WO2009151338A1

WO2009151338A1 - Temperature measuring pressure sensor

Info

Publication number: WO2009151338A1
Application number: PCT/NO2009/000215
Authority: WO
Inventors: Per Hassel SØRENSEN
Original assignee: Roxar Flow Measurement As
Priority date: 2008-06-10
Filing date: 2009-06-10
Publication date: 2009-12-17
Also published as: NO20082678L; NO332873B1

Abstract

The present invention relates to a temperature sensor for measuring temperature of a pressure sensor wherein the temperature sensor is integrated with the pressure sensor by using a film structure (1), the film structure (1) is constituted of a PTC-element with high ohmic value.

Description

TEMPERATURE MEASURING PRESSURE SENSOR

Field of the invention

The present invention relates to a temperature measuring pressure sensor. More particularly, the invention relates to a temperature measuring pressure sensor designed as a piezoelectric crystal resonator for use in a subsea environment.

Background of the invention and prior art

A venturi tube is often used for flow measurements of gas/fluid flow in subsea sensors. By measuring the differential pressure over the venturi tube and using the known density of the gas/fluid, it is relatively simple to calculate the flow velocity of the gas/fluid.

Good accuracy of measurements is achieved by using differential pressure sensors (DPS), but such pressure sensors have some disadvantages that make them unsuitable for certain applications. The risk for damage of the differential pressure sensor will increase with large variation of the flow rates and clogging of the pressure sensor connections to the DPS. By replacing the differential pressure sensor with two absolute pressure sensors (APS) capable of withstanding maximal pressure in the gas/fluid, such risk for damage will not be present. The pressure drop over the venturi tube will then be as follows:

P(APSl) -P(APS2) A disadvantage of absolute pressure sensors is that they will always measure between 0 to 100% of the maximum pressure, while a differential pressure sensor will measure 0 to 100% of the pressure difference over the venturi, which normally is much smaller than maximum pressure. Thus, to achieve the same resolution, a much larger accuracy is required, and thus the APS must be very accurate when used in differential pressure measurements.

An absolute pressure sensor providing the desired accuracy can for instance be a pressure sensor designed as a piezoelectric crystal resonator. Such a pressure sensor is disclosed by the patent publication US 3,561,832. As mention in US 3,561,832, a quartz based pressure sensor can provide very high accuracy, typically ±0,001 psi. This accuracy is provided on the condition that the temperature throughout the whole sensor is constant or is being compensated. The sensor in the above mentioned patent publication provides a constant and compensated temperature by using thermostat controlled heating elements. Unfortunately, it is difficult to keep the temperature in a subsea sensor constant. The outside of the sensor is cooled down by the sea water, and the inside of the sensor is heated by the flowing gas/fluid. The temperature differences can typically be 0,1 - 1 7ΔT over the subsea sensor, but the temperature inside the subsea sensor can vary from -10 to +200° if it does not have active heating/cooling elements. The temperature variations can be eliminated by active heating/cooling elements, but these elements have some disadvantages in that they bring along an increased energy consumption, reduced lifetime and requirements of stabilization time before the sensor has a stable temperature. Temperature control of the sensor is vital for accurate measurements, thus there is a need for measuring and compensating the temperature in the sensor.

Temperature compensation in a pressure sensor can be done in different ways, for example:

By use of thermostat controlled heating/cooling elements. - By use of a pressure sensor with temperature insensitive frequency. - By use of digital techniques/micro processors.

Temperature compensation by using heating elements controlled by a thermostat is disclosed in for example the patent publication US 3,561,832. The heating elements are very accurately regulated heating sources providing the sensor to be held on a constant temperature. However, such heaters result in increased power consumption and reduced reliability.

Another way of compensating the temperature is to design the pressure sensor so that the frequency is insensitive of temperature in the temperature range in which the sensor shall be used. This type of temperature compensation is disclosed in for example the patent publication US 4,550,610. Unfortunately this method can only be used for a limited temperature range, typically a few ⁰C. One of the most frequent used methods for temperature compensation in pressure sensors is to compensate measured frequency by measuring the temperature of the pressure sensor. Then, the measured pressure can be adjusted based on information about the temperature dependency of the pressure sensor element from previous calibration by means of digital techniques/micro processors. Such temperature compensation is disclosed in the patent publication US 3,355,949 and in the patent publication US 4,802,370. This sensor utilizes the fact that changes of temperature in the material in which quarts crystals are located will change the output frequency of each of the crystals, and consequently any change in the output frequency will be indicated in a readout, thereby indicating the exact temperature change of the material. The solution disclosed in US 3,355,949 has a separate swing element in a separate module used as a temperature reference, and the solution in US 4,802,370 has a separate swing element in the same sensor module as the temperature reference.

