Single Use Plastic Pressure Sensor

Abstract

The polymer fluid sensor (100) includes an inlet (102) and an outlet (104) configured to receive a fluid. A polymer tube (134) is fluidly interposed between the inlet (102) and the outlet (104) and has a first sidewall thickness (G 0 a first sensing location (152) having a second sidewall thickness (G 1 ), spaced from the first sensing location (152) and 0 A sleeve (142) is disposed around the polymer tube (134). The first sidewall thickness (G 0 1) is the second side wall thickness (G 0 2), and the first sensing element (122) is located at the first location (152) and the second sensing element (150) is located at the second location. In another example, the first and second sidewall thicknesses are the same, and a fluid restriction (236) is located within the polymer tube (134) between the first and second sensing locations (152, 154).

Description

Background Art

[0001] Many processes require the use of sensors to control the process. In some industries, these processes may include biological or hygienic processes, and thus contamination must be prevented. Examples of sensors used in such processes include pressure sensors, temperature sensors, flow sensors, pH sensors, conductivity sensors, and the like.

[0002] Biopharmaceutical manufacturers are increasingly using single-use disposable tools to increase efficiency, make equipment more versatile, and reduce the risk of cross-contamination. Similarly, in the hygiene and food processing industries, due to high cleaning costs and concerns about maintaining hygiene, the use of single-use or disposable measuring equipment is becoming increasingly desirable. In many of these applications, high precision is not required, so the use of materials other than metal is facilitated, resulting in the highest cost-effectiveness and ease of manufacture. Furthermore, in some cases, in order to avoid potential contamination of the process stream, it may be essential in the design that there is no filling fluid.

Summary of the Invention

Means for Solving the Problems

[0003] The polymer fluid sensor includes an inlet and an outlet configured to receive a fluid. A polymer tube is fluid-coupledly inserted between the inlet and the outlet and has a first sensing position with a first side wall thickness and a second sensing position having a second side wall thickness spaced apart from the first sensing position. A sleeve is disposed around the polymer tube. The first side wall thickness is thinner than the second side wall thickness, a first sensing element is disposed at the first position, and a second sensing element is disposed at the second position. In another example, the thicknesses of the first and second side walls are the same, and a fluid restriction is disposed within the polymer tube between the first and second sensing positions.

Brief Description of the Drawings

[0004] [Figure 1] FIG. 1 is a schematic diagram of a single-use plastic pressure sensor according to one embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view of a non-invasive and non-invasive pressure sensor according to an embodiment of the present invention; [Figure 3] 1 is a graph showing stress versus cycle at various temperatures showing Woehler curves for polycarbonate and ABS at different temperatures. [Figure 4] 1 is a chart showing the fatigue limits of polycarbonate and ABS at various temperatures. [Figure 5] FIG. 1 is a schematic diagram of a single-use plastic mass flow sensor, according to one embodiment of the present invention. [Figure 6] FIG. 1 is a schematic diagram of a single-use plastic fluid sensor according to another embodiment of the present invention. [Figure 7] FIG. 1 is a schematic diagram of a single-use plastic fluid sensor according to another embodiment of the present invention. [Figure 8] 1 is a schematic diagram of a system for measuring fluid properties, according to one embodiment of the present invention. [Figure 9] 1 is a flow diagram of a method for providing fluid characteristics in a biopharmaceutical or sanitary fluid processing system according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0005] In sanitary applications, there is commonly a concern of process batch contamination from the sensors used to monitor the process. Using organic fill fluids in the sensors (sometimes required for pressure sensing) can sometimes address toxicity issues, but does not eliminate the possibility of contamination, leading to loss of the entire process batch. Oil-less designs of pressure sensors can address this to some extent, but still require costly cleaning. Disposable, single-use sensors reduce cleaning costs, but this must be offset by competitive disposable costs and ease of use.

[0006] Some designs use a plastic film interface between the process fluid and the metal isolating diaphragm of the pressure sensing system, but even these designs are susceptible to damage and can cause contamination and compromise performance if the isolating fluid from the pressure sensing system is present within the sensor.

