Brambilla, E.

Ceresoli, M.

Sandroni, S.

English

Archive of European Integration

COMMI- SSION  OF THE  EUROPEAN  COMMUNITIES · environment and quality of life 1976 A  CALIBRATION DEVICE FOR  LONG-PATH SENSORS OF ATMOSPHERIC POLLUTANTS EUR  5472  e LEGAL  NOTICE This  document  was  prepared  under  the  sponsorship  of the  Commission  of the European Communities. Neither the Commission  of the  European  Communities,  its contractors nor any  per-son  acting on their behalf: make  any  warranty  or  representation,  express  or  implied,  with  respect  to  the  ac-curacy, completeness, or usefulness of the information contained  in  this document, or that the use  of any  information, apparatus,  method  or process  disclosed  in  this document may  not infringe privately owned rights; or assume  any  liability with  respect  to  the  use  of,  or for damages  resulting  from  the use of any information, apparatus, method or process disclosed in this document. This report is on  sale at the addresses given  on  page 3 of cover. Copyright CECA, CEE,  CEEA, Luxembourg 1976 ~ . Printed in the United Kingdom. [cOMMISSION  OF  THE  EUROPEAN  COMMUNITIES Jo±nt  Nuclear Research  Centre - Ispra Establishment ~,  CALIBRATION  DEVICE  FOR  LONG-PATH  SENSORS  OF ATMOSPHERIC  POLLUTANTS  ,.---eJ  ol s.  SANDRONI  (~)~  E.  BRAMBILLA  (*), 'I M.  CERESOLI  (**) (*)  Chemistry Dir.  Euratom Joint Research  Centre,  Ispra {**)  Design  and Fabrication Dir.  Euratom Joint Research Centre,  Ispra Edited by the  Dire~torate General "Scientific and Technical  Information and  Information Management" 1976 ABSTRACT The  calibration of  long-path optical sensors  of  atmospheric pollutants like the Barringer correlation spectrometer is obtained by  a  device in which the optical pathlength is varied. By using the device  here described the calibration procedure requires  about  6  min.  for the full length expansion of  the cell equivalent  to concentration-pathlength variation of 1  :  250. The  device  is simple  and  can  be  installed on  an optical bench i.e. for simulation of polluted atmosphere. CONTENTS I  The  need of a  calibration device  5 II  The  variable pathlength cell  6 III  Detailed description  7 IV  Mode  of operation  8 Figures  10 5 I. THE  NEED  OF  A CALIBRATION  DEVICE Within the framework  of  a  research activity aiming at the application of optical methods  to remote  sensing of air pollution,  some  difficulties have  been experienced in calibrating instruments based  on  resonance  absorption, and suitable for air quality measurements  over  a  long pathlength.  Instruments for  this application are correla-tion spectrometers  (i.e. Barringer  {x)  CospecTM)which  uses a  modulated  Xe  lamp  for  average  concentration measurements of  so2  at  around  31 o  nm  and  of  No2  at  around 440  run,  and tunable  laser analysers which use specific absorption of infrared light emitted by  a  diode  laser or  by other tunable laser device(1). The  classical ways  of calibrating these  absorption  analysers are:  1)  by inserting two  optical standards  and  extrapola-ting a  linear relationship to a  zero level or 2)  by introducing some  mixtures  of  gases  at different  concen-trations in a  cell having  a  fixed length.  