GB2066947A

GB2066947A - Gas measuring apparatus with adjustable path length, and method for operation and standardization therefor

Info

Publication number: GB2066947A
Application number: GB8038213A
Authority: GB
Original assignee: Measurex Corp
Current assignee: Honeywell Measurex Corp
Priority date: 1980-01-09
Filing date: 1980-11-28
Publication date: 1981-07-15
Also published as: SE8100053L; DE3100082C2; GB2066947B; DE3100082A1; JPS56104236A

Abstract

Apparatus for measuring particle content of a flowing gas comprises a source emitting a beam of radiation of a frequency absorbed by the gas and a detector aligned therewith and means for adjusting the path length of the beam in the gas. Measurements are taken before and after the adjustment to calculate the content. Standardisation of apparatus involving measurements at frequencies absorbed and non- absorbed by the gas and adjustment of the path length is described. <IMAGE>

Description

SPECIFICATION Gas measuring apparatus with adjustable path length, and method for operation and standardization therefor The present invention relates to an apparatus and a method for measuring the amount of gas. The present invention also relates to a standardization means for such a gas measuring device. Typically, these gases are the exhaust gases, emitted through stacks, produced as a result of combustion.

Gas measuring apparatus for monitoring the output of combustion at the stack is well known; see for example, U. S. Patent 4,076,425, or United Kingdom Patent 1,327,377. Typically, these devices operate in harsh environments and in locations that are not easily accessible. Some of the problems caused thereby are: lamp aging, drift in electronics and dirt build-up on the window. Thus, to operate effectively, i.e., maintain accuracy and repeatability, these devices must have self-contained standardization means.

Heretofore, one standardization means is described in U. S. Patent 3,836,237. That reference teaches, inter alia, the use of air curtains to keep windows clean. However, despite this practice of air curtains, dirt does build up on the window and must be accounted for in the standardization process. U. S. Patents 3,838,925 and 4,076,425 teach the use of alternative optical paths to correct for lamp aging and drift in electronics.

These references, however, do not teach the correction of other factors, such as dirt on the windows.

U. S. patent applications No.919,442 and 919,237 teach standardization means in gas measuring apparatus. However, those disclosures teach housing means with openings and means to close those openings and to purge gas from the housing means. Moreover, the gas is physically enclosed. These are cumbersome additions to the apparatus.

U. S. patent application No. 8,865; filed February 2, 1979, teaches another standardization means in gas measuring apparatus. However, that reference does not teach the adjustment of the path length of the gas measuring apparatus.

An apparatus to measure select properties of gas particles comprises a source capable of emitting a beam of radiation at a frequency which is absorbed by said gas and a detector capable of measuring the radiation.

The beam is aligned to impinge the gas particles positioned in a path between the source and the detector, to pass through the path, and to impinge the detector. Means for adjusting the length of the path along the direction of said beam passing through said source, and means for calculating the amount of gas based upon the length of the path and the amount of radiation measured are provided.

A method of using such as gas measuring apparatus comprises emitting the beam of radiation with the gas in the path. The amount of radiation received by the detector is measured. The length of the path is adjusted. The amount of radiation received by the detector after the adjustment of the length of the path is detected. The amount of gas particles is calculated based upon the amount of radiation measured, the amount of radiation detected and the amount by which the length of the path is adjusted.

Some embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa side view of the gas measuring apparatus of the present invention.

Figure 7a is a portion of Figure 1 showing the apparatus of the present invention with the operation of the means to adjust the length of the path.

Figure 2 is a side view of another gas measuring apparatus of the present invention.

Figure 2a is a portion of Figure 2 showing the apparatus adjusted for the length of the path.

Figure 3 is a pictorial view of the use of the apparatus of the present invention in a stack to monitor the exhaust gas from combustion.

Figure 4 is a graph of the absorption spectrum of a typical gas as a function of the frequency.

