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Autocompensation photometer for long-term process monitoring
ELSEVIER
Chemical Engineering and Processing 36 (1997) 407-410
Autocompensation
photometer
for long-term
process monitoring
G. Kloos Techn.
Biiro J
Optik
und Elektrovwchnnik,
Rntyfeisenstr.
15, D-50259
Pullzeim,
Geimtinq~
Received 25 October 1996; received in revised form 27 February 1997
Abstract
A design of a processphotometer is proposedthat combinesfeatures of standard double-beamspectrophotometerswith a compensationfor window soiling that is used in industrial applications. Emphasisis laid on devising a reliable and robust monitoring device while keeping the photometer simple and inexpensive. by switching betweentwo optical paths. 0 1997Elsevier ScienceS.A. Keywords:
Absorption photometry; Process monitoring;
A separation
of main and reference signal is obtained
Optical null method
1. Introduction Photometers are used in chemical engineering as instruments for process control. A typical application is the monitoring of the absorption of a liquid before and after passing an oil separator. Main requirements on the performance of a photometer used to supervise industrial processes are accuracy, long-term stability and robustness. There are three well-known error sources that might affect the photometer read-out in the long term: (i) fluctuations and aging of the light source, (ii) drift of the detector, (iii) deterioration of the transparency of the cell windows due to deposits. A standard approach in scientific spectrophotometry to deal with the two first-mentioned problems consists in double-beam designs that allow one to compensate for drifts of the light source and the detector [l-7]. Often, an optical null method is realized employing an optical wedge to balance both light paths ([3] p. 123, [7] p. 269). As a solution of the third problem, photometers with an additional cell to compensate soiling of the sample cell window are used in industry [8,9]. Lock-in amplification [lo] is an effective means to obtain a stable and reliable read-out also under adverse conditions and is quite common in scientific photometry. A very elegant way to perfoml an absorption measurement using the lock-in technique is the two-frequency method [Ill: Sample and reference beam are chopped at different frequencies and the signal is read 0255-2701/97/$17.00 0 1997 Elsevier Science S.A. All rightz reserved. PIIs0255-2701(97)00014~-7
out using one detector and two lock-in amplifiers tuned to different frequencies. Here, the schemeof a photometer is presented that is optimized for long-term use. The lay-out is influenced by the study of scientific spectrophotometer designsand by an idea realized in the set-up described in references [8,9]. An attempt was made to develop a design with a minimal number of optical components and electronic instruments to obtain a monitoring instrument that is compact, robust and economical.
2. Photometer
design
A scheme of the proposed photometer and its mechanical parts and electronics is given in Figs. 1 and 2. The light from the source is divided by a beamsplitter and directed to the measurement arm and to the reference arm of the double-beam photometer. One half of the light intensity is transmitted through the absorption cell, reflected by a mirror and then measured by a photodiode. The other half passesan optical wedge and a reference cell used for compensation before being reflected to the photodiode. In order to implement a dynamic measurement principle, the intensity of the light source is modulated electronically. The modulation signal also serves as a reference for the lock-in amplifier that measures the signal from the photodiode. For the present purpose, -an-e= and compact lock-in module is suffi-
G. Kloos /Chemical
408
Engineering
and Processing
36 (1997)
407-410
error signal Motor
Control
/
-
photometer
read-out
Switching Mirror
n Light Source Mirror
@+
Beamsplitter Photodiode
I
Modulator
Absorption Cell 1 Additional Absorption Cell (optional) for the compensation of window soiling 2 Contact-free Switch
Storage and Switching
Fig. 1. Autocompensation
Unit
photometer and associated electronics.
cient. A simple device tuned to the modulation frequency of the light source responds to the needs of the given monitoring task. To discriminate a signal from the measurement arm against a signal from the reference arm, a slowly rotating chopper is inrroduced into the photometer set-up as switching device. The technical demands on the chopper motor and wheel are rather low, because the wheel can rotate at very low velocities in this application. The signal from the lock-in amplifier that corresponds to an absorption value can then be evaluated in two steps: (i) First, the reference arm of the photometer is blocked by the chopper. A contact-free switch signals to the switching timer that the absorption value of the sample cell can be registered and stored.