Practically for a  correct calibration following method  2),  some  difficulties are experienced,  for  example  a)  it is necessary to pre-pare  and  to store large volumes  of  gas  mixtures  at very low concentration,  b)  the analysis  of  gas  mixtures  at  a  concen-tration of ppm  or  lower requires  large volumes  of  gases  or some  sophisticated  t~chniques and  c)  the wall effect in the cell and  in the  container may  modify the mixture  compo-sition. For  these reasons we  studied a  device which  by varying the length could permit  a  calibration by using  a  gas  mixture only. 6 II. THE  VARIABLE  PATHLENGTH  CELL The  device we  realised and its schematic  are  shown  in fig.1 and  2  respectively.  Essentially, it consits of  two  cylin-drical teflon bellows cells  (c1,  c2),  having  the left head fixed  on  the frame  holder  of  the device  and  the right one  moving  on  three bars;  the cells are  connected  by silicone tubes  in order to have  a  constant pressure.  The  cells are mounted  one  upon  the other for use  on  an  optical bench. A motor  (M)  operated by a  button on  the front  of  the holder moves  by means  of  a  driving  orum  (B)  one  of  the right heads towards  the fixed  one,  and  the other in the  opposite direc-tion.  The  PVC  heads  hold the optical windows  (W);  the left heads  hold the optical windows  by  a  teflon cylinder some  em  in length placed inside the bellows  to allow  a zero thickness  in the cell when  compressed.  In  one  connec-ting tube,  an inlet valve  (!)  for  gas filling and  a  teflon fan  (F)  operated by a  motor  are  inserted.  To  wash  the  gas circuit,  an exit valve  (V)  is inserted in one  left head. The  mode  of operation is quite simple:  the circuit is flushed with the  gas mixture to be  used for  several minutes; once  (I)  and  (V)  have  been closed by simply pushing the buttons for  (M)  and  (F)  the cell length can  be varied from about  zero to the maximum  value of  b  possible.  The  path-length b  of  one  cell is given by a  indicator on  a  meter scale on  the front  of  the  frame  holder;  in a  more  sophisti-cated way,  the b  values  can  be  transferred to a  recorder  (R) via a  potentiometer  connected to  (B).  A two-pen recorder can display simultaneously the  gas  signal and  b  values. In  our device  the maximum  pathlength is 50  em  with a minimum  of 0.2  corresponding to a  concentration path lenqth (c.b)  range  of 1  :  250.  By  introducing i.e.  the  commercial gas mixture so2/N2  at  100  ppm  (in vol.)  we  can  get  c  •  b values  from  50  ppm-m  down  to 0.2 ppm-m.  These  data are equivalent  over  a  1000 m.  optical path to concentrations from  50  to 0.2 ppb.  The  concentration range 1  :  250  fully covers  the concentration range  of most  of the pollutants 7 in the  atmosphere. The  device  operates at  atmo?pheric pressure but  a  somewhat lower pressure  can be used.  It is quite  simple,  easy to realize  and  can  be  installed on  an  optical bench,  i.e. for simulation of polluted atmosphere. III. DETAILED  DESCRIPTION  (fio.2) -·~·-~-~-Cells  ( c1 , c2)  They  consists  of  teflon bellows  (fig  .4),  600  mm. length,  external diameter  120 mm•,  internal diameter  80  mm. with 45  coils.  The  bellows  are made  by direct melting.  The minimum  length one  can obtain is 120 mm.  At  one  side of  the cell a  teflon cylinder 113  mm  length,  78  mm  diameter is inserted.  At  the internal end of the  teflon cylinder the optical windows  (w)  are welded  by Araldite type  Ay  101. For  operation in the ultraviolet/visible spectral region, which is the  case  of Barringer Cospec  analyser,  Tetrasil A discs 78  mm  diameter,  6  mm  thickness  have  been used.  In  the infrared region KRS-5  windows  could be used. Fan  (F)  It is a  mixin~!- teflon fan  moved  by  an electric motor Crouset  type  82160,  3000 rpm.  