Referring now to Figure 1 there is shown a gas measuring apparatus of the present invention. The gas measuring apparatus 10 comprises a first enclosure 12 and a second enclosure 14. The gas particles 16 to be measured, (shown greatly exaggerated) are positioned in a path (the length of the path is shown as "a") between the first enclosure 12 and the second enclosure 14. The first enclosure 12 to one side of the gas particle 16 has a source 18 capable of emitting a beam of radiation 20 (shown as a dash-dot-dash line), at a frequency which is absorbed by said gas particles 16 and is detected by detector 22 in the second enclosure 14.

The beam of radiation 20 is aligned to impinge the gas particles 16 to pass through the path, and to impinge the detector 22. The apparatus 10 further comprises a first hollow member 24, attached to the first enclosure 12. The first hollow member 24 is substantially tubular in shape and encloses a first portion of the beam of radiation 20. A second hollow member 26, also substantially tubular in shape, encloses a second portion of the beam of radiation 20. The second member 26 is within the first member 24 and is capable of being moved along the length of the path. The second member 26 is capable of being moved in the direction of the length of the path by a first motor 28 on a first set of wheels 30. A third hollow member 32, substantially tubular in shape, encloses a third portion of the beam of radiation 20.The third member 32 is attached to the second enclosure 14. Afourth hollow member 34, also substantially tubular in shape, encloses a fourth portion of the beam of radiation 20. The fourth member 34 is within the third member 32 and is capable of being moved along the length of the path by a second motor 36 on the second wheels 38. A first blower 40 is adapted to blow uncontaminated gas into the first member 24 to prevent the gas particles 16 from entering into the first member 24 and the first enclosure 12. The function of the first blower 40 is, of course, to keep the source and the associated electronics free from dirt and other contaminants. Similarly, a second blower 42 blows uncontaminated gas into the third hollow member 32 to prevent the gas particles 16 from entering into the third member 32 and the second enclosure 14.A first window 44 is attached to the second hollow member 26 and serves to prevent gas particles 16 from entering into the second hollow member 26 and the first enclosure 12. Similarly a second window 46 attached to the fourth member 34 serves to prevent the gas particles 16 from entering into the fourth member 34 and the second enclosure 14.

In all respect, the first hollow member 24, the second hollow member 26, the first window 44, the first blower 40, the first motor 28, and the first wheels 30 function and operate in much the same manner as the components and parts attached to the second enclosure 14, i.e., the third hollow member 32, the fourth hollow member 34, the second glass 46, the second blower 42, the second motor 36 and the second wheels 38.

Referring to Figure 1a, there is shown a portion of the apparatus 10 of the present invention, during its operation in which the length of the path is adjusted. In Figure 1a it is seen that the length of the path is decreased from "a" to "b". In operation of the apparatus 10 shown in Figure 1 a, the second hollow member 26 and the fourth hollow member 34 are brought closer together, thereby decreasing the length of the path in which the gas particles 16 are positioned. This is accomplished by activating first motor 28 and second motor 36, pushing the second hollow member 26 and the fourth hollow member 34 toward one another.

Referring to Figure 2 there is shown yet another apparatus 110 of the present invention. The apparatus 110 comprises a first enclosure 112 and a second enclosure 114. A source 118 is within the first enclosure 112, while a detector 122 is within the second enclosure 114. The gas particles 16 to be measured, (shown greatly exaggerated) are positioned ion a path (the length of the path is shown as "a") between the first enclosure 112 and the second enclosure 114. The source 118 is capable of emitting a beam of radiation 20 (shown as a dash-dot-dash line), at a frequency which is absorbed by the gas particles 16 and is detected by the detector 122 in the second enclosure 114. The apparatus 20 further comprises a first hollow member 124, substantially tubular in shape, encloses a second portion of the beam of radiation 120.The second hollow member 126 is attached to the first motor 128 along the first wheels 130. A first glass 144 covers the first hollow member 124 and prevents the gas particles 16 from entering into the first enclosure 112. Near the second enclosure 114 is the third hollow member 132. The third hollow member is substantially tubular in shape and encloses a third portion of the beam of radiation 20. A fourth hollow member 134 also substantially tubular in shape encloses a fourth portion of the beam of radiation 120. The fourth member 134 is attached to the second enclosure 114. The third hollow member 132 is capable of being moved along the length of the path by the second motor 136 along the second wheels 138. A second glass 146 encloses the open end of the third hollow member 132 and prevents gas particles 16 from entering into the second enclosure 114.

