EP2486398A2 - Micro-thermal conductivity detector, method to fabricate such and chromatography system using such - Google Patents

Abstract

A heat flux sensor, a micro-scale gas chromatograph comprising the heat flux sensor, and method for measuring the thermal conductivity of a fluid using such are provided. The heat flux sensor comprises a chamber, a heating element suspended in the chamber, at least two current contacts configured to exchange a current with the heating element, and at least two measurement contacts configured to measure a voltage change along the heating element indicative of the thermal conductivity of the fluid.

Description

21.1864

MICRO-THERMAL CONDUCTIVITY DETECTOR, METHOD TO FABRICATE

SUCH AND CHROMATOGRAPHY SYSTEM USING SUCH

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is based on and claims priority to U.S. Provisional Application No. 61/250,310, filed October 9, 2009.

TECHNICAL FIELD

[0002] The invention relates to the field of sensors for use in analytical chemical instruments. More particularly, the present invention relates to micro-thermal conductivity detectors for use in chromatography, and even more particularly, in micro chromatography.

BACKGROUND

[0003] In analytical chemistry, chromatography is the premier tool used to separate and detect different compounds in a sample mixture. One of the common methods for performing chromatography uses chromatography separation columns to separate the sample fluid into its constituent compounds. The interior surface of the chromatography column is typically an inert material that is coated with, or has adsorbed onto it, a material referred to as the "stationary phase". The sample fluid is introduced into the chromatography column through a sample inlet device and is transported through the chromatography column using an inert carrier fluid, which is referred to as the "mobile phase". When the sample fluid encounters the stationary phase, the different components in the sample fluid are attracted differently to the stationary phase, causing the different components in the sample fluid to travel through the system at different speeds. Separation occurs by the differential retardation of sample components through interaction with the stationary phase as they are driven through the chromatography column by the mobile phase. Each sample component will have a characteristic delay between the time it was introduced into the chromatographic system and the time that it is detected after it elutes from the chromatography separation column. This characteristic time is called "retention time". Some minimum amount of difference in retention time allows differentiation of sample components chromatographically. Several chromatography methods are commonly used in analytical chemistry, and among others liquid and gas chromatography, wherein the carrier fluid is respectively an inert liquid or gas. 21.1864

[0004] One or more detectors at the exit of the chromatography column detect the different compounds when they elute from the chromatography column and provide an output signal proportional to the amount of the sample component. The different components are shown as "peaks" on a chromatogram where the height and area beneath the peak correspond to the amount of the compound.

[0005] Various types of detectors may be used, but detectors that provide an output signal corresponding to the thermal conductivity of the fluid contained therein is advantageous because of sensitivity to all fluids of such detectors.

[0006] Thermal conductivity detectors (TCDs) have been used as detectors in gas or liquid chromatographs for many years to provide the output signal referred to. In a simple form, a thermal conductivity detector includes a cell having a heated element, usually an electrically heated wire or thermistor, arranged in a chamber and held in a stream of flowing fluid. The temperature of the heated element varies depending upon the thermal conductivity of the fluid flowing around it. As the output from the chromatography column flows through the chamber, the rate at which heat flows from the heated element to the wall of the chamber varies with the thermal conductivities of the fluids in the chamber. The thermal conductivity of the carrier fluid differs from the thermal conductivities of the sample fluids, and the thermal conductivities of the sample fluids mixed with carrier fluid vary with the concentration of the sample fluids in the carrier fluid. Changes in thermal conductivity are typically detected as changes in electrical resistance in the heated element and measured as voltage changes.

[0007] The fluid properties can be determined by using one of two different operating modes. In constant voltage mode, the output of the detector is related to the temperature change of the heating element. As a less thermally conductive fluid is exposed to the detector, less heat is transported away, increasing the heating element temperature. In constant temperature mode, the heating element is maintained at a predetermined operating temperature. The change in power required to maintain this predetermined temperature is measured.

[0008] At present, gas chromatography is used primarily in laboratory testing. Since the mid-1970's scientists and engineers have been working to expand the uses of gas chromatography by developing micro-scale gas chromatography systems. According to the 21.1864 scientific publication by S. C. Terry, J. H. Jerman and J. B. Angell, "A gas chromatograph air analyzer fabricated on a silicon wafer," IEEE Trans. Electron Devices, vol. ED-26, p. 1880, 1979., the first micro-fabricated gas chromatography system was developed at Stanford Electronic Laboratories. This system comprised a column integrated with a separately fabricated thermal conductivity detector. This device had poor chromatographic resolution when compared to standard gas chromatography techniques of the time, most likely due to the liquid deposition process used for the stationary phase of the chromatography column.

[0009] Micro-chromatography systems have the potential of being portable, robust and, due to batch fabrication and economies of scale, cheaper to produce than traditional chromatography systems. These systems may be applied in fields ranging from health services and homeland security to industry process control and geological exploration. An example of a miniaturized gas chromatograph which is particularly designed for use in the oil and natural gas industry is taught by European Patent Publication No. 2 065 703 Al to Applicant/Assignee, which is hereby incorporated herein by reference in its entirety.

[0010] In the push for novel gas sensing and detection mechanisms, little effort has been put into enhancing thermal conductivity detectors. However, thermal conductivity detectors are particularly suited for miniaturization, since they are sensitive to the concentrations of substances within the sample, and not to the total mass of a sample as in flame ionization detectors (FIDs). Therefore, miniaturization of thermal conductivity detectors does not affect functional sensitivity and, due to reduced mass, may simultaneously reduce power consumption and increases mechanical robustness.

[0011] In the limited micro-thermal conductivity detector (μΤΟϋ) literature, some reports indicate attempts to improve thermal isolation, and therefore sensitivity, to gas concentration. The scientific publication by Y. E. Wu, K. Chen, C. W. Chen and K. H. Hsu, "Fabrication and characterization of thermal conductivity detectors (TCDs) of different flow channel and heater designs," Sensors and Actuators A 100, pp 37-45, 2002, Wu et al. relates to an investigation of the flow channel and heater designs and their effects on the performances of the detector. They fabricated a solid membrane over a cavity and a membrane between two flow channels in an attempt to increase thermal isolation. However, even a thin membrane is significantly more thermally conductive than a gas, causing a huge loss in potential sensitivity. 21.1864

[0012] Further, the scientific publication by D. Cruz et al. "Microfabricated thermal conductivity detector for the micro-ChemMab," Sensors and Actuators B 121, pp. 414-422, 2007, relates to the design, computational prototyping, fabrication and characterization of a micro-fabricated thermal conductivity detector in order to increase the detector detection sensitivity. A suspended square silicon nitride pad was described. This structure was obtained by etching a pyramidal cavity, thus creating a large dead volume, which would affect performance in gas chromatography.

