 1B) 

Where: Nstd = Normality of HCl working solution (mol/liter) 

MWHCl = Molecular Weight of HCl (g/mol) 

4.4.1.3 Molarity of Working Solution

Calculate the molarity of HCl in the diluted solution according to Equation 2. 

HCl

ws

ws MW

C M

3

1 10−= × (Equation 2) 

Where: Mws = Molarity of the working solution (mol/liter) 

 

3

1 10−× = Conversion factor (g/mg) 

4.4.1.4 Conversion Between mg and μl of Working Solution

For dilute HCl working solutions in general, and for gravimetric feeding in particular, the units 

of mg and μl may be used interchangeably. A density of 1.000 g/mL can be assumed when 

calculating μl /minute injected into the evaporator. Similarly, in considering alternatives to the 

gravimetric/volumetric dilution procedure of 4.4.1.1, the final “volume” of the working solution 

may be determined by weight by substituting grams for ml in the equation of 4.4.1.2. 

Commercially available standard HCl solutions ≤ 0.1N, have densities that are nearly 1.00 g/mL. 

4.4.2 Uncertainty of the Working Solution

The gravimetric and volumetric standard uncertainties are combined with the stock solution 

standard uncertainty to derive the combined, standard uncertainty of the working solution 

concentration. 

(a) The uncertainty of the aliquot weight (UA) is determined by dividing three timesb

 the 

estimated readability of the balance by the measured aliquot weight, and converting the 

result to a percentage. 

(b) The uncertainty of the final working solution volume (UF) is estimated by dividing the 

readability of the certified volumetric glasswarec by the volume of the flask, and 

converting the result to a percentage. 

 

b

 Based on guidance from EPA metrology experts, the uncertainty of weights determined using a balance can be 

estimated based on the readability of the balance and a multiplier of 3. For example, for a 3-place balance, the 

readability is ± 0.001 g. Therefore, the uncertainty of measurements made with a 3-place balance is 3 times 

± 0.001 g, or ± 0.003 g. 

c

 For example, the readability of a one-liter Class A volumetric flask is ± 0.40 mL. 
- 18 - 

(c) Alternatively, the uncertainty of the final working solution weight (UF) is determined by 

dividing three times the estimated readability of the balance by the final flask weight, and 

converting the result to a percentage. 

(d) The uncertainty of the commercial standard solution value is the percent accuracy of the 

solution concentration, as specified by the supplier. 

(e) The combined standard uncertainty of the working solution concentration is calculated 

using Equation 3. 

2 2

2

2

A F

CS

WS U U

U

U  + +











= (Equation 3) 

Where: UWS = Uncertainty in the concentration of the working solution (%) 

 UCS = Expanded uncertainty of the commercial standard, with an 

assumed k=2 coverage factor (%) 

 UA = Uncertainty of the aliquot weight or volume (%) 

 UF = Uncertainty of the final, diluted standard weight or volume (%) 

(f) Example: A 10 g aliquot of a standard HCl solution is weighed using a 3-place balance. 

The exact weight of the aliquot is 10.015 g. The aliquot is diluted to 1.0 liter using a 

Class A volumetric flask. The specified accuracy of the standard solution is ± 1.0%. 

Calculate the uncertainty of the working HCl solution. 

The uncertainty of the aliquot weight (UA), is (0.003 g/10.015 g) x 100, or 0.03%. The 

uncertainty of the final solution volume (UF) is (0.40 mL/1000 mL) x 100, or 0.04%. The 

uncertainty of the commercial standard solution (UCS) is equal to its specified accuracy, 

i.e, 1.0%. Substituting the values of UA, UF, and UCS into Equation 3 gives a combined 

uncertainty for the working solution of 1.0012%, which rounds to 1.00%. For this 

example, the preparation of the working solution adds negligible additional uncertainty. 

The uncertainty of the diluted working solution is dominated by the uncertainty of the 

commercial standard solution. 

(g) Equation 3 calculates the uncertainty with a coverage factor value of 1-sigma, which is 

appropriate for an intermediate uncertainty that will be subsequently combined with other 

uncertainties and expanded. The uncertainty of a commercially available NIST-traceable 

HClstandard is assumed to have a coverage factor of 2.

4.5 Gas Flow Rates 

Every gas/vapor blending device, regardless of configuration, must measure one or more gas 

flows to calculate its output concentration. This measurement can be made many ways, but is 

commonly provided by an electronic mass flow controller (MFC). This controller accepts input 

from the blending device firmware or software to establish its setpoint flow. Using its own 

internal feedback control loop, the MFC attempts to maintain the flow at the setpoint, and 
- 19 - 

delivers a signal (analog or digital) indicating the as delivered flow. For a NIST-traceable HCl 

gas/vapor blending device, it is this as delivered flow signal that is used to calculate the device’s

expected concentration. 

Many blending devices use quadratic or even 3rd order curves to linearize both the setpoint and 

the as delivered signals, as displayed and recorded by the blending device software. The use of 

these curves can significantly reduce the uncertainty of the gas flow measurement, but require 

considerable expertise to ensure that the correct factors are stored in the blending device. Simple 

mistakes like using the wrong units (e.g., liters/min versus ml/min) to calculate the curve 

coefficients or applying the wrong curve (e.g., the setpoint curve versus the as delivered curve), 

can cause huge errors related to the MFC operation and measurements. Since the internal 

function of blending device firmware/software is beyond the scope of this protocol, it will simply 

be assumed that any linearization curves are applied transparently to the operator and the 

recorded data stream. The linearity of the blending device’s gas flow measurements will be 

verified as part of the certification process, and any residual non-linearity (i.e., after any as 

delivered curve is applied) will become part of the uncertainty. 

4.5.1 Gas Flow Rate Comparison to NIST-Traceable Reference Standard

The number and level of gas flow rates to be measured will depend on the intended application. 

The range of flows should cover the expected operating range of the device, and the number of 

flows (no less than 3) should be sufficient to minimize the impact of interpolation uncertainty for 

in-use flows that don’t exactly match the measured flows. Measure the gas flow from the device 

according to the procedures in paragraphs (a) through (e) of this section. As noted in the final 

step, this measurement process must include at least 2 independent experiments (on different 

days, to assess reproducibility uncertainty), each covering all of the flow rates that will be used 

to calculate certified concentrations. Alternatively, if the flow device manufacturer’s certification 

includes uncertainty data for the full range of flows, it may be used as one of the independent 

experiments for purposes of calculating reproducibility. 

(a) Install a primary laboratory standard device for flow rate measurement at the outlet of the 

calibrator (e.g., a digital bubble meter, piston displacement device, or other device). The 

device must have: (1) an uncertainty of 1.0% or better; and (2) a NIST-traceable 

calibration. Operate the reference flow rate measurement device in accordance with the 

manufacturer’s instructions. Make sure that the reference device is used within its 

specified operating range and conditions. 

(b) Prepare and operate the HCl gas/vapor blending device according to the manufacturer’s 

instructions. The operating conditions should match the conditions under which the 

device will be certified, but with the liquid flow(s) and any other gas flow disabled. For 

each operating condition, allow the device to run for a sufficient period of time to 

equilibrate and achieve steady operation. 

Note: If the design of the blending device prevents the disabling of other liquid or gas 

flows, the NIST-traceable reference standard may be inserted into the blending device’s 
- 20 - 

gas feed upstream of where it mixes with the other flows, provided the device’s hardware 

is accessible for this purpose. 

(c) For each gas flow rate level, record the expected reading of the device’s gas flow 

measurement (i.e., the as delivered flow after processing through the device’s 

linearization curve, if used), and the concurrent output of the NIST-traceable reference 

standard. Record at least ten pairs of readings. If applicable, adjust the reference flow 

rate measurements to standard temperature and pressure conditions. 