[0007] The embodiments described below generally provide a non-invasive and non-invasive pressure sensor constructed from polymer (and preferably plastic) tubing and using multiple conductively coated areas on its outer diameter to provide capacitive measurements for a contamination-free, single-use sensing solution. The sensor is designed so that a portion of the tubing flexes under pressure, reducing the gap between the tubing and the contact point on the outer sleeve. This gap is monitored capacitively and can be referenced to a pseudo-fixed-gap capacitor located in a region of the tubing that is stiffer and does not significantly deform with changes in internal pressure. Because the sensor is developed from polymer tubing, the sensor can be easily coupled to tubing wiring to direct process flow past the sensor and can have its outlet coupled to additional process tubing wiring to return the process flow to the process. Furthermore, it should be noted that the designs provided in the following embodiments generally do not include any electronics. Therefore, the sensors provided herein can withstand sterilization cycles using steam and / or gamma irradiation, which are often used in pharmaceutical and sanitary processing applications.

[0008] FIG. 1 is a schematic diagram of a single-use plastic pressure sensor according to one embodiment of the present invention. Pressure sensor 100 generally includes a pair of tubing sections 102, 104 configured to couple to biopharmaceutical or sanitary process tubing or piping. In the illustrated example, tubing section 106 is coupled to tubing section 102 so that process fluid flowing therethrough generally does not include obstructions or intrusions within the fluid's flow path. Similarly, tubing section 108 is coupled to tubing section 104 so that process fluid essentially flows through the tubing section and the sensor in a substantially unimpeded, non-invasive manner. As described in more detail below, sensor 100 includes multiple electrical structures positioned to measure and characterize tubing wall deflection to provide an associated process fluid pressure. Because it is the deflection of the wall itself that is measured, no pressure sensors or other structures are inserted into the flow path. Additionally, measurement circuitry 110 is coupled to various electrical structures within sensor 100 by one or more conductors 112 such that measurement circuitry 110 can identify or otherwise detect changes in the electrical properties of such structures, which changes are indicative of process fluid pressure.

[0009] 2 is a schematic cross-sectional view of a non-invasive and non-stretching pressure sensor that utilizes a polymer tube (including a conductively coated or deposited region on its exterior) to provide a capacitive measurement for a single-use sensing solution, according to one embodiment of the present invention. The sensor 100 is designed so that a section of the tube, generally designated 120, can deflect under pressure, and this deflection is achieved by a gap G between a tube contact 122 and a sleeve contact 124. 0 1. This gap G 0 1 is referenced to a pseudo-fixed gap capacitor (schematically designated by reference numeral 126) that can be measured and monitored by capacitance measurement (e.g., using measurement circuit 110 (shown in FIG. 1)) and can be formed in a region 127 of the tube that is stiff and does not deform significantly in response to changes in internal pressure.

[0010] As shown in FIG. 2 , the sensor 100 includes a pair of polymer tubular sections 102, 104, which facilitate coupling the sensor to plastic wiring or conduit. The tubular sections 102, 104 may be formed of any suitable polymer material and are coupled to a polymer tube 134. As can be appreciated, the shape and configuration of the tubular sections 102, 104 can vary widely to accommodate a variety of different techniques for coupling the sensor to the tubular wiring or conduit. Furthermore, each of the tubular sections 102, 104 can be sealingly coupled to the tube 134 in any suitable manner. However, the tube 134 is preferably formed from the same material as the tubular sections 102, 104. Furthermore, the tube 134 and the tubular sections 102, 104 are preferably formed as a single, integral piece from the same material. For example, the tubular sections 102, 104 and the tube 134 can be formed as a single piece of 3D printed or injection-molded plastic.

[0011] The tube 134 includes a pair of collars 136, 138 having an outer diameter 140 sized to receive the sleeve 142. The sleeve 142 may be attached to the outer diameter 140 of the collars 136, 138 in any suitable manner, including ultrasonic welding. Alternatively, it is expressly contemplated that the sensor may be constructed using additive manufacturing techniques (e.g., 3D printing), and the tube and sleeve may be formed as a single, integral component. When the sleeve 140 is coupled to the collars 136, 138, a pair of gaps G are formed between the inner surface 144 of the sleeve 142 and the outer surface 146 of the tube 134. 0 1. G 0 2 is formed. Furthermore, a first gap G 0 A second gap G is formed between the relatively thin section 148 of the sleeve 142 and the inner surface 144. 0 2 is formed between a relatively thick portion 127 of tube 134 and an inner surface 144 of sleeve 142. Conductive electrodes or plates 122, 150 are attached to the outer surface of tube 134 at locations 148, 127, respectively. Similarly, conductive electrodes or plates are attached to the inner surface 144 of sleeve 142 at locations 152, 154.

