MEASURING INSTRUMENT EQUIPPED WITH A FLAME IONIZATION DETECTOR

Abstract

An instrument (100) for measuring at least one volatile organic compound in a gas mixture, said instrument comprising inlets (24, 26, 28) for air, H2 or H2-He gas, and a sample of the gas mixture, a pump (36), and a flame ionization detector (34), said detector being connected to the air and H2 gas inlets (24, 26) and configured to generate a flame that ionizes one or more volatile organic compounds contained in said sample, and to emit an electronic signal (46) as a function of the quantity of the volatile organic compound(s) in said sample, characterized in that it further comprises at least one damping chamber (48) which is connected by a pressure rail (40) to the detector (34) and which is configured to contain a predetermined volume of gas at a predetermined pressure. Figure for the abstract: Figure 8

Description

Title of the invention: MEASURING INSTRUMENT EQUIPPED WITH A FLAME IONIZATION DETECTOR Technical field of the invention

[0001] The invention relates to a measuring instrument equipped with a flame ionization detector. Technical background

[0002] A flame ionization detector, also called a FID (Flame Ionization Detector), is a detector that uses a hydrogen flame burning in excess oxygen. This flame ionizes organic molecules present in the gas being measured. These molecules must be volatile, i.e., in a gaseous state, to be measured. In the presence of an electric field, the generated carbon ions can be captured by a collection electrode. The resulting electric current is very weak (on the order of pA), and it is necessary to amplify it for measurement. The detector's response signal is proportional to the quantity of carbon ions generated by the flame. It therefore depends on the concentration of organic molecules in the sample gas and the gas flow rate to be analyzed. The measured signal also depends on the different flow rates of the gases feeding the detector, namely the hydrogen (H2 or H2-He) flow rate and the air flow rate.

[0003] This type of detector can for example be used to detect and quantify gaseous hydrocarbons (methane, propane, butane, for example) in a mixture of gases, typically escaping from a gas exhaust stack of an incineration plant.

[0004] A flame ionization detector is a mass detector, meaning that the response signal is proportional to the amount of volatile organic carbon passing through the FID detector per unit time. This means that the concentration of volatile organic carbon is not the only important parameter; the sample gas flow rate is also important.

[0005] Among the volatile organic compounds measured in the stacks of industrial facilities such as municipal waste incinerators, hydrocarbons are usually by far the most abundant. Methane, in particular, represents a large proportion of the carbon present. It is therefore common practice for volatile organic compounds (VOCs) to also be referred to as total hydrocarbons (THCs), and separated into non-methane hydrocarbons (NMHCs) and methane (CH4).

[0006] To deduce a concentration from the amount of carbon measured in the FID detector, the ideal is to keep the sample flow rate to the detector constant. However, in practice, maintaining this constant flow rate presents a real technical challenge. The analyzer is generally equipped with a pneumatic diaphragm pump, which operates periodically and generates pressure variations. When analyzing VOCs from a chimney gas mixture, this gas mixture is typically at a high temperature, which is maintained up to the detector to prevent condensation of water vapor and heavy VOCs, which would distort the measurement. The pump technologies currently available for this type of application, while being both inexpensive and compact, are relatively limited and all suffer from the drawback of not providing a constant flow rate and constant outlet pressure.

[0007] In the current technique, solutions are therefore used to limit or inhibit the impact of this pressure variation on the measurement, the sample gas being taken by the pump at a relatively low pressure (typically 200hPa or 200mbar relative to atmospheric pressure).

[0008] In general, the objective is to connect the detector to the pump via what we will subsequently call a "pressure rail," which is a pipe or set of pipes in which the sample pressure is kept constant. A restrictor at the outlet of the pressure rail generally allows a constant flow rate to be produced to the detector from this constant pressure. The problem, therefore, is to regulate the pressure in the pressure rail, while the pump naturally emits pulsations due to its periodic operation.

[0009] A first solution for regulating the pressure in the pressure rail is to use a backpressure regulator. This regulator functions as a safety valve by releasing the excess pressure produced by the pump through a vent. The pressure rail extends between the pump and the backpressure regulator. The flow rate at the vent corresponds roughly to the pump flow rate minus the flow rate drawn by the sensor. The pump delivers a gas flow in pulses, and between two pulses, the backpressure regulator closes, and the sensor continues to be supplied by the residual pressure in the pressure rail.

[0010] Another solution involves using an electronic pressure regulator connected to an auxiliary source of pressurized purified air. The pressure rail is supplied with purified air, which maintains a constant pressure in the pressure rail even between two pump flow pulses. However, this solution has drawbacks: (1) significant air consumption, generally more than twice the sample flow rate, and (2) reduced operational reliability. It becomes very difficult to detect a problem with the sample pump or partial or total clogging of the sample filter because these are entirely compensated for by the auxiliary air source. In extreme cases, the analyzer may end up measuring the auxiliary air instead of the sample in the event of a total pump failure, without this being noticeable by the analyzer.