(d) Calculate the relative standard deviation (RSD) of the gas flow rate determination 

replicates at each level. If any RSD value is greater than 2.0%, take corrective action and 

repeat those pairs of readings. Record the RSD as the first-day gas flow uncertainty (Uν1) 

at each level. 

(e) Shut down the HCl gas/vapor blending device according to the manufacturer’s 

instructions. On another day, repeat the measurement experiment and record the RSD at 

each level as Uν2. The experiment may be repeated more than once, provided all data are 

recorded, and should result in lower uncertainty. 

4.5.2 Combining Data and Correlating Expected to Certified Values

Once the comparison measurements are done, the dataset will consist of a number (I ≥ 10) of 

data points, for each of J flow conditions, each compared on a number (K>1) of days. The singleday/single-flow averages (νjk) have already been calculated as part of the RSD check during the 

comparisons. 

∑=

=

I

i

i

jk I 1

ν

ν 

( )

jk

i jk

I

i

jk I

U RSD

ν

ν ν

ν

100%

( )1

2

1 ×

−

Σ −

= =

=

Calculating one reference number for each flow condition means simply averaging the νjk values, 

while the single-flow uncertainty (Uνj) involves a combination of averages and uncertainties at 

various levels of the comparison process. 

∑=

=

K

k

jk

j

1 K

ν

ν 

( )

100%

100% ( )1

1

2

2

2

1

1

2

2

2

×

− ×

Σ −

+ 













− 

 +











=

=

=

∑

j

jk j

K

k

K

k

RFS jk

j

K

U

K

U K

U

ν

ν ν

ν

ν

where URFS is the uncertainty of the reference flow standard with an assumed coverage factor of 

k=2. If the blending device is properly applying its linearization curves, the single-day/singleflow averages (νjk) should correlate well with the corresponding expected flow values (xjk) as 

recorded by the blending device. To verify this, derive a linear equation which, when applied to 

the expected gas flow rate, predicts the corresponding reference flow rate. This linear equation 

may be either: (1) a factor based on the ratio of actual gas flow rate to indicated gas flow rate 
- 21 - 

 

jk

a

x

jk

Σ

Σ

=

ν

(Σ without bounds denotes all νjk and xjk averages are included)

or (2) a combination of such a factor and an adjustment for zero offset. 

( )2

jk

2

jk jk

-

-

a

JK x x

JK x x

jk

jk jk

Σ⋅ Σ

⋅Σ Σ ⋅Σ

=

ν ν

( )2

jk

2

jk jk

2

-

-

b

JK x x

x x x

jk

jk jk jk

Σ⋅ Σ

Σ Σ⋅ Σ Σ⋅

=

ν ν

The term JK is, in fact, the product of the number of comparison test days with the number of 

test conditions, and represents the total number of data points for the correlation (usually 

expressed as N in a typical least-squares formula). After determining the correction equation, the 

data points should be compared to the line. If any point is more than 2% above or below the line, 

take corrective action and repeat the comparisons. 

4.5.3 Interpolation

As a practical matter, the delivered flow during operation will rarely match one of the calibrated 

points exactly. In order have a continuum of certified points over the full range of operation, 

interpolation is necessary. The formula for linear interpolation is straightforward. 

( ) ( ) 















−

−

≈ + −

















−

−

= + −

±

±

j j

j j

j gas j

gas j

gas j

gas j gas j

x x

x x

x x

x x

1

ν 1 ν

ν

ν ν

ν ν

where xj,νj is the certified point nearest the xgas value to be interpolated, and xj±1,νj±1 is an 

adjacent certified point chosen so that xgas is between, xj and xj±1. The uncertainty arising from 

this calculation is primarily related to the assumption that the point xgas,νgas lies on a straight line 

between xj,νj and xj±1,νj±1. This line, along with the correction equation used in 4.5.2 to verify 

response linearity, defines an estimated uncertainty envelope for interpolation slope. 

( ( ) ( ) ( ))

j j

j j j j j j j j

Max gas j

gas j

x x

Max a x b a x b a x x

x x −

− − ⋅ + ⋅ + − ⋅ −

=

















−

−

±

± ± ± ±

1

1 1 1 1 ν ν ν ν ,ν , ν ,

( ( ) ( ) ( ))

j j

j j j j j j j j

Min gas j

gas j

x x

Min a x b a x b a x x

x x −

− − ⋅ + ⋅ + − ⋅ −

=

















−

−

±

± ± ± ±

1

1 1 1 1 ν ν ν ν ,ν , ν ,

These maximum and minimum values define an uncertainty component related to the linear 

interpolation. 
- 22 - 

100%

12

2

×

































−

−

−

















−

−

=

Min gas j

gas j

Max gas j

gas j

li

x x x x

U

ν ν ν ν

ν

For any interpolated point within the range of gas flow measurements, the combined uncertainty, 

including the uncertainty of interpolation, is 

( )

2

2 2 2 2

gas

vli gas j vj j

gas

U x x U

U

ν

− + ⋅ν

=

As an alternative approach to interpolation, the linear equation from 4.5.2 may be applied as a 

correction equation to all points within the range of gas flow measurements. 

a x b ν gas = ⋅

gas +

( ( ))

( ) 100%2

+ + + 2 - -

2 2 2 2

×

−

Σ Σ ⋅ Σ Σ Σ

=

JK

JKb a x ab x b x a x

U

jk jk jk jk jk jk jk jk

gas

ν ν ν ν

Note: While this alternative does simplify the algorithm for calculating the certified output, it 

may yield a higher uncertainty than interpolation, particularly at the low end of the flow range. 

Whichever calculation is used, interpolation or the linear correction, must be permanently 

recorded and incorporated into the device’s calculation of its expected output HCl concentration. 

Maintain documentation of the experimental measurements, derivation of the linear correction 

and interpolation equations, if used, and the uncertainty determinations for both the compared 

flow points and the interpolated/corrected points. 

4.6 Liquid Feed Rates 

The liquid feed function of gas/vapor blending devices varies significantly in implementation. 

Where bottled gas humidification devices may use some type of reservoir with continuous ondemand replenishment, an evaporative HCl standard usually has some type of variable-speed 

pump. Regardless of design, all devices must measure one or more liquid flows to calculate the 

output concentration. Like the gas flows, it is the as delivered flow that is important, not the 

setpoint. For liquid flows, these are less easily confused, because any component that attempts to 

set the flow (e.g., a positive displacement pump) is usually separate and distinct from how the 

flow is measured. Some devices do have a factor (e.g., a “tube factor” for a peristaltic pump) that 

is used to predict how much flow will result from a given pump setting, but these factors tend to 

change over time, resulting in dramatic changes to the relationship between setpoint and as 

delivered flow (hence the importance of an independent flow measurement to calculate the 

output concentration). 
- 23 - 

Like the gas flow, the gas/vapor blending device might use a quadratic or 3rd order curve to 

linearize the as delivered liquid flow signal(s), as displayed and recorded by the device software. 

It is assumed that, if used, these curve coefficients are properly calculated and applied, and that 

the application of the curve(s) is transparent to the operator and the recorded data stream. The 

linearity of the blending device’s liquid flow measurements will be verified as part of the 

certification process, and any residual non-linearity will become part of the uncertainty. 

4.6.1 Liquid Flow Rate Comparison to NIST-Traceable Reference Standard

Based on the expected output moisture content and (for evaporative HCl standards) the solution 

concentration, determine the range of liquid feed rates that are necessary to generate the required 

output concentrations, taking into account the carrier gas flow rates and other applicable factors. 