[0013] Traditionally heat flux sensors are heated by running a current through the heating element. The same contacts used to supply power to the heating element are also used to measure the voltage across the device and detect the output signal. This setup leads to error and noise in the measurement of the voltage.

[0014] The principle of operation for a thermal conductivity detector may be summarized as: energy is put into heating a mass and the amount of thermal energy carried away is measured. As explained above, combined with a chromatography separation column, a thermal conductivity detector may identify a chemical and its concentration within a sample.

[0015] Prior art heat flux sensors show several drawbacks such as insufficient levels of isolation, poor resolution, inacceptable errors and noise in the measurement of the voltage, and therefore loss of accuracy during in the measurement of the thermal conductivity of a fluid and in chromatography analysis.

SUMMARY OF INVENTION

[0016] In one aspect, the present disclosure relates to a heat flux sensor for measuring the thermal conductivity of a fluid, comprising: a chamber, a heating element suspended in the chamber, at least two current contacts configured to supply a current to the heating element, and at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.

[0017] In another aspect, the present disclosure relates to a heat flux sensor for measuring the thermal conductivity of a fluid comprising a chamber defining a direction of fluid flow therein, a first heating element suspended in the chamber and configured to preheat incoming fluid to a predetermined temperature, and a second heating element suspended in the chamber and positioned downstream of the first heating element in the direction of the fluid flow, 21.1864 wherein the second heating element is configured to detect heat flux variation along the second heating element caused by the flowing fluid.

[0018] In another aspect, the present disclosure relates to a micro-scale gas chromatography system comprising at least one injector adapted to provide a fluid sample, which has a plurality of analytes, into the micro-scale gas chromatography system, at least one separation column adapted to separate at least a portion of the plurality of analytes of the fluid sample, and a heat flux sensor adapted to detect at least a portion of the plurality of analytes separated in the at least one separation columns, the heat flux sensor comprising a chamber, a heating element suspended in the chamber, at least two current contacts configured to exchange a current with the heating element, and at least two measurement contacts configured to measure the thermal conductivity of at least a portion of the plurality of analytes of the fluid sample.

[0019] In another aspect, the present disclosure relates to a method for measuring the thermal conductivity of a fluid, comprising providing a fluid to be analyzed, providing a heat flux sensor comprising a chamber and a heating element suspended in the chamber, supplying a current to the heating element near at least two current contacts, and measuring a voltage change indicative of a heat flux change caused by the fluid along the heating element near at least two measurement contacts.

[0020] In another aspect, the present disclosure relates to an apparatus and method for measuring the thermal conductivity of a fluid comprising a chamber defined by a plurality of walls, a first heating element suspended in the chamber and spaced a first distance from at least one of the chamber walls, the first heating element being configured to detect a first heat flux variation of the fluid along the first heating element; and, a second heating element suspended in the chamber and spaced a second distance, different from the first distance, from the at least one of the chamber walls, the second heating element being configured to detect a second heat flux variation of the fluid along the second heating element.

[0021] In another aspect, the present disclosure relates to a method for fabrication of a heat flux sensor for measuring the thermal conductivity of a fluid comprising providing a chamber, providing a heating element, housing the heating element in the chamber so that the heating element is suspended in the chamber, providing at least two current contacts configured to supply a current to the heating element, and providing at least two 21.1864 measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.

[0022] In another aspect, the present disclosure relates to a method for fabrication of a micro-gas chromatography system comprising providing a chamber, providing at least one separation column, providing a heating element, housing the heating element in the chamber so that the heating element is suspended in the chamber, providing at least two current contacts configured to supply a current to the heating element, and providing at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.

[0023] In yet another aspect, the present disclosure relates to an apparatus and method for analyzing a fluid sample (such as a natural gas sample) comprising a plurality of analytes. The method comprising the steps of providing a micro-scale gas chromatograph, injecting the fluid sample into the micro-scale gas chromatograph wherein at least a portion of the plurality of analytes are separated in the at least one separation column, and detecting at least a portion of the plurality of analytes separated in the at least one separation columns as a function of the thermal conductivity of the detected analytes, the heat flux change caused by the fluid sample, the thermal resistance (K/W) of the chamber, or the Knudsen number change in the chamber.

[0024] Other aspects and advantages of the disclosure will be apparent from the following detailed description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is an electric -to-thermal circuit model that simulates the operation of a heat flux sensor according to embodiments of the present disclosure.

[0026] Figure 2 is a schematic illustration of a top view of a heat flux sensor according to one embodiment of the present disclosure.

[0027] Figure 3 is a top view of a heat flux sensor according to one embodiment of the present disclosure.

[0028] Figure 4 is a schematic illustration of several heat flux sensors according to one embodiment of the present disclosure. 21.1864

[0029] Figure 5a is a schematic representation of a heat flux sensor according to one embodiment of the present disclosure.

[0030] Figure 5b is an enlarged view of the heat flux sensor as show in Figure 5a.

[0031] Figure 5c is a cross-sectional view of one embodiment of Figure 5a taken along the line Va-Va.

[0032] Figure 5d is a cross-sectional view of another embodiment of Figure 5a taken along the line Va-Va.

[0033] Figures 6a, 6b, 6c, and 6d are schematic illustrations of fabrication steps of a heat flux sensor according to one embodiment of the present disclosure.

[0034] Figures 7a, 7b, 7c, 7d, 7e, 7f, and 7g are schematic illustrations of fabrication steps of a chromatography separation column and a heat flux sensor according to one embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Embodiments disclosed herein are directed to heat flux sensors for measuring the thermal conductivity of a fluid. In particular, embodiments disclosed herein are directed to heat flux sensors comprising a chamber, and at least one heating element suspended in the chamber wherein at least one of the heating elements is configured to measure the thermal conductivity of the fluid in the chamber. In some embodiments of the present disclosure, the heat flux sensor further includes at least two current contacts configured to exchange current with the heating element, and at least two measurement contacts configured to measure a voltage change along the heating element indicative of the thermal conductivity of the fluid. In other embodiments of the present disclosure, the heat flux sensor includes at least two heating elements: a first heating element and a second heating element, wherein at least one of the first or second heating elements is configured to detect heat flux change along the heating element. Even other embodiments of the present disclosure are directed to a gas chromatography system incorporating micro-scale components, either partially or completely.

