WO1992018847A1

WO1992018847A1 - Method and apparatus for determining the concentration of a particular constituent of a fluid

Info

Publication number: WO1992018847A1
Application number: PCT/GB1992/000670
Authority: WO
Inventors: Nigel Keith Tester; James Stuart Osprey
Original assignee: Servomex (Uk) Ltd.
Priority date: 1991-04-12
Filing date: 1992-04-13
Publication date: 1992-10-29
Also published as: GB9107815D0

Abstract

To determine the concentration of a particular constituent of a test fluid electromagnetic radiation is provided from a source. The radiation has components having first and second reference wavelengths and a measure wavelength. The measure wavelength is such that radiation at that wavelength is absorbed by the particular constituent of the test fluid. The first and second reference wavelengths are such that radiation at those wavelengths is not absorbed by the particular constituent of the test fluid. The radiation is then passed through a sample of the test fluid and the intensity of the radiation at the three wavelengths after the passage through the sample is measured. From the measured intensities at the first and second reference wavelengths, the intensity at the measure wavelength in the absence of any absorption by the sample is determined. The determined intensity at the measure wavelength is compared with the measured intensity at the measure wavelength to establish the concentration of the particular constituent in the test fluid.

Description

METHOD AND APPARATUS FOR DETERMINING THE CONCENTRATION OF A PARTICULAR CONSTITUENT OF A FLUID

The present invention relates to improvements in a single beam photometer.

Photometers are used to measure the concentration of a particular component of a fluid. This is done by passing a beam of electromagnetic radiation through a sample of the fluid. The beam radiation has a wavelength which is known to be absorbed by the component in question. The concentration of that component may be calculated from the degree of absorbence which is calculated as follows:

Absorbence = log (I /I) where I is the intensity of the beam after passing through the sample and I is the intensity the beam would have were it not for the attenuation by the sample.

Because I is affected by effects such as obscuration which may change from sample to sample it is necessary to be able to estimate I . It is known to pass a further reference beam through the sample for this purpose. This reference beam has a wavelength which is known not to be absorbed by any component of the fluid. From the received intensity of the reference beam, I may be estimated, and the absorbence established.

The main elements of a known non-dispersive photometer operating as outlined above are as follows. An appropriate source of radiation is provided which supplies the beam of radiation used in the measurement. This beam is modulated by a mechanical chopper which in the case of a single beam photometer carries a first filter (the reference filter) tuned to a wavelength at which no component in the sample has any absorption; and a second filter (the measure filter) which is tuned to a main absorption line of a component in the sample to be analysed.

The two filters alternately impede the path of the radiation from the source thus eliminating major components of the radiation and passing only a selected band of radiation as defined by the filter imposed at that time on the radiation beam. It is also possible to add more measure filters tuned to other components of interest in the sample. The now filtered beam is admitted to a suitable sample cell into which the fluid of interest or a reference fluid or a calibration fluid may be admitted at any given time.

The amount of energy arriving at the end of the cell is detected by a suitable detector and knowledge of which output relates to which filter and the correct timing of events is ensured by a synchronizer.

The following information is therefore provided to a signal processing unit, corresponding to the calibration of the photometer: i) A received energy level set by having a non- absorbing (zero) fluid in the cell whilst the reference filter is engaged. ii) A received energy level set by having a non- absorbing (zero) fluid in the cell whilst the measure filter is engaged. iii) A received energy level set by having an absorbing (span) fluid in the cell of known composition (calibrated) whilst the reference filter is engaged, iv) A received energy level set by having an absorbing (span), fluid in the cell of known composition (calibrated) whilst the measure filter is engage -

This information gives the basic constants needed to allow an analysis of a fluid which may be a liquid or a gas with an unknown concentration of an absorbing component. As mentioned above a common problem encountered in the field is obscuration along the path of the beam of radiation. There is however enough information in the signal processor to be able to compensate for this, and by ratioing the detected signals the effects of a large degree of obscuration may be nearly eliminated.