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[0018] the measurement of the sample pressure. Other solutions still exist and consist for example of replacing the purified air source of the last solution with a pump, and the electronic regulator with a mechanical or backpressure regulator. The technologies currently available to guarantee a constant pressure in the pressure rail of the measuring instrument are therefore not satisfactory, from the point of view of gas consumption, reliability, and also in terms of complexity and cost of maintenance. Summary of the invention The present invention provides a simple, effective and economical passive solution to the aforementioned need. The invention relates to an instrument for measuring at least one volatile organic compound in a gas mixture, this instrument comprising: - an air inlet, - an inlet of H2 or H2-He gas, - an inlet for a sample of the gas mixture, - a pump, - a pressure rail connected via the pump to the inlet of sample and configured to convey said sample, - at least one flame ionization detector, this detector being connected to the air and H2 gas inlets and to said pressure rail, this detector being configured to generate an ionization flame of the volatile organic compound(s) (VOC) contained in said sample, and to emit an electronic signal proportional to the quantity of the volatile organic compound(s) in this sample, and - an electronic control and processing unit for said signal, characterized in that it further comprises: - at least one damping chamber configured to contain a predetermined volume of gas at a predetermined pressure, this chamber being connected to said pressure rail and being configured to dampen pressure variations in said rail. The invention proposes replacing the regulator of the prior art with a chamber that dampens pulsations or pressure variations. Existing compressed air injection-based regulators provide damping that could be described as "active," due to the need for an auxiliary regulator to set the pressure of this supplementary air. The chamber provides "passive" damping, requiring no additional equipment. Furthermore, this passive damping is significantly less expensive than a backpressure regulator, which is also passive.

[0019] One of the advantages of the invention is that, unlike the regulator of the prior art, no special maintenance of the chamber is required. Either the air regulation system at the sample setpoint pressure or the backpressure regulator can be eliminated simply by adding the damping chamber, which can be very inexpensive.

[0020] The instrument according to the invention may comprise one or more of the following features, taken individually or in combination with each other:

[0021] - said enclosure is connected to the pressure rail, either directly or via an minus a flow restrictor;

[0022] - said detector is connected to said pressure rail by at least one flow restrictor;

[0023] In this application, the term “flow restrictor” or “restrictor” means a A device that allows the gas flow to be limited or reduced; this can be done in several ways and the simplest way is to reduce the cross-section of the gas through a pipe; a narrowing of the cross-section in a pipe forms a flow restrictor; the connection of two pipes by a pipe of smaller internal diameter also forms a flow restrictor; a restrictor is for example formed by a capillary; a flow restrictor can be used to ensure a known gas flow from a known pressure.

[0024] - said enclosure forms part of said pressure rail;

[0025] - said enclosure and / or said pressure rail is / are connected by at least one res flow restrictor at said pump and by at least one other flow restrictor at a vent port;

[0026] - the instrument further comprises a heated and / or thermostatically controlled cavity, for example at a temperature above 150°C and preferably above or equal to 180°C, said detector being located in said cavity; the heating temperature of the cavity can be around 100°C in the case of use of the instrument for measuring VOCs in ambient air (and will rather be around 180°C or more for use in an exhaust chimney);

[0027] - said enclosure is located at least partly outside said cavity;

[0028] - the instrument further comprises an inlet for a standard gas mixture for the purpose of the calibration of the instrument;

[0029] - said enclosure is configured to contain a volume of gas greater than or equal to 50mL, preferably greater than or equal to 200mL, and more preferably greater than or equal to 500mL; this volume is preferably adapted according to the sample flow rate coming out of the pump, the pump speed, and the desired damping ratio;

[0030] — the instrument is configured to draw a sample flow from said rail of pressure which represents less than 50% of the total flow through said pressure rail, preferably less than 20% of the total flow through said pressure rail, more preferably less than 10% of the flow through said pressure rail and even more preferably less than 5% of the flow through said pressure rail;

[0031] - the instrument further comprises a pressure sensor configured to measure the pressure in said enclosure;

[0032] - said detector is connected to said pressure rail at a connection point and said The chamber is located upstream or downstream of this connection point relative to the flow of the sample in the pressure rail; the larger the volume of the damping chamber, the more effective the damping of pressure variations can be; the smaller this volume, the faster the measurement by the detector can be. A compromise must therefore be chosen between effective damping (large volume chamber) and a reduced response time (small volume chamber); placing the damping chamber downstream of the sampling point eliminates this constraint; moreover, the response time of the instrument is not affected, and any condensation does not interfere with the measurement.

