Guide on dynaic ution ethods for O, O and SO at liit values January 04 Written by: Jari Walden (Finnish Meteorological Institute atiana Macé, hristohe Sutour, Jérôe ouette (Laboratoire national de étrologie et d'essais Hans-Peter Haerri, Bernhard iederhauser (Federal Institute of Metrology
able of ontent. OEX... Error! Bookark not defined.. OBJEIVES... 3 3. DYAMI DILUIO MEHOD... 4 3.. Princile... 4 3.. Flow easureent instruents... 6 3.. Mass flow controllers and eters... 6 3.. ritical nozzles... 6 3..3 Lainar flow eleents... 7 3..4 oarison of the flow easureent ethods... 7 3.3. Ileentation of dynaic ethod... 8 3.3. Dilution syste... 8 3.3. High concentration gas ixtures... 0 3.3.3 Purity of the ution gas... 3.3.4 Regulating and easuring the gas flows (san and ution flow rates... 3.3.5 Hoogeneity of dynaic standard gas ixtures... 3 3.3.6 Stabilization tie... 4 3.4. Uncertainty of dynaic ution ethod... 5 3.4. alculation of the exanded uncertainty... 5 3.4. Standard uncertainty on... 6 4. GAS PHASE IRAIO (GP MEHOD... 7 4.. Princile... 7 4.. Ileentation of GP ethods... 0 4.. GP syste... 4.. High concentration gas ixtures... 3 4..3 Purity of the ution gas... 4 4.3. Uncertainty of dynaic ution ethod... 4 5. OLUSIO... 5 6. REFEREES... 5 Annex : alculation of the cobined standard uncertainty for the dynaic ution ethod... 7 Annex : alculation of the cobined standard uncertainty for the critical nozzle... 8 Annex 3: alculation of the cobined standard uncertainty for the Lainar flow eleent... 9 Annex 4: Partial derivatives for the uncertainty calculations of the concentrations for the gas hase titration ethod (GP... 3
. OEX Historically, the ain air ollution roble in both develoed and raidly industrialising countries has tyically been high levels of soke and sulhur dioxide (SO eitted fro the cobustion of sulhur-containing fossil fuels such as coal and heavy oil, used for doestic and industrial uroses. hese days, the ajor threat to clean air is osed by traffic eissions. Petrol and dieselengined otor vehicles eit a wide variety of ollutants, rincially carbon onoxide (O, nitrogen oxides (O x, volatile organic coounds (VOs and articulate atter (PM0, which have an increasing iact on urban air uality. Photocheical reactions resulting fro the action of sunlight on nitrogen dioxide (O and VOs, tyically eitted fro road vehicles, lead to the foration of ozone (O 3 which often iacts rural areas far fro the original eission site as a result of long-range transort. Because of their otential iacts on huan health, welfare and the natural environent, abient concentrations for nuerous ollutants are easured with different saling and analytical ethods at a wide range of rural and urban onitoring sites throughout the world. In the fraework of a Euroean Joint Research Prograe (JRP of EURAME naed Metrology for heical Pollutants in Air (MAPoll, one task ais to rovide haronized rearative ution ethods of air ollutant gases for calibration and uality control uroses in air uality onitoring with a need for iroved ethodologies.. OBJEIVES his guide focuses on the descrition for the generation of SO, O and O standard gas ixtures at concentration levels corresonding to the liit values given in the Euroean Directive on Abient air uality and cleaner air for Euroe (008/50/E for the easureent of abient air ollutants []. itrogen oxide (O is studied as well because it is norally easured in cobination with nitrogen dioxide for the easureent of O x. he ethods that are alied in this docuent are the dynaic ution ethod and the gas hase titration (GP ethod. he latter ethod is based on the cheical reaction of O and O 3 foring O. he ethod is alicable for calibration uroses of O and O 3 analyzers and also to deterine the O converter efficiency in the cheiluinescence analyzers. he docuent gives guidance on how to generate SO, O and O standard gas ixtures alied with different flow easureent ethods and sources of gas standards in concentration ranges and with associated uncertainties in accordance with the secifications given in the Euroean Directive 008/50/E and relevant E standards [][3] (cf. able. oonent Liit values (LV fro the Euroean Directive 008/50/E Scoe of the guide oncentration range (nol/ol Exanded uncertainty (% O - 0 to 00 3 O SO One hour: 05 nol/ol* alendar year: nol/ol* One hour: 3 nol/ol* One day: 47 nol/ol* alendar year: 8 nol/ol** 0 to 00 3 40 to 50 3 able : Liit values and scoe of the guide for SO, O and O standard gas ixtures generated with dynaic ethods (* air uality directive (008/50/E Annex XI and art. 3; ** air uality directive (008/50/E Annex XIII and art. 4 3
3. DYAMI DILUIO MEHOD his guide is originally based on the international standard ISO 645 (arts, 6 and 7 [4][5][6] and is coleted with newer sources, the knowledge and the exerience of the articiating laboratories in the fraework of the MAPoll roject. 3.. PRIIPLE Dynaic ution ethods are widely used to ute gases fro sources with higher concentration to lower concentration ranges for various alications in air uality easureents. hey can be alied for the rearation of standard gas ixtures for calibrating and testing of analyzers for air uality easureents and for inter-laboratory coarisons. he gas sources used for dynaic ution ethods are gravietrically reared gas standards (certified reference aterials, RM and high urity gases. For controlling and easuring the calibration gas or reference gas ixture and the ution gas flows, various tyes of flow eleents are used: lainar flow eleents, theral ass flow controllers or eters and critical nozzles or cobinations of these. he ution ethods are also regulated in standards ade by e.g. the International Standardization Organization (ISO. In this guide the focus of the alication is for abient air uality easureents but the ethods are also alicable to calibrate analyzers for eission easureents for exale. Since for all three tyes of flow eleents the calibrated flow values are usually given as (standard volue / (tie unit e.g. L/in (usually at 0 and 03.5 hpa, the colete ixing forula is established with the concentrations B of the Analyte B first. orresonding abbreviations are introduced afterwards. he colete ixing forula for ution i can be exressed as follows: Bi B san san B ( With Bi the concentration of analyte B in ution i B san B the concentration of analyte B in gas (san gas, standard S, RM the volue flow of san gas the concentration of analyte B in gas (ution gas the volue flow of ution gas With the euation of the ideal gas the relation between the concentration B and the aount of substance fraction x B of a substance B is: Bi n V Bi x Bi R ( With Bi the concentration of analyte B in ution i n Bi the aount of substance B in ution I V the volue x Bi the aount of substance fraction B in ution i the total ressure R the gas constant of ideal gas the absolute teerature 4