The number and level of liquid feed rates to be verified (no less than 3) will depend on the 

application. The range of flows should cover the expected operating range of the device, and the 

number of flows should be sufficient to minimize the impact of interpolation uncertainty for inuse flows that don’t exactly match the measured flows. Measure the liquid flow from the device 

according to the procedures in paragraphs (a) through (k) of this section. As noted in the final 

step, this measurement process must include at least 2 independent experiments (on different 

days, to assess reproducibility uncertainty), each covering all of the flow rates that will be used 

to calculate certified concentrations. Alternatively, if the flow device manufacturer’s certification 

includes uncertainty data for the full range of flows, it may be used as one of the independent 

experiments for purposes of calculating reproducibility. 

(a) Zero an appropriate three-place (or more accurate) balance and then check its 

calibration against NIST-traceable weights in the range of measurement necessary for 

each liquid feed rate determination. 

(b) Connect the balance to a data logging device that is capable of recording its reading in 

real time with an accurate timestamp corresponding to each reading. Software that plots 

the readings, and even the rate of change, would be very useful for these comparisons. It 

would also be beneficial to assure that the logging device clock is synchronized with the 

blending device. 

(c) Set up the HCl gas/vapor blending device according to the manufacturer’s instructions, 

allowing all temperatures to equilibrate. Place the HCl working solution (or DI water, as 

applicable) in a reservoir on the balance. Position the liquid uptake or delivery tube in 

such a manner that it does not affect the weighing process. 

(d) Operate the HCl gas/vapor blending device according to the manufacturer’s 

instructions. The operating conditions should match the conditions under which the 

device will be certified, including a representative gas flow. Allow the device to run for 

a sufficient period of time to equilibrate and achieve steady operation. 

Note: A significant period may be required for systems with peristaltic pumps to ensure 

that all gas bubbles are removed from the apparatus and steady operation is achieved. 
- 24 - 

(e) Once steady operation is indicated, allow the device to run for at least 5 minutes while 

the balance readings are being logged (more time generates more data points , which 

lowers uncertainty). Change to the next operating condition, making sure the exact time 

of the change is recorded (either in a notebook or by a flag or comment in the balance 

logging file). Allow the blending device to equilibrate at the new flow setting(s). 

(f) Continue operating the device at different operating conditions, recording at least 5 

minutes of balance data after each condition stabilizes. For each condition, record the 

expected reading of the device’s liquid flow measurement (i.e., the as delivered flow 

after processing through the device’s linearization curve, if used), and the matching 

timestamps from the balance logging. 

(g) Shut down the HCl gas/vapor blending device according to the manufacturer’s 

instructions. Shut down the balance logging and retrieve the data file. 

(h) Parse the balance data into segments of time (t) and mass (m) readings, each 

representing a single operating condition as recorded after stable operation. Calculate 

least-squares linear slope, intercept, and standard error of slope for each data segment. 

/ min

-

-

a 2

jk

2

jk jk

jk g

N t t

N t m t m

m

seg

jk

seg

jk

seg seg

jk

seg













⋅ Σ Σ

⋅ Σ Σ ⋅ Σ

& = = grams

N t t

m t t t m

seg

jk

seg

jk

seg seg

jk

seg

jk

seg

2

jk

2

jk jk

2

-

-

b













⋅ Σ Σ

Σ ⋅ Σ Σ ⋅ Σ

=

( )

/ min

2 -

+ + + 2 - -

2

jk

2

2 2 2 2

g

N N t t

N m Nb a t ab t b m a t m

stderr

seg

jk

seg

jk jk

seg

jk

seg

jk

seg

jk

seg

jk

seg

ajk



























 − ⋅ Σ Σ























 Σ Σ ⋅ Σ Σ Σ

=

where the indexes j and k identify the specific operating condition out of J total, and the 

comparison day (out of K>1), similar to the gas flow comparisons in 4.5.1. 

(i) Calculate the relative standard error (Uṁjk) of each liquid flow rate determination 

= ×100%

jk

ajk

mjk m

stderr

U

&

&

(j) If any Uṁjk value is greater than 1.0%, take corrective action and repeat the affected 

operating conditions. If no operational problem is detected, consider increasing the 

interval for recording balance readings. 

(k) On another day, repeat the entire comparison experiment, and record the Uajk for all test 

conditions. The experiment may be repeated more than once, provided all data are 

recorded, and should result in lower uncertainty. 
- 25 - 

4.6.2 Combining Data and Correlating Expected to Certified Values

Once the comparison measurements are done, calculating one reference number for each flow 

condition means simply averaging the ṁjk values, while the single-flow uncertainty (Uṁjk) 

involves a combination of averages and uncertainties at various levels of the comparison process. 

∑=

=

K

k

jk

j

K

m

m

1

&

& 

( )

100%

100% ( )1

1

2

2

1

1

2

2

×

− ×

Σ −

+ 













− 

=

=

=

∑

j

jk j

K

k

K

k

jkm

mj

K m

U m m

K

K

U

&

& &

&

&

If the blending device is properly applying its linearization curves, the single-day/single-flow 

averages (ṁjk) should correlate well with the corresponding expected flow values (xjk) as 

recorded by the blending device. To verify this, derive a linear equation similar to the gas 

correlation in 4.5.2. After determining the linear equation, the data points should be compared to 

the line. If any point is more than 2% above or below the line, take corrective action and repeat 

the comparisons. 

4.6.3 Interpolation

During normal operation, the delivered liquid flow, to an even greater extent than the gas flow, 

will rarely match one of the calibrated points exactly. As such, linear interpolation is one of the 

two options for having a continuum of certified points over the full range of operation. 

( ) ( ) 















−

−

≈ + −

















−

−

= + −

±

±

j j

j j

j liq j

liq j

liq j

j liq j

x x

m m

m x x

x x

m m

m m x x

1

1

& &

&

& &

& &

making sure that point xj, ṁ j is the certified point nearest the xgas value to be interpolated, and 

xj±1, ṁ j±1 is an adjacent certified point chosen so that xgas is between, xj and xj±1. The uncertainty 

calculation for this approach is similar to section 4.5.3. 

( ( ) ( ) ( ))

j j

j j j j j j j j

Max gas j

liq j

x x

Max m m m a x b a x b m a x x

x x

m m

−

− − ⋅ + ⋅ + − ⋅ −

=

















−

−

±

± ± ± ±

1

1 1 1 1

& & & & , & , & ,

( ( ) ( ) ( ))

j j

j j j j j j j j

Min gas j

liq j

x x

Min m m m a x b a x b m a x x

x x

m m

−

− − ⋅ + ⋅ + − ⋅ −

=

















−

−

±

± ± ± ±

1

1 1 1 1

& & & & , & , & ,

100%

12

2

×

































−

−

−

















−

−

=

Min gas j

gas j

Max gas j

gas j

mli

x x

m m

x x

m m

U

& & & &

&
- 26 - 

( )

2

2 2 2 2

liq

lim liq j mj j

liq

U x x U

U

ν

− + ⋅ν

=

& &

where ṁ can be either ṁsol or ṁw (for working solution injection or DI water injection). 

The alternative approach of applying the linear equation from 4.6.2 as a correction equation, 

within the range of liquid flow measurements, is also available. 

m a x b &

liq = ⋅liq +

( ( ))

( ) 100%2

+ + + 2 - -

jk

2 2 2 2

×

−

Σ Σ ⋅ Σ Σ Σ

=

JK

m JKb a x ab x b m a x m

U

jk jk jk jk jk

liq

& & &

Note: It bears repeating that this alternative may yield a higher uncertainty than interpolation, 

particularly at the low end of the flow range. 

Whichever calculation is used, interpolation or the linear correction, must be permanently 

recorded and incorporated into the device’s calculation of its expected output HCl concentration. 

Maintain documentation of the experimental measurements, derivation of the linear correction 

and interpolation equations, if used, and the uncertainty determinations for both the compared 

flow points and the interpolated/corrected points. 