[0033] - the pump has a variable speed and is controlled by said control unit; this allows for precise regulation of the average pressure value in the pressure rail; the pump operating speed can also be an important factor in improving the efficiency of passive damping with the enclosure; the damping ratio can be of the type: [°°341 V draws _ D v - a' V

[0035] with D the air flow rate at the pump outlet, co the pump speed, Vpuixe the sample volume pulsed by the pump at each revolution and Vmort the volume of the damping chamber; in our application, this ratio is preferably less than or equal to 2%, ideally less than or equal to 1%;

[0036] — said control unit is connected to said pressure sensor and regulates the operating regime of the pump so as to maintain constant pressure in said enclosure;

[0037] — the instrument comprises two or more damping enclosures mounted in series side by side;

[0038] — the pressure rail includes a restrictor at each of its ends;

[0039] — the pressure rail comprises a main pipe connected by a T-fitting to a branch line, the main line having one end connected to the enclosure and the branch line being connected to the detector;

[0040] — the bypass pipe is equipped with a restrictor;

[0041] — at least one end of the main pipe is equipped with a restrictor;

[0042] — one end of the main pipe is connected to a venting port;

[0043] — said pump is a pneumatic diaphragm pump;

[0044] — said diaphragm pump is equipped with at least one first inlet valve of gas, and at least one gas outlet valve;

[0045] — said pump comprises a diaphragm connected to a mechanical piston driven by a motor via an eccentric.

[0046] The invention further relates to equipment for measuring at least one volatile organic compound in a gas mixture, this equipment comprising:

[0047] - an instrument as described above,

[0048] - a sampling rod for said sample, this rod being for example configured to be inserted into a gas exhaust chimney, and

[0049] - a heating unit mounted between the sampling tube and the inlet sample of the instrument, and configured to heat the sample to a predetermined temperature.

[0050] The invention further relates to a method of implementing the instrument as described above, comprising:

[0051] - an instrument start-up step in which the enclosure is powered by the pump with said gas mixture until the pressure in the enclosure is greater than or equal to a predetermined value, then

[0052] - a step of analyzing the sample by said detector.

[0053] Advantageously, the pressure is greater than or equal to 100mbar, preferably greater than or equal to 150mbar, and more preferably greater than or equal to 200mbar.

[0054] The pressure variations in said pressure rail preferably have an amplitude less than or equal to 2% of the average pressure in said pressure rail, and more preferably to 1%.

[0055] Advantageously, the cavity is heated and / or thermostated to a predetermined temperature during the start-up step. Brief description of the figures

[0056] Other features and advantages of the invention will become apparent upon reading the detailed description that follows, for an understanding of which reference should be made to the accompanying drawings in which:

[0057] [Fig-1] [Fig.1] is a schematic perspective view of a measuring instrument according to the invention, equipped with a flame ionization detector,

[0058] [Fig.2] [Fig.2] is a schematic view of a compound(s) measuring equipment volatile organic(s) (VOCs) in a gas mixture, this equipment including a measuring instrument of the type of that in [Fig. 1],

[0059] [Fig.3] [Fig.3] is a schematic view of a measuring instrument according to the technique prior to the present invention,

[0060] [Fig.4] [Fig.4] is a schematic view of a flame ionization detector (FID),

[0061] [Fig.5] [Fig.5] is a schematic graph showing the evolution of the sample gas flow rate supplied by the pump of a measuring instrument, as a function of time,

[0062] [Fig.6] [Fig.6] is a graph similar to that of [Fig.5] and showing the effect of an additional regulator in the measuring instrument,

[0063] [Fig.7] [Fig.7] is a very schematic view of a measuring instrument according to a method of embodiment of the invention,

[0064] [Fig.8] [Fig.8] is a very schematic view of a measuring instrument according to another embodiment of the invention,

[0065] [Fig.9] [Fig.9] is a very schematic view of a measuring instrument according to a another embodiment of the invention,

[0066] [Fig. 10] [Fig. 10] is a schematic perspective view of the pump and the housing of the measuring instrument of [Fig. 9], and

[0067] [Fig. 11] [Fig. 11] is a schematic cross-sectional view of the pump and the housing of the measuring instrument of [Fig. 9], the section being made along line XI-XI of [Fig. 10]. Detailed description of the invention

[0068] Reference is first made to [Fig. 1] which illustrates a measuring instrument 10 equipped with a flame ionization detector or FID detector.

[0069] The instrument 10 is presented here in the form of a parallelepiped-shaped case 12 which can be placed on a support or mounted with other measuring instruments in a cabinet provided for this purpose.

[0070] The instrument 10 comprises several components which will be detailed below with reference to [Fig.3] and can be equipped with a screen 14, possibly touch-sensitive, for displaying information such as measured values.

[0071] The screen 14 is located here on a front face of the housing 12 which includes a rear face with connectors and ports for connecting the instrument 10 for its operation.