[0036] Referring to Figure 2, a heat flux sensor, according to some embodiments of the present disclosure, is shown. The heat flux sensor comprises a heating element 218 disposed on a support pad 220, two current contacts 226, 228 to supply a current to the heating element 21.1864

218, and two measurement contacts 222, 224 that measure the voltage so that a voltage change along the heating element 218 can be determined. The heating element 218 and support pad 220 are suspended in the channel 212 of the heat flux sensor chamber 238. The heating element 218 may be fabricated in any shape. In some embodiments, as shown in Figure 2, the heating element 218 has a meandering shape.

[0037] In one embodiment of the present disclosure, voltage probes 214, 216, connected to the measurement contacts 222, 224, measure and record the voltage change throughout the heat flux sensor. A current flows from a power source 202 through the sensor into a power sink 204. A current meter 206 is used to measure and control the current flow.

[0038] In another embodiment of the present disclosure, probe lines connect the sensor to the voltage probes 214, 216 via the measurements contacts 222, 224. The probe lines may be connected directly to the heating element 218 suspended in the channel 212 of the heat flux sensor chamber 238 as shown in Figure 2. Alternatively, the probe lines may be connected to the heating element 218 through intermediate lines, outside of the channel 212 of the heat flux sensor. The probe lines according to embodiments of the present disclosure are not intended to carry current, so the probe lines do not disturb in any manners the electrical responses sent from the heat flux sensor to the voltage probes 214, 216.

[0039] Temperature variations in the operating environment of the heat flux sensors described will cause fluctuations in the impedance of the connection lines 208, 210. These impedance fluctuations are difficult or impossible to decouple from the sensor response to an analyte. The provision of different contacts, namely contacts intended to supply/exchange current (current contacts 226, 228) with a power source and contacts intended to measure a voltage (measurement contacts 222, 224) advantageously removes this source of noise and error.

[0040] In yet another embodiment, the number of measurement contacts may be increased and they may be disposed in a sequence along the heat flux sensor. This provides enhanced information about the voltage and corresponding temperature distribution on the sensor. According to one embodiment of the disclosure, the use of more than two measurement contacts along the heat flux sensor enables the determination of the flow rate of a fluid. In some embodiments, the heating element comprises at least two zones, wherein at least two measurement contacts are disposed along each zone and are configured to measure a voltage 21.1864 change in the respective zone, the voltage change being indicative of the flow rate of the fluid. The zones may be electrically connected or electrically independent of one another. As such, an embodiment of the present disclosure relates to a method for measuring the flow rate of a fluid employing a heat flux sensor comprising: a chamber; a heating element suspended in the chamber; at least two current contacts configured to exchange a current with the heating element; and at least two measurement contacts configured to measure a voltage change along the heating element indicative of the flow rate of the fluid.

[0041] Figure 3 shows a heat flux sensor according to one embodiment of the present disclosure. The image is a top-down view showing a heater pad suspended inside a flow channel 310 formed within the support layer 302. The heater pad comprises a heating element 316, having a meandering shape and disposed on a support pad 308. Two current contacts 314, connected to the heating element 316 via power legs 312 and disposed at either end of the heating element 316, provides a current to the heating element 316. A different pair of contacts, measurement contacts 304, is connected to the heating element 316 through measurement legs 306 and is positioned along the central portion of the heating element 316 for detecting voltage changes along the central portion of the heating element 316.

[0042] Other embodiments of the present disclosure relate to heat flux sensors for measuring the thermal conductivity of a fluid which comprise a chamber and at least one heating element comprising at least two zones, wherein at least two measurement contacts are disposed along each zone and are configure to measure a voltage change in the respective zone. The voltage changes are indicative of the flow rate of the fluid.

[0043] Yet other embodiments of the present disclosure are directed to heat flux sensors for measuring the thermal conductivity of a fluid which comprise a chamber defining a direction a fluid flow therein, and a first heating element suspended in the chamber and configured to preheat incoming fluid to a predetermined temperature, and a second heating element suspended in the chamber and positioned downstream of the first heating element in the direction of the fluid flow, wherein the second heating element is configured to detect heat flux variation along the second heating element caused by the flowing fluid. In yet other embodiments, the second heating element comprises at least two current contacts configured to supply a current to the second heating element, and at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid. 21.1864

[0044] The range of the heating element temperature may vary from the ambient to the highest temperature that the heating element can handle. In one embodiment, at least two heating elements are used in series with the first heating element being used to preheat the incoming fluid to establish the thermal gradient orthogonal to the flow direction. This way, subsequent elements do not see the effects of convection heat flux and therefore they detect only the conductive heat flux effects.

[0045] Referring to Figure 4, a detection chamber 402 comprising more than one heating element 406, 407, 408 enables additional functionalities for the heat flux sensor such as, for example, flow rate detection, flow invariance, and detection of molecular species. More than one heating element in a detection chamber 402 in accordance with embodiments of the present invention improves the overall sensitivity of the system where individual heating elements are used to detect narrow peaks eluting from a chromatography column. In such a configuration, the various heating elements may be composed of various types of suitable materials, such as, for example, silicon dioxide, silicon nitride, or any other dielectric material compatible with fabrication processes.

[0046] As shown on Figure 4, as an analyte band is moved by a fluid flow 404 past multiple heating elements 406, 407, 408, the analyte band is first detected by the first heating element 406 and then by the following heating elements 407, 408. The time delay between detection at each heating element can be divided by the space between the heating elements to determine the flow rate of the analyte band at the heating elements. In one embodiment, at least two heating elements are used in series relative to the flow direction 404 and they may be spaced apart by a predetermined distance. In yet another embodiment in order to determine the flow rate of a fluid, a single heating element may be used provided that at least two sets of measurement contacts, each set comprising at least two measurement contacts, are used along the length of the sensor element.