Figure 2a shows the operation of the apparatus 20 of the present invention in which the first hollow member 124 and the second hollow member 132 are moved toward one another to decrease the length of the path in which the gas particles 16 are positioned. The apparatus 110 of Figure 2 operates and functions in much the same manner as the apparatus 10 of Figure 1, except that the adjustment of the length of the path of Figure 1 is accomplished by the movement of the second hollow member 26 and the fourth hollow member 34 moving with respect to the first hollow member 24 and the third hollow member 32; whereas in Figure 2 the adjustment of the path length is accomplished by the relative movement of the first hollow member 124 with respect to the second hollow member 126 and the relative movement of the third hollow member 132 with respect to the fourth hollow member 134.In addition, in either apparatus 10 or 110, the path length may be adjusted by either increasing or decreasing the length of the path.

In general, the first window 44, and the second window 46 of the apparatus 10 of the present invention are not needed. However, if the windows 44 and 46 were present and there is a tight seal between the first hollow member 24 and the second hollow member 26, then the first blower 40 and the second blower 42 are not needed. Moreover, any suitable moving means, such as actuators, may be used in place of the first motor 28 and second motor 36. Similarly in the apparatus 110, the first glass 144 and the second glass 146 are not crucial to the present invention. In addition, any suitable moving means may be used in place of first motor 128 and the second motor 136.

One use of the apparatus 10 of the present invention is in monitoring the exhaust gas 16 from a stack 52 as shown in Figure 3. Typically, the first enclosure 12 and the second enclosure 14 are on opposite sides of the stack 52, with the exhaust gas 16 flowing in a stream through the stack 52. The beam 20 or 120 is aligned to impinge the gas 16 at a direction substantially perpendicular to the direction of flow of said gas 16. In such application, the apparatus 10 is termed an in-situ gas analyzer and is useful for monitoring the exhaust gas 16 to ensure compliance with applicable environmental standards, such as EPA. In these applications, the apparatus 10 may operate as an opacity sensor, with the source 18 or 118 emitting a beam 20 or 120 of visible light. In such environment, a rigid member (not shown) such as disclosed in U. S. Patent 3,838,926 may also be used to hold the first enclosure 12 and the second enclosure 14 in rigid alignment.

In the method of the present invention, the source 18 emits a beam 20 of radiation at a frequency (shown as V1 in Figure 4) which is absorbed by the gas 16. The beam 20 passes through the gas 16 in the stack 52 and is absorbed as it travels to the detector 22. The intensity of the beam 20, received by the detector 22 is dependent on the amount of absorption, i.e., the greater the absorption, the lower the intensity of the beam 20 received by the detector 22, and vice versa. This is shown as 13 in Figure 4. The path length is then adjusted, either increased or decreased, by, for example, moving the second hollow member 26 and the fourth hollow member 34 closer to one another. After the adjustment of the length of the path, the intensity of the beam of radiation 20 received by the detector 22 is measured once again.Based upon the amount of adjustment of the path length, the amount of radiation received by the detector 22 measured prior to the path length being adjusted, and the amount of radiation received by the detector 22 after the path length is adjusted, the amount of gas, i.e., the concentration of the gas 16 in units suchas parts per million, is calculated.