[0072] Fig. 2 shows a measuring device for volatile organic compound(s) (VOC) in a gas mixture, this device comprising a measuring instrument 10 of the type of that in Fig. 1.

[0073] Reference numeral 16 designates a gas exhaust stack of a municipal waste treatment plant, for example. This stack 16 includes an opening through which a sampling probe 18 is inserted. The length of the probe 18 depends in particular on the diameter of the stack, especially if the sampling is to take place in the center of the stack.

[0074] The rod 18 is connected by a conduit 22 to the instrument 10. The rod 18 is heated by a casing 20 so that the gas collected is heated and conveyed to the instrument 10.

[0075] The gases contained in the stack 16 are generally combustion gases. These gases are at a relatively high temperature, and their temperature must be maintained at a relatively high level to prevent the water they contain from condensing after sampling. Gases to be measured could end up in the condensate, which would distort the measurement by the instrument 10. It is therefore important to prevent any condensation of the sampled gases upstream of the instrument 10.

[0076] Instrument 10 comprises at least three inputs:

[0077] - an air inlet 24, - an inlet 26 of H2 gas or an H2-He mixture and - an entry 28 of the sample taken by the rod 18.

[0078] The instrument 10 may further include a calibration gas inlet 30 for calibration and a zero air inlet for adjusting the zero point of the instrument 10 in a manner known to a person skilled in the art.

[0079] The instrument 10 may further include a vent port 32 and a condensate drain port 33 from the instrument's FID detector(s).

[0080] Fig. 3 is a schematic view of instrument 10 and shows that the three inputs 24, 26 and 28 are connected to the instrument's FID detector 34.

[0081] The instrument 10 includes a pump 36 linking the inlet 28 to a pressure rail 40.

[0082] A filter 35 may be provided upstream of the pump 36, i.e., between the pump 36 and the inlet 28. The filter 35 is, for example, a sintered stainless steel filter having a filtration capacity of between 2 and 100 µm³, and preferably on the order of 5 µm³.

[0083] Fig. 3 shows a technology prior to the present invention in which the instrument 10 is equipped with a backpressure regulator 38.

[0084] This regulator 38 functions as a safety valve by releasing excess pressure produced by the pump 36 to the vent port 32. The pressure rail 40 extending between the pump 36 and the regulator 38 is formed by a tube with a large internal diameter connected to the detector 34, which is supplied with a constant flow rate via a restrictor C4. The instrument 10 may include additional restrictors C1-C5.

[0085] The detector 34, the filter 35, the pump 36, the regulator 38, and the various restrictors and tubes carrying the sample are housed in a thermostatically controlled cavity 4L. This cavity 41 can be a thermally insulated enclosure combined with heating means. For example, it is configured to be maintained at a temperature sufficient for... greater than or equal to 180°C.

[0086] Fig. 4 is a schematic view of the FID 34 detector. This type of detector is well known to those skilled in the art.

[0087] The detector 34 is supplied with three gases (air, sample, and H2) and is configured to generate a hydrogen flame 42 burning in excess oxygen in order to ionize the organic compound(s) contained in the sample. The ions thus generated, in the presence of an electric field, are captured on a collection electrode 44. The electrical signal 46 is amplified and is representative of the number of carbon atoms contained in the sample. This signal 46 is processed by an electronic control and processing unit 47, which is connected to the screen 14 for displaying measurement values.

[0088] The graph in [Fig. 5] illustrates the behavior of the pump 36 of the measuring instrument 10. The pump 36 operates periodically and delivers a gas flow in the form of successive pulses 36a. [Fig. 5] thus shows that the pressure flow rate (Q) at the outlet of the pump 36 changes from a maximum value to zero or a very low value periodically over time (t).

[0089] Pump 36 is, for example, a pneumatic diaphragm pump equipped with valves. This is, for example, the pump marketed by GARDNER DENVER THOMAS GMBH® under the reference Diaphragm Pump 7010 DVKT / 230V-50Hz.

[0090] The diaphragm is generally connected to a piston driven by an eccentric. The diaphragm closes a sealed cavity which is connected to two valves, one for gas inlet and one for gas outlet. The movement of the piston results in an elastic deformation of the diaphragm. When the diaphragm is stretched, the pressure in the cavity decreases and gas is drawn into the cavity through the inlet valve. When the diaphragm is released, the gas is expelled from the cavity through the outlet valve.

[0091] The graph in [Fig. 6] illustrates the behavior in the pressure rail 40 when the measuring instrument 10 is equipped with a regulator 38 and an auxiliary air source, as mentioned above. This regulator 38 ensures a constant pressure 36b in the pressure rail 40 despite the irregular effect of the pump 36.

