- 1 - 

# 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- 2 -

1.0 INTRODUCTION

For regulatory purposes, hydrochloric acid (HCl) ca libration gas standards of known concentration and known uncertainty are needed to q uality-assure data recorded by HCl continuous emission monitoring systems (CEMS) and m easurements made with instrumental HCl reference test methods. This EPA traceability protocol reflects the current state of development of procedures for quantifying hydrochlo ric 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 k ey components of HCl gas/vapor blending devices; (2) determining the uncertainty contribute d by each of these components; (3) calculating certified output concentrations and expanded, combi ned uncertainty values for the gas standards produced by the devices, and (4) verifying the outp ut concentrations of the evaporative through comparison to a NIST traceable gaseous HCl referenc e standard. This protocol assumes a minimum “target” expanded, combined uncertainty val ue of 5% for HCl gas/vapor blending devices (see Section 4.7) unless otherwise specifie d 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 trac eable to NIST standards by means currently acceptable to EPA.

Section 2, “Overview,” discusses the different appr oaches to delivering HCl gas standards, explains the certification process for gas/vapor bl ending devices, and describes the two types of blending devices that are used for HCl gas standard s. Section 3, “Qualification,” contains provisions designed to demonstrate the adequacy of a particular model prior to performing the actual certification tests. Section 4, “Certificat ion,” explains specific procedures for quantitatively determining certified output concent rations from each type of device and for calculating and reporting the expanded, combined un certainties of the HCl output concentrations. Section 5, “Verification,” shows how the certified device output is verified based on measurements of the actual output concentration. Se ction 6, “Quality Assurance,” details procedures for periodic data quality assessments, w hich provide assurance of continued acceptable HCl gas/vapor blending device performanc e.
The technical specifications and requirements for H Cl 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 issu es may be addressed during formal rulemaking processes or by the regulatory agency re sponsible for implementing the standards, and are not addressed in this or other EPA traceabi lity 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.- 3 -

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 concentrat ion, (2) a device that quantitatively blends a bottled dry gas with water vapor or humidified zero gas, and (3) an evaporative HCl standard, which produces gas standards of known concentration by vaporization of aqueous solutions of HCl and quantitatively blending the resultant vapor wit h 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 chall enges 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 de sign, 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 requiremen ts. In order for the output concentration of a device to be NIST-traceable, however, all of the in puts must be calibrated using NIST-traceable standards, and the uncertainties of these inputs mu st be combined to calculate the uncertainty of the resulting gas standard.

Qualification tests are required for any blending d evice that is designed to produce NISTtraceable gas standards. For the purposes of this p rotocol, 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 operat e reliably over a stated range of conditions (e.g., line voltages, back pressures, ambient tempe ratures, delivery back pressures, and other applicable factors). These tests may either be perf ormed on each individual device or on selected devices that are representative of the model. The m anufacturer 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 c an be expected.

This EPA Traceability Protocol specifies the proced ures for establishing the NIST traceable quantitative output of HCl gas/vapor blending devic es. 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 manufactur er or equipment supplier performing the certifications must maintain documentation of all c ertifications. 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 regulat ory officials.

The final step in the process of establishing the a cceptability of a blending device as a calibration standard is to verify that the measured output conc entration 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 o utlet undeterred and unaltered; it is a pass/fail test which does not result in any change to the cer tified output concentrations.- 4 -

Following the initial certification of a blending d evice, 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 as surance (QA) checks and/or quality control (QC) procedures are required to verify that the dev ice continues to provide accurate and reliable calibration standards. Recertification of a device is required periodically to maintain NIST traceability. Recertification may also become neces sary 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 HC l gas standards through calibration lines and sample lines (i.e., umbilical) and through other sy stem 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 severa l 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 st ream 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 f low 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 co ncentration and uncertainty. Liquid feed rates are established by using a balance to gravimetrically c alibrate the feed rate meter relative to NISTtraceable mass standards. The HCl gas standard flo w 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-tr aceable calibration (see Sections 4.2 and 4.7.1).

2.2 Evaporative HCl Standard

In an evaporative HCl standard, a liquid stream con taining a known concentration of HCl is metered and then dynamically mixed with a known flo w rate of a carrier gas. The liquid feed is nebulized into small droplets, evaporated in a heat ed 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. Thi s principal of operation assumes that all of the HCl in the feed solution is emitted from the evapor ator without re-condensing or reacting chemically with any of the device components or oth er feed streams. The output HCl concentration is calculated from the measured input s: the HCl solution concentration, the liquid feed rate(s), and the carrier gas flow rate (see Se ction 4.2).

Certification of evaporative HCl standards requires NIST-traceability for the inputs listed above. The HCl concentration of the working liquid feed so lution is established through the use of commercially available NIST-traceable HCl liquid st andards and, if necessary, the quantitative dilution of these standards. Like the simpler blen ding devices described in Section 2.1 above, liquid feed rates are established by using a balanc e to gravimetrically calibrate the feed rate meter relative to NIST-traceable mass standards, an d the carrier gas flow rate is established by- 5 -

calibrating the gas flow rate meter relative to a p rimary laboratory standard that has a NISTtraceable calibration.

2.3 Uncertainty

In addition to the mass balance calculations that e stablish 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 conc entration 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. Exp anded uncertainty is simply the combined uncertainty multiplied by a coverage factor (k=2) t o 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 cer tified concentration(s). Information about the limitations of the device must be provided to the e nd user by means of a “Manufacturers Disclosure” (see Section 3.2) defining the limits o f 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. Documentat ion of the qualification test procedures and results must be available upon request for inspecti on 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/contr ol devices, evaporation chamber, temperature controllers, temperature or pressure me asurement and compensation devices, and other components;

(b) The physical arrangement of principal components;

(c) The specific electronics configuration and signal p rocessing; and

(d) The specific software/firmware version and data pro cessing algorithms, as implemented by a particular manufacturer and subject to an iden tifiable quality assurance system.- 6 -

Note: The following changes to a gas/vapor blending devic e do not constitute a model change. However, the manufacturer must document all such ch anges:

• Minor changes to software, firmware, or data output s 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 referen ce 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 b lending device that provides for the internal control of sp ecific components or adjustment of the device output in a manner consistent with the manuf acturer's design and its intended operation. Intrinsic adjustments are either non-el ective or are configured according to factory recommended procedures; they are not subjec t 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 performa nce, 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 adjustmen ts are made at the election of the end user but may be subject to various regulatory requi rements.

3.2 Manufacturer’s Disclosure

As part of each gas/vapor blending device’s certifi cation, a “Manufacturer’s Disclosure” is required. This disclosure must include documentatio n and other relevant information for the blending device model, in accordance with subsectio ns 3.2.1 through 3.2.5, below. This information shall be provided to each end-user in t he 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 compensatio n 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 concentr ation range(s) (minimum and maximum HCl concentrations in units of ppm) over which the blending device model has been- 7 -

demonstrated to meet all of the manufacturer’s spec ifications. The description of the range should explain the parameter values that must be ch anged 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 operati onal range or limitations within which the device model can be expected to operate reliably an d meet all of the manufacturer’s specifications for:

3.2.3.1 Line Voltage

The acceptable line voltage limits over which the d evice can provide reliable HCl gas concentrations.