[0047] One embodiment of the present disclosure relates to a heat flux sensor comprising two or more heating elements in series or in parallel inside the chamber 402. This results in rendering the heat flux sensor less sensitive to flow rate changes as compared to existing heat flux sensors which are highly sensitive to any variation in the flow rate of the mobile phase likely due to changes in convective heat losses. According to embodiments of the disclosure, the heat flux sensor shows improvement in its response to changing flow rates. In essence, the heat flux sensor is not affected by changing fluid flow rates. A heat flux sensor with 21.1864 multiple independent heating elements enables flow rate independence for the central section of the heat flux sensor by having adjacent elements that may be controlled to a predetermined temperature. In one embodiment as shown in Figure 4, three heating elements, a first heating element 406, a second heating element 407 and a third heating element 408, may be used in series with the first heating element 406 being used to preheat the incoming fluid to establish a thermal gradient. This way, subsequent elements do not see the effects of convection heat flux and therefore they detect only the conductive heat flux effects. As a consequence, the second and third heating elements 407, 408 are flow rate independent.

[0048] Referring to Figures 2 and 3, a heating element is suspended within a chamber 238, 302 of a heat flux sensor. In the present description and in the appended claims, the expression "suspended heating element" and all its equivalents are used to indicate the fact that the heating element is preferably supported through legs, at least one leg being connected with at least one support arranged outside the chamber. Preferably, the heating element 218, 316 is connected via power legs 234, 236, 312, preferably two power legs, to a power supply 202 and a power sink 204 and via measurement legs 230, 232, 306, preferably two measurement legs, to voltage probes 214, 216. The heater pad, comprising the heating element 218, 316 and the support pad 220, 308 is connected to the chamber 238, 302 via support legs 308 disposed on a support pad 220, 308 of the heater pad.

[0049] The chamber of the heat flux sensor according to embodiments of the present disclosure may be a structure which is at least partially hollow. This structure may be formed with a single piece or with multiple pieces of material. The material is preferably capable of dispersing the heat generated by the heating element. Materials with a high thermal conductivity are preferred. The material may be any adequate material known to a person of ordinary skill in the art, and preferably any one of silicon, glass or metal.

[0050] According to some embodiments of the present disclosure, the chamber of the heat flux sensor comprises a first support layer and a second support layer arranged on the first support layer, the heating element being interposed between the first and second support layers. Further, according to some embodiments, the heating element is disposed on a support pad comprising at least one sustaining layer.

[0051] In some embodiments of the present disclosure, as shown in Figures 6d and 7g, the chamber of the heat flux sensor 622, 722 may have a structure composed of two pieces 602, 21.1864

616, 702, 716 formed with the same or a different material. In some embodiments, the chamber may comprise a first support layer 602, 702 and a second support layer 616, 716 arranged on the first support layer 602, 702. The heater pad 614, 714 is interposed between the first and second support layers 602, 702, 616, 716.

[0052] Still referring to Figure 6d and 7g, in some embodiments of the present disclosure, a heater pad 614, 714 may comprise at least one support pad 604, 704 and a heating element arranged on the support pad 604, 704. The support pad 604, 704 may comprise at least one sustaining layer which may be composed of at least one layer of silicon nitride, silicon dioxide or any other dielectric material.

[0053] The heating element, in some embodiments, may comprise one or more adhesion layers 606, 706 and one or more metal layers 608, 708 provided on the adhesion layer 606, 706. The adhesion layer 606, 706 may be a layer which facilitates the adhesion of the metal layer 608, 708 to the sustaining layer 604, 704. The adhesion layer 606, 706 may comprise one or more elements which enhance the bonding strength between the sustaining layer 604, 704 and the metal layer 608, 708. The adhesion layer may be a chromium or titanium layer. The metal layer 608, 708 may comprise one or more elements of any conductive material that changes resistance with temperature, and may be a nickel layer.

[0054] In some embodiments, the surface area of the heating element of the heat flux sensor may be at least partially covered by selectively depositing or growing nanotubes in order to enhance heat transfer and/or alter mass transport.

[0055] The heat flux sensor according to embodiments of the present disclosure may be a micro-thermal conductivity detector. The heat flux sensor may be integrated in a chromatography system, which may be a portable or micro-chromatography system. Generally, the cross sectional dimensions of the heat flux sensor described are less than 1 mm making the heat flux sensor a micro-heat flux sensor.

[0056] The heat flux sensor as described in embodiments of the present disclosure allows the processing of a non-destructive method substance detection and thus may be used in series with other heat flux sensors of this disclosure or with any other type of detectors.

[0057] The heat flux sensor according to embodiments of the present disclosure may be integrated with, but not limited to, an injector, a pressure source, a separation column and 21. 1864 operating software which includes an algorithm and database to quantitatively identify the components in the sample fluid.

[0058] To understand and simulate the operation of the heat flux sensors, the system may be modeled with electrical circuit elements. Thermoresistance may be measured by applying a known voltage and current to the heat flux sensor heating element and calculating the electrical resistance of the heating element. Power dissipation in the heating element causes joule heating (measured in watts), increasing the electrical resistance of the heating element. When the heating element temperature is known, the heat flow and resistance to heat flow may be calculated. To perform these calculations, input power (voltage x current) may be adjusted until the predetermined temperature is reached.

[0059] The thermal energy domain may be mapped to the electrical domain, where temperature (T) is equivalent to voltage (V), thermal heat flux (Q) replaces current (I) and thermal resistance (RT) is comparable to electrical resistance (R). Thus, the equation V = IR may be comparable to T = QRT. In this case Q is equal to V*I, the input energy in watts. Q and T are known, so total thermal resistance may be calculated by RT = T/IV = T/Q.

[0060] RT is the total thermal resistance for the system, a parallel combination of all the possible heat flow pathways. Figure 1 shows a circuit diagram representing the relation between the electrical and thermal domain. This system may be modeled as an electric -to- thermal transducer 102 with an electric circuit comprising power supply 108, a thermal circuit with a dependent current source 104, and a mass 120.

[0061] The electric -thermal circuit model comprises a resistive element 106 having a thermal resistance RTCD corresponding to the heat flux sensor heating element. The change in resistance of this heating element is proportional to the Thermal Coefficient of Resistance (TCR) of the heating element metal. This depends on the temperature of the heating element since the TCR effects resistance can be defined as RTCD = RO (1 + «R (T_r + T₀)) where Ro is the resistance at a base temperature To, TR is the operating temperature and (XR is the metal TCR.