(ii) In the second step of the cycle, the measurement arm is obscured and the switch indicates that the absorption value of the reference cell can now be recorded. At the end of one cycle the reference value is subtracted from the value that has been stored first. The resulting value is fed to the control of the optical wedge as an error signal. As long as the signal is not equal to zero the optical wedge, which serves as an attenuating element, is moved in such a way that the absorption values of both arms are made equal. In a state where both arms of the photometer are balanced, the position of the optical wedge forms the photometer read-out, i.e. a direct measure for the absorption of the liquid in the absorption cell corrected for drift of both the light
G. Kloos / Chewical
error
Engineering
signal
Lock-in Amplifier
I
I
and Processing
36 (1997)
407-410
309
Fig. 2. Storage and switching unit.
source and the photodiode. The set-up has the usual characteristics of an optical bridge, i.e. an optical design based on a null method: Errors that influence the signals in both arms to the same extent are cancelled out by the subtraction and balancing process. Additionally, independence from non-linearities of the transfer functions of the measurement electronics is achieved. Under the assumption that the windows of both cells are soiled in the same way, the photometer read-out is also corrected for the spurious influence that a deterioration of the cell windows has on the measured signal
WI. The total extinction E’ in the reference arm of the compensation photometer is the superposition of the variable extinction E of the optical wedge, a contribution to the extinction caused by window soiling, EW’, and the extinction of the liquid in the reference cell,
of the substance-dependent molar extinction coefficient, C, the concentration, c, and the Iength of the corresponding cell, I, or l2 [12]: EC’ = EC/,
(4)
Therefore, Eq. (3) can be written as: c=&(E-AElil.)
(5)
Here, the dependence of the concentration on the extinction induced by the optical attenuator is of interest. Maximum sensitivity is obtained if the following derivative takes its maximal value: de dE=E(IZ
1
(6)
E’ = E + Ew’ + EC’
(1)
The corresponding relation for the total extinction, of the measurement arm of the photometer is:
E2,
This corresponds to the case that I, and I2 are equal. Eq. (5) can further be simplified if it is assumed that the windows in both arms are deteriorated to the same extent. In this case, the relation AEW = 0 holds.
(2)
3. Conclusion
EC’.
E’
= Ew2
+
EC2
where Ew2 is the extinction originating from soiling of the windows of the sample cell and EC2 corresponds to the absorbance of the liquid in the sample cell. If both arms of the null instrument are balanced (El = E2) by putting the optical wedge in the ap,propriate position, one has: E = AEW + EC2 -
EC’
(3) where the abbreviation AEW = Ew2 - Ew’ has been introduced. The extinction of the liquid is the product
Starting from classical schemes of photometry, an alternative optical configuration and a sequential electronic read-out scheme are proposed. The number of moving electromechanical components is reduced compared to other modulation techniques which employ vibrating mirrors. This makes the system more robust and cheaper. The on-line photometer that is presented is not able to follow quick reactions. Its use can be seen in accu-
410
G. Kloos / CAemica/
Engineering
rate and reliable long-term monitoring. The combination of a modulation technique with an optical null method leads to an instrument that is relatively insensitive to disturbances and has the advantages of a device based on an autocompensation principle. Most parts of the proposed set-up are simple and robust and can be integrated in a compact design. The only exception is formed by the optical wedge. Its positioning mechanics and control electronics are demanding from a technical point of view. The asymmetric flow geometry represented in Fig. 1 is suitable only for by-pass monitoring. Eq. (6) suggests that it is convenient to have photometer arms with equal path lengths. This can be realized by monitoring a single flow tube at two different points. In such a configuration the instrument can also be used for inline monitoring. Due to the correction for spurious effects that might affect a long-term measurement, the proposed photometer is able to monitor processes reliably.