Another common problem is drift in the baseline necessitating frequent calibration. A main factor causing this is the fluctuations in the temperature of the source which provides the radiation. Techniques used in the past to minimize this include control of the voltage, or the current, driving the source; the detection of the source temperature and setting up a control system to maintain it constant; and the diversion of a fraction of the beam to a discrete sensor and using its output to maintain constant conditions. Thus these have generally been attempts to ensure a constant temperature of the source.

The present invention proposes apparatus and a method which by a simple addition of an extra reference wavelength gives enough information to allow the processor to allow for the change in the source intensity thus reducing the effect of output drift on the value of concentration also and allows a better compensation for obscuration as it takes into account the latest operating temperature of the source. Thus the present invention proposes to take account of any source temperature changes which do occur, rather than specifically attempting to prevent such charges.

The present invention provides a method for the determination of the concentration of a particular constituent of a test fluid comprising: providing, from a source, electromagnetic radiation having components having first and second reference wavelengths and a measure wavelength, said measure wavelength being such that radiation at that wavelength is absorbed by said constituent of the test fluid and said first and second reference wavelengths being such that radiation at those wavelengths is not absorbed by said constituent of the test fluid; passing said electromagnetic radiation through a sample of said test fluid; measuring the intensity of the radiation at the first and second reference and measure wavelengths after the passage through the sample; determining, from the measured intensities at the first and second reference wavelengths, the intensity at the measure wavelength in the absence of any absorption by the sample; and comparing the determined intensity at the measure wavelength with the measured intensity at the measure wavelength to establish the concentration of said constituent in said test fluid.

The present invention also provides an apparatus for the determination of the concentration of a particular constituent of a test fluid comprising: a source of electromagnetic radiation, said radiation having components having first and second reference wavelengths and a measure wavelength, said measure wavelength being such that radiation at that wavelength is absorbed by said constituent of the test fluid and said first and second reference wavelengths being such that radiation at those wavelengths is not absorbed by said constituent of the test fluid; means arranged to contain a sample of said test fluid and for passing said electromagnetic radiation through said sample of said test fluid; measuring means arranged to measure the intensity of the radiation at the first and second reference and measure wavelengths after the passage through the sample; means for determining, from the measured intensities at the first and second reference wavelengths, the intensity at the measure wavelength in the absence of any absorption by the sample; and means for comparing the determined intensity at the measure wavelength with the measured intensity at the measure wavelength to establish the concentration of said constituent in said test fluid.

In order that the present invention be more readily understood preferred embodiments thereof will now be described by way of example with reference to the accompanying drawing which shows an exemplary photometer in diagrammatic form.

As discussed above, two major factors apart from absorption by the component being tested which affect the intensity of radiation received at particular frequencies are obscuration and source temperature. It is known from observation, as well as from the Plane equation that when a given source changes temperature, not only does the overall intensity of the emitted radiation change but the relative intensity of various wavelengths within the overall emission changes. It will therefore be appreciated that the reading from a single reference beam cannot compensate both for variation in source temperature and the effect of obscuration.

If it is assumed that obscuration has the same effect on all wavelengths within the radiation, ie. all wavelengths are attenuated by the same factor, it will be appreciated that the ratio of received intensities at two given wavelengths is not affected by obscuration. It is found that such a ratio varies with source temperature and may therefore be taken as a characteristic of the source temperature. The present invention uses two reference wavelengths in addition to the measure wavelength, and from the information regarding the received intensities at these three wavelengths the absorption at the measure wavelength and hence the concentration of the particular component in the fluid can be ascertained.

Thus if a second reference filter is introduced into the filter and modulator section described above, a second reference signal can be generated and the two reference signals as seen by the detector section can be employed to determine the change in source intensity at the measure wavelength due to source temperature change.

The information regarding the intensities at the reference wavelengths is used to determine what the intensity at the measure wavelength would be in the absence of any absorption by the sample.

This information allows the output of the photometer to be compensated at the measurement wavelength for fluctuations in the source temperature; it also enables a more accurate compensation for the effects of obscuration.