[0092] To overcome the drawbacks of the prior art, the present invention proposes equipping the measuring instrument 10 with a damping chamber 48 configured to contain a predetermined volume of gas at a predetermined pressure. This chamber 48 is connected to the pressure rail 40 and is configured to dampen pressure variations induced by the pump 36.

[0093] Figure 7 illustrates a first embodiment of the measuring instrument 100 according to the invention, wherein the elements already described above are designated by the same references.

[0094] The enclosure 48 here includes an inlet 48a connected to the outlet of the pump 36, preferably by a restrictor 50. The enclosure 48 includes an outlet 48b which is connected to the pressure rail 40, also preferably by a restrictor 52. The pressure rail 40 extends from the enclosure 48 to the vent port 32, which may also be equipped with a restrictor 54. Finally, the detector 34 is connected to the pressure rail 40, also preferably by a restrictor 56.

[0095] More specifically, the pressure rail 40 comprises a main line 40a connected by a T-fitting 40b (which forms a sampling or connection point) to a branch line 40c. The main line 40a has one end connected to the enclosure by the restrictor 52 and an opposite end connected to the port 32 by the restrictor 54. The branch line 40c is connected by the restrictor 56 to the detector 34, this restrictor being, for example, a capillary tube limiting the gas flow supplying the detector 34 to less than 100 mL / min, and for example to 40 mL / min.

[0096] Given the position of the enclosure 48 in this embodiment, this enclosure 48 must be housed in the thermostatically controlled cavity 41 of the instrument 100, which may limit the choice of its volume.

[0097] The instrument 100 is preferably also equipped with a pressure sensor 58 which is configured to measure the pressure in the enclosure 48 or in the pressure rail 40.

[0098] The pressure sensor 58 is connected to the processing unit 47 of the instrument, which controls the pump 36 based on the information transmitted by this sensor. The pressure in the pressure rail 40 and in the chamber 48 is therefore regulated by the servo control of the speed of the pump 36.

[0099] This unit 47 may include a microprocessor which performs the processing of acquisitions, calculations, automation and control of interfaces, and an analog / digital converter which receives, for example via a multiplexer, the signals provided by the detector 34 as well as those delivered by the pressure sensor 58 or even other types of sensor of the instrument 10, for example temperature.

[0100] The vent port 32 allows for rapid renewal of the air in the pressure rail 40, in order to ensure a sufficiently short response time of the instrument 100.

[0101] During operation, the gas flow rate drawn by detector 34 is, for example, on the order of a few tens of milliliters per minute, while the gas flow rate exiting port 32 is on the order of a few liters per minute. It is therefore understood that the gas drawn by detector 34 does not have a significant impact on the pressure inside pressure rail 40.

[0102] The damping efficiency may depend on the speed or pump speed 36. More precisely, a representative parameter of passive damping can be the ratio: [01031 _ D v ~ wV F dead dead

[0104] with D the air flow rate at the outlet of pump 36, co the pump 36 speed and VmOrt the volume of the damping chamber.

[0105] The flow rate D of the pump is for example between 10 and 200L / h, and preferably between 45 and 120L / h.

[0106] With a relative pressure setpoint of 220 hPa measured by sensor 58 at pressure rail 40 and generated by pump 36 at a frequency of 10 Hz, the experimental results are as follows: Damping chamber volume 48 (Vmort) Restrictor diameter 50 (RI) Restrictor length 50 (RI) Pressure pulsation amplitude in pressure rail 40 1.5 mL 1 mm 50 mm 64% 3 mL 1 mm 300 mm 32% 23.5 mL 1 mm 300 mm 8.3% 33.5 mL 1 mm 300 mm 4.9% 51.5 mL 1 mm 300 mm 1.5% 2 chambers in series of 32 mL each 1 mm 300 mm 0.8%

[0107] In the aforementioned examples, which are by no means limiting, damping is effective (pulsation amplitude less than or equal to 2%, or even 1%) using an enclosure with a volume greater than or equal to 50 mL or two enclosures in series, each with a volume greater than 25 mL. Integrating two or more enclosures in series inside the thermostatically controlled cavity 41 may be simpler than integrating a single enclosure of the same volume, due to the limited space available inside this cavity 4L.

[0108] Fig. 8 illustrates another embodiment of the measuring instrument 100 according to the invention in which the elements already described above are designated by the same references.

[0109] The enclosure 48 here includes an inlet 48a which is connected by the pressure rail 40 to the outlet of the pump 36. A restrictor 50 is mounted at the outlet of the pump 36, for its connection to the pressure rail 40.

[0110] The enclosure 48 includes an output 48b which is connected, also preferably by a restrictor 54, at vent port 32. The detector 34 is connected to the pressure rail 40, also preferably by a restrictor 56.