4.7 Calculation of Certified HCl Concentrations and Uncertainties 

Section 4.2 describes how HCl gas vapor blending devices calculate and report the expected

output concentration. These calculations are typically done automatically by the device’s 

microprocessor using the specific software/firmware version and data processing algorithms as 

implemented by the particular manufacturer. When certifying or using any blending device 

model for the first time, the calculations should be checked independently, according to standard 

engineering practices, to verify that the units of all inputs, the formulas used, and the units of the

device output are all correct. 

Defining the certified output concentrations, however, is a process that may or may not be 

readily and fully incorporated into the device itself. If the device software/firmware is robust 

enough to incorporate the algorithms of sections 4.6 and 4.7, it is likely that it can be configured 

to actually report the certified concentration as its expected concentration. Otherwise, an 

accessory program or macro will be needed to import raw or processed data from the device and 

calculate the certified concentration for use in calibrating or otherwise quality-assuring emissions 

measurement equipment. 

4.7.1 Bottled HCl Gas Humidifier

The certified output concentration of a bottled HCl gas humidifier is calculated by one of two 

formulas, depending on whether there is a carrier gas stream (νadd) used to deliver the moisture. 
- 27 - 

w dry

dry dry

wet m

C

C

ν

ν

⋅ +

⋅

=

.001244 &

 

w dry add

dry dry

wet m

C

C

ν ν

ν

⋅ + +

⋅

=

.001244 &

Where Cwet ≡ output HCl concentration, ppm 

Cdry ≡ HCl concentration of dry gas standard, ppm 

νdry ≡ volumetric flow rate of dry HCl standard, liter/min 

νadd ≡ volumetric flow rate of inert carrier gas (if used), liter/min 

ṁw ≡ mass flow rate of humidification water, mg/min 

Cdry comes straight from the bottled gas certificate of analysis, where νdry and νadd are each 

determined by the procedures in section 4.5, and ṁw follows section 4.6. The combined 

uncertainty incorporates these values and their uncertainties. 

( )

( )

( )2

2 2 2 2

2 2 2

2

.001244

(.001244 )

2

w dry add

add add mw w

wet

Cdry dry dry

wet

dry dry wet

dry

Cwet

m

U U m

C

U C

C

C C

U

U

ν ν

ν

ν ν

ν ν

⋅ + +

+ + 































+ 













 −

=

&

&

&

Where Uνdry comes from the section 4.5 Ugas calculation, % 

UCdry comes from the dry gas standard certificate of analysis, % 

Note: A coverage factor of k=2 is assumed

Uνadd comes from the section 4.5 Ugas calculation, % 

Uṁw comes from the section 4.6 Uliq calculation, % 

4.7.2 Evaporative HCl Gas Standard

The certified output concentration of an evaporative HCl gas standard is calculated by one of two 

formulas, depending on whether there is an additional liquid stream of deionized or distilled 

water (ṁw). 

sol add

ws sol

wet m

M C

ν

ν

⋅ +

⋅ ⋅

=

.001244 &

22.414

 ( ) sol w add

ws sol

wet m m

M C

ν

ν

⋅ + +

⋅ ⋅

=

.001244 & &

22.414

where 

 Cwet ≡ output HCl concentration, ppm 

Mws ≡ molarity of HCl working solution, mol/liter 

νsol ≡ volumetric flow rate of HCl working solution, μl/min 

ṁsol ≡ mass flow rate of HCl working solution, mg/min 

ṁw ≡ mass flow rate of additional water, mg/min 

νadd ≡ volumetric flow rate of inert carrier gas, liter/min 

Mws comes either from the solution provider’s certificate of analysis, or from section 4.4 if the 

solution was prepared from a more concentrated standard solution. Values for ṁsol and ṁw are 

each determined by the procedures in section 4.6, which also provides that νsol = ṁsol because the 
- 28 - 

working solution density is 1.000 mg/μl. The value of νadd comes from section 4.5. The 

combined uncertainty incorporates these values and their uncertainties. 

( ) ( )

( ) ( ) ( ) ( ) 2

2 2 2 2 2

2 2

2 2 2 2 2 2

.001244

(.001244)

.001244

22.414 .001244 22.414

sol w add

add add mw w

wet sol w add

msol sol ws wet Mws ws sol

Cwet

m m

U U m

C m m

U m M C U M U

ν

ν

ν

ν ν

⋅ + +

+

+

⋅ + +

⋅ − ⋅ + ⋅

=

& &

&

& &

&

& &

Where Uṁsol comes from the section 4.6 Uliq calculation, % 

UMws comes from the section 4.4 Uws calculation, % 

Uνadd comes from the section 4.5 Ugas calculation, % 

Uṁw comes from the section 4.6 Uliq calculation, % 

Note: If a commercially available working solution is used instead of diluting a more 

concentrated commercial standard, look for the uncertainty on the certificate of analysis and 

divide this number by an assumed coverage factor of k=2 to get the value of UMws. 

4.7.3 Expanded Uncertainty

The performance specification for uncertainty will be based on expanded uncertainty. To 

calculate this quantity, multiply the combined uncertainty value from 4.7.1 or 4.7.2 by a 

coverage factor. Unless specified otherwise in the applicable regulation, a coverage factor of k=2 

should be used. 

4.8 Alternative Approach: Direct Comparison to a Traceable Reference Gas Standard 

As an alternative to relying on the calculations of section 4.7 to determine certified output 

concentrations, a blending device may instead undergo a bracketing procedure that provides a 

direct comparison to a certified gas standard. In a bracketing procedure, the outputs from two 

sources (in this case, a gas/vapor blending device and a certified gas standard) are introduced 

alternately to a measurement system or detector so that each response from the candidate

(blending device) is bracketed by a pair of responses from the reference (certified gas standard). 

At each concentration, the bracketing procedure requires a minimum of four responses from the 

reference and three responses from the candidate. An example of a bracketing procedure 

injection sequence, as performed at a single output concentration, is provided in Table 5. 

Table 5: Example of Bracketing Procedure Injection Sequence 

Source Response ID 

Reference Standard RS1 

Candidate C1 

Reference Standard RS2 

Candidate C2 

Reference Standard RS3 

Candidate C3 

Reference Standard RS4 
- 29 - 

In addition to the minimum of three candidate injections as shown above, the bracketing 

procedure must be performed at a minimum of three concentrations (covering the blending 

device’s usable range), for a 3x3 or larger bracketing matrix. Also, this entire comparison 

sequence must be repeated at least once on a different day after the blending device has been 

shut down and restarted by the manufacturer’s recommended procedures. The bracketing 

comparisons are used to calculate an average concentration ratio between the candidate and 

reference gases, and the repetition of the 3x3x2 (or larger) certification matrix (see Table 6) is 

used to assess the repeatability and reproducibility of the certification results. 

Table 6: Example Bracketing Certification Matrix 

Concentration Day 1 Day 2 

Zero 1Z1 2Z1

High 1H-RS1 2H-RS1

High 1H-C1 2H-C1

High 1H-RS2 2H-RS2

High 1H-C2 2H-C2

High 1H-RS3 2H-RS3

High 1H-C3 2H-C3

High 1H-RS4 2H-RS4

Mid 1M-RS1 2M-RS1

Mid 1M-C1 2M-C1

Mid 1M-RS2 2M-RS2

Mid 1M-C2 2M-C2

Mid 1M-RS3 2M-RS3

Mid 1M-C3 2M-C3

Mid 1M-RS4 2M-RS4

Low 1L-RS1 2L-RS1

Low 1L-C1 2L-C1

Low 1L-RS2 2L-RS2

Low 1L-C2 2L-C2

Low 1L-RS3 2L-RS3

Low 1L-C3 2L-C3

Low 1L-RS4 2L-RS4

Zero 1Z2 2Z2

4.8.1 Stepwise Bracketing Procedure

(a) Turn on all components of the HCl gas/vapor blending device, the HCl measurement 

system (analyzer), and any associated equipment; allow them to reach thermal 

equilibrium and stable operation. 