[0062] Further, the electric-thermal circuit model comprises a resistive element 112 having a thermal resistance R_rad corresponding to the heat flux from thermal radiation, i.e. heat exchanged between the heat flux sensor heating element and the channel wall through radiative heat transfer. Radiative heat flux is defined as IQ = OSB F₁₂ A (T2⁴ - Ti⁴) where IQ is 21.1864 heat current in watts, OSB is the Stefan-Boltzman constant (5.67xl0^~8 W/m²-K⁴), F₁₂ is a constant between 0 and 1 which determines energy absorption of the surface, A is the heating element area in m² and, for the heat flux sensor, T₂ is the heated detector element temperature and Ti is the channel walls temperature. Linearizing to convert to a thermal resistance gives

[0063] The electric -thermal circuit model also comprises a resistive element 1 14 having a thermal resistance Rc_0nv corresponding to the heat flux for forced convection caused by fluid in the flow channel, i.e., heat carried away from the heated element by fluid convection. Rc₀nv may be defined as Rc_0nv = l/(h_Piate A) where A is the heating element area and hpi_ate is a coefficient depending on the Reynolds number, the flow rate, the type of fluid surrounding the element, and the mechanical structure of the heated element and the channel. Reynolds number equals Re = U* 1 / v where U is the average flow rate, 1 is the characteristic length, and v is the kinematic viscosity for the fluid.

[0064] The electric -thermal circuit model further comprises a resistive element 116 having a thermal resistance Ri_eg corresponding to the conductive heat flux through the heating element mechanical support structure. Ri_eg may be defined as Ri_eg = L/ κΑ where L is the length of the material, A is the cross-sectional area and κ is the thermal conductivity.

[0065] Finally, the model comprises a resistive element 118 having a thermal resistance Rfiuid corresponding to the conductive heat flux through the fluid to the channel walls. R_fluid has the same governing equation as Ri_eg with the conduction medium being fluid and the heat storage capacity Cthermai 1 10 of the heat flux sensor defined as Ct ermai = c "' Pm V where r

Cthermai is the thermal capacitance, >» is the specific heat of the material, p_m is the material density and V is the volume.

[0066] The sensitivity of a heat flux sensor depends on the energy flow in the system. When a fluid sample is flowing past the heat flux sensor heating element, the value of Rc_0nv and Rfiuid is determined through the sample thermal conductivity. Maximum sensitivity occurs when R_fluid is the prevailing heat loss pathway and all other heat loss pathways are minimized.

[0067] By controlling the temperature difference between the heat flux sensor and the heating element (ΔΤ), the performance of a heat flux sensor can be advantageously measured 21.1864 and modeled. All necessary calculations depend on measurement of thermal resistance defined as R_T = ΔΤ / IV = ΔΤ / Q.

[0068] Figure 5a shows a schematic representation of the positioning of a heater pad 514 in a chamber 520 of a heat flux sensor 524 according to one embodiment of the present disclosure. Figure 5b is an enlarged schematic view of the heater pad 514 as shown in Figure 5a. Figure 5c is a cross-sectional view of one embodiment of the heat flux sensor 524 shown in Figure 5a along the line Va-Va. Figure 5d is a cross-sectional view of another embodiment of the heat flux sensor 524 shown in Figure 5a along the line Va-Va.

[0069] It is possible to further improve the performance of a heat flux sensor 524 by using the chamber 520 and heater pad 514 designs to increase the sensitivity of the sensor 524. Changes in the physical structure and location of the heater pad 514 in relation to the chamber 520 of the sensor 524 changes the value of R_fi_ui_d- Thus a control of the type and location of the heater pad 514 allows for control of the heat flux and therefore improves the performance of the heat flux sensor 524. Specially, the spacings 502, 504, 506, 508 between the chamber walls and the heater pad 514 may be selected in order to enhance the performance of the heat flux sensor 524.

[0070] Furthermore, another way to enhance the heat flux sensor sensitivity may be to increase the thermal isolation of the heating element. In some embodiments, the increase of the thermal isolation may be achieved by decreasing the heat condition capacity of the support legs 230, 232, 234, 236 by increasing support length and decreasing the support pad width and thickness. In yet other embodiments, the enhancing of the sensor sensitivity may be achieved through inductive power transfer to the heating element. For example, by removing metal traces on and off of the heating element, the heat transferred along the support legs is decreased.

[0071] Band broadening corresponds to the widening of the analyte band before the latter reaches the sensor; this effect decreases the resolution of an analysis. According to some embodiments of the present disclosure, the minimization of band broadening may be achieved by matching the chamber dimensions to connection tubes. In more particular embodiments, the matching of the chamber dimensions to connection tubes is realized by installing deformable connection tubes whose shape is adjustable by actuators. The cross- sectional area of the connection tubes may be changed to provide a substantial match to the 21.1864 cross-sectional areas of a range of chromatography separation columns. In some micro-scale chromatography systems according to yet other embodiments, the heating element may be placed in the flow channel of a chamber which has a cross-sectional area that matches the cross-sectional area of at least one chromatography separation column. The matching of channel sizes may prevent band broadening and likely improves chromatographic performance. It should be understood, that the term "matching" as used herein with reference to sizes should take into consideration manufacturing tolerances in the order of about 1%, 5%, 10% or more of the cross-sectional area between the separation column, the flow channel, the connection tubes, the heating element, and/or the heat flux sensor integrally formed together.

[0072] Referring to Figures 5a to 5d, if the heat flux sensor is integrated in a chromatography system, the channel 522 may have a height 510 and width 512 such that the height 510 and width 512 match the cross-sectional area of a given chromatography separation column, ranging from 10 μιη² in some embodiments to 1 mm² in other embodiments.

[0073] Still referring to Figures 5a to 5d, the spacings 502, 504, 506, 508 between the heater pad 514 and the channel/chamber walls may be designed such that the heater pad 514 is suspended in the chamber 520. The spacings 502, 504 are shown to be equal to the total chamber height 510 less the thickness of the heater pad 514. By using micro-fabrication and the thermal models created, the spacings 502, 504 may be dimensioned to provide maximum response to analytes creating a still more sensitive sensor. Further, the channel 522 may have any width 512 such that the heater pad 514 does not touch the chamber walls and fits within the chamber width 512.