The theoretical basis for the operation of the apparatus and the method of the present invention is as follows. The absorption of the beam of radiation 20 by the detector 22 as it passes through the gas particles 16 is in accordance with Beer's law, i.e.,

where I is the intensity of the beam of radiation measured by the detector 22 after it has passed through the gas particle 16 based upon a path length equal to 4. lo is the intensity of the beam of radiation 20 measured by the detector 22 based upon the path length equal to zero, CL is the absorption coefficient 1 (PPM-CM) e is the path length of gas in centimeters, and C is the concentration of the gas (PPM) or the amount of gas.In the case where 13 represents the intensity of the beam 20 received by the detector 22 with the path length e equal to a, the equation reduces to 13 = loe-PCa The difficulty of accurate measurement of the amount of gas has always been to attempt to find the value of the intensity of the beam 20 received by the detector 22 with the path length e equal to zero, i.e., lo. In the instant case, to determine lo two measurements with two different path lengths of the gas particles 16 are taken by the detector 22.The two resulting equations are 12 = loe-Pcb and 13 = Ige-pCa or lo = 12e"cb (1) lo = 13era (2) 13eSlCa =l2ecb (3) solving for C, we obtain

Typically, the frequency yl is in the infrared region and the curve shown in Figure 4 is the absorption band of carbon dioxide. The advantage of the apparatus and method of the present invention is that the measurements (i.e., before and after adjustment of the length of the path) are performed under substantially the same conditions. In each case, the measurement uses the same source and electronics, follows the same optical path and is subject to the same environment as the other measurement.This provides for greater accuracy and reliability than has been achieved heretofore.

Moreover, unlike the invention disclosed in U. S. patent applications No.919,442, 191,237, and 8,865, the method and apparatus of the present invention does not require that the exact value of lo (i.e., when the path length is equal to zero) be known or measured. Unlike the invention disclosed in those applications, in which lo is measured directly in which tight seals to purge the gas from the housing member must be provided, the invention of the present application does not require that lo be measured directly. Thus, seals and blowersto purge the gas from the housing member are not provided. The present invention, however, does not require that the concentration of the gas, or the amount of the gas, remain a constant before and after the adjustment of the length of the path.The invention assumes that the amount of the gas is the same duringthe two measurements. Thus, the present invention is most applicable to situations in which the concentration of gas or the amount of the gas does not change rapidly and that two measurements (differing by only the length of the path) are made when the concentration of the gas or the amount of gas is or nearly is identical.

In the method of the prior art, measurements were made based upon a beam of radiation at two different frequencies - one which is absorbed by the gas 16 and another which is not absorbed. The source 18 emits a beam of radiation 20 at a first frequency V1 which is absorbed by the gas 16 and a second frequency V2 which is not absorbed by the gas 16. The detector 22 receives the beam of radiation 20 after it passes through the gas 16. The detector 22 measures the amount of first frequency yl received, i.e., 13, and measures the amount of second frequencyv2 received, i.e., Ii. Calculation of the amount of gas 16 is made based upon 13 and Ii in accordance with Beer's law, based upon the assumption that Ii is the same as lo.However, it should be noted from Figure 4, that even though the second frequency V2 is chosen such that it is not absorbed by the gas 16, the amount of second frequency V2 received may not be exactly the same as the amount of first frequency yl received but with length of the path equal to zero, i.e., lr may not necessarily be exactly the same as lo. There are many possible causes for this, including drift in electronics, since V2 is a frequency different from V1. This is clearly a source of error.