With eual values for and for the reference and ution gas, resectively, e. becoes x x R Bi B san B R san x With x B the aount of substance fraction of analyte B in the san gas x B the aount of substance fraction of analyte B in the ution gas Under the above conditions the colete ixing forula with aount fractions can be written: x Bi x B san san x B (3 (4 If x B is (or were zero, the ution factor f d can be introduced: f d san san (5 e. 4 can then be written as: xbi (6 fd xb he san and ution flows are either exressed as a volue flows ( V or a ass flows ( deending on the alied calibration ethod. Various realisations of dynaic ution devices using the different flow control and easureent ethods exist. oercials ution devices integrate all coonents into one unit, as shown in Figure. Figure : 5
3.. FLOW MEASUREME ISRUMES 3.. Mass flow controllers and eters heral ass flow controllers (MF or eters (MFM are widely used for the controlling and easuring of gas flow rates. he flow ranges are within aroxiately 5 l/in u to 500 l/in. Both theral ass flow controllers and eters are designed in such a way that the flow is divided into two lainar flows: a sensor flow and a byass flow. he sensor tube contains two theral sensors, the first one (ustrea is heated and the second one easures the gas teerature. he gas flow assing the first, heated eleent causes a heat loss thus leading to a teerature difference between the two sensors. his teerature difference is roortional to the ass flow and the theral roerties of the gas. herefore the indicated ass flow is deendent on the theral roerties of the easured gas. Only the sensor flow goes through the theral sensors u to 5 l/in of the full scale of the flow in order to have correct oeration of the teerature sensors. Since the ratio of the sensor to the byass flow is constant it allows the correct calculation of the total flow. Mass flow controllers are designed to kee the flow rate constant at a re-set value with the hel of the ressure difference between the inut and outut flow, while the ass flow eter easures the flow in the revailing conditions. he erforance characteristics of ass flow controllers and eters have been studied by a nuber of research grous [7][8] in which the uncertainty of the devices has also been evaluated [9][0]. In addition, the ISO 645/7 standard for the use of ass flow controllers for calibration uroses is available [6]. 3.. ritical nozzles An other ethod for obtaining standard gas ixtures by the dynaic ution ethod is the use of critical orifices. he oeration of the orifices is based on the characteristics of gas flow through an orifice (nozzle at the velocity of sound. An ISO standard is available for the set u of critical orifices [5]. he erforance characteristics of critical orifices have not been evaluated as widely in the literature as MFs. Provided that the critical conditions across the orifice are fulfilled, i.e., the ratio of ustrea ressure to downstrea ressure exceeds the value of and the ratio of the diaeter of the orifice to the diaeter of the ustrea tube reains below 0., the ass flow ( assing isotroically through the sonic orifice can be exressed in the for []: Acd M R (7 With ass flow A c the cross-section area of the sonic nozzle d the concentration coefficient of the gas jet (discharge coefficient ustrea ressure, absolute (hpa ustrea teerature (K M olar ass of the gas R gas constant of an ideal gas ratio of the ass theral caacities c /c v 6
Euation (7 can be exressed in the for: Ac d * (8 Where in euation (8 the coefficient * is: * M R (9 Fro euation (8 one can see that the ass flow through the sonic nozzle deends on the ustrea teerature and ressure, and on the coefficients A c, d and *, which deend on the geoetry of the nozzle, characteristics of the flow dynaics (Reynolds nuber, on the theral roerties of the gas (the ratio of c /c v and on its olar ass, resectively. he discharge araeter is a sooth function on Reynolds nuber and deends on the geoetric factors of the orifice. On coercial gas utors the anufacturer has defined the geoetric factor, the conditions for the discharge coefficient and the gas roerties. he ressure and teerature conditions should be controlled by the user. 3..3 Lainar flow eleents he lainar flow theory is well established which serves the way to define the gas behavior accurately. An alication for deterining the flow rate based on lainar flow theory is ade coercially available e.g. by DH Instruents, Inc., USA (Molbloc/Molbox flow easureent syste. he Molbloc/Molbox syste achieves unrecedented levels of stability and recision by alying today's odern ressure sensors, atheatical odeling and data rocessing technologies to the lainar flow rincile. Following the Hagen-Poiseuille's law [], the ass flow of a coressible ediu through an annular athway in the lainar regie can be exressed as: ( ( P, (0 g ( P, P With the ass flow eual to ( + / the ustrea absolute ressure the downstrea absolute ressure ρ the gas density under P, conditions the standard teerature of gas the gas coressibility factor under standard conditions the absolute teerature of the gas (P, the gas coressibility factor under P, conditions η (P, the dynaic gas viscosity P the standard ressure g the geoetric factor, defined exerientally Standard condition of teerature and ressure, 0 and 0.3 kpa 3..4 oarison of the flow easureent ethods he high accuracy of flow easureent is ostly due to the high erforance of the flow control and easureent systes used for the dynaic ution ethod. In the resent case the best erforance for gas flow easureents is achieved with the lainar flow eleents with a standard uncertainty of 0. % and u to.0 % for ass flow controllers. he rather new solution of 7