3.2.3.2 Gas Supply

The minimum and maximum supply pressure of air, nit rogen, or dry HCl gas standard, the allowable variation in supply pressure, the require d 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 th ermal 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 h as been tested, and can deliver reliable HCl gas concentrations.

3.2.4 Operation, Maintenance, and QA

The manufacturer shall provide written procedures f or 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 s pecific conditions and factors that require recertification of the device, including, as applic able, known malfunctions or failures, exposure- 8 -

to unacceptable operating conditions, component rep lacements, component recalibrations, or other conditions and/or factors.

3.3 Qualification Tests

Manufacturers shall perform and pass a series of st andardized qualification tests for each HCl gas/vapor blending device model. These tests will a ssess the repeatability of the delivery concentration, and evaluate the effect of changes i n 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 devi ce. The HCl detector need not be precisely calibrated with NIST-traceable gases, but must be c apable of drift-free operation and generate a continuous, linear signal proportional to the HCl c oncentration.

3.3.1 Repeatability Test

Prior to all other qualification tests, the repeata bility of the gas/vapor blending device must be assessed over the range of possible delivery concen trations and operating conditions. Table 1 shows an example list of test conditions, represent ing 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/vap or blending device and the HCl measurement system must be operational and stabilized in a cont rolled 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 displayin g the expected concentration.

(b) Continue feeding until the HCl measurement response stabilizes and an average response is recorded. Proceed to the next test condition, an d repeat until a response is recorded for each condition.

(c) After the first set of responses is recorded, repea t from the beginning to record a second set of responses. For each matched pair of response s ( 1 x , 2 x ), calculate a relative percent difference.- 9 -



## ( ) %

100 2 2 1 2 1 × + −
= 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 t est for the full set of conditions.

3.3.2 Line Voltage Test

Since HCl measurement equipment often operates in a n industrial environment which can be prone to electricity voltage sags and surges, the o peration of gas/vapor blending devices must be verified over a range of supply voltages. These tes ts may be conducted under controlled laboratory conditions using a simple variable trans former (e.g., Variac®) to adjust the line voltage up and down from its nominal value. Like th e repeatability test, the output concentration of the gas/vapor blending device will be measured w ith a suitable detector during the test. The high and low voltage test conditions may be set bas ed on the manufacturer’s disclosure, but must include the range shown in Table 2. If the device i s 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 conditi ons at the high and low limits of each flow measurement (gas and liquid) that is incorporated i nto the device.

(a) Plug the device into the variable transformer set t o 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 displayin g 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 lowes t voltage corresponding to the nominal voltage in Table 2. Allow at least 5 minutes for an y shifted response to stabilize, and record an average response for this voltage.

(e) Slowly adjust the variable transformer to the highe st voltage corresponding to the nominal voltage in Table 2. Allow at least 5 minute s for any shifted response to stabilize, and record an average response for this voltage.- 10 -

(f) Using the highest and lowest response averages ( highest x and lowest x , from among the three recorded responses), calculate a relative percent d ifference.



# ( ) %

100 2 × + −
=
lowest highest lowest highest

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 voltag e settings in any order, making sure to allow at least 5 minutes for responses to shift bef ore recording the response. Calculate a RPD for this condition.

(h) Once all operating conditions have been tested, rev iew 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 con trolled 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 dis closure. 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 condition s must be set based on the manufacturer’s disclosure, if a range is given. If the manufacture r 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 inpu t 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 specificatio ns, 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 sa me settings that were used for the repeatability test, or a subset thereof, but must i nclude conditions at the high and low limits of each flow measurement (gas and liquid) that is inco rporated 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 th at 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.- 11 -

(b) Use the blending device’s user interface to set one of the test conditions and activate. Verify that the device is calculating and displayin g 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 su pply pressure. Allow at least 5 minutes for any shifted response to stabilize, and record a n average response for this supply pressure.

(e) Using the response averages for the high and low pr essure settings ( highP x , lowP x ), calculate a relative percent difference.



# ( ) %

100 2 × + −

=
lowP highP lowP highP

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 respons es to shift, if they do.

(g) Once all operating conditions have been tested, rev iew 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 environmenta l chamber or enclosure large enough to contain all components of the gas/vapor blending de vice, with penetrations for electrical and tubing connections. This chamber must be able to ma intain temperatures at the high and low extremes listed on the manufacturer’s disclosure. S imilar to other tests, the output concentration of the gas/vapor blending device will be measured w ith a suitable detector or measurement system located in a climate controlled laboratory s pace or control room outside of the environmental chamber. For this test, because it ta kes 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 b e 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 thereo f, but must include conditions at the high and low limits of each flow measurement (gas and liquid) th at is incorporated into the device. Ideally, these test conditions will be pre-programmed into t he device, or activated remotely, in order to minimize intrusions into the environmental chamber during the test.

(a) Place the device into the environmental chamber, ro uting the HCl gas output and all power and signal wires outside the chamber as neces sary. Set the chamber to a- 12 -

representative ambient temperature (e.g., 20-25°C). If the chamber has humidity controls, set them to a dewpoint temperature below the low te mperature 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 displayin g 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 conditi on, allow the reading to stabilize for at least 5 minutes, and repeat until a response is rec orded for each test condition.

(d) Change the environmental chamber controls to the lo w temperature setting (i.e., 5°C or the low end of the manufacture-specified range, whi chever is lower). Allow at least 4 hours for the device to equilibrate at this tempera ture.

(e) Perform any necessary calibrations or drift adjustm ents to the HCl detector to assure that its responses will be comparable to the earlier mea surements. Set the gas/vapor blending device to each of the test conditions, as before, a nd record the detector responses to each test condition.

(f) Change the environmental chamber controls to the hi gh temperature setting (i.e., 40°C or the high end of the manufacture-specified range, wh ichever is higher). Allow at least 4 hours for the device to equilibrate at this tempera ture.
(g) Perform any necessary calibrations or drift adjustm ents to the HCl detector to assure that its responses will be comparable to the earlier mea surements. Set the gas/vapor blending device to each of the test conditions, as before, a nd record the detector responses to each test condition.

(h) Process the detector response data. For each gas/va por blending device operating condition, identify the lowest and highest response average ( highest x and lowest x , from among the three recorded response averages), and calculat e a relative percent difference.



# ( ) %

100 2 × + −
=
lowest highest lowest highest

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 sta bility test.

3.3.5 Delivery Back Pressure Test

The delivery back pressure test may require some mo dification of the tubing connection between the gas/vapor blending device and the HCl measureme nt system. By inserting one or more valves or back pressure regulators into the detector and/o r vent lines, create a system where the back pressure at the HCl gas/vapor blending device outle t may be varied and maintained- 13 -

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 othe rwise, the low back pressure test condition will be ambient pressure, or whatever the minimum b ack 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 pr essure test condition will be 28 kPa. If the manufacturer documentation provides specific warnin gs against too much back pressure, the high back pressure test condition may be set as muc h as 10% below the manufacturer-specified maximum. The delivery back pressure test may be con ducted at the same HCl gas/vapor blending device settings that were used for the rep eatability test, or a subset thereof, but must include conditions at the high and low limits of ea ch flow measurement (gas and liquid) that is incorporated into the device.

(a) Connect the device, through the back pressure arran gement, 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 displayin g 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 a n average response for this delivery back pressure.