(b) While awaiting warm-up, check that all data acquisition and logging systems associated 

with the blending device and the analyzer are operational and properly configured to 

record data at a sufficient time and concentration resolution, so that complete 

documentation of the certification test procedures is available for future inspection. 

(c) After equilibrium has been established, inject zero gas and make any necessary 

adjustments to the zero response. Then, calibrate the detector by injecting the high-level 
- 30 - 

reference gas standard and making any necessary span adjustments. Repeat these steps 

as necessary to optimize the analyzer calibration relative to the reference standard. 

Note: Adjusting the analyzer to provide the correct responses to the reference gas 

standard is considered to be a valuable diagnostic technique to detect changes in the 

measurement system that can adversely affect the bracketing procedure. 

(d) Before beginning the bracketing procedure, ensure that the blending device, the 

analyzer, and any support equipment are operating within the prescribed limits for all 

key parameters as specified by the respective manufacturers. 

(e) Set the blending device to the first set point (i.e., the high-level or low-level point, as 

deemed appropriate) and allow the device operation and output to stabilize. 

(f) Turn on the reference gas standard that is closest in concentration to the blending device 

output, and feed this gas standard to the analyzer inlet and obtain a response (RS1). It is 

recommended that the recorded response at each test point be based on at least 5 

minutes of stable readings. 

(g) Shut off the reference gas standard and direct the output of the HCl gas/vapor blending 

device to the analyzer. Allow the HCl measurements to stabilize and record an average 

response (C1), again based on at least 5 minutes of stable readings. 

(h) Switch back to feeding the reference gas standard to the analyzer and record an average 

response (RS2). 

(i) Repeat steps (g) and (h) at least two more times at this concentration to provide three or 

more candidate responses, each preceded and followed by a reference standard 

response. 

Note: If, at any time during the process, test results demonstrate that equilibration is 

not being achieved, or that any key parameter is not within acceptable limits, stop the 

experiment, investigate and correct the problem. Valid data collected up to that point in 

the test may be retained. Discard all data collected at non-equilibrium conditions or 

with any key parameter outside of prescribed limits; such data are not valid and may 

not be used for certification of the blending device. 

(j) Repeat steps (e) through (i) for at least two more concentrations. Be sure to allow the 

blending device to reach equilibrium at each new setting before recording any 

responses. 

(k) Inject the zero gas to the analyzer and record an average response without making any 

adjustments. 

(l) Shut down the HCl gas/vapor blending device according to the manufacturer’s 

instructions. Also shut down all data logging and any other equipment that is not 

designed to be left on overnight. 
- 31 - 

(l) On another day, repeat steps (a) through (k). The bracketing procedure may be repeated 

more than once, provided all data are recorded, and should result in lower uncertainty. 

4.8.2 Bracketing Data Processing

Determining the certified output of the blending device starts with correcting each response for 

any zero offset of the analyzer. For each test day, calculate the interpolated zero offset based on 

the time when a specific response was recorded as follows: 

 ( ) ( )

( ) 











−

−

= + −

1 2

1 2

1 1

t t

Z Z

Z Z t t

i i

 (Equation 4) 

Where: Zi = Calculated zero offset at ti

 Z1 = Initial zero response 

 Z2 = Final zero response 

 ti = Time of response to be zero corrected 

 t1 = Time initial zero response was recorded 

 t2 = Time final zero response was recorded 

 

Then, correct the response at ti for the zero offset using the following equation: 

Yci = Yi − Zi (Equation 4) 

Where: Yci = Response at ti corrected for zero offset 

 Yi = Response at ti as recorded during bracketing

 Zi = Zero offset at ti

The data set in Table 7 provides an example of how each response is corrected for zero offset 

over the course of a test period. 

Table 7: Zero Corrections for an Example Data Set 

Time 

(min) 

Zero 

Response 

Interpolated 

Zero 

RSi

Response 

RSci

Zero-Cor.

Ci 

Response

Cci 

Zero-Cor. 

0 0.00 

10 0.04 10.00 9.96 

20 0.08 9.85 9.77 

30 0.12 10.20 10.08 

40 0.16 9.96 9.80 

50 0.20 10.25 10.05 

60 0.24 10.00 9.76 

70 0.28 10.40 10.12 

80 0.32 
- 32 - 

Next, for each zero-corrected value of CCi, calculate the output ratio Ri, using the formula shown 

in Equation 5 for the first candidate injection response. 

 



































 +

=

2

1 2

1

1

C C

C

RS RS

C

R (Equation 5) 

 

 Where: R1 = Output ratio for first candidate device injection 

 CC1 = First response to candidate device, zero corrected 

 RSC1 = First response to reference gas standard, zero corrected 

 RSC2 = Second response to reference gas standard, zero corrected 

Table 8 illustrates these calculations for the example data set presented in Table 7. 

Table 8: Example Calculation of Output Ratio 

Time 

(min) 

RSi

Response 

RSci

Zero-Cor. 

Ci 

Response 

Cci 

Zero-Cor. 

Output Ratio 

(Ri) 

10 10.00 9.96 

20 9.85 9.77 0.975 

30 10.20 10.08 

40 9.95 9.80 0.974 

50 10.25 10.05 

60 10.00 9.76 0.968 

70 10.40 10.12 

For each of J ≥ 3 concentrations on each of K ≥ 2 days, calculate the average ratio Rjk and the 

relative standard deviation (RSD) of the individual output ratio measurements. 

∑=

=

I

i

i

jk I

R

R

1

 

( )

jk

i jk

I

i

Rjk I R

R R

U RSD 100%

( )1

2

1 ×

−

Σ −

= =

=

 Where: RSD = Relative standard deviation, percent 

 Rjk = Arithmetic average of the output ratios 

 Ri = Individual, “ith” output ratio 

 I = Number of output ratio measurements 

Note: The RSD is also the bracketing “repeatability” uncertainty, which will be later combined 

with other components to calculate the overall uncertainty of the output concentration. 

The RSD must not exceed 1.0%. If the RSD value is exceeded, the test at that concentration is 

invalid and must be repeated. The calculated RSD value for the output ratios in Table 8 is 0.4%, 

and is therefore acceptable. 
- 33 - 

Finally, the output concentration is calculated by averaging the Rjk ratios at each concentration 

and multiplying those averages by the corresponding concentration of the certified reference gas. 

∑=

=

K

k

jk

j

K

R

R

1

 Cj = Rj ×Cjref

Where: Rj = Overall average ratio for concentration j bracketing 

Rjk = Average ratio measured at concentration j on day k

 K = Number of days of bracketing experiments 

 Cj = Certified concentration from blending device

Cjref = Certified concentration of reference gas used during 

bracketing at concentration j

4.8.3 Concentration Uncertainty from Bracketing

When using bracketing to certify calibration gases, the calculation of uncertainty is complicated 

by the fact that bracketing is an inherently indirect comparison (i.e., the analyzer cannot measure 

both outputs at the same time, so it must function as an intermediate). Appendix A of this 

document (which is based on another protocol that uses bracketing1) shows all of the uncertainty 

components and how they are combined to calculate overall concentration uncertainty.

4.8.4 Operational Monitoring Requirement

As a condition of using this alternative approach to certification, the operational parameters of 

the device must be continuously monitored for shifts that could impact the output concentration. 

These operational parameters include, minimally, all gas and liquid flows, each of which must 

remain within ±2.0% of the value recorded during certification. Because the bracketing 

certification values are related specifically to the original operating conditions, any shift in those

operating conditions outside of the ±2.0% allowance triggers the need to recertify and reverify 

the device output. 

5.0 VERIFICATION 

To verify the output of the device, perform a direct comparison of the output to compressed gas 

cylinders containing HCl with NIST-traceable concentrations. The comparison measurements 

must be done by an HCl measurement system or detector that is subject to regular QA/QC 

activities to verify its stable operation. 