[0111] More specifically, the pressure rail 40 comprises a main line 40a connected by a T-fitting 40b (which forms a sampling or connection point) to a branch line 40c. The main line 40a has one end connected to the enclosure 40 and an opposite end connected to the pump by the restrictor 50. The branch line 40c is connected by the restrictor 56 to the detector 34, this restrictor being, for example, a capillary tube limiting the gas flow supplying the detector 34 to less than 100 mL / min, and for example to 40 mL / min.

[0112] The enclosure 48 can be equipped with a drain 60 or condensate drain.

[0113] The instrument 100 is preferably also equipped with a pressure sensor 58 which is configured to measure the pressure in the enclosure 48 or in the pressure rail 40. The average value of the pressure in the pressure rail 40 and in the enclosure 48 is regulated by the speed control of the pump 36, and the pressure pulsations are dampened by the enclosure.

[0114] In this embodiment, the enclosure 48 can be considered as forming an integral part of the pressure rail 34. This pressure rail can have a relatively large diameter. The larger this diameter, the more uniform the pressure damping is within the volume of the enclosure and the pressure rail.

[0115] Unlike the previous embodiment, it is not the gas contained in the chamber 48 that is measured by the detector 34. This chamber is in fact located downstream of the T-fitting 40b, which draws the sample towards the detector. Therefore, there is no need to prevent condensation of the gases inside the chamber 48. Any condensate in the chamber 48 can be drained through its drain 60 using a peristaltic pump, which is inexpensive and only needs to be switched on occasionally.

[0116] One advantage of this embodiment is that the enclosure 48 does not need to be housed within the thermostatically controlled cavity 41 of the instrument 100, and therefore its volume is not limited by this constraint. It is thus possible to dimension the enclosure 48 in such a way as to significantly dampen pressure variations in the pressure rail 40.

[0117] The enclosure 48 can thus have a volume greater than or equal to 50 mL, preferably greater than or equal to 200 mL, and more preferably greater than or equal to 500 mL. In a particular embodiment of the invention, the enclosure 48 can have a volume greater than or equal to IL.

[0118] Furthermore, the use of a large-volume enclosure 48 does not affect the response time of the detector 34 because it is the gas contained in the pressure rail 40, which is sampled and analyzed by the detector, and not the gas contained in enclosure 40.

[0119] Figures 9 to 11 illustrate another embodiment of the measuring instrument 100 according to the invention in which the elements already described above are designated by the same references.

[0120] The instrument in Figures 9 to 11 is similar to that in [Fig.8] and differs from it in the following characteristics.

[0121] The pump 36 is here separated from its drive motor 36' and is housed in the cavity 41 heated by heating means 62 controlled by the control unit 47. The motor 36' is housed outside this cavity 41 and in the housing of the instrument 100 which may be of the type shown in [Fig.1].

[0122] As mentioned above, the actuation of the pump 36 diaphragm by the motor 36' is achieved by a piston 36a visible in [Fig. 10]. This piston 36a extends between the pump 36 and the motor 36' and passes through one of the insulating walls of the cavity 41 (see [Fig. 9]). Spacers 36b also extend between the pump 36 and the motor 36' and pass through this wall to stiffen the connection between the pump 36 and the motor 36' and to secure them to this wall.

[0123] The pump 36 has its inlet or inlet valve which is connected to the inlet 28, as schematically illustrated in [Fig.9]. Its outlet or outlet valve is connected to the enclosure 48.

[0124] In the example shown, instrument 100 includes two FID 34 detectors, one specifically for the detection of methane CH4 and the other for all volatile organic compounds.

[0125] Fig. 9 also shows the peristaltic pump 64 which can be housed in the instrument housing 100 next to the enclosure 48, in order to collect and remove to the outside the condensates which are likely to appear and accumulate in the enclosure 48.

[0126] Figures 9 to 11 further show an example of an embodiment of enclosure 48 which here comprises two parts.

[0127] The first part 48a of the enclosure 48 is located outside the cavity 41 and is in the form of a glass bowl 66 sealed by a lid 68. The bowl 66 is mounted in the instrument housing so that its opening faces the cavity 41 and the X-axis of this opening is positioned horizontally. This bowl 66 can be held tightly against a bottom wall of the housing by a strap system 70, for example, which allows the bowl 66 to be easily disassembled and removed from the housing during maintenance.

[0128] The lid 68 is screwed onto the edge of the opening of the bowl 66 and includes a first passage for the watertight mounting of the sensor 58 and a second passage for the watertight installation of drain 60.

[0129] The cover 68 also includes a central passage for connection to a second part 48b of the enclosure 48.

[0130] The second part 48b of the enclosure 48 is located in the cavity 41a and is presented here in the form of a block 72 comprising an internal chamber 74 ([Fig. 11]) connected to the pressure rail 40. The chamber 74 can also be considered as forming part of the pressure rail 40.