ass flow controllers by MOS techniue iroves the standard uncertainty down to 0.4 %. he standard uncertainty of the critical nozzles lies soewhere between the uncertainty ranges above. echnical solution of critical orifices can vary fro sohisticated coercial solutions by accurate ressure control to a sile flow ort ade by drilling or by haer the etal tube. he cost of the flow easureent syste alying the dynaic ethod can also vary orders of agnitude and ostly is connected with the erforance characteristics of the flow easuring device, controlling unit and data acuisition syste. In this resect lainar flow eleent syste is ost exensive. Deending on the technical reuireents cheaer solutions are available by ass flow controllers or single critical nozzles. 3.3. IMPLEMEAIO OF DYAMI MEHOD he ileentation of the dynaic ethod to generate low concentrations of O, O (below 00 nol/ol and SO (below 50 nol/ol reuires soe recautions to achieve the target uncertainty of 3%. he instructions are addressed both for the laboratory as well as for the transfer syste alying the dynaic ution ethod. he various criteria to consider are listed below: Dilution syste o ature of the aterials in contact with the different gases used to avoid reactions between aterials and the different gases o Purging and conditioning rocedure o Leak detection High concentration gas ixture o he concentration range to be used and the associated uncertainty o he ressure regulator (aterial, flushing and evacuation ossibility Quality (urity of the ution gas Regulation and easureent of the gas flows (san and ution flow rates o alibration of gas flow eter Hoogeneity of the generated standard gas ixture Stabilization tie to obtain a reference gas ixture with a stable concentration ote : Soe guidances are given in the encycloedia fro Air Liuide [] and on the website of Sectragases [3]. 3.3. Dilution syste 3.3.. ature of the aterials Before generating a standard gas ixture by dynaic ution, it is advised to further check the suitability of the laboratory and the transfer syste against the ossible cheical reactions of ixture coonents. Secial consideration shall be given to the attack by corrosive gases such as SO and O on etals and ossible reactions with elastoers and greases used, for exale, in the valve seats and seals. Such reactions should be revented by using only aterials that are inert to all coonents of the ixture. If this is not ossible, easures shall be taken to iniize corrosive attack on the aterials with which the gases are in contact so as to revent any significant effect on ixture coosition and any danger in storage and use. Inforation on the coatibility of gases with container aterials is given in gas saling guidelines, corrosion tables and gas sulier handbooks. 8
Inforation on the ost coonly used gases and their level of coatibility with a range of aterials is outlined in able. When using olyeric transfer lines, it is also iortant to consider their ereability by certain gases and water vaour. For exale, PFE is very inert and suitable for a large range of gases; however, the ereability of this aterial akes it unsuitable for use with water-soluble gases, as water vaour can easily ereate into the transfer line. Material Gas Stainless steel eflon Glass/uartz 36L Silica-lined stainless steel Polytetrafluoroethylene (PFE Perfluoroalkoxy coolyer resin (PFA Fluorinatedethene-roene (FEP O S S S a S a S a S O Possible c S S a Possible c Sa b S SO S S a S a S able : Material coatibility, S: Satisfactory for use with the intended gas with suitable conditions at roo teerature, : ot satisfactory for use with the intended gas, a use is ossible but the concentration of the reference gas ixture ay be influenced by ereation of analyte, b ossible use with high flow rates, c satisfactory but corrosive in resence of water excet for high uality stainless steel. It is advised to use short transfer lines in order to: iniize ereation, liit the leaks in the transfer syste between the cylinder and the analyser, take care of the connections, decrease the stabilization tie for obtaining a stable concentration. It is recoended to use all etal sealed connections. 3.3.. Purging and conditioning rocedure he transfer line integrity, including the leak tightness and cleanliness of all the coonents (ressure regulator, valves, transfer line, connections, etc. shall be guaranteed to avoid containation of the syste and of the gas cylinder. In order to guarantee this, aroriate euient, aterials choice and urging rocedure shall be used. For reactive gases it is advised to urge the transfer line with a flow higher than the flow used after to generate the reference gas ixture. For adsorbing olecules like O and SO conditioning the entire gas syste with the gas ixture for aroriate tie (axiu two hours saturates the adsorbing sites of the walls, resulting in ore stable coositions. It is recoended to seuentially ressurise, vent and evacuate all coonents of the transfer syste with the gas ixture to be used by a ebrane u (ideally to 0 bar. his ethod reuires tyically three cycles. In case evacuation is not ossible, five to eight ressurising cycles are recoended. he insertion of a sto valve after the ressure regulator is recoended for safety reasons. An additional conditioning eriod of tyically one to two hours followed by a last ressurising/venting cycle (not evacuation further ensures the stability of the gas coosition. htt://www.swagelok.co/downloads/webcatalogs/de/ms-06-64.df e.g. SEMI F0-0305, Secification for 36L Stainless Steel Bar,Forgings, Extruded Shaes, Plate, and ubing for oonents used in General Purose, High Purity, and Ultra-High Purity Seiconductor Manufacturing Alications For coating anufacturers see e.g. www.restek.co or www.silcoek.co 9