(e) Using the response averages for the high and low ba ck pressure settings ( highBP x , lowBP x ), calculate a relative percent difference.



# ( ) %

100 2 × + −

=
lowBP highBP lowBP highBP

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 f or responses to shift, if they do.

(g) Once all operating conditions have been tested, rev iew 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- 14 -

parameters (e.g., evaporator temperature for an eva porative calibration gas standard). Qualification tests for these parameters shall be s tructurally 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 t hereof, but must include conditions at the high and low limits of each flow measurement (gas and li quid) that is incorporated into the device.

4.0 CERTIFICATION

4.1 Introduction

For each candidate HCl gas/vapor blending device, t his section provides procedures and calculations to determine certified reference value s at specific operating conditions and to calculate the combined, expanded uncertainties of t hose reference values. Documentation that a particular HCl gas/vapor blending device has been e valuated according to these procedures, and has met the applicable acceptance criteria, satisfi es 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 v ary based on the type of device and what it measures. Table 4 shows four device designs, and th e 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

dry w dry dry

m C

ν ν

+
⋅ ⋅

& 001244

. add dry w dry dry

m C

ν ν ν

+ +
⋅ ⋅

& 001244

. add sol sol ws m M

ν ν

+
⋅ ⋅ ⋅

& 001244

. 414

. 22 

## ( ) add sol w sol ws m m M

ν ν

+ +
⋅ ⋅ ⋅

& & 001244

. 414

. 22

dry C Concentration of dry HCl gas, ppm

dry ν Flow rate of dry HCl gas, liters/min ws M Working solution Molarity, mol/liter

sol ν Working solution flow, μ l/min a

sol m & Working solution flow, mg/min a

w m & Flow rate of liquid water, mg/min

add ν Flow rate of inert carrier gas, liters/min 001244 . liters/mg Specific volume of water vapor at stan dard conditions (0°C, 1 atm)
414

. 22 liters/mol Molar volume of ideal gas at standar 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 calculat ing μ l/min injected into the evaporator when the actual measur ement 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 la boratory, and is provided as an input parameter to the blending device. The gas flow and liquid flo w rates, however, must be measured in real time and used by the device to calculate the expect ed HCl concentration. In order to translate an expected concentration into a NIST-traceable concen tration, the calculation inputs must be NIST-traceable, and the uncertainties must be propa gated through all calculations to the final “certified” HCl concentration and uncertainty. Thes e final calculations may be performed internally or externally to the device’s firmware o r software.

4.3 Dry HCl Gas Concentration

The NIST-traceability of dry HCl gas standards is b eyond the scope of this protocol. For the purpose of certifying devices that use these gas st andards 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 standar ds. 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 a nd stored under normal laboratory conditions.

Note: Standards with accuracy specifications of ± 0.1 t o 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 HCl standard solution is preferred. However, subsequen t 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 ra tes, and other applicable factors that are necessary to generate each of the desired HCl gas c oncentrations. These target concentrations may be supplied by the device manufa cturer.- 16 -

(b) If necessary, prepare each target working solution by diluting the NIST traceable HCl solution to the target concentration using the proc edure 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 knowledgeabl e 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 volumetr ic preparation techniques. An aliquot of the commercial stock solution is added to a tared volum etric 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, eit her a 3-place or a 4-place balance may be used. For aliquots less than 5g, a 4 place balance is req uired. 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 o f measurement necessary to prepare each target working solution concentration.

b) Tare an empty Class A volumetric flask (e.g., 100 m L 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 w eight. 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 s olution:



## ( ) 

# ( ) ( )( )

std total std std

ws V

C C 

## w

ρ × × = 3 10 1 (Equation 1A)

Where:
ws

C = HCl concentration of the working solution (mg/li ter)

std C = HCl concentration of the standard solution (g/lit er) 

# w std = Weight of standard solution (g)

total V = Final total volume of working solution (mL)

std 

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

3 10 1 × = Conversion factor (mg/g)- 17 - 

# ( ) ( ) ( ) ( )( )

std total HCl std std

ws V MW

N C 

## w ρ

× × = 3 10 1 (Equation 1B)

Where: std

N = Normality of HCl working solution (mol/liter)

HCl MW = Molecular Weight of HCl (g/mol)

4.4.1.3 Molarity of Working Solution

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

HCl ws
ws MW

C M 3 10 1 − × = (Equation 2)

Where:
ws

M = Molarity of the working solution (mol/lite r)

3 10 1 − × = Conversion factor (g/mg)

4.4.1.4 Conversion Between mg and μ l of Working Solution

For dilute HCl working solutions in general, and fo r gravimetric feeding in particular, the units of mg and μ l may be used interchangeably. A density of 1.000 g/mL can be assumed when calculating μ l /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 f or 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 uncertainti es are combined with the stock solution standard uncertainty to derive the combined, standa rd uncertainty of the working solution concentration.

(a) The uncertainty of the aliquot weight (U A ) is determined by dividing three times b the estimated readability of the balance by the measure d aliquot weight, and converting the result to a percentage.

(b) The uncertainty of the final working solution volum e (U F ) is estimated by dividing the readability of the certified volumetric glassware c 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 c an be estimated based on the readability of the balance a nd a multiplier of 3. For example, for a 3-place b alance, the readability is ± 0.001 g. Therefore, the uncertain ty 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 (U F ) is determined by dividing three times the estimated readability of t he 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 supplie r.
(e) The combined standard uncertainty of the working so lution concentration is calculated using Equation 3.

2 2 2



### 2 F

A CS WS 

### U

U U U +

+      

= (Equation 3)

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

CS = Expanded uncertainty of the commercial standard , with an assumed k=2 coverage factor (%) U A = Uncertainty of the aliquot weight or volume ( %) U F = Uncertainty of the final, diluted standard we ight or volume (%)

(f) Example: A 10 g aliquot of a standard HCl solution is weighe d using a 3-place balance. The exact weight of the aliquot is 10.015 g. The a liquot is diluted to 1.0 liter using a Class A volumetric flask. The specified accuracy o f the standard solution is ± 1.0%. Calculate the uncertainty of the working HCl soluti on.
The uncertainty of the aliquot weight (U A ), is (0.003 g/10.015 g) x 100, or 0.03%. The uncertainty of the final solution volume (U F ) is (0.40 mL/1000 mL) x 100, or 0.04%. The uncertainty of the commercial standard solution (U CS ) is equal to its specified accuracy, i.e, 1.0%. Substituting the values of U A , U F , and U CS into Equation 3 gives a combined uncertainty for the working solution of 1.0012%, wh ich rounds to 1.00%. For this example, the preparation of the working solution ad ds 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 covera ge factor value of 1-sigma, which is appropriate for an intermediate uncertainty that wi ll be subsequently combined with other uncertainties and expanded. The uncertainty of a c ommercially available NIST-traceable HCl standard is assumed to have a coverage factor of 2.

4.5 Gas Flow Rates

Every gas/vapor blending device, regardless of conf iguration, must measure one or more gas flows to calculate its output concentration. This m easurement can be made many ways, but is commonly provided by an electronic mass flow contro ller (MFC). This controller accepts input from the blending device firmware or software to es tablish 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 th e 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 3 rd 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 uncertain ty of the gas flow measurement, but require considerable expertise to ensure that the correct f actors are stored in the blending device. Simple mistakes like using the wrong units (e.g., liters/m in 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 be yond the scope of this protocol, it will simply be assumed that any linearization curves are applie d 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 uncertai nty.