[0131] The block 72 is connected to the bowl 66 by a straight conduit 76 which passes through the wall of the cavity 4L. The conduit 76 has an internal bore that is aligned with the X-axis of the bowl 66. The chamber 74 has a generally tubular shape and its axis of revolution is also substantially aligned with the X-axis of the bowl 66. It is thus understood that the chamber 74 communicates directly with the interior of the bowl 66 via the bore of the conduit 76. The conduit 76 can also be considered as forming part of the pressure rail 40. The combined internal volumes of the bowl 66, the conduit 76, and the chamber 74 represent the total volume of the enclosure 48 as defined in the invention. We also understand that part of this volume is heated in cavity 41 (this is the case for the volume of chamber 74) and that another part of this volume is not heated (this is the case for the volume of bowl 66).Any condensate in bowl 66 is removed by drain 60 and pump 64, as mentioned above.

[0132] As can be seen in [Fig. 11], chamber 74 is connected to an inlet port 74a which is connected by the pressure rail 40 to the outlet of pump 36, and an outlet port 74b which is connected by the pressure rail 40 to the inlet of at least one restrictor which supplies a FID detector 34 with sample gas. The inlet port 74a is located here on the X-axis, on the opposite side to the pipe 76 and the bowl 66. The outlet port 74b is located on one side of the chamber 74 and as close as possible to the inlet port 74a, which allows for a very short response time between the entry into the pump 36 of a sample gas particularly concentrated in VOCs, the sampling of a portion of the sample and the analysis by the FID detectors 34. The block 72 thus forms the aforementioned sampling point and replaces the T-fitting 40b mentioned above.

[0133] The pressure rail 40 may include one or more restrictors as mentioned above.

[0134] The unit 47 is configured to take a sample flow rate from the pressure rail 40 which represents less than 50% of the total flow rate through the pressure rail, preferably less than 20% of this total flow rate, more preferably less than 10% of this flow rate and even more preferably less than 5% of this flow rate.

[0135] Although the instrument according to the invention has been described in the context of sampling from a chimney using a rod, it is possible to use this instrument in another context and for example for measuring VOCs in the air ambient. In this case, it would be useful to heat enclosure 41 to limit condensation and evaporate any potential pollutants in the pipes (such as organic cutting oils used for machining parts, for example), but it could be heated to a lower temperature, for example around 100°C.

[0136] In the following, we present experimental results obtained on an analyzer 100 in the configuration described in Figures 1 to 11. The sample pressure is measured in the damping chamber 48 and is regulated around an average value of 200 mbar. The peak-to-peak amplitude of the pressure pulses is expressed as a percentage of the average pressure value.

[0137] With a volume in the damping chamber 48 equal to 300mL, a pump 36 operating at 23Hz (1380 rpm) and a pressure rail 40 with an internal diameter of 6mm, a pulsation amplitude of 1.6% is obtained.

[0138] When increasing to a volume of 1000mL, the amplitude is 1%.

[0139] Without the damping chamber but retaining the rest of the configuration shown in Figures 8 to 11, the pressure pulsations at the T-fitting 40b reach 20% of the nominal pressure. The damping chamber 48 is therefore particularly advantageous in this configuration for ensuring a stable VOC measurement.

[0140] The absence of a restrictor between the pump 36 and the pressure rail 40 can cause pressure pulsations in the enclosure representing 4.5% of the average pressure, even with a large damping volume of 1000 mL. This is less effective than what is obtained with this restrictor and the same volume. The presence of the restrictor in the pressure rail 40 between the pump and the pressure rail therefore improves the damping.

[0141] With a backpressure regulator set at 200 mbar instead of the pair of damping and control chambers for the sample pump, the pressure pulsations are on the order of 3.5 mbar, or 1.7% of the nominal pressure. The performance of the invention is therefore very satisfactory compared to this prior technology.

[0142] The invention offers numerous advantages over the prior art:

[0143] a. The pump pulsation damping is achieved without the need for pressurized air or an additional pump for supplemental air. A problem with the sample pump or the sample filter is therefore instantly visible in the sample pressure. This prevents supplemental air from even partially replacing the sample and distorting the measurement if such a problem occurs. Significant air consumption is also saved, as the sample pump itself creates this pressure reservoir using excess sample gas. b. The damping of the pump pulsations is achieved without a mechanical or electronic regulator. It is done simply with the sample pump (whose The electronic power supply is servo-controlled, which was already necessary, and a passive enclosure that is inexpensive and maintenance-free. This represents savings both in purchase price and maintenance. The absence of a heated mechanical regulator allows for a reduction in the volume of the heated cavity. c. The sample can be partially dried (by natural condensation) in the enclosure 48, which reduces the risk of fouling in the outlet restrictor 54. d. When the damping chamber is outside the thermostatically controlled cavity, the size of this cavity can be reduced, resulting in lower electricity consumption for heating. This freed-up volume can be occupied by the pump, for example. e. In the case of a dual architecture, where the instrument comprises two parallel channels, each with a pump and an FID detector, the damping chamber could be common to both channels, since mixing from these channels within the chamber would not risk reaching the detectors. This would eliminate the need for a pressure sensor, and both pumps would then be driven by the same speed setpoint. The outlet restrictor 54 would be calculated for double the flow rate. f. When the damping chamber is placed directly at the pump outlet, this can lead to an increase in response time and an increase in the volume of the heated cavity. This is avoided by placing at least part of this chamber downstream of the 40b T-fitting or the sensor sampling point(s).