he choice of a low-volue ressure regulator for the high concentration ixtures is recoended. Reactive gases such as SO and O reuire extensive urging of the wetted areas of the transfer line and, in articular, the low-volue ressure regulator. he use of secial regulators for ultraclean gases with searate ressure, vent and evacuation orts on the high ressure side are recoended. In the laboratory the dynaic ution syste is colete and ready to use. It is recoended to leave the syste ressurized in order to avoid the difficulties which ay haen in transfer systes. However, when changing the Priary Reference gas Mixture (PRM, during the aintenance of the syste or any tie when the syste has been in contact with the laboratory air the instructions given above are followed. 3.3. High concentration gas ixtures 3.3.. ye of gas ixtures It is recoended to use a Priary Reference gas Mixture (PRM reared by a ational Metrological Institute (MI and/or by a accredited laboratories according to the standard ISO 705 [4] and to the ISO Guide 34 [5]. his allows the use of gravietric gas ixtures as described in the standard ISO 64 [6] assuring traceability to the International Syste (SI with adeuate uncertainties even at low concentrations. Moreover in the certificate an exiry date is given according to the ISO Guide 34 secifying the stability tie of the concentration of the gas ixture. 3.3.. oncentration range and uncertainties o generate gas ixtures with aount fractions below 00 nol/ol for O, O and below 50 nol/ol for SO it is recoended to use gravietric gas ixtures in cylinders with concentrations and exanded uncertainties given in able 3. oonent oncentration (µol/ol Exanded uncertainty (% O 5 to 0.0 O 5 to 0.0 SO 5 to 0.0 able 3: oncentration ranges and exanded uncertainties of gas ixtures in cylinders with high aount fractions 3.3..3 Precautions with the gas cylinders he ileentation of the high concentration gas ixture reuires soe recautions. oncerning the choice of the ressure regulator care shall be taken with resect to aterial coatibility. Moreover reactive gases such as SO and O reuire the use of low-volue regulators which reduces the wetted surface area and hence reduces the level of urging reuired. Re-connecting cylinders always has the danger of creating a leaking connection which shall be avoided for several reasons. In an otiu set-u, the ressure regulator should reain on the cylinder and the cylinder ust be left with a iniu ressure inside. hese easures will hel to reduce the nuber of urge cycles for the ressure regulator rior to analysis. Environental air diffuses back into the ressure regulator over tie, even with this otiu set-u and if it is ressurized. A iniu oerating ressure secified by the RM roducer indicates, if alicable, the ressure value below which the gas should not be used. It has been reorted that below a certain ressure, 0
gas olecules that were attached to the cylinder wall coe off when the ressure dros in soe cases, resulting in higher ole fraction. he recaution actions that the oerators should take care of are resented in able 4. oonent Pressure regulator Miniu oerating ressure (bar Storage teerature ( O Stainless steel Low dead volues Seals coatible with O 0 or higher (according to secification of anufacturer according to secification of anufacturer O Stainless steel Low dead volues Seals coatible with O 0 or higher (according to secification of anufacturer according to secification of anufacturer SO Stainless steel Low dead volues Seals coatible with SO 0 or higher (according to secification of anufacturer according to secification of anufacturer able 4: Ileentation of the high concentration gas ixture 3.3..4 Storage he storage and oeration of gas ixtures in cylinders are often liited by the anufacturer to certain teerature ranges. If the cylinder is stored at or exosed to teeratures below the stated range given by the anufacturer, soe coonents ay condense and thus leading to a ossible change in the coosition of the ixture and voiding the values given in the certificate by the anufacturer. If this haen a ore detailed guide is rovided in 5..5. It is recoended to avoid high teeratures (heating close to the cylinder, e.g. fro ovens or other intensive heat sources. High teeratures will result in higher ressures leading to otentially hazardous situations. In addition, elevated teeratures ay result in decoosition of therally unstable secies. 3.3..5 onditioning of gas cylinders Mixtures ay reuire to be hoogenized again if stored for longer eriods of tie and if exosure to teeratures below the condensation oint of one or ore substances cannot be excluded. his ay be done by bringing the cylinders u to abient teerature and rotating the horizontally for an aroriate eriod of tie, which ay deend on the atrix gas and the coonents. It is further advised to store the cylinders for iniu 4 h at laboratory teerature rior to their use. 3.3.3 Purity of the ution gas
Analytes with x B ax values in zero air SO O O O Bz o Xy H 4 H 3 Ozone H S O O H O Standard (nol/ol (µol/ol E 4 for O and O 4 50 E 4 for SO 0.05 00 50 Both criteria - - 0.05 00-4 50 able 5: Maxiu allowed aount of substance fractions x B ax in zero air for abient air easureents and the values in Euroean standards (E for the secified analytes. a Bz: benzene, o: toluene Xy: xylene. For benzene the standard sets the criteria for zero air to be below the detection liit of the used ethods, the lowest value is indicated. If the sae zero air is to be used to easure all listed analytes, all criteria have to be fulfilled. 3.3.4 Regulating and easuring the gas flows (san and ution flow rates he regulation and the easureent of the gas flows are crucially iortant for the accuracy and the stability of the concentration of dynaically roduced standard gas ixtures. 