5.1 Comparison Concentrations

This verification will be done at two different output concentrations from among those routinely 

used by the device in operation. One of the two concentrations must have the blending device 

operating at its most-used operating condition, which for CEMS applications will likely be the 

concentration used for daily calibration error checks. One of the bottled HCl gas standards needs 
- 34 - 

to be within ± 30% of that concentration. The other gas standard may be higher or lower, but 

must have a concentration between 40% and 250% of the first bottle in order to assess the 

response linearity of the HCl measurement system. Both bottled HCl gas standards must have an 

uncertainty of ±2.0% or better. 

5.2 Verification Setup

Assemble a manifold for the HCl measurement system inlet that allows easy switching between 

the gas/vapor blending device and the compressed HCl gas standards. This manifold design must 

ensure that 

• There are no changes in manifold pressure or flow to the detector when switching valves. 

• Output flow from the gas vapor blending device is unrestricted, even when the manifold 

valve from that device is closed. 

• There is no leakage through the valves when they are closed, even if the bottled gases exert 

pressure against their valves. 

• Materials of construction are inert to HCl gas. 

The manifold may also include a valve for zero gas, and may use solenoid valves for automated 

switching. 

5.3 Stepwise Verification Procedure

(a) Inject zero gas to the HCl measurement system, and make necessary adjustments to bring 

the response to zero. 

(b) With the measurement system connected to the switching manifold, inject the higher of 

the two compressed gases, and make necessary adjustments to make the response match 

the certified concentration. 

(c) Inject the other compressed gas, and check that the response matches the certified 

concentration within ±2.0% of the higher gas value. If this tolerance is exceeded, take 

corrective action and repeat from step (a). 

(d) Switch the manifold to the gas/vapor blending device operating at its most used operating 

condition. Allow at least one minute for the response to begin changing, and wait for it to 

stabilize at the new value. Record the average stable response. 

(e) Switch back to the bottled gas whose concentration is closest to the blending device 

concentration. Allow at least one minute for the response to begin changing, then wait for 

a stable response before recording the average value. 

(f) Continue alternating between the bottled gas and the blending device until at least three 

blending device responses have been recorded, each one immediately preceded and 

followed by a bottled gas response (i.e., a minimum of four bottled gas responses). 
- 35 - 

(g) For each blending device response average, divide this response by the average of the 

two adjacent bottled gas responses (i.e., before and after) and to get a response ratio. 

( ) m m 2

m

Bottle-Before Bottle-After

Device

+

Ri =

(h) After calculating all of these ratios, calculate the relative standard deviation of the ratios. 

( )

100%

R

1

 R

2

1

×

−

Σ −

=

=

I

R

RSD

i

I

i

 Where: 

 RSD ≡ Relative standard deviation, percent 

 R ≡ Arithmetic average of the output ratios 

 Ri ≡ Individual output ratio 

 I ≡ Number of output ratios (≥ 3) 

(i) The RSD must not exceed 1.0%. If the RSD value is exceeded, the test is invalid and 

must be repeated. 

(j) Use the average response ratio to calculate a verification value 

CVerification = R ×CBottle

(k) Switch to the other bottled gas, and record a stable response as before. 

(l) Select the routinely-used device operating condition that produces an output closest to the 

bottle concentration, and repeat the procedure from step (d) to get a verification value. 

5.4 Verification Tolerance

For each of the two verified points, calculate the verification tolerance as the combined 

uncertainty of the device output and the verification value (ignoring the uncertainty of the 

response ratio, which should be small). 

2 2

Tolerance = 2× UCwet + R ×UBottle

Where UCwet is the device output uncertainty calculated in section 4.7.1 or 4.7.2, and UBottle is the 

bottle concentration uncertainty (divided by its coverage factor). If the difference between the 

device’s certified output concentration and the verification value exceeds this tolerance, take 

corrective action and repeat the verification. 
- 36 - 

6.0 QUALITY ASSURANCE 

This section specifies: (1) the periodic data quality assessments that are required following initial 

certification of an HCl gas/vapor blending device, to provide assurance of continued acceptable 

performance; and (2) the required frequency of device recertification. 

6.1 Periodic Data Quality Assessments 

Periodic performance evaluations of each blending device shall be conducted at the following 

frequency: 

(a) At least once each calendar quarter; and 

(b) Following any malfunction, repair, or corrective action that may reasonably be expected 

to affect the device’s output, but does not necessitate recertification of the device (see 

Section 6.2, below). 

Any of the data quality assessment procedures (Options A through C) described in Sections 6.1.1 

through 6.1.3 below, may be used for the performance evaluations. 

If the HCl gas/vapor blending device does not meet the acceptable performance criteria, suspend 

the device certification until either: 

(a) Another performance evaluation is successfully completed, or 

(b) The device is recertified in accordance with the procedures in Section 4, and the output is 

verified by the procedure in Section 5. 

6.1.1 Option A: Single-Point Verification of Inputs 

For periodic performance evaluations that are not precipitated by a device malfunction, the 

evaluation may take the form of a simple check of the input measurements that are used to 

calculated the certified output. Where a device malfunction has occurred, this option is only to be 

used when the malfunction was confined to one of the measurement components (i.e., gas flow 

or liquid flow), and that component has been recertified (or replaced and recertified) using the 

process in Section 4.5 or 4.6. 

This check will be done with the device operating at the condition that is used most often by the 

measurement system(s) to which the blending device is connected. For CEMS applications, this 

will likely be the HCl concentration that is used for daily calibration error checks. 

(a) For bottled HCl gas humidifiers, measure each of the gas and liquid flow rates with ±1% 

NIST-traceable flow standards. 

 
- 37 - 

(b) For evaporative HCl gas standards, perform the checks in (a) above, then collect a sample 

of the HCl feed solution from the input reservoir, and have the sample analyzed for HCl 

concentration by an independent laboratory. 

The acceptable performance criterion for each of the measurements is a difference ≤ 2.0% of the 

reference value. 

6.1.2 Option B: Operational Envelope Linearity Verification 

For devices where the liquid and gas flows can be independently set and maintained without 

substantially limiting each other’s range, acceptable performance may be verified by exercising 

the blending device near the boundaries of its operational envelope (i.e., the maximum and 

minimum liquid and gas flows recommended by the manufacturer). The evaluation metric will 

be the linearity of the output HCl, as determined by the connected measurement system, its 

detector (i.e., without the sample conditioning and delivery system), or another HCl detector that 

is subject to regular QA/QC activities to verify its response linearity. 

Like Option A above, this option is not to be used when the performance evaluation is 

precipitated by a device malfunction, unless the malfunction is confined to one of the 

measurement components (i.e., gas flow or liquid flow), and that component has been recertified 

(or replaced and recertified) using the process in Section 4.5 or 4.6. 

For this check, define an operating envelope consisting of the highest and lowest settings that 

each flow measurement can maintain without imposing limits on the range of the other flow 

measurement(s) beyond the manufacturer’s specifications. 

(a) With the liquid flow at its highest setting within the operating envelope, operate the 

blending device such that the gas flows at its highest, lowest, and at least one 

intermediate setting (nominally equally spaced between the highest and lowest settings), 

all within the operating envelope, while the liquid flow remains constant. 

(b) Allowing the HCl detector response to stabilize at each condition, record the average 

response and the reported HCl concentration from the blending device at each operating 

condition, resulting in at least three pairs of data points. 

(c) Calculate a best-fit line for the data points collected in step (b). If any point is more than 

3% above or below the line, take corrective action and repeat the check. 

(d) Adjust the blending device so that the liquid flow is at its lowest setting within the 

operating envelope, and repeat steps (a) through (c) at this setting. 

(e) If the device uses a second gas flow, repeat steps (a) through (d) for that measurement. 