It is also known to have an external temperature sensor outside the sensor housing. However, by using an external temperature sensor the measured temperature does not sufficiently correspond to the real temperature inside the sensor, and has not a sufficient good thermal coupling. In addition, an external unit results in increased complexity of the sensor.

Another solution is to monitor the temperature inside the sensor as disclosed in the patent publication US 6, 131 ,462. The invention described in US 6, 131 ,462 compensates for varying temperature on the sensor by placing a major portion of the side wall of a pressure crystal immediately adjacent an inner wall of the sensor housing, separated by only a small gap. Alternatively, the pressure crystal can be separated from the inner wall of the sensor housing by an electrically insulating element if this is required to prevent electrical grounding of the crystal to the metal of the sensor housing. However, it does not provide sufficient accurate temperature compensation because it is dependent on the location of where the temperature measurements are taken in relation to the direction of the temperature gradient.

Summary of the invention It is therefore an object of the present invention to provide a high reliability temperature measuring pressure sensor designed as a piezoelectric crystal resonator which can measure the temperature and compensate for temperature changes throughout the whole sensor without significantly increased power consummation or complexity of the sensor. It is also an object of the invention to provide a temperature measuring pressure sensor that gives accurate measurement of the temperature, and where the location of the temperature measurements does not affect the accuracy of the temperature compensation.

These objects are obtained by using a sensor for measuring temperature of a pressure sensor as characterized as specified in the accompanying claims.

The objectives set forth above are achieved by providing, in a first aspect of a preferred embodiment of the present invention, a temperature sensor for measuring temperature of a pressure sensor wherein the temperature sensor is integrated with the pressure sensor by using a film structure, the film structure is constituted of a PTC-element with high ohmic value. The film structure is dependent on temperature, but neglectably dependent on pressure, so that temperature can be accurately measured. By designing the temperature sensor as a film structure, the mechanical design and the size of the sensor assembly will be simpler than for example a pressure sensor with an external temperature sensor.

According to the invention the temperature sensor the PTC-element is connected with a low pass filter, preferably a film low pass filter. This low pass filter allows the temperature to be measured in the temperature sensor through DC (or possibly low frequency) currents, while the pressure sensor provides measurements at higher frequencies used for PTC pressure sensors. The pressure sensor is isolated from the rest of the electronics by means of pressure solid electric penetrators. There is a risk for leakage at such a penetrator, but by connecting the PTC-element coupled together with the temperature sensor with a low pass filter one can read off temperature and pressure through the same penetrator.

In one embodiment of the temperature sensor according to the present invention, the pressure sensor is a piezoelectric crystal resonator comprising a swing element. In still an alternative embodiment of the temperature sensor according to the present invention, the film structure is arranged on one or both of the ends of the pressure sensor. This makes it possible to compensate for temperature gradients over the pressure sensor housing.

In further an alternative embodiment of the temperature sensor according to the present invention, the film structure is arranged on the swing element of the pressure sensor. This makes it possible to compensate for temperature gradients over the pressure sensor housing.

In still a further alternative embodiment of the temperature sensor according to the present invention the film structure is arranged on the pressure sensor by means of vacuum deposition, adhesive or printing. These are well known methods for applying film structures on sensors, and provide good and stable attachment of the film structure to the pressure sensor.

In further an alternative embodiment of the temperature sensor according to the present invention the film structure is a thin film structure or a thick film structure. The choice of thin or thick film structure is dependent on needs, the desired function, time of delivery, availability etc. The thin film structure requires use of vacuum equipment when applying the film structure on the sensor, which also is more expensive than the equipment required for thick film. Application of the thin film structure is a substantially controlled process which has less variation than thick film. The thick film structure is a more cost effective and simpler process that requires cheaper equipment, such as for instance silk printing. The variation is larger with a thick film structure than when applying the thin film structure, so that an additional step must be performed in arranging the thick film structure on the sensor.

In still an alternative embodiment of the temperature sensor according to the invention, the temperature is measured by using direct current (DC).

In an alternative embodiment of the temperature sensor according to the present invention, pressure is measured by using alternating current (AC). In further an alternative embodiment of the temperature sensor according to the invention, the film low pass filter is a thin film low pass filter or a thick film low pass filter.

In an alternative embodiment of the temperature sensor according to the present invention, the temperature sensor comprises a filter arranged to filter AC components. This way, the AC-components do not influence the swing element, and vice versa.

In a further alternative embodiment of the temperature sensor according to the present invention, a surface acoustic wave temperature sensor is arranged on the swing element of the pressure sensor. This avoids analogue measurement of the temperature. The surface acoustic wave temperature sensor will typically be used in downhole electronics requested to be as simple as possible and to have a long lifetime at high temperatures.