[0074] In other embodiments, still referring to Figures 5a to 5d, the heater pad 514 may have a length 516 ranging from several millimeters to a few microns. The heater pad 514 may have a length 516 of about 2 mm in some embodiments to about 20 μιη in other embodiments. While longer lengths provide higher sensitivity, shorter pads enable detection of shorter band. In particular embodiments, the chamber 520 may have a length 518 greater than that of the heater pad 516, preferably of from 100 μιη to several centimeters.

[0075] In further embodiments, referring specifically to Figure 5d, multiple heater pads 514, 514a, 514b are shown suspended in the channel 522, and can be used to detect when the 21.1864 thermal conductivity of a fluid sample changes due to local Knudsen number (Kn), a dimensionless number representing the ratio of the molecular mean free path and the physical dimensions of the chamber 520. When the device dimensions are larger compared to the mean free path of the gas, for example, the thermal conductivity, a physical gas property, is constant. In Figure 5d, the chamber 520 is shown to include a plurality of chamber walls 520a, 520b, 520c, and 520d. The array of heater pads 514, 514a, 514b with different distances 502, 502a, 502b from the chamber walls may be used to determine the components/analytes of a fluid sample. When the distances 502, 502a, 502b are decreased to the point that they are within an order of magnitude of the molecular mean free path, the thermal conductivity of the fluid begins to decrease. This decrease depends on the mean free path of the gas. Since all gases have different mean free paths, the Kn, given fixed device parameters, depends on the gas in the chamber. For example in a binary gas mixture, like hydrogen and water vapor, at least two heater pads 514, 514a can be used to detect hydrogen versus water vapor by taking the differential signal from the two heating pads 514, 514a with different gap distances. Hydrogen has a much larger mean free path, so the Kn of a given chamber for that gas is much higher than the Kn for the same chamber for water vapor. The two heater pads would have a similar response to the water vapor, but a much different response to the hydrogen.

[0076] In the embodiment shown in Figure 5d, each heater pad 514, 514a, 514b is spaced a different distance 502, 502a, 502b from at least one of the chamber walls (i.e., a cover 520a and a base 520c) for measuring the thermal conductivity of a fluid in an effort to detect and identify molecular components of the fluid. The heater pad 514 is preferably configured to detect a first heat flux variation of the fluid and is spaced a first distance 502 from the cover 520a; the heater pad 514a is preferably configured to detect a second heat flux variation of the fluid and is spaced a second distance 502a from the cover 520a; and the heater pad 514b is preferably configured to detect a third heat flux variation of the fluid and is spaced a third distance 502b from the cover 520a. Although three heater pads are shown and described herein, the present invention should not be limited to such implementation. There may be a plurality of heater pads in the channel depending on a number of factors, such as, for example, the fluid sample to be analyzed, the temperature of the detector filament, the size of the chamber/channel, the material used for the heater pad or heating element, and the like. The heater pads 514, 514a, 514b are preferably spaced a predetermined longitudinal distance from the cross-sectional plane Va-Va, wherein the channel defines a longitudinal axis of the 21.1864 chamber; however, the heater pads 514, 514a, 514b may be fabricated in a manner to dispose each heater pad 514, 514a, 514b within the same cross-sectional plane. The distances 502, 502a, 502b may be achieved by etching the channels at different depths. The distances 504, 504a, 504b between the heater pad 514, 514a, 514b and the base 520c may also be used for optimizing the detection and identification of the molecular components of the fluid. By using two or more heater pads 514, 514a, 514b in series with different distances 502, 502a, 502b and 504, 504a, 504b, when the distances are within 2 orders of magnitude of the mean free path, the local Kn will affect thermal conductivity of the gas in the chamber 520 in a given ratio. As such, the ratio of responses from the heater pads can be used to identify the gas species. Another advantage of the present embodiment is that the fluid in the chamber may be either flowing or may be stagnant.

[0077] Further, in another aspect of the present disclosure, a micro-scale gas chromatography system may comprise at least one separation column, an injector, and a heat flux sensor. The heat flux sensor may comprise a chamber, at least one heating element suspended in the chamber, at least two current contacts configured to supply a current to the heating element, and at least two measurement contacts configured to measure a voltage change along the heating element indicative of the thermal conductivity of the fluid. In some embodiments, the at least one separation column and the heat flux sensor are integrally formed. In some embodiments, the cross-sectional area of the chamber of the heat flux sensor matches the cross-sectional area of the at least one separation column. The micro-scale gas chromatography system may be used in a laboratory for analysis of various analytes, such as those found in a natural gas sample, and particularly analytes of a natural gas sample having molecular masses lower than hexane. The micro-scale gas chromatography system may be implemented at the surface of a well-site, or may be contained within a downhole tool adapted to be deployed down a wellbore and connected to a rig via wireline, a drill string, tool string, or tubing.

[0078] In some embodiments, as shown in Figure 7g, the heat flux sensor 722 comprises a chamber and a heater pad 714. The chamber may have a structure in two pieces 702, 716 formed with the same or a different material. The chamber may comprise a first support layer 702 and a second support layer 716 arranged on the first support layer 702. The heating element is then interposed between the first and second support layers 702, 716. In these embodiments, the at least one separation column 728 shares the first and second support 21.1864 layers 702, 716 with the chamber. As shown in Figures 7a to 7g, the heat flux sensor 722 is preferably integrated with the separation column 728 of the micro-gas chromatography system and both are micro-fabricated on the same support layer 702.

[0079] According to a further aspect of the present disclosure, there is provided a method for measuring the thermal conductivity of a fluid, comprising providing a fluid to be analyzed, providing a heat flux sensor comprising a chamber and a heating element suspended in the chamber, supplying a current to the heating element near at least two current contacts, and measuring a voltage change indicative of a heat flux change caused by the fluid flowing along the heating element near at least two measurement contacts.

[0080] According to a further aspect of the present disclosure, there is provided a method for fabrication of a heat flux sensor for measuring the thermal conductivity of a fluid.