The  gas  tightness is assured by Viton type  a-ring-s resistent to most  of  corrosive  gases. C<?EE~Etin9"  tu.E.~2__(T)  These  are  silicone tubes  IDv/55  type, 14  mm  external diameter. Motor  (11)  It is a  Isotermic  Swiss  speed reducer with  a reduction ratio 405  :  1  with  5,9 rpm.  The  motor moves the screw cams  (B)  by  gears. Ba:~~?_.(B)  Both the  screw  cams  and  t1'1_P  fixed bars  are  in stainless steel.  The  sc:r·ew  cams  hl.ove  on  ball bearings and their position is regulated by  two  microswitches  • .  Po!~!l-_!3:-omete£_  (P)  It is a  Helipot  type potentiometer with an  accuracy of  0.25%  a.nd  a  resistence of 0.5  K  ~-!.. Recorder  (R)  It is a  two  pen recorder 10  mV  full scale v1e  use  a  Philips type  8221  PN  recorder. 8 Va.lves(V  and  three ways  valve Y)  are teflon or  glass valves. The  device holder is in alurainium  AG-3:  the front  panel has two  buttons for  (M)  and  (F)  and  a  meter for the direct reading of  the·  lower cell length.  The  bellows  are moved by the motor  (M)throu two  gears;  when  one cell reaches the maximum  optical length of  500·mm,  the other reaches the minimum  optical length of  2  mm  (obviously these distan-ces refer to the internal walls  of the windows).  The  speed of pathlength variation is 82  ram/min.,  the full expansion being obtained in about  6  minutes.  The  length of the  lower cell can  be read  (a)  directly on  the meter scale or  (b) on  the recorder simultaneously vrith  the  gas  signal.  So one  can directly verify the relationship between path-length and  absorption. IV.  HODE  OF  OPERATION The  mode  of  operation of this  dP.vice  is quite simple. First the  .flushing oB  the system is obtained in the following manner;  the va.lve  (I)  is connected to a  gas  supply and (V)  is opened•  The  gas  circulates alternatively via  (F) and  by  (c1)  and  (c2),  and  then  on  closing (I)  and  (V)  the fan  (F)  is started for  some  minutes,  after which the  gas is removed  by  (v).  This  operation is repeated several times until a  stable instrumental response is obtained. Once  filled the cells  and  closed  (I)  and  (V),  the variation of  the pathlength is obtained by pushing the button for (N)  on  the front  panel.  In about  6  minutes  the pathlength varies  from  2  to 500  mm  continously and  the  length b  is recorded  togeth~r with the  gas  signal~  fJ·.  4) 9 ACKNOWLEDGMENTS we  are  indebted to Mr.  G.  Giovanelli  and  G.  Cesari,Istituto di Fisica dell 1Atmosfera del C.N.R.  Bologna for  the helpful discussions in the realisation of the device. Fig. 1 -The calibration device B Fig. 2 -Schema~ic R C1-2 Gas cells M Motors F Fan I Inlet valve W Optical windows V Vacum connection B Bars TB Teflon bellows R T p Recorder Connecting tubes Potentiometer I Fig. ~ -A teflon bellow 'lT:l;:!'wl I I'.' '. ' 1! ~::_+i~H,,11:tt --q i• > I·! I' ' ·, •,, . . I ; . r• ,,, , ''f+t'' ' : I I --r-: --~ I j. --:·l -~ I I L..L l l ;. i. I I ' , • 1·1 1 ··" r f • t l I I I 'I j f ; I f-l I 1 ~ I,., ~ ~ ! lt-1 f -~~~ ; ·: j !·I : j-' I ·-•. ! . f!! I .. '·~-~-~H~ ; i fll: L: :r ; :: • t ~ ; t t v. ~--~tr:-r--t t-1--·--·1· t t 'it t'• t i j 'll; J! i·l; !tT ' I I ! I t t ' v' t ·t -~.-! .. ; I , I , I t i > I' I i j _t 1 __ ,! t ' I l ; > l· I ; l--i· ..:...i r-,.....,. -:· I-~ ',~·I. . ! ·I ' .-r·r-~ rT"' . I ' 1 ~ I • ; ! 1 r , 1 1 I 1 ·1 t l l 1 j I J . . ' ~ . j t ! ' ' l II .. ~ j . '_j ' .. 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A calibration device for long-path sensors of atmospheric pollutants. EUR 5472 e. Environment and quality of life

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