3.3.4. oon criteria for all flow eleents he ethod for controlling the flow deends on the secific roerties of the gas (e.g. theral caacity, viscosity. herefore the flow eleents need to be calibrated for the sae secific gas as it is used. In order to be able to guarantee the stability and the coosition of the flow soe additional actions need to be taken care of. he dead volues of the flow regulator eleents (MFs, sonic nozzle, valves should be ket as sall as ossible in order to iniize containation. he inner stainless steel etal surfaces of the eleents and tubings are referably all electro olished to iniise gas adsortion. For the use, the ounting and the oeration of the flow eleents, the instructions given by the anufacturers should be followed. Electronic eleents dissiating heat should be laced above the gas tubings, the housing should be vented. he use of solenoid valves is discouraged, neuatic valves are recoended. All coonents and fittings (seals should be evacuatable to enable the uch ore efficient flushing cycles with vacuu and ressure instead of ressure only. Furtherore the construction of the ution syste should allow calibrating the flow eleents in its oerating ounting and configuration without e.g. the need for disassebling the hardware. his guarantees the validity of the forerly erfored leak test a crucial reuireent esecially for the tubing arts between the flow eleents and the ixing zone. alibration of the flow eleents should be conducted at least once a year by an accredited laboratory in the oerating range and under defined oerating conditions (teerature and ressure. It should be noted that the values of the flows usually given in certificates are at standard conditions, norally at 0 (73.6 K and 03.5 hpa. he easured values need to be converted to standard conditions. o be sure that there is no atrix effect during calibration of the flow eleents, calibration of the flow eleents should be ade by the sae gas atrix as for generating the dynaic standard gas ixtures. he flow eleent for high concentration gas ixture should be calibrated by the arent gas of the cylinder (nitrogen and the flow eleent for
ution should be calibrated by ution gas (synthetic air. In soe of the flow eleents the oerator can change the setting for the gas in use even when calibrated with different gas. he oerator needs to be careful when using these otions in case of confusions. It is also recoended to coare the whole dynaic ution syste against the other ethod between the flow calibrations in order to avoid drift of the flow eleents. ot only the correct calibration gas but also the calibration conditions should be the sae as oeration conditions, see below. In case if a lainar flow eleent is used as flow eter it should be calibrated with the sae gas coosition as for generation of the dynaic standard gas ixture. 3.3.4. MF s It is recoended to use the flow eters not below 0 % of their axiu flow and to kee the iniu flow of the high concentration gas ixture above 5 l/in to liit retention in the transfer syste and to allow roer urging of the transfer lines. he foreline ressure set by the reducing valve is generally sufficiently stable but should be as secified during the calibration and for the use. Due to the uch faster resonse tie of MF s with sensor based on the oleentary Metal Oxide Seiconductor (MOS technology and conseuently better stability and reroducibility, this new tye is referred where ever ossible. Flushing with inert and dry gas after each use is necessary to avoid corrosion. It is recoended to take care of the coatibility between the aterials of the MF s and the gas assing through the MF s. 3.3.4.3 Lainar flow eleents hey need to be oerated with controlled ustrea ressure and with additional MF s for setting the flow. hese reuireents ake their use laborious and ractically restricting it to laboratory alications. It is recoended to relace the initial EFLO anufacturing tubing between the lainar flow eleent and the ressure sensor by a silica-lined stainless steel tubing. he different eleents of lainar flow easureent syste (lainar flow eleent and ressure sensors should be calibrated together. It is recoended to correct the resonse of the ressure sensors by using the tare function before each easureent according to the anufacturer rocedure. If necessary the ressure sensors of the eleents can be calibrated and adjusted. 3.3.4.4 ritical nozzles he stability of the flow deends on the u- and downstrea ressure regulation. onseuently it s iortant to take care of the choice of robust ressure regulation eleents (echanical or electronic. Moreover due to the gas exansion the nozzle housings and tubings should be teerature stabilized to allow reasonable stabilisation ties. In coercial devices any nozzles are cobined in various ways with the flow of the gas standard and the ution gas, resectively, in order to obtain a great nuber of evenly distributed ution ratios. It is worth to ention that the total flow for the different ution ratios reains constant and that the oeration of the nozzles is echanical (no electronic signal is needed and that the flow follows the hysical theory. he calibration should be ade at all discrete ution ratios to avoid the need for interolating values. 3.3.5 Hoogeneity of dynaic standard gas ixtures It is essential that a gas ixture is hoogeneous before it is analysed or used as a standard gas ixture. Hoogeneity is defined in ISO 7504:00 [7] as the state of a gas ixture wherein all of its coonents are distributed uniforly throughout the volue occuied by the gas ixture. Inhoogeneous ixtures anifest by long and unstable readings of the easureent instruent without obvious drifts. 3
Soe recautions should be taken to ensure the hoogeneity of the dynaic standard gas ixtures. It is recoended to use: urbulent flow eleents after the ixing oint, like a eandering tube. A ixing chaber (aterial: glass, stainless steel or coercial ixing nozzles. he rincile is injecting the (low flow of a high concentration gas ixture erendicular into the lainar to turbulent ain flow of the ution gas through fine nozzles into a ixing chaber (length l R with a sufficiently large diaeter d R. he necessary diensions of the reciient (e.g. a tubing with a larger diaeter than the inlet and outlet for various flows can be calculated fro the diffusion tie of the analyte gas B in the ution gas A in each eleent with the finite eleent ethod. An estiation for the necessary tube diaeter d R can be given by alying Einstein s diffusion euation (e.. he diffusion tie t D of the analyte olecule B for the sallest diension d R of the chaber (erendicular to the ain flow direction and the residence tie t R of the ution gas in the chaber (e. are calculated. his estiation is for the worst case not considering turbulence and convection. dr td ( D BA, with D B,A the diffusion coefficient of substance B in substance A t R V d l V 4V R R R ( with V R, the volue of the ixing reciient and l R its length he necessary criterion for hoogeneous ixing by diffusion only is that t D t R As an exale: for O diffusing in at 0 and 00 kpa for a distance d R of 0.4 c a t D of 0.7 s results. For a ixing chaber volue of L and a volue flow of 400 L/in, t R is 3.3 s, thus the criterion is well fulfilled. For further details and references see the ublication by Haerri [8]. 3.3.6 Stabilization tie It s iortant to use dynaically uted gas ixtures for calibrating analysers only when their concentrations are stable over tie; otherwise the analysers will be calibrated with wrong concentrations resulting in erroneous abient air easureents. he stabilization ties for obtaining standard gas ixtures with a stable concentrations are linked to: he retreatent of the cylinders, the nature and the ileentation of gases he transfer syste (design, diension, flows he ixing syste and the coonents used (aterials, volues Orders of agnitude for stabilization ties are given in able 6. 4