Brief description of the drawings

The invention will now be further described in more detail in the following detailed description by reference to the appended drawings in which

Fig. 1 illustrates the temperature measuring pressure sensor where the temperature sensor is arranged on the end surfaces of the sensor.

Fig. 2 illustrates the temperature measuring pressure sensor where the temperature sensor is arranged on the swing element.

Fig. 3 illustrates the temperature measuring pressure sensor where the PTC- element is connected to a low pass filter.

Fig. 4 illustrates the temperature measuring pressure sensor where the PTC- element is integrated with a low pass filter.

Fig. 5 shows a top view of the temperature measuring pressure sensor where the temperature sensor is a SAW temperature sensor. Detailed description of the drawings

As mentioned above, a piezoelectric crystal resonator provides very high accuracy, typically +/-0,001 psi. An example of such a piezoelectric crystal resonator is disclosed in the patent publication US 3,561,832. The principle of measurement disclosed in that publication providing a pressure and temperature insensitive sensor element is the same as can be used in the temperature measuring pressure sensor according to the present invention. Thus, accuracy sufficient for flow measurement over a normal venturi tube is achieved. Such a pressure sensor can for example have diameter and length of approximately 10 — 15 mm.

Such a piezoelectric crystal resonator comprises a swing element dependent on both pressure and temperature. To provide temperature measurements and compensation, an additional functionality must be applied to the pressure sensor. This additional temperature functionality can be provided to the pressure sensor by arranging a temperature sensor, in the form of a film structure 1, on one or both of the end surfaces of the pressure sensor as shown in fig. I₅ 3 and 4, or on the swing element itself as shown in fig. 2. It is simpler to arrange the temperature sensor on the end surface of the pressure sensor, as the ends are easy to access, but it is also possible to arrange the temperature sensor on the swing element itself. The swing element itself has already a film structure, but it might be a problem regarding the signals when arranging the temperature sensore on the swing element itself. The best solution would be to arrange the temperature sensor on both the swing element and the ends, but sufficient measurements are obtained by arranging the temperature sensor on the end surfaces. The end surface of the pressure sensor are easy to access and it is therefore feasible to apply the film structure on the ends and. Application on the ends gives sufficient contact which gives the desired measurements. By arranging the temperature on both end surfaces, the temperature will be measured on both ends, and a average of the temperature can be found.

The film structure 1 can be a thin film structure or a thick film structure. The choice of thin or thick film structure is dependent on needs, the desired function, time of delivery, availability etc. The thin film structure requires use of vacuum equipment when applying the film structure on the sensor, which also is more expensive than the equipment required for thick film. Application of the thin film structure is a substantially controlled process which has less variation than thick film. The thick film structure is a more cost effective and simpler process that requires cheaper equipment, such as for instance silk printing. The variation is larger than when applying the thin film structure, so that an additional step must be performed. A thin film structure can have a thickness of for example less than 1 μm, and a thick film structure can have a thickness of for example lOμm.

The film structure 1 is an electrically conductive film layer that can be arranged on the pressure sensor by means of vacuum deposition. Vacuum deposition is a well known method for applying conductive film structures 1 on sensors, and provides good and stable attachment of the film structure 1 to the pressure sensor. Alternatively film structures 1 can be applied on sensors by way of adhesive or printing of PTC (positive temperature coefficient), preferably on the ends of the sensor. A temperature sensor designed as a film structure 1 applied to the pressure sensor gives the pressure sensor a simpler mechanical design and reduced size of the sensor housing compared to for instance a pressure sensor with an external temperature sensor. By placing the temperature sensor on or in the pressure sensor, as shown in fig. 1 - 4, it will be possible to compensate for temperature gradients over the pressure sensor element.

Measurement on the sensor element when the temperature is varying provides a better compensation and better accuracy of the pressure measurements (compared to indirect measurement). The film structure 1 provides an increased thermal coupling and is constituted by a PTC-element (Positive Temperature Coefficient) with high resistance. The PTC-element of the film structure 1 provides the temperature sensor with a resistance that is dependent on temperature, but neglectably dependent on pressure. A person skilled in the art will know that PTC refers to materials that have an increase in electrical resistance when their temperature is increased. The temperature sensor in the form of a film structure 1 can for example be made of platina. The PTC element which is an electrically conductive film structure 1 is coupled in parallel with the crystal coupling in the pressure sensor. As the Q-factor of a quarts crystal pressure sensor is very high, the change of the relatively high parallel resistance, constituted by addition of the film structure 1 with high ohmic PTC-element, will not influence the frequency of the pressure sensor. As illustrated in fig. 3, the film structure 1 can be connected to the temperature sensor through a film low pass filter 4. This is especially advantageous if the pressure sensor is isolated from the rest of the electronics by means of pressure solid electric penetrators, and if there is a risk for leakage at such a penetrator. Then, by connecting the PTC-element (film structure 1) with a low pass filter 4 one can read off temperature and pressure through the same penetrator, and AC-components will not influence the swing element, and vice versa. The low pass filter 4 can be realized as a series inductor. Fig. 4 illustrates an embodiment where the PTC-element (film structure 1) is integrated with the low pass filter 4.