Acknowledgements I thank JosC Koelman, Technische Universiteit hoven, for her kind help.
Eind-
and Processing
36 (1997)
407-410
References Ul D.F. Hornig, G.E. Hyde, W.A. Adcock, A ratio-recording
double-beam infra-red spectrophotometer with automatic slit control, J. Opt, Sot. Am. 40 (1950) 497-503. 121A. Savitzky, R.S. Halford, A ratio-recording double beam infra-red spectrophotometer using phase discrimination and a single detector, Rev. Sci. Inst. 21 (1950) 203-212. [31 R.P. Bauman, Absorption Spectroscopy, Wiley, New York, 1962, pp. 116-139. [41 G. Kortiim, Kolorimetrie, Photometrie und Spektrometrie, Springer-Verlag, Berlin, 1962 (in German). PI J.F. James, R.S. Sternberg, The design of optical spectrometers, Chapman and Hall, London, 1969, pp. 175-179. 161H. Giinzler, H. B&k, IR-Spektrometrie - Eine Einfiihrung, VCH, Weinheim, 1990. pp. 58-66 (in German). [71 W. Przygocki, Metody iizyczne badali polimer6w, Paristwowe Wydawnictwo Naukowe, Warszaw, 1990, pp. 269-271 and pp, 315-317 (in Polish). Procesfotometers bewaken continu PI W.A. Dirkzwager, veiligheid en kwaliteit, Procestechnologie, 4 (1990) 77-84 (in Dutch). PI G. Schwedt and J. Schreiber, Taschenbuch der Analytik, Georg Thieme Verlag, Stuttgart and New York, 1996, pp. 210-211 (in German). [lo] M.L. Meade, Lock-in Amplifiers: Principles and Applications, Peter Peregrinus, London, 1983. [llj Anonymous, Lock-in Applications Anthology, EC&G Princeton Applied Research, Princeton, NJ, 1986, LIA Note 22. [12] D.A. Skoog, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1984, pp~ 161- 162.
Chemical Engineering and Processing 36 (1997) 407-410
Autocompensation
photometer
for long-term
process monitoring
G. Kloos Techn.
Biiro J
Optik
und Elektrovwchnnik,
Rntyfeisenstr.
15, D-50259
Pullzeim,
Geimtinq~
Received 25 October 1996; received in revised form 27 February 1997
Abstract
A design of a processphotometer is proposedthat combinesfeatures of standard double-beamspectrophotometerswith a compensationfor window soiling that is used in industrial applications. Emphasisis laid on devising a reliable and robust monitoring device while keeping the photometer simple and inexpensive. by switching betweentwo optical paths. 0 1997Elsevier ScienceS.A. Keywords:
Absorption photometry; Process monitoring;
A separation
of main and reference signal is obtained
Optical null method
1. Introduction Photometers are used in chemical engineering as instruments for process control. A typical application is the monitoring of the absorption of a liquid before and after passing an oil separator. Main requirements on the performance of a photometer used to supervise industrial processes are accuracy, long-term stability and robustness. There are three well-known error sources that might affect the photometer read-out in the long term: (i) fluctuations and aging of the light source, (ii) drift of the detector, (iii) deterioration of the transparency of the cell windows due to deposits. A standard approach in scientific spectrophotometry to deal with the two first-mentioned problems consists in double-beam designs that allow one to compensate for drifts of the light source and the detector [l-7]. Often, an optical null method is realized employing an optical wedge to balance both light paths ([3] p. 123, [7] p. 269). As a solution of the third problem, photometers with an additional cell to compensate soiling of the sample cell window are used in industry [8,9]. Lock-in amplification [lo] is an effective means to obtain a stable and reliable read-out also under adverse conditions and is quite common in scientific photometry. A very elegant way to perfoml an absorption measurement using the lock-in technique is the two-frequency method [Ill: Sample and reference beam are chopped at different frequencies and the signal is read 0255-2701/97/$17.00 0 1997 Elsevier Science S.A. All rightz reserved. PIIs0255-2701(97)00014~-7
out using one detector and two lock-in amplifiers tuned to different frequencies. Here, the schemeof a photometer is presented that is optimized for long-term use. The lay-out is influenced by the study of scientific spectrophotometer designsand by an idea realized in the set-up described in references [8,9]. An attempt was made to develop a design with a minimal number of optical components and electronic instruments to obtain a monitoring instrument that is compact, robust and economical.