[0012] The conductive electrodes or plates 122, 150 may be considered sensing elements because they form part of a variable capacitor with a capacitance based on the gap. The conductive electrodes or plates may take any suitable shape, including, but not limited to, circular pads, elliptical pads, and annular bands extending a selected amount (e.g., 270 degrees) around a surface. Thus, a first variable capacitor is formed between the plate or conductive electrode 122 at location 148 and plate 152. A reference capacitance is formed between plate 150 and plate 154. Due to the relatively thick portion of the tubing wiring at location 127, the reference capacitance does not deflect significantly with pressure changes within tubing 134. In a preferred embodiment, the gap G 0 1 and G 0 2 is equal to when the sensor is unpressurized (i.e., at zero pressure). This characteristic is provided by using different sidewall thicknesses for the sleeves. However, other factors that affect capacitance (e.g., changes in temperature and thermal expansion and contraction of sensor 100) are generally relatively common to both capacitors. In this way, contrasting the response of sensing capacitor C1 with the response of reference capacitor C2 provides a direct indication of pressure that is substantially independent of other factors.

[0013] To quantify how the outer diameter of a thick-walled pipe changes (represented as ΔD0) when pressure P is introduced into the pipe, several assumptions are helpful. The first assumption is that the environment outside the pipe is atmospheric pressure, and therefore the pressure is a gauge pressure. Under these assumptions, the change in outer diameter (ΔD0) can be calculated from Equation 1:

[0014]

number

[0015] where E is the Young's modulus of the pipe, D iand D0 are the inner and outer diameters of the pipe, respectively. Due to the large thermal expansion of plastic and plastic-like materials, it is desirable to passively compensate for temperature changes by a ratiometric or differential scheme. The example shown in Figure 2 is a sensor that can passively compensate for temperature effects, particularly at zero pressure.

[0016] To achieve passive temperature compensation, the electrode pair is arranged to measure the gap change at two different locations, as shown in FIG. 2. The gap faces are plates of a parallel capacitor, decreasing as the pressure in the tube increases. Note that the tube at locations C1 and C2 has different thicknesses 159 and 161 at locations 148 and 127, respectively. Each of the plates is connected to an electrical lead operably coupled to a measurement circuit (shown in FIG. 8) to enable the controller to determine the capacitance (and therefore the pressure). Although the tube expansion with temperature at these two locations is the same, the amount of gap change with pressure is different due to the unequal tube wall thicknesses. In this way, by subtracting the signals from the two locations, a pressure signal can be extracted that is unaffected by steady-state temperature changes.

[0017] Since the gap is proportional to the inverse of the capacitance, taking the difference between each capacitance fraction is effectively taking the difference between two gaps G1 and G2, as shown in Equation 2 below.

[0018]

number

[0019] where K is a constant. By design, G1 = G2 = G0, i.e., the gap at zero gauge pressure is the same, but the change in gap with pressure is not the same due to different tube wall thicknesses. Since the gap is the same at zero gauge pressure, the change with temperature will be the same. Therefore, there is no change with temperature at zero gauge pressure. Furthermore, in Equation 3 (below), it can be shown that ΔG has the following approximate pressure-dependent form:

[0020]

number

[0021] where t1 and t2 are the wall thicknesses of the two diameters at locations C1 and C2, respectively. Thus, the pressure P can be obtained from the power measurements via equations 2 and 3, i.e.,

[0022]

number

[0023] The sensor designs described herein preferably use non-toxic, rigid plastics (e.g., polycarbonate or acrylonitrile butadiene styrene (ABS)) as their primary components. Before discussing the design details, the issue of plastic creep must be addressed. To manage this limitation, the maximum stress encountered during sensor use must be kept below an upper level so that the sensor withstands beyond what is known as its fatigue limit. Figures 3 and 4 provide data on the fatigue limits of polycarbonate and ABS plastics.

[0024] Figure 3 is a graph displaying stress in megapascals versus number of cycles at various temperatures, showing the stress-life or Wöhler curves for polycarbonate and ABS at different temperatures. Additionally, Figure 4 shows the various fatigue limits (expressed in megapascals) for polycarbonate and ABS at various temperatures. As can be seen, the data suggests that designs using polycarbonate plastic that maintain stress levels below 10.3 megapascals (approximately 1500 PSI) should be able to withstand long-term cyclic stress operation without creep. Therefore, it should be possible to build pressure sensors out of plastic using 3D printing or injection molding methods, provided the stress is below the fatigue limit level.