(f) Choose one or more intermediate liquid flow rates, and operate the blending device such 

that the gas flow is at its highest and lowest setting within the operating envelope. 
- 38 - 

(g) Similar to before, allow the HCl detector response to stabilize at each condition, and 

record the average response. Combine these data points with the highest and lowest 

liquid and gas flow points from step (b), so there are at least three highest gas flow 

conditions, and three lowest gas flow conditions. 

(h) Calculate two best-fit lines from the step (g) data points, each representing the reported 

HCl versus measured HCl at a constant gas flow and variable liquid flow (one for all the 

highest gas flow points, and one for all the lowest gas flow points), where the blending 

device output varies solely due to differences in the liquid flow. If any point is more than 

3% above or below the line, take corrective action and repeat the check. 

(i) If the device uses a second liquid flow (e.g., to control moisture independently of HCl), 

repeat steps (f) through (h) for that measurement, but include the highest and lowest 

liquid flows in step (f) so that the check includes the full range of liquid flows. 

(j) For evaporative HCl gas standards, perform all of the checks above, then collect a sample 

of the HCl feed solution from the input reservoir, and have the sample analyzed for HCl 

concentration by an independent laboratory. 

6.1.3 Option C: Comparison to a Dry Bottled HCl Gas Standard 

Evaluate the performance of a gas/vapor blending device by direct comparison to a compressed 

gas cylinder containing HCl with a NIST-traceable concentration. Like Option B above, the 

comparison measurements may be done by the connected measurement system, its detector (i.e., 

without the sample conditioning and delivery system), or another HCl detector that is subject to 

regular QA/QC activities to verify its response linearity. 

Similar to Option A above, this check will be done with the blending device operating at its 

most-used operating condition, which for CEMS applications will likely be the concentration 

used for daily calibration error checks. In order to have an effective comparison, the bottled HCl 

gas standard needs to be within ± 30% of that concentration, and have an uncertainty ≤ ±2.0%. 

(a) Connect the blending device and the bottled gas to a manifold similar to the one used for 

verification in section 5. Connect this manifold to the measurement system or detector in 

a way that does not restrict the output flow or allow air in-leakage. 

(b) Feed bottled gas through the manifold to the measurement system/detector. Allow the 

response to stabilize and record the average stable response. 

(c) Switch the manifold to the gas/vapor blending device. Allow at least one minute for the 

response to begin changing, and wait for it to stabilize at the new value. Record the 

average stable response. 

(d) Continue alternating between the bottled gas and the blending device until at least three 

blending device responses have been recorded, each one immediately preceded and 

followed by a bottled gas response (i.e., a minimum of four bottled gas responses). 
- 39 - 

(e) For each blending device response average, divide this response by the average of the 

two adjacent bottled gas responses (i.e., before and after) and to get a response ratio. 

( ) m m 2

m

Bottle-Before Bottle-After

Device

+

Ri =

(f) After calculating all of these ratios, calculate the relative standard deviation of the ratios. 

( )

100%

R

1

 R

2

1

×

−

Σ −

=

=

I

R

RSD

i

I

i

 Where: 

 RSD ≡ Relative standard deviation, percent 

 R ≡ Arithmetic average of the output ratios 

 Ri ≡ Individual output ratio 

 I ≡ Number of output ratios (≥ 3) 

(g) The RSD must not exceed 1.0%. If the RSD value is exceeded, the test is invalid and 

must be repeated. 

(h) Multiply the average ratio by the certified HCl concentration of the bottled gas to get the 

reference concentration. If the reported concentration from the blending device software 

differs from this reference concentration by more than the concentration uncertainty limit 

from the applicable regulation (or 5.0%, whichever is less), take corrective action and 

repeat the test. 

6.1.4 Option D: Comparison to Another Certified HCl Gas/Vapor Blending Device 

This option is functionally similar to Option C, except instead of a bottled gas, a field reference 

blending device is used. In order to qualify for field reference applications, a device must be 

annually recertified to an uncertainty of ≤ ±2.0%, and undergo the same quarterly quality 

assurance checks as an in-service device connected to a measurement system. 

6.2 Recertification 

Following initial certification, all HCl gas/vapor blending devices must be periodically 

recertified to maintain NIST traceability. At a minimum, devices shall be recertified at least 

once every 8 calendar quarters in accordance with the procedures in Sections 4 and 5. Devices 

shall also be recertified whenever (1) any event, condition, or factor that triggers recertification 

(as identified in the Manufacturer’s Disclosure) occurs, or (2) device performance is suspected to 

be the cause of a CEMS RATA failure. 
- 40 - 

6.2.1 Full Recertification 

Any recertification that involves full implementation of the procedures in Sections 4 and 5 is a 

full recertification, which resets the “once every 8 calendar quarters” interval. 

6.2.2 Cumulative Certification 

If, during the first day of flow comparisons for the recertification procedure (see Sections 4.5.1 

and 4.6.1), it becomes clear that the HCl gas/vapor blending device performance compares very 

well to the most recent certification, there is an option to simply incorporate the day’s data with 

the data from the previous comparisons for purposes of calculating reproducibility. For a very 

stable device, this could substantially improve the overall uncertainty, and reduce the level of 

effort to maintain certification. If, however, using the old comparison data causes excess 

reproducibility uncertainty, that data may be abandoned in favor of a full recertification with an 

additional day (or more) of comparison tests. Regardless of whether a cumulative or full 

recertification is performed, the process must include the Section 5 verification process. 

6.2.3 Partial Recertification 

If a recertification is triggered by a malfunction or other event that is identified in the 

Manufacturer’s Disclosure, and the cause is confined to one specific measurement component, a 

partial recertification may be performed. In this type of recertification, the Section 4 procedures 

are only performed on the affected measurement component, and the traceable concentrations 

from the previous certification are recalculated with the new data substituted. This partial 

recertification does not change the expiration of the earlier certification (i.e., no reseting of the 

“once every 8 calendar quarters” interval), and it does require a Section 5 verification process. 

7.0 REFERENCES

1. Companion Document, “Treatment of Elemental Mercury Gas Generator Certification 

Data and Calculation of Uncertainty in Support of the Interim EPA Traceability Protocol 

for Qualification and Certification of Elemental Mercury Gas Generators”; U.S. EPA; 

July 2009. 
- 41 - 

Appendix A – Concentration Uncertainty from Bracketing Alternative 

When assigning concentration values to blending device operating conditions by bracketing, five 

sources of uncertainty have been identified: 

 

 Calibration Linearity 

 Measurement Stability 

 Repeatability 

 Reproducibility 

 Reference Uncertainty 

The first three uncertainty components apply specifically to a bracketing procedure. The other 

two components apply to the overall process of transferring traceability from the reference 

gas(es) to the candidate. 