In another method of the present invention, this error is eliminated by standardizing the value of 1a, i.e., determining the quantitative relationship between 11 and io. To standardize the value of 1a, a standardization factor based upon Ii and lo is determined, i.e., K = lo 11 The standardization factor, K is determined by emitting the beam of radiation 20 at a first frequency V1 which is absorbed by the gas 16 and a second frequency V2 which is not absorbed by the gas 16.The amount of radiation received by the detector 22 at the second frequency V2 is determined, i.e.,11. The amount of radiation received by the detector 22 at the first frequency v1 is measured, with the gas 16 in the path, and the path length at e = a, i.e., 13. The path length is then adjusted (for example to e = b). The concentration of the gas 16, C, is determined as previously discussed. Based upon the value of C, the value of lo may be calculated using equation (1) or (2). The ratio of lo to Ii is the standardization factor. Thereafter, in the measurement of the amount of gas 16 using a first frequency V1 and a second frequency V2, the calculation of the amount of gas 16 is based upon 13, Ii and K in accordance with

or

In this method, the path length need not be adjusted upon every measurement. Instead, the adjustment of the path length is used to standardize the apparatus 10 and to correlate Ii to lo.

Claims

1. An apparatus for measuring select properties of gas particles having a direction of flow in a gas stream, with standardization means, comprising: a source, capable of emitting a beam of radiation at a frequency which is absorbed by said gas particles; a detector, capable of measuring the amount of said radiation; said beam aligned to impinge said gas particles positioned in a path between said source and said detector, to pass through said path, and to impinge said detector; means for adjusting the length of said path along the direction of said beam passing through said path; and means for calculating the amount of gas based upon the amount of radiation measured and the amount of path length adjusted.

2. The apparatus of Claim 1 wherein said adjusting means comprises: afirst hollow member, substantially tubular in shape, enclosing a first portion of said beam, wherein said member is stationary; a second hollow member, substantially tubular in shape, enclosing a second portion of said beam, wherein said second member is within said first member and is capable of being moved along the length of said path.

3. The apparatus of Claim 2 wherein said source is located to one side of said stream; said detector is located on side substantially opposite said one side; and said beam aligned to pass at a direction substantially perpendicular to the direction of flow of said gas stream.

4. A method of measuring the amount of gas particles using an apparatus having a source, capable of emitting a beam of radiation at a frequency which is absorbed by said gas; a detector, capable of measuring the amount of said radiation; said gas particles positioned in a path interposed between said source and said detector; said beam aligned to impinge said gas particles, to pass through said path and to impinge said detector; means for adjusting the length of said path; said method comprises: emitting said beam of radiation with said gas in said path; measuring the amount of radiation received by said detector; adjusting the length of said path; detecting the amount of radiation received by said detector after said adjustment; calculating the amount of gas based upon said amount of radiation measured, said amount of radiation detected, and said amount of path length adjusted.

5. The method of Claim 4 wherein said adjusting step is increasing the length of said path.

6. The method of Claim 4 wherein said adjusting step is decreasing the length of said path.

7. A method of standardizing a gas measuring apparatus having a source capable of emitting a beam of radiation at a first frequency which is absorbed by said gas and at a second frequency which is not absorbed by said gas; a detector, capable of detecting said first frequency and said second frequency; said gas particles positioned in a path interposed between said source and said detector; said beam aligned to impinge said gas particles to pass through said path and to impinge said detector; means for adjusting the length of said path; said method comprises: emitting said beam of radiation at said first frequency and said second frequency with said gas in said path; measuring the amount of radiation at said first frequency received by said detector; detecting the amount of radiation at said second frequency received by said detector; adjusting the length of said path; sensing the amount of radiation at said first frequency received by said detector after said adjustment; standardizing said apparatus based upon said first frequency measured, said first frequency sensed, said second frequency detected, and said amount of path length adjusted.

8. A method of Claim 7 wherein said adjusting step is increasing the length of said path.

9. The method of Claim 7 wherein said adjusting step is decreasing the length of said path.

10. An apparatus for measuring select properties of gas particles having a direction of flow in a gas stream substantially as herein before described with reference to and as illustrated in Figures 1 and 1 a or 2 and 2a of the accompanying drawings.

11. A method of measuring the amount of gas particles as claimed in Claim 4 substantially as hereinbefore described.

12. A method of standardizing a gas measuring apparatus as claimed in Claim 7 substantially as hereinbefore described.