The second reference filter is preferably chosen to have an absorption wavelength which does not overlap an absorption wavelength of the components in the sample fluid and has at the same time a distinctly different wavelength from that of the first reference filter.

A first preferred method using the invention is as follows: a) Calibration of reference interference filter transmission.

An initial source working temperature T is inputted (measured) to the processing unit which has also been given the nominal wavelength of all filters. The unit calculates:-

P , the predicted intensity at the output of the detector electronics for a nominal source temperature T and at the wavelength \ of the first reference filter, worked out from the Plane equation; P , the predicted intensity at the output of the detector electronics for the nominal source temperature To and at the wavelength X2 of the second reference filter, worked out from the Plane equation;

R , the received intensity as given by the detector electronics when the reference filter F is engaged in the path of the beam; and

R 2, the received intensity as given by the detector electronics when the reference filter F 2 is engaged in the path of the beam.

The filter transmission ratio (FTR) is defined by:

FTR = _* X J_* (1)

2 1

This information is held in a store in the signal processor.

In addition a table of a range of ratios of the intensities at the nominal reference wavelengths and is calculated in suitable temperature steps between expected lower and higher values of the source working temperature and stored in the signal processor, b) Normal Operation

During the normal running of the instrument and at any given source operating temperature at that given time the received intensities (Rιx and Rax) as given by the detector electronics when the reference filters F Z_ and F2 are engaged, are used through the FTR to remove the difference in the reference filter transmission and provide a reference intensity ratio (RIR) which is given by:

RIR = R ^/FTR ₍₂₎

2X

The resultant ratio is compared with the ratio values stored in the table to locate two ratio between which it falls. Linear interpolation methods are then used between these two located ratios to work out the corresponding operating temperature T . The source intensity at the measure wavelength is then calculated, followed by the measure intensity change (MIC) :

Calculated measure intensity using the current source temperature (T )

MIC = ...(3)

Calculated measure intensity using original source temperature (To) .

This allows the final outputted value of measurement to be corrected for the fluctuation in source temperature.

This calculation is updated at a typical rate of 1 per second, having thus a profound impact on short term drift, c) Obscuration Compensation.

By using the information at the two reference wavelengths and correcting for any source intensity variations and averaging the results, a much improved obscuration compensation will ensue.

A second preferred method using the invention is as follows: a) At calibration or set up.

The steps described for the calibration under a) above for the first method may be followed, with an entered temperature, to establish the relevant ratios.

For a particular source and filter arrangement a table is compiled and stored in the processing means of the apparatus. The received intensities at the two reference wavelengths and the measure wavelength are measured when there is no absorbing fluid present. This measurement is made for a number of source temperatures around the nominal source temperature and preferably 256 different temperatures may be used. In the table are stored the values of R 1/R2, R1/M and R2/M for each measured temperature, where R is the received intensity at the first reference wavelength, R is the received intensity at the second reference wavelength and M is the received intensity at the measure wavelength.

Alternatively such a table may be complied using the Plane equation to generate the expected intensities at the various source temperatures.

As will become apparent it is not necessary to store the actual values of source temperature. It is only necessary to store the values of the three ratios mentioned above which correspond to each other.

b) Normal Operation

During normal operation the received intensities at the first and second reference wavelengths (R IX,R2X) are measured. The ratio RIX/R2X is compared with the values in the stored table. If the temperature values have been stored the source operating temperature may be established at this stage. In any event the values of the ratios R 1/M and R2/M which correspond to the measured value of Ri /Rax are established.

From these values of the ratios R -L/M and R2/M and the measured values R IX and R2 , two values of M are calculated. These are averaged to give the estimated value of M , designated Mox. The absorbence can then be calculated as stated above (absorbence = logιo (MOX/MX) )

Because the intensity ratios are used in this method, the effect of obscuration is automatically compensated for. It is assumed that the effect of obscuration is simply equivalent to using a source at the same temperature but having a lower overall intensity. Thus the values of the ratios are not affected.