[0025] The use of plastic sensors offers many advantages. First, there are generally no internal dead zones, step changes, or material transitions through the sensor body. Furthermore, at least with regard to polycarbonate, this plastic has a Young's modulus 50 times lower than that of 304 stainless steel. This difference significantly increases the deflection per psi. Additionally, polycarbonate is electrically insulating, with a dielectric constant of approximately 3. Furthermore, plastic sensors facilitate the incorporation of built-in overpressure protection and mounting brackets. Finally, another advantage of plastic sensors is that they are generally non-toxic.

[0026] Thus far, the embodiments have described plastic pressure sensors that provide an electrical response that is indicative of the pressure of a fluid flowing through a tube, but additional properties of the fluid can be sensed or determined using similar techniques.

[0027] FIG. 5 is a schematic diagram of a single-use plastic mass flow sensor according to one embodiment of the present invention. Sensor 200 shares many similarities with sensor 100, with similar components being similarly numbered. However, a key difference between sensor 200 and sensor 100 is that tube portion 234 of sensor 200 includes a plastic aperture plate 236 inside the tube 234 that separates two capacitance regions C1 and C2. Furthermore, unlike sensor 100, the wall thickness of the tube at locations C1 and C2 is the same on both sides of the sensor. When flow is present, a pressure difference is created across aperture plate 236, and the mass flow rate is proportional to the square root of the pressure difference. In this way, the square root of the output defined in Equation 2 (above), scaled by an appropriate constant, is proportional to the mass flow rate through tube 234. Furthermore, because both capacitances formed at locations C1 and C2 are affected by temperature in a similar manner, sensor 200 remains substantially insensitive to temperature changes.

[0028] Another fluid property that can be measured using the techniques described herein is the electrical conductivity of the fluid itself. Using alternating current (AC) excitation of the capacitor electrodes, the reactive and dissipative properties of the fluid's dielectric and conductive properties can be determined. Specifically, the fluid's electrical conductivity can be determined from measurements of capacitance, ESR, or series equivalent resistance. Bulk electrical conductivity, denoted σ, can be related to ESR measurements by Equation 5 below:

[0029]

number

[0030] where ε is the permittivity, C is the capacitance, and f is the excitation frequency. Note that the measurement assumes knowledge of the permittivity, frequency, and capacitance.

[0031] Another fluid characteristic that can be determined using the embodiments and techniques described herein is fluid flow rate. The dielectric properties of a fluid are observed to change with flow rate. For example, a paper titled "Dynamic Dielectric Effect in Liquids at Low Flow Rates" by A.A. Potapov of the Irkutsk Scientific Center, Siberian Department of the USSR Academy of Sciences (submitted April 18, 1991), Zh. Eksp. Teor. Fiz 101, 895-900 (March 1992), demonstrates this phenomenon and proposes that the galvanic effect is associated with the partial orientation of liquid molecules, such that their long axes tend to align in the direction of liquid flow. This, in turn, affects the measurable dielectric properties of the fluid. This effect, which the paper calls the dynamic dielectric effect, has been observed in both polar and nonpolar liquids. Therefore, in fluids with substantially constant conductivity, variations in the dielectric constant can be considered to indicate changes in fluid flow rate using the above effect.

[0032] FIG. 6 is a schematic diagram of a single-use plastic fluid sensor 250 according to another embodiment of the present invention. While previous embodiments have generally utilized capacitive plates to sense tube deformation (i.e., gap changes), other embodiments may be implemented that sense tube deformation in response to fluid pressure. FIG. 6 shows a pair of strain gauges 252, 254 coupled to a tube wall 256 at locations 258, 260, respectively. A strain gauge is a device whose resistance changes with strain. Strain gauges are often formed with long, thin conductive strips in a zigzag pattern of parallel lines. The strain gauges 252, 254 may be abutted against or fabricated within the tube wall 256. Furthermore, the strain gauges 252, 254 may be positioned to partially surround the tube wall 256 or may substantially completely surround the tube wall 256. However, it is preferred that the strain gauges 252, 254 extend the same amount around the tube wall. Because strain gauges are sensitive to temperature changes, the embodiment shown in FIG. 6 employs strain gauge 254 at thicker location 260 as the reference sensor because location 260 does not distort with fluid pressure (at least up to the extent of location 258). In this way, changes in the signal from strain gauge 254 are indicative of factors not related to pressure (e.g., temperature). This signal can then be subtracted or removed from the signal from strain gauge 252 to provide a compensated strain-related output.