2. Photometer
design
A scheme of the proposed photometer and its mechanical parts and electronics is given in Figs. 1 and 2. The light from the source is divided by a beamsplitter and directed to the measurement arm and to the reference arm of the double-beam photometer. One half of the light intensity is transmitted through the absorption cell, reflected by a mirror and then measured by a photodiode. The other half passesan optical wedge and a reference cell used for compensation before being reflected to the photodiode. In order to implement a dynamic measurement principle, the intensity of the light source is modulated electronically. The modulation signal also serves as a reference for the lock-in amplifier that measures the signal from the photodiode. For the present purpose, -an-e= and compact lock-in module is suffi-
G. Kloos /Chemical
408
Engineering
and Processing
36 (1997)
407-410
error signal Motor
Control
/
-
photometer
read-out
Switching Mirror
n Light Source Mirror
@+
Beamsplitter Photodiode
I
Modulator
Absorption Cell 1 Additional Absorption Cell (optional) for the compensation of window soiling 2 Contact-free Switch
Storage and Switching
Fig. 1. Autocompensation
Unit
photometer and associated electronics.
cient. A simple device tuned to the modulation frequency of the light source responds to the needs of the given monitoring task. To discriminate a signal from the measurement arm against a signal from the reference arm, a slowly rotating chopper is inrroduced into the photometer set-up as switching device. The technical demands on the chopper motor and wheel are rather low, because the wheel can rotate at very low velocities in this application. The signal from the lock-in amplifier that corresponds to an absorption value can then be evaluated in two steps: (i) First, the reference arm of the photometer is blocked by the chopper. A contact-free switch signals to the switching timer that the absorption value of the sample cell can be registered and stored.
(ii) In the second step of the cycle, the measurement arm is obscured and the switch indicates that the absorption value of the reference cell can now be recorded. At the end of one cycle the reference value is subtracted from the value that has been stored first. The resulting value is fed to the control of the optical wedge as an error signal. As long as the signal is not equal to zero the optical wedge, which serves as an attenuating element, is moved in such a way that the absorption values of both arms are made equal. In a state where both arms of the photometer are balanced, the position of the optical wedge forms the photometer read-out, i.e. a direct measure for the absorption of the liquid in the absorption cell corrected for drift of both the light
G. Kloos / Chewical
error
Engineering
signal
Lock-in Amplifier
I
I
and Processing
36 (1997)
407-410
309
Fig. 2. Storage and switching unit.
source and the photodiode. The set-up has the usual characteristics of an optical bridge, i.e. an optical design based on a null method: Errors that influence the signals in both arms to the same extent are cancelled out by the subtraction and balancing process. Additionally, independence from non-linearities of the transfer functions of the measurement electronics is achieved. Under the assumption that the windows of both cells are soiled in the same way, the photometer read-out is also corrected for the spurious influence that a deterioration of the cell windows has on the measured signal
WI. The total extinction E’ in the reference arm of the compensation photometer is the superposition of the variable extinction E of the optical wedge, a contribution to the extinction caused by window soiling, EW’, and the extinction of the liquid in the reference cell,
of the substance-dependent molar extinction coefficient, C, the concentration, c, and the Iength of the corresponding cell, I, or l2 [12]: EC’ = EC/,
(4)
Therefore, Eq. (3) can be written as: c=&(E-AElil.)