For any of the methods of this invention it is important that the wavelength of the reference beams and the measure beam are correctly chosen. There are a number of factors to be taken into account in the selection of wavelengths: i) The reference wavelengths should not be significantly attenuated by any component in the sample. ii) The reference wavelengths should ideally be separated by 10%. If they become too close the readings become noisy and the correction is less effective, iii) It is advantageous to have one reference as close as possible to the measure wavelength, providing the reference signal is not attenuated by the sample. If a single reference can be chosen close to the measure wavelength then the effect of source temperature is reduced since the effect of source temperature becomes similar as the wavelengths tend to the same value, iv) The technique is most advantageous in the region of the spectrum where source intensity charges fastest with temperature, ie in the 2 to 5 μm region rather than the 7 to 8 μm region.

Typical values for the reference wavelengths are 2.425μm and 3.95μm. These may be used in the measurement of CO 2, which uses a measure wavelength of

4.28μm, CO, which uses a measure wavelength of 4.70μm, and hydrocarbons, which use measure wavelengths in the region of 3.4μm.

It is possible to use the above method for the determination of the concentration of more than one constituent of a test fluid. In this case a corresponding number of measure wavelengths are used, in addition to the two reference wavelengths, which are respectively absorbed by the constituents of interest. The above discussed calculations are performed in relation to each of the received intensities at the measure wavelengths to determine the concentrations of each of the constituents of interest.

The accompanying drawing illustrates in diagrammatic form a preferred apparatus according to this invention which may be used in conjunction with either of the methods described above. The apparatus comprises a source of electromagnetic radiation 1 which supplies a beam of radiation, preferably infra-red, which has components at at least the measure and the two reference wavelengths. This beam is modulated by a mechanical chopper 2 which carries three filters respectively tuned to the measure and first and second reference wavelengths. The filters in turn impede the path of the beam from the source 1.

The filtered beam is then admitted to a suitable sample cell 3. The fluid of interest may be circulated in cell 3 as indicated by arrows A and B. The apparatus may work either by admitting a sample, testing it and ejecting it, or by testing while the fluid is circulating continuously.

It will be appreciated that the effect of the chopper is that successive beams of radiation at the measure, first reference and second reference wavelengths are applied in turn to the sample cell 3, and the fluid therein. The amount of energy (intensity of radiation) passing through the cell 3 is measured by detector 4 and passed to a processor 5. Synchronizer 6 ensures that it is possible to establish which received intensity corresponds to each of the wavelengths of interest.

Alternatively it would be possible to pass all of the radiation produced by source 1 through cell 3 and position the chopper 2 carrying the filters between the cell 3 and the detector 4. Another alternative would be to dispense with the chopper 2 and use three detectors each tuned to a respective one of the three wavelengths of interest.

It will be appreciated that any apparatus which allows detection of the amount of radiation at each of the three wavelengths which passes through the cell 3 may be used to implement the method of the invention. Stored in processor 5 is any table or tables required by this invention as described above which contain the information derived from the calibration steps.

If the concentration of more than one constituent of the test fluid is to be determined and therefore more than one reference wavelength is necessary, the apparatus is arranged to detect the amount of radiation which passes through cell 3 at each of the first and second reference wavelengths and each required measure wavelength. This may be achieved by using more than three filters on the chopper 2, or by using more than three detectors, each tuned to one of the three wavelengths of interest.

Once the received intensities at each of the three wavelengths of interest are passed to the processor 5, the processor uses this information in conjunction with the information stored therein to establish the concentration of the constituent of interest in the test fluid.

Claims

CLAIMS :

1. A method for the determination of the concentration of a particular constituent of a test fluid comprising: providing, from a source, electromagnetic radiation having components having first and second reference wavelengths and a measure wavelength, said measure wavelength being such that radiation at that wavelength is absorbed by said constituent of the test fluid and said first and second reference wavelengths being such that radiation at those wavelengths is not absorbed by said constituent of the test fluid; passing said electromagnetic radiation through a sample of said test fluid; measuring the intensity of the radiation at the first and second reference and measure wavelengths after the passage through the sample; determining, from the measured intensities at the first and second reference wavelengths, the intensity at the measure wavelength in the absence of any absorption by the sample; and comparing the determined intensity at the measure wavelength with the measured intensity at the measure wavelength to establish the concentration of said constituent in said test fluid.