[0033] FIG. 7 is a schematic diagram of a single-use plastic fluid sensor according to another embodiment of the present invention. Sensor 300 shares some similarities with sensors 100 and 200 described above, with similar components being similarly numbered. In particular, sensor 300 includes a pair of temperature sensors located on the exterior surface of tube 334 at locations T1 and T2. These temperature sensors may be considered sensing elements. Additionally, the thickness of the tube at locations T1 and T2 is different. This is similar to the embodiment and sensor 100 described above with respect to FIG. 2. However, integrating two thickness dimensions into a single-use pressure sensor allows for the use of heat flow measurements between two conductive plates. A simple steady-state heat flow calculation can be used to obtain information about the actual internal temperature of the fluid flowing through the sensor tube. Due to stray heat paths within the sensor, there is no simple geometric relationship that describes the thermal impedance. However, as a good approximation, the process fluid can be shown to obey the following relationship, shown in Equation 6:

[0034]

number

[0035] where T process is equal to the temperature of the internal process fluid flowing through the tube, and N is an experimentally determinable constant.

[0036] Although polycarbonate and other plastics have relatively poor thermal conductivity, there are materials that can provide better thermal conductivity to improve the time response of the measurement. One example of such a material is sold by Covestro AG of Germany under the trade name Makrolon. This is a polycarbonate material that is used as a heat sink because it provides better thermal conductivity than other plastics.

[0037] The temperature sensors provided in sensor 300 at locations T1 and T2 may take any form (e.g., thermocouples, thermistors, and RTDs), although it is expressly contemplated that the temperature sensor elements themselves may be embodied in capacitive plates. For example, the capacitive plates may be formed of a temperature-sensitive metal (e.g., copper or nickel) and etched or otherwise patterned to provide a measurable resistance that varies with temperature.

[0038] 8 is a schematic diagram of a system for measuring fluid properties according to one embodiment of the present invention. The system 400 generally includes a single-use process fluid sensor 402, shown diagrammatically as a dashed box around capacitors 404, 406 and temperature sensors 408, 410. As shown, each capacitor 404, 406 is operably coupled to a switch or other suitable multiplexing circuit 412 controlled by a controller 414. The controller 414 may be any suitable electronic device capable of operating the switch 412 and interacting with a measurement circuit 416 to detect electrical parameters of the capacitors 404, 406, and the temperature sensor 408. For example, the controller 414 may be a microprocessor coupled to or including an analog-to-digital converter capable of determining the voltage or other electrical response of an attached electrical device. By controlling the switch 412, the controller 414 may interact with and individually measure the capacitances, i.e., capacitors 404 and 406. Additionally, the controller 414 may include appropriate processing capabilities (e.g., arithmetic processing) so that the various properties described above with respect to Equations 1-6 may be calculated to provide calculated process fluid parameter outputs (e.g., fluid pressure, mass flow rate, flow rate, conductivity, and / or temperature).

[0039] FIG. 9 is a flow diagram of a method for providing fluid characteristics in a biopharmaceutical or sanitary fluid processing system according to one embodiment of the present invention. Method 500 begins at block 502, where a measurement is obtained at a first location within a plastic tube through which the fluid flows. For example, the first measurement may be a first resistance measurement indicative of strain in the plastic tube through which the process fluid flows, as indicated at reference numeral 503. Alternatively, the first measurement may be a first capacitance measurement of the gap between a pair of capacitive plates at a location such as C1 (shown in FIG. 2). This is indicated at block 504. Additionally or alternatively, the first position measurement may be a temperature measurement, as indicated at block 506. Next, at block 508, a second measurement is obtained at a second location of the plastic fluid sensor. This second measurement may be a resistance measurement of a second strain gauge, as indicated at block 509. Alternatively, this second measurement may be a capacitance-based measurement at a location (e.g., location C2 (shown in FIG. 2)) and / or a temperature measurement 512 at the same location, as indicated at block 510. Next, at block 514, fluid properties are calculated based on the measurements obtained at blocks 502 and 508. As mentioned above, the embodiments described herein may provide measurements that can be used to calculate various fluid properties. Furthermore, combinations of fluid properties may provide higher-order indicators. For example, knowing the temperature of a known fluid may allow corrections to fluid density to be determined, or even the calculation of the Reynolds number (Re). Regardless, the calculated fluid properties may include pressure, as indicated at block 516; mass flow rate, as indicated at block 518; and fluid flow rate, as indicated at block 520. Finally, at block 524, the calculated fluid properties are provided as output, which may be in the form of a local output or transmitted to a remote device, as desired.