4.5.1 Gas Flow Rate Comparison to NIST-Traceable Referenc e Standard

The number and level of gas flow rates to be measur ed will depend on the intended application. The range of flows should cover the expected operat ing range of the device, and the number of flows (no less than 3) should be sufficient to mini mize 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) throu gh (e) of this section. As noted in the final step, this measurement process must include at leas t 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. Alternativel y, if the flow device manufacturer’s certification includes uncertainty data for the full range of flo ws, it may be used as one of the independent experiments for purposes of calculating reproducibi lity.

(a) Install a primary laboratory standard device for fl ow rate measurement at the outlet of the calibrator (e.g., a digital bubble meter, piston di splacement device, or other device). The device must have: (1) an uncertainty of 1.0% or bet ter; and (2) a NIST-traceable calibration. Operate the reference flow rate measu rement device in accordance with the manufacturer’s instructions. Make sure that the re ference device is used within its specified operating range and conditions.

(b) Prepare and operate the HCl gas/vapor blending devi ce 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 f or 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 o utput 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 press ure conditions.

(d) Calculate the relative standard deviation (RSD) of the gas flow rate determination replicates at each level. If any RSD value is grea ter than 2.0%, take corrective action and repeat those pairs of readings. Record the RSD as the first-day gas flow uncertainty ( U ν 1 ) at each level.

(e) Shut down the HCl gas/vapor blending device accordi ng to the manufacturer’s instructions. On another day, repeat the measuremen t experiment and record the RSD at each level as U ν 2 . The experiment may be repeated more than once, pr ovided all data are recorded, and should result in lower uncertainty.

4.5.2 Combining Data and Correlating Expected to Certifie d Values

Once the comparison measurements are done, the data set will consist of a number (I ≥ 10) of data points, for each of J flow conditions, each co mpared on a number (K>1) of days. The singleday/single-flow averages ( ν jk ) have already been calculated as part of the RSD c heck during the comparisons.


# ∑

= 

### = I

i i jk 

### I

1 

### ν ν 

## ( 

# )

jk jk i I

i jk I RSD U ν

ν ν

ν % 100 )1 ( 2

1 ×

− − Σ

= = =

Calculating one reference number for each flow condi tion means simply averaging the ν jk values, while the single-flow uncertainty ( U ν j ) involves a combination of averages and uncertaint ies at various levels of the comparison process.



# ∑

= 

### = K

k jk j 

### K 1 

### ν ν 

# ( ) % 100 )1 ( % 100 1 2 2 2

1

1 2

2 2 × ×

− − Σ +        

− +      

= =

= 

# ∑

j j jk K

k K

k jk RFS j K U
K K U U ν ν ν
ν
ν

where U

RFS is the uncertainty of the reference flow standard with an assumed coverage factor of k=2. If the blending device is properly applying it s linearization curves, the single-day/singleflow averages ( ν

jk ) should correlate well with the corresponding expected flow values ( x jk ) as recorded by the blending device. To verify this, de rive 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 a ctual gas flow rate to indicated gas flow rate- 21 - jk a x jk Σ Σ

= ν ( Σ

without bounds denotes all ν jk and x jk averages are included)

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



# ( ) 2 jk 2 jk jk

- - a x

x JK x x JK

jk jk

jk Σ Σ⋅ Σ

⋅ Σ Σ

⋅

= ν ν 

# ( ) 2 jk 2 jk jk 2

- - b
x

x JK 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 comparis on 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 op eration will rarely match one of the calibrated points exactly. In order have a continuum of certif ied points over the full range of operation, interpolation is necessary. The formula for linear interpolation is straightforward.



# ( ) ( )        

− −

− + ≈        

− −

− + =

± ±

j j j j j gas j j gas j gas j gas j gas x x x x x x x x

1 1 ν ν ν ν ν ν ν

where x

j , ν j is the certified point nearest the x gas value to be interpolated, and x j±1 , ν j±1 is an adjacent certified point chosen so that x gas is between, x

j and x j±1 . The uncertainty arising from this calculation is primarily related to the assump tion that the point x gas , ν gas lies on a straight line between x

j , ν j and x j± 1 , ν j± 1 . This line, along with the correction equation use d in 4.5.2 to verify response linearity, defines an estimated uncertaint y envelope for interpolation slope. 

# ( ) ( ) ( ) ( )

j j j j j j j j j j

Max j gas j gas
x x x

x a b x a b x a Max

x x

− − ⋅ − +
⋅ +
⋅ − −

=        

− −

± ± ± ± ±

1 1 1 1 1 ,

, , ν ν ν ν ν ν



# ( ) ( ) ( )

( )

j j j j j j j j j j

Min j gas j gas
x x x

x a b x a b x a Min

x x

− − ⋅ − +
⋅ +
⋅ − −

=        

− −

± ± ± ± ±

1 1 1 1 1 ,

, , ν ν ν ν ν ν

These maximum and minimum values define an uncertai nty component related to the linear interpolation.- 22 - % 100 12 2

×

               

− −

−        

− −

= Min j gas j gas

Max j gas j gas

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 j vj j gas vli
gas 

### U

x x U U ν ν 

### ⋅ +

−

=

As an alternative approach to interpolation, the li near equation from 4.5.2 may be applied as a correction equation to all points within the range of gas flow measurements.

b x a gas gas +
⋅ = ν 

# ( ) 

# ( )

( ) % 100 2 - - 2 + + + 2

2 2 2 ×

− Σ

Σ Σ

⋅ Σ Σ

= JK x a x b x ab x a JKb U jk jk jk jk jk jk jk jk
gas ν ν ν ν

Note: While this alternative does simplify the algo rithm 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 calcula tion of its expected output HCl concentration. Maintain documentation of the experimental measurem ents, derivation of the linear correction and interpolation equations, if used, and the uncer tainty 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 devi ces varies significantly in implementation. Where bottled gas humidification devices may use so me type of reservoir with continuous ondemand replenishment, an evaporative HCl standard u sually has some type of variable-speed pump. Regardless of design, all devices must measur e 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) i s 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 mi ght use a quadratic or 3 rd order curve to linearize the as delivered liquid flow signal(s), as displayed and recorded b y the device software. It is assumed that, if used, these curve coefficien ts are properly calculated and applied, and that the application of the curve(s) is transparent to t he operator and the recorded data stream. The linearity of the blending device’s liquid flow meas urements will be verified as part of the certification process, and any residual non-lineari ty will become part of the uncertainty.

4.6.1 Liquid Flow Rate Comparison to NIST-Traceable Refer ence Standard

Based on the expected output moisture content and ( for evaporative HCl standards) the solution concentration, determine the range of liquid feed r ates that are necessary to generate the required output concentrations, taking into account the carr ier gas flow rates and other applicable factors. The number and level of liquid feed rates to be ver ified (no less than 3) will depend on the application. The range of flows should cover the ex pected operating range of the device, and the number of flows should be sufficient to minimize th e impact of interpolation uncertainty for inuse flows that don’t exactly match the measured flo ws. Measure the liquid flow from the device according to the procedures in paragraphs (a) throu gh (k) of this section. As noted in the final step, this measurement process must include at leas t 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. Alternativel y, if the flow device manufacturer’s certification includes uncertainty data for the full range of flo ws, it may be used as one of the independent experiments for purposes of calculating reproducibi lity.