oonent oncentration range (nol/ol Dilution ratios Stabilization tie (in O 0 to 00 /000 to /00 u to 5 O 0 to 00 /000 to /00 u to 0 SO 40 to 50 /000 to /00 u to 0 able 6: Orders of agnitude for stabilization ties (ution ratios are given for a RM with 5-0 µol/ol 3.4. UERAIY OF DYAMI DILUIO MEHOD 3.4. alculation of the exanded uncertainty he cobined standard uncertainty, u c (y of the dynaic ution ethod is calculated fro the variance of u c (y according to: c ( y i u f ( x i xi u x i (3 where y is the estiate of the easurand (Y= f(x,x,,x n and u c (y is the cobined standard uncertainty of the estiate (y. Variances (u(x i of the inut estiates (x i can be calculated or estiated fro the function f(x,x,,x n. he exanded uncertainty is obtained fro the cobined standard uncertainty according to: U k u (y (4 c where u c (y is the cobined standard uncertainty of the easurand (Y and k is a coverage factor. In this connection k = for all uncertainty calculations corresonding to a confidence level of 95 % for the easureents that follow noral distribution. he cobined standard uncertainty of the dynaic ution ethod can be exressed alying the euation (3 to the euation (. he detail of the calculation is given in Annex. san uc (Bi u (B u ( B san san (B B san (B B u (san (san (san u ( (5 If B is eual to zero, the euation (5 becoes : san uc (Bi san u (B (san B san B u (san (san u ( (6 And the exanded uncertainty, U, can be calculated as following: U u c ( Bi 5
Euation (6 is slightly colicated when several flow easureent devices are connected at the sae tie for rearation of the gas ixture for calibration urose and when iurity of the ution gas and/or in the gas standard are taken into account. he concentration Bi and the associated exanded uncertainty are given in the calibration certificate of the high concentration gas ixture. he standard uncertainty on Bi is calculated as following: u( Bi 3.4. 3.4.. U( Bi (7 Standard uncertainty on Mass flow controller If ass flow controllers (MF are alied with dynaic ution ethod to easure san and ution flows, the standard uncertainties of each MF are a cobination of the uncertainty found in the calibration certificate issued by accredited laboratories or MI s and other sources of uncertainty (influence of teerature, drift. he uncertainty can be exressed as a relative value of the full scale or by cobining the erforance characteristics (e.g. linearity, reeatability, drift of the device that give the ajor contributions to the uncertainty budget of the MF. he variables and uncertainty sources that give the ajor contributions to the exanded uncertainty can be defined by exaining the standard uncertainties of each of the variables and their resective sensitivity factors. 3.4.. ritical nozzle alibration by an accredited laboratory In the case the critical nozzles are alied with dynaic ution ethod to regulate san and ution flows, the standard uncertainties of each critical nozzle can be obtained fro the calibration certificate issued by an accredited calibration laboratory. It can be exressed as a relative value of the full scale which is a 6
Since the analytical evaluation is cubersoe and not all uantities are easurable, e.g. A c and d, the uncertainty analysis with the flows fro the certificate with a gas flow standard is ractical and widely alied. 3.4..3 Lainar flow eleent alibration by an accredited laboratory In case the lainar flow eleents for the san and ution flows are calibrated by an accredited laboratory (according to ISO IE 705 the standard uncertainties of each calibration oint are indicated in the certificates. alibration with theoretical function he flow of a ediu through a lainar flow eleent is analytically described by the laws of flow dynaics, see e. 0. In coarison with the flow through a critical nozzle ore araeters characteristic of the gas are associated and ore hysical uantities both ustrea and downstrea of the flow eleent need to be known. he variance of the cobined standard uncertainty for a single lainar flow eleent can be exressed in the following for, for details see Annex 3. u ( u( u( u( u( ( ( u( u( u(g u( g u(, u(, u(,, (0 4. GAS PHASE IRAIO (GP MEHOD he gas hase titration ethod is based on the international standard ISO 5337 [0] and this guide is coleted with the latest knowledge and the exerience of the articiating laboratories in the fraework of the MAPoll roject. 4.. PRIIPLE he gas hase titration of nitrogen onoxide by ozone is a sile gas hase biolecular reaction between O and oxygen (O. he titration can be resented as: O + O 3 O * + O, (R O : 600 n 400 n he nitrogen dioxide fored in reaction (R can be in the excited state, O *, which decays to the ground state. he wavelength, λ, of the transition energy occurs between 600 n and 400 n, with an intensity eak at 00 n. he rate constant, k R, of the reaction (R has been studied uite intensively over a wide teerature range by a nuber of research grous. he reaction rate constant of k R = (.8 x 0-4 c 3 olecule - s - at 98 K [0] is used. he reaction (R is also an iortant reaction in the atoshere. Once O is fored through the reaction (R, nitrogen dioxide dissociates into nitrogen onoxide and oxygen in the resence of sunlight; this is the ost iortant hotocheical reaction occurring in the atoshere. 7