2. A method as claimed in claim 1 wherein said step of providing electromagnetic radiation comprises providing successive beams of electromagnetic radiation having said first and second reference and measure wavelengths.

3. A method as claimed in claim 1 or 2 wherein said step of determining the intensity at said measure wavelength in the absence of any absorption by the sample, comprises calculating the ratio of the measured intensities at the first and second reference wavelengths.

4. A method as claimed in claim 3 wherein said step of determining the intensity at said measure wavelength in the absence of any absorption by the sample further comprises comparing said ratio with previously measured and stored ratios for the intensities at the first and second reference wavelengths with said source at various operating temperatures.

5. A method as claimed in claim 3 wherein said step of determining the intensity at said measure wavelength in the absence of any absorption by the sample further comprises estimating, using Plane's equation, from said ratio, the source temperature.

6. A method as claimed in any of claims 1 to 5 for the determination of the concentration of each of a plurality of constituents of a test fluid comprising: providing from said source electromagnetic radiation having components having a corresponding plurality of measure wavelengths, each measure wavelength being such that radiation at that wavelength is absorbed by a respective one of said constituents of said test fluid; measuring the intensity of the radiation at each of the measure wavelengths after the passage through the sample; determining the intensity at each of the measure wavelengths in the absence of any absorption by the sample; and comparing the determined intensity at each measure wavelength with the measured intensity at that measure wavelength to establish the concentration of each respective constituent of the test fluid.

7. An apparatus for the determination of the concentration of a particular constituent of a test fluid comprising: a source of electromagnetic radiation, said radiation having components having first and second reference wavelengths and a measure wavelength, said measure wavelength being such that radiation at that wavelength is absorbed by said constituent of the test fluid and said first and second reference wavelengths being such that radiation at those wavelengths is not absorbed by said constituent of the test fluid; means arranged to contain a sample of said test fluid and for passing said electromagnetic radiation through said sample of said test fluid; measuring means arranged to measure the intensity of the radiation at the first and second reference and measure wavelengths after the passage through the sample; means for determining, from the measured intensities at the first and second reference wavelengths, the intensity at the measure wavelength in the absence of any absorption by the sample; and means for comparing the determined intensity at the measure wavelength with the measured intensity at the measure wavelength to establish the concentration of said constituent in said test fluid.

8. An apparatus as claimed in claim 7 further comprising three filters which are successively interposed between the source of electromagnetic radiation and the means for containing a sample of the test fluid thereby to provide successive beams of electromagnetic radiation having said first and second reference and measure wavelengths.

9. An apparatus as claimed in claim 7 or 8 wherein said means for determining the intensity at said measure wavelength in the absence of any absorption by the sample comprises means for calculating the ratio of the measured intensities at the first and second reference wavelengths.

10. An apparatus as claimed in claim 9 wherein said means for determining the intensity at said measure wavelength in the absence of any absorption by the sample further comprises means for comparing said ratio with previously measured and stored ratios for the intensities at the first and second reference wavelengths with said source at various operating temperatures.

11. An apparatus as claimed in claim 9 wherein said means for determining the intensity at said measure wavelength in the absence of any absorption by the sample further comprises means for estimating, using Plane's equation, from said ratio the source temperature.

12. An apparatus as claimed in any of claims 7 to 11 for the determination of the concentration of each of a plurality of constituents of a test fluid wherein: the source of electromagnetic radiation has components having a corresponding plurality of measure wavelengths, each measure wavelength being such that radiation at that wavelength is absorbed by a respective one of said constituents of said test fluid; the measuring means is arranged to measure the intensity of the radiation at each of the measure wavelengths after the passage through the sample; the means for determining determines the intensity at each of the measure wavelengths in the absence of any absorption by the sample; and the means for comparing compares the determined intensity at each measure wavelength with the measured intensity at that measure wavelength to establish the concentration of each respective constituent of the test fluid.