(a) Zero an appropriate three-place (or more accurate) balance and then check its calibration against NIST-traceable weights in the r ange of measurement necessary for each liquid feed rate determination.

(b) Connect the balance to a data logging device that i s 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. Positio n 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 representativ e gas flow. Allow the device to run for a sufficient period of time to equilibrate and achi eve steady operation.

Note: A significant period may be required for sys tems 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 devic e to run for at least 5 minutes while the balance readings are being logged (more time ge nerates more data points , which lowers uncertainty). Change to the next operating c ondition, 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 equilib rate at the new flow setting(s).

(f) Continue operating the device at different operatin g conditions, recording at least 5 minutes of balance data after each condition stabil izes. 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 accordi ng to the manufacturer’s instructions. Shut down the balance logging and ret rieve the data file.

(h) Parse the balance data into segments of time ( t ) and mass ( m ) readings, each representing a single operating condition as record ed 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
t t N m

t m t N

m

seg jk seg jk

seg seg jk seg 

     Σ Σ

⋅ Σ

⋅ Σ Σ

⋅

= = & grams
t t N 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
t t N N m t a m b t ab t a Nb m N 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 ( U ṁ jk ) of each liquid flow rate determination

% 100 × = jk ajk jk m m stderr U

& &

(j) If any U

ṁ jk 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 experi ment, and record the U ajk for all test conditions. The experiment may be repeated more tha n once, provided all data are recorded, and should result in lower uncertainty.- 25 -

4.6.2 Combining Data and Correlating Expected to Certifie d Values
Once the comparison measurements are done, calculat ing one reference number for each flow condition means simply averaging the ṁ jk values, while the single-flow uncertainty ( U ṁ jk ) involves a combination of averages and uncertaintie s at various levels of the comparison process.



# ∑

= 

### = K

k jk j 

### K m m

1 

### &

& 

# ( ) % 100 )1 ( % 100 1

2 2

1

1 2

2 ×

×

− − Σ +        

− = =

= 

# ∑

j j jk K

k K

k jkm j m m K m m U
K K U

& & &

&

&

If the blending device is properly applying its lin earization curves, the single-day/single-flow averages ( ṁ

jk ) should correlate well with the corresponding expected flow values ( x jk ) as recorded by the blending device. To verify this, de rive 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 bel ow 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 exac tly. As such, linear interpolation is one of the two options for having a continuum of certified poi nts over the full range of operation.



# ( ) ( )        

− −

− + ≈        

− −

− + =

± ±

j j j j j liq j j liq j liq j liq j x x m m x x m x x m m x x m m

1 1 & &

& & &

& &

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

x j±1 , ṁ j±1 is an adjacent certified point chosen so that x gas is between, x

j and x j±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 j gas j liq

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

x x m m

− − ⋅ − +
⋅ +
⋅ − −

=        

− −

± ± ± ± ±

1 1 1 1 1 ,

, , & & & & & &



# ( ) ( ) ( )

( )

j j j j j j j j j j

Min j gas j liq

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

x x m m

− − ⋅ − +
⋅ +
⋅ − −

=        

− −

± ± ± ± ±

1 1 1 1 1 ,

, , & & & & & &

% 100 12 2

×

               

− −

−        

− −

= Min j gas j gas

Max j gas j gas

li m x x m m

x x m m

U & & & &

&- 26 - 

# ( )

2 2

2 2 2

liq j j m j liq lim liq 

### U

x x U U ν ν 

### ⋅ +

−

= &

&

where ṁ can be either ṁ sol or ṁ w (for working solution injection or DI water inject ion).

The alternative approach of applying the linear equ ation from 4.6.2 as a correction equation, within the range of liquid flow measurements, is al so available.

b x a m liq liq +
⋅ = & 

# ( ) 

# ( )

( ) % 100 2 - - 2 + + + jk

2 2 2 2 ×

− Σ Σ Σ

⋅ Σ Σ

= JK m x a m b x ab x a JKb 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 calcula tion of its expected output HCl concentration. Maintain documentation of the experimental measurem ents, derivation of the linear correction and interpolation equations, if used, and the uncer tainty determinations for both the compared flow points and the interpolated/corrected points.

4.7 Calculation of Certified HCl Concentrations and Unc ertainties

Section 4.2 describes how HCl gas vapor blending de vices calculate and report the expected output concentration. These calculations are typica lly done automatically by the device’s microprocessor using the specific software/firmware version and data processing algorithms as implemented by the particular manufacturer. When ce rtifying or using any blending device model for the first time, the calculations should b e 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 itse lf. 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 g as stream ( ν add ) used to deliver the moisture.- 27 - dry w dry dry
wet m

C C ν

ν

+
⋅ ⋅

=

& 001244

. add dry w dry dry
wet m

C C ν

ν ν

+ +
⋅ ⋅

=

& 001244

.

Where C wet ≡ output HCl concentration, ppm

C dry ≡ HCl concentration of dry gas standard, ppm

ν dry ≡ volumetric flow rate of dry HCl standard, liter/mi n
ν add ≡ volumetric flow rate of inert carrier gas (if used ), liter/min ṁ w ≡ mass flow rate of humidification water, mg/min

C dry comes straight from the bottled gas certificate of analysis, where ν dry and ν add are each determined by the procedures in section 4.5, and ṁ w follows section 4.6. The combined uncertainty incorporates these values and their unc ertainties.



# ( ) ( )



# ( ) 2 2 2 2 2 2 2 2
2

001244

. ) 001244 (. 2

add

dry w w w m add add

wet dry dry Cdry

wet wet dry dry dry

Cwet m m U U C C U
C C C U
U ν ν ν ν ν

ν ν

+ +
⋅ +

+                 +        

−

=

& &

&

Where U ν dry comes from the section 4.5 U gas calculation, % U C dry comes from the dry gas standard certificate of an alysis, % Note: A coverage factor of k=2 is assumed U ν add comes from the section 4.5 U gas calculation, % U ṁ w comes from the section 4.6 U liq calculation, %

4.7.2 Evaporative HCl Gas Standard

The certified output concentration of an evaporativ e HCl gas standard is calculated by one of two formulas, depending on whether there is an addition al liquid stream of deionized or distilled water ( ṁ w ).

add sol sol ws wet m

M C ν

ν

+
⋅ ⋅ ⋅ =

& 001244

. 414

. 22 

## ( ) add w sol sol ws wet m

m M C ν

ν

+ +
⋅ ⋅ ⋅ =

&

& 001244

. 414

. 22

where C wet ≡ output HCl concentration, ppm M ws ≡ molarity of HCl working solution, mol/liter

ν sol ≡ volumetric flow rate of HCl working solution, μ l/min ṁ sol ≡ mass flow rate of HCl working solution, mg/min ṁ w ≡ mass flow rate of additional water, mg/min

ν add ≡ volumetric flow rate of inert carrier gas, liter/m in

M ws comes either from the solution provider’s certific ate of analysis, or from section 4.4 if the solution was prepared from a more concentrated stan dard solution. Values for ṁ sol and ṁ w are each determined by the procedures in section 4.6, w hich also provides that ν sol = ṁ sol because the- 28 -

working solution density is 1.000 mg/ μ l. The value of ν add 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