The temperature measuring pressure sensor also provides a possibility to detect breakage or crack formations in the crystal structure of the pressure sensor. Breakage or crack formations is detected when a change in the connection is detected, i.e. a irregular change in the measured signals.

The measurement of temperature can be done preferably via direct current (DC) or possibly low frequency alternating current and pressure measurements are done via alternating current (AC) at a higher frequency suitable for the chosen pressure sensor. The temperature is measured at the temperature terminals 3 and the pressure is measured at the pressure terminals 2, as shown in fig. 1 - 4. The temperature is compensation by means of digital signal processing (Multiple Control Unit, MCU) supplied with data from previous calibration of the temperature measuring pressure sensor. The sensor has been exposed to varying pressure and temperature, and data from the relationship between the PTC-value and the temperature can be derived from the calibration data. From that, the correct pressure is derived by measuring the PTC- value giving the temperature, and using both the temperature and the frequency from the sensor to derive the exact pressure.

As a DC-measurement will require an accurate A/D-converter (Analog-to-Digital Converter), this embodiment is most suitable for locations where good and cost effective A/D-converters exist, such as subsea or topside locations. But with future, improved A/D-converters the same embodiment can be used in downhole applications as well. In another embodiment of the present invention, the swing element in the pressure sensor can be provided with a surface acoustic wave temperature sensor 5 (SAW temperature sensor). The pressure sensor with the SAW temperature sensor 5 is shown in fig. 5. A SAW temperature sensor 5 avoids analogue measurement of the temperature. The surface acoustic wave temperature sensor 5 will typically be used in downhole electronics requested to be as simple as possible and to have a long lifetime at high temperatures.

Today, some pressure sensors are already equipped with two different metals arranged on the sensor by means of vacuum deposition. Therefore, applying another film structure to the sensor does not imply increased production complexity. The amount of calibrating of the sensor will neither be increased, which is a cost saving effect.

Thus the invention relates to a temperature sensor for measuring temperature of a pressure sensor wherein the temperature sensor is integrated with the pressure sensor constituted by a film structure (1). The film structure (1) is a PTC-element with high ohmic value, and is coupled to the temperature sensor (3) with a low pass filter (4). Thus a common conductor may be provided for connection to the external readout instruments and a temperature compensated pressure signal may be obtained without increasing the risk of leaks in when used in a pressurized fluid..

Reference numerals

1. Film structure

2. Pressure terminal

3. Temperature terminal

4. Low pass filter

5. SAW temperature sensor

Claims

1. A temperature sensor for measuring temperature of a pressure sensor wherein the temperature sensor is integrated with the pressure sensor by using a film structure (1), the film structure (1) is constituted of a PTC-element with high ohmic value, and wherein the PTC-element is connected to the temperature sensor (3) with a low pass filter (4).

2. A temperature sensor according to claim 1, wherein the pressure sensor is a piezoelectric crystal resonator comprising a swing element.

3. A temperature sensor according to claim 1 or 2, wherein the film structure (1) is arranged on one of the ends of the pressure sensor.

4. A temperature sensor according to claim 2, wherein the film structure (1) is arranged on the swing element of the pressure sensor.

5. A temperature sensor according to claim 3 or 4, wherein the film structure (1) is arranged on the pressure sensor by means of vacuum deposition, adhesive or printing.

6. A temperature sensor according to claims 1 — 5, wherein the film structure (1) is a thin film structure or a thick film structure.

7. A temperature sensor according to claim 1, wherein the low pass filter is a film low pass filter integrated in the film structure.

8. A temperature sensor according to claim 7, wherein temperature is measured by using direct current (DC).

9. A temperature sensor according to claim 7, wherein pressure is measured by using alternating current (AC).

10. A temperature sensor according to claim 7 -9, wherein the film low pass filter (4) is a thin film low pass filter or a thick film low pass filter.

11. A temperature sensor according to claim 1 , wherein the temperature sensor comprises a filter arranged to filter AC components.

12. A sensor according to claim 2, wherein a surface acoustic wave temperature sensor (5) is arranged on the swing element of the pressure sensor.