A.1 Calibration Linearity 

[Reserved] 

A.2 Measurement Stability 

The measurement stability uncertainty calculation applies a statistical test to the assumptions that 

are built into the bracketing technique (e.g., no non-linear drift, wavering responses, or 

excessively “noisy” measurements). This uncertainty component is quantified in terms of the 

residual error estimate for these two lines: 

( ) [ ( )( )]

( )

( )2 Cand

2

Cand Cand

2

2 Cand Cand Cand Cand

Cand Cand

Cand

−

















−

=

∑

∑ ∑ −

− −

−

n

m m

m m

t t

t t

SE

( ) [ ( )( )]

( )

( )2 Ref

2

Ref Ref

2

2 Ref Ref Ref Ref

Ref Ref

Ref

−

















−

=

∑

∑ ∑ −

− −

−

n

m m

m m

t t

t t

SE

where Cand t and Ref t are the timestamps associated with the candidate and reference 

measurements. The t units and the t = 0 origin can be anything, as long as they are consistent 

within each standard error calculation. The bold m and n values refer to all of the points of the 

regression lines. The standard errors for these lines are used to calculate the standard error of the 

mean of each individual measurement interval. 
- 42 - 

( )

Ref -B

Ref

Ref -B

n

m

SE SE = ( )

Cand

Cand

Cand

n

m

SE SE = ( )

Ref -A

Ref

Ref -A

n

m

SE SE =

where m is the mean for just that measurement interval, and n is the number of individual 

measurements in that interval. The subscript notations (i.e., Ref-B, Cand, and Ref-A) relate to the 

before-candidate reference measurement, the candidate measurement, and the after-candidate

reference measurement (respectively). For each ratio calculation, these standard errors are used 

to calculate a combined stability uncertainty for this ratio 

Ref -B Ref -A

Cand

m m

m

Ref A Ref -B

Cand Ref -B

Ref A Ref -B

Ref -A Cand

⋅ ⋅

=

−

−

+

−

−

− −

t t

t t

t t

t t

Ri

( ) ( ) ( ) ( )

2

Ref-A

Ref-A

2

Cand

Cand

2

Ref-B

Ref-B

1

m

m

m

m

m

m

Cand Ref -B

Ref -A Ref -B

Ref A Cand

Ref -A Ref -B

























×

+ 















+

























×

= ×

−

−

−

−

−

t t

t t

SE SE

t t

t t

SE

u σ Ri stability Ri 

where Ri is the measured ratio for one of I ≥ 1 individual brackets performed at concentration j

on day k (out of K ≥ 1 days). 

Measurement stability is one of three uncertainty components (the first being detector linearity) 

that relate only to a single bracketing procedure, with average ratio 

I

R

R

I

i

i

jk

∑= =

1

and measurement stability uncertainty 

( ) ( )i stability

I

i

stability jk u R

I

u R

2

1

1 2

1

=

σ = Σ

where I is the number of brackets in this th j set. 

Note: Whenever a “jk” subscript appears in this appendix, the term represents an average for 

the jth concentration on the kth bracketing day. When the “j” subscript appears alone, the term 

represents the average for the jth concentration, inclusive of all test days. 

A.3 Repeatability 

The most straightforward way to calculate repeatability is a simple standard deviation of the 

individual bracket ratios. 
- 43 - 

( )

( )

( )1

1

2

−

∑

=

=

−

I

R R

s R

I

i

i jk

i

This approach, however, creates the possibility of “double-counting” where measurement 

instability contributes to imprecision. To allow for this, the experiments are structured as a 3-

level nested design (see chapter 2 of NIST’s “Engineering Statistical Handbook” 

http://www.itl.nist.gov/div898/handbook/mpc/section4/mpc433.htm), as follows: 

Level 1: Individual ratio uncertainty 

Level 2: Repeatability within set (sometimes called “within day” uncertainty) 

Level 3: Set reproducibility (sometimes called “between day” uncertainty) 

In a nested experimental design, standard errors are pooled at each level, and the level-specific 

contributions to uncertainty are computed from these pooled standard errors. Starting with the 

pooled level-1 standard error 

( )i stability

I

i

R u R

I

s = ∑

=1

2

1

1

 (subscript R denotes a Ratio uncertainty component) 

and the level-2 standard deviation 

( )

( )1

1

2

2

−

∑

=

=

−

I

R R

s

I

i

i jk

R

The repeatability uncertainty of the ratio measurements is computed using 

( )

( )





















×

∑

−

−

∑

= 











= − ×

=













=

−

I L

R

I

R R

s Max

L

u R Max s

I

i

stability jk

I

i

i jk

i R R

1

2

1

2

2

1

2

1 repeatability 2

u

( 1) ,

0

1

 , 

0 σ

where L represents the average number of individual detector readings used in each 

measurement average and ratio determination 

= ∑( ) + +

=

I

i

i i i

I

L

1

Ref -B, Cand, Ref -A, n n n

1

The uncertainty of the set-average ratio is 

( ) ( )





















×

∑

−

−

∑ −

= = ×

=













=













I L

R

I

R R

Max

I

u R

I

u R

I

i

stability jk

I

i

i jk

jk i

1

2

1

2

1 repeatability 1 repeatability

u

( 1) ,

0

1 1

σ σ
- 44 - 

The “Max” function holds this term at zero when short-term precision uncertainty is dominated 

by measurement instability. 

A.4 Reproducibility 

Reproducibility is an uncertainty term applied to the calculated candidate concentrations 

(Cjk = Rjk ×Cjkref ). It may be estimated differently depending on what type of device is being 

certified or how many times the bracketing procedure is repeated. 

A.4.1 Statistical Approach 

A 3-level nested design2 assumes that the ratio measurement is done I > 1 times on each of K > 1 

days. Using the same technique as for the repeatability uncertainty 

( ) 











= − ×

2

2

2

3 reproducibility 1

1

,

jk 0 C C

s

K

u C Max s σ

where 

× ∑ −

−

=

=













K

k

C Cjk Cj

K

s

1

2

3

1

1

 (subscript C denotes Concentration uncertainty) 

K

C

C

K

k

jk

j

∑

=

=1

and Rs2 values (from A.3) are converted to Cjk errors and pooled as follows 

× ∑[ − × ( ) × ]

Σ −

=

=

K

k

k jk ref R

k

C

I C s

I K

s

1

2

2 , 2

( )1

1

As with the repeatability uncertainty, the “Max” function holds this term at zero when the 

deviation among sets is dominated by short-term effects. The reproducibility uncertainty of the 

mean candidate concentration is 

( ) 











= − ×

2

2

2

3 reproducibility 1

1

,

0

1

j C C

s

K

Max s

K

u σ C

A.4.2 Bound on Bias (BOB) Approach 

When the number of bracketing data sets are few (≤5 sets), a shorthand approach, described in 

detail elsewhere3, can be used to compute the reproducibility standard error. This is necessary 

because small sample sizes don’t provide a lot of information about population distribution. The 

BOB approach assumes a uniform distribution (as opposed to a normal distribution), and sets the 

bounds at the highest and lowest Cjk values among the data at each concentration. The estimated 

standard deviation for a uniform distribution is 
- 45 - 

3

a

suniform =

where a is half the width of the distribution. Applying this formula to all the bracketing data 

yields 

( )

12

( ) ( )

2 3

( ) ( )

reproducibility 1

jk jk jk jk

j

Max C Min C Max C Min C

u C

−

=

×

−

σ =

A.5 Combined Uncertainty 

Once calculated, all of the uncertainties are combined using propagation of error. First, the 

combined uncertainty of each day’s bracketing procedure is calculated 

( ) ( ) ( ) ( )

repeatability

2

stability 1

2

1

2

linearity 1

Ref

Cand

1

0 jk jk

assumed

default jk jk

jk

u R u R u R u R

C

C

u

σ σ

σ σ + +













= = ≈ 















Then, these uncertainties are combined into a single comparison uncertainty 

( ) ∑[ ] ( )

=

= ×

K

k

jk ref jk comparison j C u R

K

u C

1

2

1 2 , 1

1

σ σ

The reference gas uncertainty component is calculated based on the comparison ratio and the 

reported uncertainty of the reference gases. 

( ) = × ∑[ ] × ( )

=

K

k

j jk Cjk ref R u

K

u C

1

2

certification 1 ,reference 1

1

σ σ

Note: In cases where there is no change of reference gas from one bracketing day to the next, the 

above pooling formula reduces to the quadratic mean of the day-average comparison ratios 

times the reference gas uncertainty. 

The overall combined uncertainty for the candidate gas certification is 

( ) ( ) ( ) ( )

reproducibility

2

1 reference

2

1 comparison

2

1 1 j j j combined

u σ Cj = u σ C + u σ C + u σ C 

and the expanded certification uncertainty (k=2) is

( ) ( ) ( )

combined j combined j anded

u σ Cjk u1σ C u1σ C

exp 1 = × = 2×