Claims

Demands

1. An instrument (100) for measuring at least one volatile organic compound in a gas mixture, said instrument comprising: - an air inlet (24), - an H2 or H2-He gas inlet (26), - an inlet (28) for a sample of the gas mixture, - a pump (36), - a pressure rail (40) connected via the pump (36) to the sample inlet (28) and configured to convey said sample, - at least one flame ionization detector (34), this detector being connected to the air and H2 gas inlets (24, 26) and to said pressure rail (40), this detector (34) being configured to generate an ionization flame of the volatile organic compound(s) contained in said sample, and to emit an electronic signal (46) proportional to the quantity of the organic compound(s) volatile(s) in this sample, and - an electronic control and processing unit (47) for said signal,characterized in that it further comprises: - at least one damping chamber (48) configured to contain a predetermined volume of gas at a predetermined pressure, this chamber (48) being connected to said pressure rail (40) and being configured to dampen pressure variations in said rail.

2. Instrument (100) according to claim 1, wherein said enclosure (48) is connected to the pressure rail (40), either directly or via at least one flow restrictor (52).

3. Instrument (100) according to claim 1 or 2, wherein said detector (34) is connected to said pressure rail (40) by at least one flow restrictor (56).

4. Instrument (100) according to any one of the preceding claims, wherein said enclosure (48) is part of said pressure rail (40).

5. Instrument (100) according to any one of the preceding claims, wherein said enclosure (48) and / or said pressure rail is / are connected by to less one flow restrictor (50) to said pump (36) and by at least one other flow restrictor (52, 54) to a vent port (32).

6. Instrument (100) according to any one of the preceding claims, wherein it further comprises a cavity (41) heated and / or thermostatically controlled, for example at a temperature above 150°C and preferably above or equal to 180°C, said detector (34) being located in said cavity (41).

7. Instrument (100) according to the preceding claim, wherein said enclosure (48) is at least partly located outside said cavity (41).

8. Instrument (100) according to any one of the preceding claims, wherein it further comprises an inlet (30) of a standard gas mixture for the purpose of calibrating the instrument.

9. Instrument (100) according to any one of the preceding claims, wherein said enclosure (48) is configured to contain a volume of gas (Vmort) greater than or equal to 50mL, preferably greater than or equal to 200mL, and more preferably greater than or equal to 500mL.

10. Instrument (100) according to any one of the preceding claims, wherein it further comprises a pressure sensor (58) configured to measure the pressure in said enclosure (48).

11. Instrument (100) according to any one of the preceding claims, wherein said detector (34) is connected to said pressure rail (40) at a connection point and said enclosure (48) is located upstream or downstream of this connection point with respect to the flow of the sample in the pressure rail (40).

12. Instrument (100) according to any one of the preceding claims, wherein the pump (36) is variable speed and is controlled by said control unit (47).

13. Equipment for measuring at least one volatile organic compound in a gas mixture, said equipment comprising: - an instrument (100) according to any one of the preceding claims, - a sampling probe (18) for said sample, this probe (18) being for example configured to be inserted into a gas exhaust chimney (16), and - a heating box (20) mounted between the sampling probe (18) and the sample inlet (28) of the instrument (100), and configured to heat the sample to a predetermined temperature.

14. Method of implementing the instrument (100) according to one of the claims instructions 1 to 12, comprising: - a start-up step of the instrument (100) in which the chamber is supplied by the pump (36) with said gas mixture, until the pressure in the chamber (48) is greater than or equal to a predetermined value, then - a sample analysis step by said detector (34).

15. A method according to the preceding claim, wherein the pressure is greater than or equal to 100 mbar, preferably greater than or equal to 150 mbar, and more preferably greater than or equal to 200 mbar.

16. A method according to claim 14 or 15, wherein the pressure variations in said pressure rail (40) have an amplitude less than or equal to 2% of the average pressure in said pressure rail, and preferably 1%.

17. A method according to any one of claims 14 to 16, wherein, the instrument (100) being as defined in claim 6 or 7, said cavity (48) is heated and / or thermostated to a predetermined temperature during the start-up step.

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