Claims

1. 1. A polymer fluid sensor comprising: an inlet configured to receive a fluid; Exit, a polymer tube fluidly interposed between the inlet and the outlet, the polymer tube having a first sensing location having a first sidewall thickness and a second sensing location having a second sidewall thickness spaced from the first sensing location; a sleeve disposed about the polymer tube; and the first sidewall thickness is less than the second sidewall thickness, a first sensing element is disposed at the first sensing location, and a second sensing element is disposed at the second sensing location. The polymer fluid sensor.

2. 2. The polymer fluid sensor of claim 1, wherein the first sensing element is a capacitive plate forming a first variable capacitor with opposing capacitive plates disposed on an inner surface of the sleeve, the first variable capacitor having a capacitance that changes with deflection of the polymer tube at the first sensing location.

3. 3. The polymer fluid sensor of claim 2, wherein the second sensing element is a capacitive plate forming a second variable capacitor with opposing capacitive plates disposed on an inner surface of the sleeve, the second variable capacitor having a capacitance that changes with deflection of the polymer tube at the second sensing location.

4. The polymer fluid sensor of claim 3 , wherein the first and second variable capacitors have the same capacitance at zero pressure.

5. The polymer fluid sensor of claim 3 , wherein the distances between opposing plates of the first and second variable capacitors are equal at zero pressure.

6. The polymer fluid sensor of claim 1 , wherein the first and second sensing elements are temperature sensors.

7. The polymer fluid sensor of claim 6 , wherein the temperature sensor is embedded within a capacitive plate.

8. The polymer fluid sensor of claim 1 , wherein the polymer tube is constructed from polycarbonate.

9. The polymer fluid sensor of claim 1 , wherein the polymer tube is constructed from ABS.

10. The polymer fluid sensor of claim 1 , wherein the first and second sensing elements are strain gauges.

11. 1. A polymer fluid sensor comprising: an inlet configured to receive a fluid; Exit, a tube fluidly interposed between the inlet and the outlet, the tube having a first sensing location with a first sidewall thickness and a second sensing location spaced from the first sensing location with a second sidewall thickness the same as the first sidewall thickness, the tube having a flow restriction disposed between the first sensing location and the second sensing location; a sleeve disposed around the tube; Equipped with a first sensing element disposed at the first sensing location; and a second sensing element disposed at the second sensing location; The polymer fluid sensor.

12. The polymer fluid sensor of claim 11 , wherein the sleeve includes a plurality of capacitive plates that form a capacitor with the first and second sensing elements.

13. The polymer fluid sensor of claim 11 , wherein the tube is constructed from polycarbonate.

14. The polymer fluid sensor of claim 11 , wherein the tube is constructed from ABS.

15. The polymer fluid sensor of claim 11 , wherein the first and second sensing elements are strain gauges.

16. 1. A method for sensing a fluid property, comprising: directing the fluid through a polymer tube having a plurality of sensing locations; sensing an electrical characteristic of a first sensing element disposed at a first one of the plurality of sensing locations; sensing an electrical characteristic of a second sensing element disposed at a second one of the plurality of sensing locations; calculating a fluid parameter based on the sensed electrical properties of the first and second sensing elements; providing the calculated fluid parameters as an output; and driving the first and second sensing elements with an alternating excitation signal and determining fluid conductivity; The method comprising the steps of:

17. 17. The method of claim 16, wherein the polymer tube has a first sidewall thickness at a first sensing location and a second sidewall thickness at a second sensing location, the second sidewall thickness being thicker than the first sidewall thickness.

18. The method of claim 16 , wherein the calculated fluid parameter is a fluid pressure.

19. The method of claim 16 , wherein the calculated fluid parameter is temperature.

20. The method of claim 19 , wherein the sensed electrical property is provided to a heat flow calculation.

21. The method of claim 16 , wherein the calculated fluid parameter is a fluid flow rate.

22. 17. The method of claim 16, wherein the polymer tube has a first sidewall thickness at a first sensing location and a second sidewall thickness at a second sensing location, the second sidewall thickness being the same as the first sidewall thickness, and the polymer tube has a flow restriction between the first and second sensing locations, and the calculated fluid parameter is mass flow rate.

23. The method of claim 22 , wherein the calculated fluid parameter is a combination of a plurality of calculated fluid parameters.

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