. 414

. 22 001244

. 414

. 22

add w sol w w m add add

add w sol wet sol ws Mws wet ws sol sol m Cwet m m m U U

m m C M U C M m U U ν ν

ν ν ν + +
⋅ + + + +
⋅ ⋅

+
⋅ − ⋅
=

& & &

& & &

& &

Where U ṁ sol comes from the section 4.6 U liq calculation, % U Mws comes from the section 4.4 U ws calculation, % U ν add comes from the section 4.5 U gas calculation, % U ṁ w comes from the section 4.6 U liq calculation, %

Note: If a commercially available working solution is used instead of diluting a more concentrated commercial standard, look for the unce rtainty on the certificate of analysis and divide this number by an assumed coverage factor of k=2 to get the value of U Mws .
4.7.3 Expanded Uncertainty

The performance specification for uncertainty will be based on expanded uncertainty. To calculate this quantity, multiply the combined unce rtainty 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 Trace able 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 under go 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 respons es from the reference (certified gas standard). At each concentration, the bracketing procedure req uires 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 RS 1 Candidate C 1 Reference Standard RS 2 Candidate C 2 Reference Standard RS 3 Candidate C 3 Reference Standard RS 4- 29 -

In addition to the minimum of three candidate injec tions as shown above, the bracketing procedure must be performed at a minimum of three c oncentrations (covering the blending device’s usable range), for a 3x3 or larger bracket ing matrix. Also, this entire comparison sequence must be repeated at least once on a differ ent day after the blending device has been shut down and restarted by the manufacturer’s recom mended procedures. The bracketing comparisons are used to calculate an average concen tration ratio between the candidate and reference gases, and the repetition of the 3x3x2 (o r larger) certification matrix (see Table 6) is used to assess the repeatability and reproducibilit y of the certification results.

Table 6: Example Bracketing Certification Matrix

Concentration Day 1 Day 2
Zero 1Z 1 2Z 1 High 1H-RS 1 2H-RS 1 High 1H-C 1 2H-C 1 High 1H-RS 2 2H-RS 2 High 1H-C 2 2H-C 2 High 1H-RS 3 2H-RS 3 High 1H-C 3 2H-C 3 High 1H-RS 4 2H-RS 4 Mid 1M-RS 1 2M-RS 1 Mid 1M-C 1 2M-C 1 Mid 1M-RS 2 2M-RS 2 Mid 1M-C 2 2M-C 2 Mid 1M-RS 3 2M-RS 3 Mid 1M-C 3 2M-C 3 Mid 1M-RS 4 2M-RS 4 Low 1L-RS 1 2L-RS 1 Low 1L-C 1 2L-C 1 Low 1L-RS 2 2L-RS 2 Low 1L-C 2 2L-C 2 Low 1L-RS 3 2L-RS 3 Low 1L-C 3 2L-C 3 Low 1L-RS 4 2L-RS 4 Zero 1Z 2 2Z 2

4.8.1 Stepwise Bracketing Procedure

(a) Turn on all components of the HCl gas/vapor blendin g device, the HCl measurement system (analyzer), and any associated equipment; al low them to reach thermal equilibrium and stable operation.

(b) While awaiting warm-up, check that all data acquisi tion and logging systems associated with the blending device and the analyzer are opera tional 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 t he detector by injecting the high-level- 30 -

reference gas standard and making any necessary spa n adjustments. Repeat these steps as necessary to optimize the analyzer calibration r elative 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 br acketing procedure.

(d) Before beginning the bracketing procedure, ensure t hat the blending device, the analyzer, and any support equipment are operating w ithin the prescribed limits for all key parameters as specified by the respective manuf acturers.

(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 (RS 1 ). 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 (C 1 ), again based on at least 5 minutes of stable read ings.

(h) Switch back to feeding the reference gas standard t o the analyzer and record an average response (RS 2 ).

(i) Repeat steps (g) and (h) at least two more times at this concentration to provide three or more candidate responses, each preceded and followe d by a reference standard response.
Note: If, at any time during the process, test resu lts demonstrate that equilibration is not being achieved, or that any key parameter is no t within acceptable limits, stop the experiment, investigate and correct the problem. Va lid data collected up to that point in the test may be retained. Discard all data collect ed 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 devic e.
(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 se tting before recording any responses.
(k) Inject the zero gas to the analyzer and record an a verage response without making any adjustments. (l) Shut down the HCl gas/vapor blending device accordi ng to the manufacturer’s instructions. Also shut down all data logging and a ny other equipment that is not designed to be left on overnight.- 31 -

(l) On another day, repeat steps (a) through (k). The b racketing 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 de vice 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 f ollows:



## ( ) ( ) ( )      

−

−

− + =

2 1 2 1 1 1 t

t Z Z t t Z Z i i (Equation 4)

Where: Z

i = Calculated zero offset at t i Z 1 = Initial zero response Z 2 = Final zero response
t i = Time of response to be zero corrected
t 1 = Time initial zero response was recorded
t 2 = Time final zero response was recorded
Then, correct the response at t i for the zero offset using the following equation:

i i ci Z Y Y

− = (Equation 4)

Where: Y

ci = Response at t i corrected for zero offset Y i = Response at t i as recorded during bracketing Z i = Zero offset at t i
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 RS i

Response RS ci

Zero-Cor. C i Response C ci 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 C Ci , calculate the output ratio R i , using the formula shown in Equation 5 for the first candidate injection res ponse.

   

 

   

 



     + =

2 2

1 1 1 C

C C RS RS C R (Equation 5)

Where: R 1 = Output ratio for first candidate device injec tion C C1 = First response to candidate device, zero correcte d RS C1 = First response to reference gas standard, zero c orrected RS C2 = Second response to reference gas standard, zero c orrected

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

Table 8: Example Calculation of Output Ratio

Time (min) RS i Response RS ci

Zero-Cor. C i Response C ci Zero-Cor. Output Ratio

( R i ) 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 R jk and the relative standard deviation (RSD) of the individual output ratio measurements.



# ∑

= 

### = I

i i jk 

### I R R

1 

## ( 

# )

jk jk i I

i Rjk R

I R R RSD U % 100 )1 ( 2

1 ×

− − Σ

= = =

Where: RSD = Relative standard deviation, percen t

R jk = Arithmetic average of the output ratios R i = Individual, “i th ” 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 uncer tainty of the output concentration. The RSD must not exceed 1.0%. If the RSD value is ex ceeded, the test at that concentration is invalid and must be repeated. The calculated RSD va lue for the output ratios in Table 8 is 0.4%, and is therefore acceptable.- 33 -

Finally, the output concentration is calculated by averaging the R jk 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 jref j j 

### C R C × =

Where: R

j = Overall average ratio for concentration j bracketing R jk = Average ratio measured at concentration j on day k K = Number of days of bracketing experiments C j = Certified concentration from blending device C jref = Certified concentration of reference gas used d uring 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 indire ct 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 us es bracketing 1 ) shows all of the uncertainty components and how they are combined to calculate ov erall 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 shift s that could impact the output concentration. These operational parameters include, minimally, al l gas and liquid flows, each of which must remain within ±2.0% of the value recorded during cer tification. Because the bracketing certification values are related specifically to th e 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 direc t comparison of the output to compressed gas cylinders containing HCl with NIST-traceable concentr ations. The comparison measurements must be done by an HCl measurement system or detecto r 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 outpu t concentrations from among those routinely used by the device in operation. One of the two conce ntrations must have the blending device operating at its most-used operating condition, whi ch for CEMS applications will likely be the concentration used for daily calibration error chec ks. 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 th e first bottle in order to assess the response linearity of the HCl measurement system. Bo th 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 in let 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 unr estricted, 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 m ake necessary adjustments to bring the response to zero.