(5)
Here, the dependence of the concentration on the extinction induced by the optical attenuator is of interest. Maximum sensitivity is obtained if the following derivative takes its maximal value: de dE=E(IZ
1
(6)
E’ = E + Ew’ + EC’
(1)
The corresponding relation for the total extinction, of the measurement arm of the photometer is:
E2,
This corresponds to the case that I, and I2 are equal. Eq. (5) can further be simplified if it is assumed that the windows in both arms are deteriorated to the same extent. In this case, the relation AEW = 0 holds.
(2)
3. Conclusion
EC’.
E’
= Ew2
+
EC2
where Ew2 is the extinction originating from soiling of the windows of the sample cell and EC2 corresponds to the absorbance of the liquid in the sample cell. If both arms of the null instrument are balanced (El = E2) by putting the optical wedge in the ap,propriate position, one has: E = AEW + EC2 -
EC’
(3) where the abbreviation AEW = Ew2 - Ew’ has been introduced. The extinction of the liquid is the product
Starting from classical schemes of photometry, an alternative optical configuration and a sequential electronic read-out scheme are proposed. The number of moving electromechanical components is reduced compared to other modulation techniques which employ vibrating mirrors. This makes the system more robust and cheaper. The on-line photometer that is presented is not able to follow quick reactions. Its use can be seen in accu-
410
G. Kloos / CAemica/
Engineering
rate and reliable long-term monitoring. The combination of a modulation technique with an optical null method leads to an instrument that is relatively insensitive to disturbances and has the advantages of a device based on an autocompensation principle. Most parts of the proposed set-up are simple and robust and can be integrated in a compact design. The only exception is formed by the optical wedge. Its positioning mechanics and control electronics are demanding from a technical point of view. The asymmetric flow geometry represented in Fig. 1 is suitable only for by-pass monitoring. Eq. (6) suggests that it is convenient to have photometer arms with equal path lengths. This can be realized by monitoring a single flow tube at two different points. In such a configuration the instrument can also be used for inline monitoring. Due to the correction for spurious effects that might affect a long-term measurement, the proposed photometer is able to monitor processes reliably.
Acknowledgements I thank JosC Koelman, Technische Universiteit hoven, for her kind help.
Eind-
and Processing
36 (1997)
407-410
References Ul D.F. Hornig, G.E. Hyde, W.A. Adcock, A ratio-recording
double-beam infra-red spectrophotometer with automatic slit control, J. Opt, Sot. Am. 40 (1950) 497-503. 121A. Savitzky, R.S. Halford, A ratio-recording double beam infra-red spectrophotometer using phase discrimination and a single detector, Rev. Sci. Inst. 21 (1950) 203-212. [31 R.P. Bauman, Absorption Spectroscopy, Wiley, New York, 1962, pp. 116-139. [41 G. Kortiim, Kolorimetrie, Photometrie und Spektrometrie, Springer-Verlag, Berlin, 1962 (in German). PI J.F. James, R.S. Sternberg, The design of optical spectrometers, Chapman and Hall, London, 1969, pp. 175-179. 161H. Giinzler, H. B&k, IR-Spektrometrie - Eine Einfiihrung, VCH, Weinheim, 1990. pp. 58-66 (in German). [71 W. Przygocki, Metody iizyczne badali polimer6w, Paristwowe Wydawnictwo Naukowe, Warszaw, 1990, pp. 269-271 and pp, 315-317 (in Polish). Procesfotometers bewaken continu PI W.A. Dirkzwager, veiligheid en kwaliteit, Procestechnologie, 4 (1990) 77-84 (in Dutch). PI G. Schwedt and J. Schreiber, Taschenbuch der Analytik, Georg Thieme Verlag, Stuttgart and New York, 1996, pp. 210-211 (in German). [lo] M.L. Meade, Lock-in Amplifiers: Principles and Applications, Peter Peregrinus, London, 1983. [llj Anonymous, Lock-in Applications Anthology, EC&G Princeton Applied Research, Princeton, NJ, 1986, LIA Note 22. [12] D.A. Skoog, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1984, pp~ 161- 162.