During the GP, nuber of reactions with nitrogen dioxide and ozone ay take lace with the wall aterial of the reaction chaber. hese reactions have been studied in the literature []. In order to avoid the other reactions than (R the reaction chaber and the tube aterials in the easureent syste need to be ade of inert aterial to avoid detectable loss of reactants. he reaction (R is allowed to roceed in an excess concentration of nitrogen onoxide coared to the concentration of ozone, in order to consue the ozone coletely in the reaction. he reverse situation, ozone in excess of nitrogen onoxide, would leave ozone in the syste. As a reactive olecule it would react again with the nitrogen onoxide after the O O conversion causing nonlinear behaviour of the results. Fro the conservation of nitrogen in the reaction (R, the su of nitrogen onoxide and nitrogen dioxide is a conservative uantity, i.e., [O] + [O ] = [O] o + [O ] o = [O] o ( On the right-hand side of the euation, an initial situation [O ] o = 0 has been used. he reaction (R takes lace in a stochioetric condition, i.e., the changes in the ozone concentration euals to the changes in the nitrogen onoxide concentration: Δ[O 3 ] = [O 3 ] o [O 3 ] = [O] o [O] = Δ[O] ( he titration takes lace norally in a reaction chaber where the reaction tie ust be long enough to allow the titration reaction to go to coletion. he rate constant of reaction (R is welldocuented in the literature over a wide teerature range []. he rate of change of nitrogen onoxide in reaction (R using the condition of Euation ( can be calculated according to: d O k 3 3 0 0 dt O O ko O O O Integration of Euation (3 with the initial condition of Euation ( gives the reaction tie: (3 t k ( O O ln O 0( O3 O 0 0 O O 3 0 3 0 0 O (4 where t the tie of reaction (in k the rate constant (nol/ol - in - [O 3 ] o the initial O 3 concentration (in nol/ol [O] o the initial O concentration (in nol/ol [O] the final O concentration (in nol/ol An iortant asect of the GP ethod with nitrogen onoxide is that the aount of ozone introduced into the syste can be calculated fro the change of nitrogen onoxide. his gives the ossibility of tracing the ozone concentration to a reference standard of nitrogen onoxide ade by a gravietric ethod. However, there are soe robles along the way. First, the reaction (R has to go to coletion with resect to ozone. he aterials of the GP syste have to be inert to ozone as well as to nitrogen coounds, in order to revent the loss of reactants with the wall and tube aterials. he best way is to build a very coact syste of inert aterial and use high concentrations of both gas coonents to iniize the reuired delay tie according to Euation (4 for the colete reaction (R. 8
Laboratory studies have been erfored to deonstrate GP as a ethod to trace the ozone reference standard to a gravietrically-reared nitrogen onoxide standard. In order to reach a colete reaction (R, an eirical araeter, the so-called dynaic araeter [] is used. his araeter, P R, is a roduct of the concentration of nitrogen onoxide and the residence tie, t R, of the reactants in the reaction chaber. It should fulfil the criteria: P P R O t O.75ol / ol in R R R f V f ozone O (5 where V R is the volue of the vessel (reaction chaber, and f ozone and f O are the flows of the ozone and the nitrogen onoxide, resectively. he volue of the reaction chaber shall designed in such a way that t R > t, where the reaction tie, t, is calculated fro Euation (4. he coercial utors which are euied with the otions for erforing the GP, can be designed in such a way that the concentration of nitrogen onoxide in the reaction vessel, [O] R = [O] SD, i.e., no ution with zero air takes lace before the reaction with ozone. When the concentration of O is high, the reaction tie is short enough to ake the reaction (R colete with ozone. Dilution of the reactants should take lace after the reactions in the ution chaber. As an exale of the coercial gas utors euied with facilities for conducting the GP, the flow rate of ozone can be f ozone = 50 to 00 l/in and f O varies deending on the ution rate, being fro 3 to 00 l/in. he size of the reaction chaber can vary fro 0 c 3 u to coule of hundreds of c 3 and the concentration of the gas standard of nitrogen onoxide can be 50 µol/ol or higher. he dynaic araeter, P R is thus 0 [μol/ol in] taking the lowest values of each variables fulfilling the criterion of Euation (5. It can also be lower than the criterion of Euation (5 deending on the cobination of the values for each of the variables. Dilution of the nitrogen dioxide roduced by the reaction (R, need to be ade raidly after the coleted reaction. he concentration of the nitrogen dioxide roduced can be calculated with the hel of Euations ( and (: O O O O O rod O SD fo O3 SD fozone o 3 o 3 Oactual (6 f f f f O he two siilarities on the right-hand side of Euation (6 describes the roduction of [O] o and [O 3 ] o for reaction of O + O 3 in the GP syste. he [O] SD and [O 3 ] SD are the sources for rearation of the initial concentrations of [O] o and [O 3 ] o by ution ethod fro gravietric standard of O and fro the ozone source (e.g. by Standard Reference Photoeter or by other UV-la. he flow rates f O. f ozone and f are the flow rates used for rearation of the gas concentrations. Dilution flow needs to be large enough to have excess of flow at least of 0 % of the total flow of the GP syste including the sale flow of the analyzer(s. In the GP syste the ozone is consued coletely and therefore [O 3 ] = 0 in Euation (6. However, [O]=[O] actual, is not zero and is the actual concentration which is not reacted with the ozone i.e. is not titrated. In Figure a tyical set u for erforing the GP to calibrate ozone analyzer or to deterine the O converter efficiency of the cheiluinescence O-O x analyzer is resented. ozone 9