(b) With the measurement system connected to the switch ing manifold, inject the higher of the two compressed gases, and make necessary adjustm ents 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 stabl e response.
(e) Switch back to the bottled gas whose concentration is closest to the blending device concentration. Allow at least one minute for the resp onse to begin changing, then wait for a stable response before recording the average valu e.
(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 t his response by the average of the two adjacent bottled gas responses (i.e., before and after ) and to get a response ratio.



## ( ) 2

m m m

After

- Bottle Before

- Bottle Device

+ = i R

(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

i R ≡ Individual output ratio
I ≡ Number of output ratios ( ≥ 3)

(i) The RSD must not exceed 1.0%. If the RSD value is ex ceeded, the test is invalid and must be repeated.

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

Bottle on Verificati C R C × =

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

(l) Select the routinely-used device operating conditio n 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 v erification tolerance as the combined uncertainty of the device output and the verificati on value (ignoring the uncertainty of the response ratio, which should be small).

2 2 2 Bottle

Cwet U R U Tolerance ×

+ × =

Where U Cwet

is the device output uncertainty calculated in sec tion 4.7.1 or 4.7.2, and U Bottle is the bottle concentration uncertainty (divided by its co verage factor). If the difference between the device’s certified output concentration and the ver ification value exceeds this tolerance, take corrective action and repeat the verification.- 36 -

6.0 QUALITY ASSURANCE

This section specifies: (1) the periodic data quali ty assessments that are required following initial certification of an HCl gas/vapor blending device, t o provide assurance of continued acceptable performance; and (2) the required frequency of devi ce recertification.

6.1 Periodic Data Quality Assessments

Periodic performance evaluations of each blending d evice shall be conducted at the following frequency:

(a) At least once each calendar quarter; and

(b) Following any malfunction, repair, or corrective act ion that may reasonably be expected to affect the device’s output, but does not necessi tate recertification of the device (see Section 6.2, below).

Any of the data quality assessment procedures (Optio ns 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 t he acceptable performance criteria, suspend the device certification until either:

(a) Another performance evaluation is successfully compl eted, or

(b) The device is recertified in accordance with the pro cedures 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 p recipitated by a device malfunction, the evaluation may take the form of a simple check of t he input measurements that are used to calculated the certified output. Where a device mal function 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 recerti fied (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 i s 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 check s in (a) above, then collect a sample of the HCl feed solution from the input reservoir, a nd have the sample analyzed for HCl concentration by an independent laboratory.

The acceptable performance criterion for each of th e measurements is a difference ≤ 2.0% of the reference value.

6.1.2 Option B: Operational Envelope Linearity Verificati on
For devices where the liquid and gas flows can be ind ependently set and maintained without substantially limiting each other’s range, acceptab le performance may be verified by exercising the blending device near the boundaries of its oper ational envelope (i.e., the maximum and minimum liquid and gas flows recommended by the manu facturer). 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 r esponse linearity.

Like Option A above, this option is not to be used whe n the performance evaluation is precipitated by a device malfunction, unless the ma lfunction is confined to one of the measurement components (i.e., gas flow or liquid flo w), 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 l imits on the range of the other flow measurement(s) beyond the manufacturer’s specificat ions.

(a) With the liquid flow at its highest setting within th e operating envelope, operate the blending device such that the gas flows at its highe st, lowest, and at least one intermediate setting (nominally equally spaced betwe en the highest and lowest settings), all within the operating envelope, while the liquid f low remains constant.

(b) Allowing the HCl detector response to stabilize at ea ch 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 dat a points.

(c) Calculate a best-fit line for the data points colle cted in step (b). If any point is more than 3% above or below the line, take corrective action a nd 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, a nd operate the blending device such that the gas flow is at its highest and lowest sett ing 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 poi nts with the highest and lowest liquid and gas flow points from step (b), so there a re 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 var iable 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 t he liquid flow. If any point is more than 3% above or below the line, take corrective action a nd repeat the check.

(i) If the device uses a second liquid flow (e.g., to co ntrol 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 th e checks above, then collect a sample of the HCl feed solution from the input reservoir, a nd have the sample analyzed for HCl concentration by an independent laboratory.

6.1.3 Option C: Comparison to a Dry Bottled HCl Gas Stand ard
Evaluate the performance of a gas/vapor blending de vice by direct comparison to a compressed gas cylinder containing HCl with a NIST-traceable con centration. Like Option B above, the comparison measurements may be done by the connecte d 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 linea rity.

Similar to Option A above, this check will be done with the blending device operating at its most-used operating condition, which for CEMS applic ations will likely be the concentration used for daily calibration error checks. In order t o have an effective comparison, the bottled HCl gas standard needs to be within ± 30% of that concen tration, 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 measur ement 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 stabi lize 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 t his response by the average of the two adjacent bottled gas responses (i.e., before and after ) and to get a response ratio.



## ( ) 2

m m m

After

- Bottle Before

- Bottle Device

+ = i R

(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

i R ≡ Individual output ratio
I ≡ Number of output ratios ( ≥ 3)

(g) The RSD must not exceed 1.0%. If the RSD value is ex ceeded, the test is invalid and must be repeated.

(h) Multiply the average ratio by the certified HCl conc entration of the bottled gas to get the reference concentration. If the reported concentrat ion from the blending device software differs from this reference concentration by more t han the concentration uncertainty limit from the applicable regulation (or 5.0%, whichever i s less), take corrective action and repeat the test.

6.1.4 Option D: Comparison to Another Certified HCl Gas/V apor Blending Device

This option is functionally similar to Option C, exc ept 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 bl ending devices must be periodically recertified to maintain NIST traceability. At a min imum, devices shall be recertified at least once every 8 calendar quarters in accordance with th e procedures in Sections 4 and 5. Devices shall also be recertified whenever (1) any event, co ndition, or factor that triggers recertification (as identified in the Manufacturer’s Disclosure) occ urs, 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 implementatio n of the procedures in Sections 4 and 5 is a full recertification, which resets the “once every 8 cal endar 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 o ption 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 aband oned in favor of a full recertification with an additional day (or more) of comparison tests. Regar dless of whether a cumulative or full recertification is performed, the process must incl ude 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 comp onent, and the traceable concentrations from the previous certification are recalculated wit h 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 Mer cury Gas Generators”; U.S. EPA; July 2009.- 41 -

Appendix A – Concentration Uncertainty from Bracket ing Alternative

When assigning concentration values to blending dev ice 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 specif ically to a bracketing procedure. The other two components apply to the overall process of trans ferring traceability from the reference gas(es) to the candidate.
A.1 Calibration Linearity

[Reserved]