[0081] The method comprises providing a chamber, providing a heating element, housing the heating element in the chamber so that the heating element is suspended in the chamber, providing at least two current contacts configured to supply a current to the heating element, and providing at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.

[0082] According to some embodiments, providing a chamber comprises providing a first support layer and arranging a second support layer on the first support layer. The first and second support layers are arranged so as to define a void within which the heating element is housed.

[0083] In other embodiments, housing the heating element in the chamber comprises depositing at least one sustaining layer on the first support layer, depositing at least one adhesion layer on the at least one sustaining layer, depositing at least one metal layer over the at least one adhesion layer, and etching the first support layer and the at least one sustaining layer so as to define the heater pad and the channel for the fluid flow.

[0084] In yet other embodiments, the at least one sustaining layer is etched, prior to depositing the at least one adhesion layer, according to a predetermined pattern.

[0085] The method for fabrication of a heat flux sensor 622 is going to be described with reference to Figures 6a, 6b, 6c, 6d. 21.1864

[0086] Referring now more particularly to Figure 6a, a first support layer 602 is provided. Then a sustaining layer 604 is deposited on the first support layer 602. The sustaining layer 604 is covered by a photoresist 620, thereby creating the pattern of a mask for the deposition of an adhesion layer 606 and a metal layer 608. The sustaining layer 604 is then etched in the pattern of the mask. The adhesion layer 606 and the metal layer 608 are then deposited on the photoresist 620 and on the pre-etched sustaining layer 604, thus filling the pre-etched grooves.

[0087] As shown in Figure 6b, the photoresist 620 is then removed, leaving a nearly flush surface on the top side of the sustaining layer 604 allowing for bonding later in the method. Flow channels 610 and heater pad 614 are defined by patterning and etching through the sustaining layer 604.

[0088] On Figure 6c, the first support layer 602 is etched to form a channel 612. Finally, as may be seen on Figure 6d, the heat flux sensor 622 comprises a second support layer 616. The second support layer 616 is fabricated with a channel 618 symmetrical to the first support layer channel 412. Prior to the bonding of the first and second support layers 602, 616, an alignment of the channels 610, 612, 618 is made. This alignment may be achieved through the use of keys placed in the etched channels 610, 612, 618. The first and second support layers 602, 616 are then bonded anodically. This bonding may be performed by heating the aligned support layers 602, 616 and applying a voltage to a hotplate and a contact probe.

[0089] According to a further aspect of the present disclosure, a method is provided for fabrication of a micro gas chromatography system comprising a heat flux sensor and at least one separation column.

[0090] The method comprises providing a chamber, providing at least one separation column, providing a heating element, housing the heating element in the chamber so that the heating element is suspended in the chamber, providing at least two current contacts configured to supply a current to the heating element, and providing at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.

[0091] According to some embodiments, providing a chamber comprises providing a first support layer and arranging a second support layer on the first support layer. The first and 21.1864 second support layers are arranged so as to define a void within which the heater pad is housed.

[0092] In other embodiments, housing the heater pad in the chamber comprises depositing at least one sustaining layer on the first support layer, depositing at least one adhesion layer on the at least one sustaining layer, depositing at least one metal layer over the at least one adhesion layer, and etching the first support layer and the at least one sustaining layer so as to define the heater pad and the channel.

[0093] According to some embodiments of the present disclosure, the at least one separation column shares the first and second support layers and the at least one sustaining layer with the chamber.

[0094] In yet other embodiments, the at least one sustaining layer is etched, prior to depositing the at least one adhesion layer, according to a predetermined pattern.

[0095] The method for fabrication of a micro gas chromatography system will be described below with reference to Figures 7a, 7b, 7c, 7d, 7e, 7f and 7g.

[0096] Referring now more particularly to Figure 7a, a first support layer 702 is provided. Then a sustaining layer 704 is deposited on the first support layer 702. Referring to Figure 7b, the sustaining layer 704 is covered by a photoresist 720, thereby creating the pattern of a mask for the deposition of an adhesion layer 706 and a metal layer 708. The sustaining layer 704 is then etched in the pattern of the mask.

[0097] As shown on Figure 7c, the adhesion layer 706 and the metal layer 708 are then deposited on the photoresist 720 and on the pre-etched sustaining layer 704, thus filling the pre-etched grooves.

[0098] On Figure 7d, the sustaining layer 704 and the metal layer 708 are patterned by a photoresist deposition 720 for channel etch. On Figures 7e and 7f, the flow channels 710, 712, 724 are etched.

[0099] Finally, as shown in Figure 7g, after the cleaning of the sustaining layer surface 704 and the metal surface 708, a second support layer 716 is bonded to the device. The second support layer 716 is fabricated with a channel 718 symmetrical to the channel 712 of the first 21.1864 support layer 702, and with a channel 726 symmetrical to the channel 724 of the separation column 728.

[00100] The methods for deposition and etching of the layers of the chamber and heater pad may be any known techniques available to a person of ordinary skill in the art. For example, the deposition of the layers may be made by low pressure chemical vapor deposition (LPCVD). Further, the etching may for example be realized in a reactive ion etcher using CF4, using a deep reactive ion etching (DRIE) combining an anisotropic etch and an exposure to SF₆.

[00101] The data obtained by a measurement method using the heat flux sensor according to the present disclosure may be combined with other data like gas chromatography, optical measurements, and mass spectroscopy.

[00102] Advantages of the disclosure may further include one or more of the following. A heat flux sensor having different contacts, namely contacts intended to supply current and contacts intended to measure a voltage, remove noise and error due to fluctuations in the impedance of the current lines. Advantageously, embodiments of the present disclosure provide an improved heat flux sensor having an increased thermal isolation, thus being very sensitive with a very fast time constant. The heat flux sensor may allow reduced carrier gas consumption.

[00103] Although various specific embodiments have been described above for purposes of illustration, the invention is not limited to the specific embodiments disclosed herein. For example, while one, two or three heating element(s) or heating pad(s), it is contemplated that four, five, six, ten, fifty, one hundred or more may be implemented, arranged, and spaced apart in various implementations consistent with embodiments of the present invention. Various modifications to the disclosed embodiments would be possible by persons skilled in this art without departing from the scope of the invention. Accordingly, the invention is defined herein only by the scope of the appended claims.