Figure : yical set u for the GP syste. he ozone generator can be an external or integrated art of the ution device. he ozone concentration can be injected directly into the ixture of high O concentration line (solid line or into the ixture of low (uted O concentration line (dashed line. It s iortant to note that the ozone concentration can be injected either with the high O concentration or with the uted concentration line. he advantage of the forer way is that the reaction tie is very short in order of seconds or less while in latter case the reaction tie is order of tens of seconds. In latter case the ixing (reaction chaber chaber needs to be large enough in order to have the reaction (R in coletion. here is also clear evidence (Viallon et al. 006 [3], anioto et al. 006 [4] that the agreeent between the erforance of the riary ozone hotoeter i.e. Standard Reference Photoeter (SRP by IS and the GP are not consistent but rather has a bias of to 3 %. he coittee "ASO" ("Absortion ross Sections of Ozone" established in sring 009 is a joint ad hoc coission of the Scientific Advisory Grou (SAG of the Global Atoshere Watch (GAW of the World Meteorological Organization (WMO and the International Ozone oission (IO 3 of the International Association of Meteorology and Atosheric Sciences (IAMAS. he andate of ASO includes: Review of the resently available ozone absortion cross sections with a riority on Huggins band. Deterination of the iact of odifying the reference ozone absortion cross sections for all of the coonly used (both ground-based and satellite atosheric ozone onitoring instruents. Recoendations whether a change needs to be ade to the resently used WMO/IO 3 standard ozone absortion cross section data (by Bass and Paur, 985. he recoendations are to be discussed between the counity of the involved exerts. Until now the final decision of the change of the cross section which will change the concentration level obtained fro the SRP has not been ade by he International Bureau of Weights and Measures (BIPM. 4.. IMPLEMEAIO OF GP MEHODS he ileentation of the GP ethod to generate low concentrations of O (between 0 to 00 nol/ol reuires soe recautions to achieve the target uncertainty of 3%. 0
he instructions which are addressed both for the laboratory as well as for the transfer syste alying the GP ethod are listed below: GP syste o ature of the aterials in contact with the ozone, nitrogen onoxide and nitrogen dioxide o Purging and conditioning rocedure o Leak detection High concentration gas ixture: O o he concentration range to be used and the associated uncertainty o he ressure regulator (aterial, flushing and evacuation ossibility Stability of the ozone source Quality (urity of the ution gas Regulation and easureent of the gas flows (O 3 flow, O flow and ution flow o alibration of gas flow eter Hoogeneity of the generated standard gas ixture of O Reaction tie for the coletion of the reaction (R in the GP syste to obtain a stable concentration of O 4.. GP syste 4... ature of the aterials Before alying the GP syste, it is advised to further check the suitability of the laboratory and the transfer syste against the ossible cheical reactions of ixture coonents. Secial consideration shall be given to avoid the attack by corrosive gases such as O on etals and ossible reactions with elastoers and greases used, for exale, in the valve seats and seals. Such reactions should be revented by using only aterials that are inert to all coonents of the ixture. If this is not ossible, easures shall be taken to iniize corrosive attack on the aterials with which the gases are in contact so as to revent any significant effect on ixture coosition and any danger in storage and use. Inforation on the coatibility of gases with container aterials is given in gas saling guidelines, corrosion tables and gas sulier handbooks. Inforation on the ost coonly used gases and their level of coatibility with a range of aterials is outlined in able 7. When using olyeric transfer lines, it is also iortant to consider their ereability by certain gases and water vaour. For exale, PFE is very inert and suitable for a large range of gases; however, the ereability of this aterial akes it unsuitable for use with water-soluble gases, as water vaour can easily ereate into the transfer line.
Material Gas Stainless steel 36L Silica-lined stainless steel Polytetrafluoroethylene (PFE eflon Perfluoroalkoxy coolyer resin (PFA Fluorinatedethene-roene (FEP Glass/uartz O S S S a S a S a S O Possible c S S a Possible c Sa b S O 3 S S a S a S able 7: Material coatibility, S: Satisfactory for use with the intended gas with suitable conditions at roo teerature, : ot satisfactory for use with the intended gas, a use is ossible but the concentration of the reference gas ixture ay be influenced by ereation of analyte, b ossible use with high flow rates, c satisfactory but corrosive in resence of water excet for high uality stainless steel. It is advised to use short transfer lines in order to: iniize ereation, liit the leaks in the transfer syste between the cylinder and the analyser, take care of the connections, decrease the stabilization tie for obtaining a stable concentration. It is recoended to use all etal sealed connections. 4... Purging and conditioning rocedure he transfer line integrity, including the leak tightness and cleanliness of all the coonents (ressure regulator, valves, transfer line, connections, etc. shall be guaranteed to avoid containation of the syste and of the gas cylinder. In order to guarantee this, aroriate euient, aterials choice and urging rocedure shall be used. For reactive gases it is advised to urge the transfer line with a flow higher than the flow used after to generate the reference gas ixture. For adsorbing olecules like O conditioning the entire gas syste with the gas ixture for aroriate tie (axiu two hours saturates the adsorbing sites of the walls, resulting in ore stable coositions. It is recoended to seuentially ressurise, vent and evacuate all coonents of the transfer syste with the gas ixture to be used by a ebrane u (ideally 0 bar. his ethod reuires tyically three cycles. In case evacuation is not ossible, five to eight ressurising cycles are recoended. he insertion of a sto valve after the ressure regulator is recoended for safety reasons. An additional conditioning eriod of tyically one to two hours followed by a last ressurising/venting cycle (not evacuation further ensures the stability of the gas coosition. In the laboratory the GP syste is colete and ready to use. It is recoended to leave the syste ressurized in order to avoid the difficulties which ay haen in transfer systes. However, when changing the Priary Reference gas Mixture (O standard, during the aintenance of the syste or any tie when the syste has been in contact with the laboratory air the instructions given above are followed.