A.2 Measurement Stability

The measurement stability uncertainty calculation a pplies a statistical test to the assumptions that are built into the bracketing technique (e.g., no n on-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

Cand Cand Cand Cand 2 Cand Cand

Cand

−        

−

= 

# ∑

∑ ∑

− − −

−

n m

m m m t t t t

SE



## ( ) ( ) ( )



# [ ] ( )
)2 ( Ref 2 Ref Ref 2

Ref Ref Ref Ref 2 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 0 = t origin can be anything, as long as they are consis tent 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 lin es are used to calculate the standard error of the mean of each individual measurement interval.- 42 -



## ( )

B

- Ref Ref B

- Ref n m SE SE = 

## ( )

Cand Cand Cand n m SE SE = 

## ( )

A

- Ref Ref A

- Ref n m SE SE =

where m is the mean for just that measurement interval, an d n is the number of individual measurements in that interval. The subscript notati ons (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 rati o calculation, these standard errors are used to calculate a combined stability uncertainty for t his ratio

A

- Ref B

- Ref Cand

m m m

B

- Ref A Ref B

- Ref Cand

B

- Ref A Ref Cand A

- Ref

⋅ ⋅ =

− − +

− −

− − t

t t t
t t t t R i



## ( ) ( ) ( ) ( ) 2

A

- Ref A

- Ref 2

Cand Cand 2

B

- Ref B

- Ref 1 m m

m m

m m

B

- Ref Cand B

- Ref A

- Ref

Cand A Ref B

- Ref A

- Ref    

 

   

 

× +         +    

 

   

 

× × =

− −

− −

− t t t t SE SE

t t t t SE R R u i

stability i σ

where R i 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 c omponents (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 

# ( ) ( ) stability i I

i stability jk R

u I R u 2

1 2 1 1

=

Σ = σ

where I is the number of brackets in this th j set.
Note: Whenever a “jk” subscript appears in this app endix, the term represents an average for the j th concentration on the k th bracketing day. When the “j” subscript appears alo ne, the term represents the average for the j th concentration, inclusive of all test days.
A.3 Repeatability
The most straightforward way to calculate repeatabi lity is a simple standard deviation of the individual bracket ratios.- 43 -



## ( ) 

# ( )

)1 ( 1 2

− ∑

= = −
I R R R s I

i jk i

i
This approach, however, creates the possibility of “double-counting” where measurement instability contributes to imprecision. To allow fo r this, the experiments are structured as a 3- level nested design (see chapter 2 of NIST’s “Engin eering Statistical Handbook” http://www.itl.nist.gov/div898/handbook/mpc/section 4/mpc433.htm), as follows:
Level 1: Individual ratio uncertainty Level 2: Repeatability within set (sometimes calle d “within day” uncertainty) Level 3: Set reproducibility (sometimes called “be tween day” uncertainty)
In a nested experimental design, standard errors ar e pooled at each level, and the level-specific contributions to uncertainty are computed from thes e pooled standard errors. Starting with the pooled level-1 standard error



## ( ) stability i I

i R R u 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 jk i

R
The repeatability uncertainty of the ratio measurem ents is computed using



## ( ) 

# ( )

  
 

  
 

×

∑

−

− ∑

=       ×

− = =      

= − L

I R

I R R Max s L s Max R u I

i stability

jk I

i jk i

R R i 1 2

1 2

2 1 2 2 ity repeatabil 1 u 1) ( , 0 1

, 0 σ

where L represents the average number of individual detect or readings used in each measurement average and ratio determination 

# ( ) ∑ + + =

= I

i i i i I L

1 A,

- Ref Cand, B,

- Ref n n n 1

The uncertainty of the set-average ratio is



# ( ) ( )   
 

  
 

×

∑

−

− ∑

− × = = =      

= 

    

L

I R

I R R Max I R u I R u I

i stability

jk I

i jk i

i jk 1 2

1 2

ity repeatabil 1 ity repeatabil 1 u 1) ( , 0 1 1

σ σ- 44 -

The “Max” function holds this term at zero when sho rt-term precision uncertainty is dominated by measurement instability.
A.4 Reproducibility
Reproducibility is an uncertainty term applied to t he calculated candidate concentrations ( jkref

jk jk C R C × = ). It may be estimated differently depending on wha t type of device is being certified or how many times the bracketing procedur e is repeated.

A.4.1 Statistical Approach
A 3-level nested design 2 assumes that the ratio measurement is done I > 1 t imes on each of K > 1 days. Using the same technique as for the repeatabi lity uncertainty 

# ( )       ×

− = 2 2 2 3 ility reproducib 1 1

, 0 C C jk s K s Max C u σ

where

∑

−

×

− =

=       K

k j jk C C

C K s 1 2
3 1 1 (subscript C denotes Concentration uncertainty)

K C C K

k jk
j ∑

= = 1

and R s 2 values (from A.3) are converted to jk C errors and pooled as follows 

# ( ) 

# [ ] ∑ × ×

− ×

− Σ =

=

K

k R ref jk k k C s C I K I s 1 2 2
, 2 )1

( 1

As with the repeatability uncertainty, the “Max” fu nction holds this term at zero when the deviation among sets is dominated by short-term eff ects. The reproducibility uncertainty of the mean candidate concentration is 

# ( )       ×

− = 2 2 2 3 ility reproducib 1 1

, 0 1

C

C j s K s Max K C u σ
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 elsewhere 3 , can be used to compute the reproducibility standa rd error. This is necessary because small sample sizes don’t provide a lot of i nformation about population distribution. The BOB approach assumes a uniform distribution (as opp osed to a normal distribution), and sets the bounds at the highest and lowest C jk values among the data at each concentration. The e stimated standard deviation for a uniform distribution is- 45 - 3 a s uniform =

where a is half the width of the distribution. Applying thi s formula to all the bracketing data yields 

# ( ) 12 ) ( ) (
3 2 ) ( ) (

ility reproducib 1 jk jk jk jk j C Min C Max C Min C Max C u −

= × −

= σ

A.5 Combined Uncertainty

Once calculated, all of the uncertainties are combi ned using propagation of error. First, the combined uncertainty of each day’s bracketing proce dure is calculated



# ( ) ( ) ( ) ( ) ity repeatabil 2
1 stability 2
1 2

linearity 1 Ref Cand 1 0 jk jk assumed

default jk jk jk R u R u R u R u C C u
σ σ σ σ + +      

≈ = =        

Then, these uncertainties are combined into a singl e comparison uncertainty 

# ( ) ( )



# [ ] ∑

= × = K

k jk

ref jk comparison j R u C K C u

1 2 1
, 2 1 1

σ σ
The reference gas uncertainty component is calculat ed based on the comparison ratio and the reported uncertainty of the reference gases. 

# ( ) ( )



# [ ] ∑ × × =

=

K

k ref

jk jk j C

u R K C u 1 2
ion certificat , 1 reference 1 1

σ σ Note: In cases where there is no change of referenc e 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 

# ( ) ( ) ( ) ( ) ility reproducib 2 1 reference 2 1 comparison 2 1 1 j j j combined j C u C u C u C u σ σ σ σ + + =

and the expanded certification uncertainty (k=2) is 

# ( ) ( ) ( )

combined j combined j anded j C u C u k C u σ σ σ 1 1 exp 1 2 × = × =