Claims

21.1864 CLAIMS What is claimed is:

1. A heat flux sensor for measuring the thermal conductivity of a fluid, the heat flux sensor comprising:

a chamber;

a heating element suspended in the chamber;

at least two current contacts configured to exchange current with the heating element; and

at least two measurement contacts configured to measure a voltage change along the heating element indicative of the thermal conductivity of the fluid.

2. The heat flux sensor according to claim 1, wherein the heating element comprises at least two zones, wherein at least two measurement contacts are disposed along each zone and are configured to measure a voltage change in the respective zone, the voltage change being indicative of the flow rate of the fluid.

3. The heat flux sensor according to claim 1, wherein the chamber comprises a first support layer and a second support layer arranged on the first support layer, the heating element being interposed between the first and second support layers.

4. The heat flux sensor according to claim 3, wherein the heating element is disposed on a support pad comprising at least one sustaining layer.

5. The heat flux sensor according to claim 3, wherein the heating element comprises at least one adhesion layer and at least one metal layer provided on the adhesion layer.

6. The heat flux sensor according to claim 1, wherein the heat flux sensor is a micro- thermal conductivity detector.

7. A heat flux sensor for measuring the thermal conductivity of a fluid, the heat flux sensor comprising:

a chamber defining a direction of fluid flow therein;

a first heating element suspended in the chamber and configured to preheat incoming fluid to a predetermined temperature; and

21.1864 a second heating element suspended in the chamber and positioned downstream of the first heating element in the direction of the fluid flow, wherein the second heating element is configured to detect heat flux change along the second heating element caused by the flowing fluid.

8. The heat flux sensor according to claim 7, wherein the second heating element comprises:

at least two current contacts configured to exchange current with at least one of the first and second heating element; and

at least two measurement contacts configured to measure a voltage change along at least one of the first and second heating element indicative of the thermal conductivity of the fluid.

9. The heat flux sensor according to claim 7, wherein the chamber comprises a plurality of walls, and wherein the first heating element is spaced a first distance from at least one of the chamber walls, and the second heating element is spaced a second distance from at least one of the chamber walls, and wherein the first distance and the second distance are different. .

10. A micro-scale gas chromatography system comprising at least one injector adapted to provide a fluid sample comprising a plurality of analytes into the micro-scale gas

chromatography system, at least one separation column adapted to separate at least a portion of the plurality of analytes of the fluid sample, and a heat flux sensor adapted to detect at least a portion of the plurality of analytes separated in the at least one separation columns, the heat flux sensor comprising:

a chamber;

a heating element suspended in the chamber;

at least two current contacts configured to exchange current with the heating element; and

at least two measurement contacts configured to measure a voltage change along the heating element indicative of the thermal conductivity of at least a portion of the plurality of analytes of the fluid sample.

1 1. The micro-scale gas chromatography system according to claim 10, wherein the at least one separation column and the heat flux sensor are integrally formed.

21.1864

12. The micro-scale gas chromatography system according to claim 11, wherein the sensor is a micro-thermal conductivity detector.

13. The micro-scale gas chromatography system according to claim 11, wherein the cross-sectional area of the chamber of the heat flux sensor matches the cross-sectional area of the at least one separation column.

14. The micro-scale gas chromatography system according to claim 10, wherein the chamber comprises a plurality of walls, and wherein the heat flux sensor comprises a second heating element suspended in the chamber spaced a distance from at least one of the chamber walls, the second heating element having at least two current contacts configured to exchange current with the second heating element, and at least two measurement contacts configured to measure a voltage change along the second heating element indicative of the thermal conductivity of at least a portion of the plurality of analytes of the fluid sample.A method for analyzing a fluid sample comprising a plurality of analytes, the method comprising the steps of:

providing the micro-scale gas chromatograph of claim 10;

injecting the fluid sample comprising a plurality of analytes into the micro-scale gas chromatograph system;

separating at least a portion of the plurality of analytes of the fluid sample;

supplying a current to the heating element at at least two current contacts; and measuring a voltage change indicative of a heat flux change caused by the fluid along the heating element at at least two measurement contacts.

15. A method of measuring the thermal conductivity of a fluid, comprising the steps of: providing a fluid to be analyzed;

providing a heat flux sensor comprising a chamber and a heating element suspended in the chamber;

supplying a current to the heating element at at least two current contacts; and measuring a voltage change indicative of a heat flux change caused by the fluid along the heating element at at least two measurement contacts.

16. A method of measuring the thermal conductivity of a fluid, comprising the steps of: providing a fluid to be analyzed;

21.1864 providing a heat flux sensor comprising a chamber having a longitudinal axis and a plurality of chamber walls laterally disposed about the longitudinal axis of the chamber;

providing a first and second heating element suspended in the chamber wherein the first and second heating element are spaced at different distances from at least one of the plurality of chamber walls;

supplying a current to at least one of the first and second heating elements at at least two current contacts; and

measuring the thermal conductivity of the fluid as a function of the predetermined distances between the chamber walls and at least one of the first and second heating elements.

17. A method for fabrication of a heat flux sensor for measuring the thermal conductivity of a fluid comprising:

providing a chamber;

providing a heating element;

housing the heating element in the chamber so that the heating element is suspended in the chamber;

providing at least two current contacts configured to supply a current to the heating element; and

providing at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.

18. The method according to claim 17, wherein providing the chamber comprises:

providing a first support layer; and

arranging a second support layer on the first support layer;

wherein the first and second support layers are arranged so as to define a void within which the heating element is housed.

19. The method according to claim 17, wherein housing the heating element in the chamber comprises:

depositing at least one sustaining layer on the first support layer;

depositing at least one adhesion layer on the at least one sustaining layer;

depositing at least one metal layer over the at least one adhesion layer; and

21.1864 etching the first support layer and the at least one sustaining layer so as to define the heating element and a channel for the circulation of the fluid.

20. The method according to claim 19, wherein the at least one sustaining layer is etched, prior to depositing the at least one adhesion layer, according to a predetermined pattern.

21. A method for fabrication of a micro-scale gas chromatography system comprising: providing a chamber;

providing at least one separation column;

providing a heating element;

housing the heating element in the chamber so that the heating element is suspended in the chamber;

providing at least two current contacts configured to supply a current to the heating element; and

providing at least two measurement contacts configured to measure a voltage change along the heating element indicative of a heat flux change caused by the fluid.