WO2010054676A1

WO2010054676A1 - Pressure transducer for process instrumentation and method for measuring pressure

Info

Publication number: WO2010054676A1
Application number: PCT/EP2008/009621
Authority: WO
Inventors: Herbert Schorb; Stefan Von Dosky
Original assignee: Siemens Aktiengesellschaft
Priority date: 2008-11-13
Filing date: 2008-11-13
Publication date: 2010-05-20

Abstract

The invention relates to a pressure transducer for process instrumentation and to a method for measuring pressure using a pressure transducer having a pressure sensor (1) with a diaphragm (2), on one side of which the pressure (P) to be measured is guided and on the other side of which a reference pressure is guided. In order to generate the reference pressure, a reference pressure chamber (4) is filled with a medium (5, 6), the pressure of which has a previously known temperature dependence. Means (8) for adjusting the temperature (T) of the medium are provided such that a position signal which represents the deflection of the diaphragm (2) corresponds to a predefined value. An evaluation device (20) determines the pressure (P) to be measured on the basis of the temperature (T) of the medium (5, 6), which temperature has been adjusted in this manner, and outputs said pressure. Since a fundamental material property which is stable over a long period of time results when using a liquid gas, the vapour pressure curve of which is used as a pressure reference, a pressure transducer which has a high degree of accuracy and particularly good long-term stability and manages without factory calibration and recalibration of the pressure in the field is obtained.

Description

description

Pressure transmitter for process instrumentation and pressure measurement

The invention relates to a pressure transmitter for process instrumentation with a pressure sensor with a membrane, on one side of the pressure to be measured and on the other side a reference pressure are guided, and with at least one sensor element for detecting a deflection of the membrane according to the preamble of claim 1 and a method for pressure measurement with such a pressure transducer.

In process engineering plants, a variety of field devices are used for process instrumentation to control processes. Transmitters are used to record process variables, such as temperature, pressure, flow rate, level, density or gas concentration of a medium. By means of actuators, the process flow can be influenced as a function of detected process variables in accordance with a strategy predetermined, for example, by a control station. As examples of actuators may be mentioned a control valve, a heater or a pump. Especially in process engineering plants, pressure transmitters are essential sensory components in the context of automated production processes. With regard to an optimal plant behavior and a permanently high product quality, high-quality transducers are necessary, which provide long-term stable and low error measured values even under extreme conditions.

From the Siemens catalog "ST FI 01-2008", chapter 1, a SITRANS P pressure transmitter for process instrumentation is known, which has sensor elements arranged on a membrane for detecting the membrane deflection as a result of the applied pressure and for generating a measurement signal representing this pressure , wherein an evaluation device determines a measured value of the pressure as a function of the measuring signal.

In order to obtain accurate measurement results, it is necessary to calibrate such transducers as part of a factory setting. In this case, the determined pressure value is compared with a reference pressure value, for which purpose the pressure signal representing the measurement signal is evaluated. This measurement signal usually produces material voltage-dependent, piezoresistive resistances, which are interconnected in a Wheatstone bridge. A disadvantage is that the measurement results over a longer periods due to aging effects of the piezoresistive resistors and the membrane or due to outgassing of a filling oil in a measuring chamber which is provided for transmission of the process pressure to be measured to the membrane, drift and after some time, the accuracy requirements are no longer sufficient, so that recalibration measures are required. These measures are expensive because usually the transmitter must be disconnected from the process equipment and recalibrated in the factory or in the laboratory. For example, different test pressures are generated by a hand spindle, which are simultaneously given to the pressure transmitter to be calibrated and a reference device. The pressure transmitter can be precisely adjusted to the measured values of the reference device.

The invention has for its object to provide a pressure transducer for process instrumentation, which is characterized by improved long-term stability and thus can be operated longer without recalibration. Another object is to find a suitable method of pressure measurement with such a pressure transducer.

To solve this problem, the pressure transmitter for process instrumentation of the type mentioned in the characterizing part of claim 1 features on. In the dependent claims advantageous developments of the pressure transducer, in claim 5, a method for pressure measurement with such a pressure transducer described.

The invention has the advantage that it can be integrated in a simple manner into a pressure transmitter with a micromechanically produced silicon diaphragm which carries a piezoresistive bridge circuit for detecting the diaphragm deflection. Such a structure is used for example in the known pressure transmitter SITRANS P. On one side of the membrane of the pressure to be measured, on the other side a reference pressure of a reference pressure chamber is performed. The reference pressure chamber is now filled with a medium whose pressure has a known temperature dependence. Advantageously, the deflection of the membrane can be detected by the piezoresistive measuring resistors of the bridge circuit. As an alternative to the use of a piezoresistive full-bridge circuit, the deflection of the diaphragm can, of course, be measured using inductive, capacitive or optical transducers. By heating or cooling, the medium is brought in the reference pressure chamber by means of a temperature change to that temperature at which the membrane is in a predetermined position, which is preferably free of mechanical stresses. There is then a pressure balance between the two membrane sides. The resulting temperature is precisely measured, for example, with a thermocouple or a temperature-dependent resistor. On the physical relationship between the pressure of the medium in the reference pressure chamber and its temperature, which ultimately depends only on the physical properties of the medium, the pressure to be measured, which can also be referred to as system or process pressure, determined and ausgege - ben. If the reference pressure chamber is designed to be diffusion-tight, then the amount of medium located in the reference pressure chamber does not change over the service life of the pressure transmitter and there is no contamination of the medium either instead of. Since the measurement of the system pressure essentially depends only on the long-term stability of the medium in the reference pressure chamber and the accuracy of the temperature measurement, which can also be performed very long-term stable, the new pressure transducer is characterized by a comparatively good long-term stability with relatively little technical effort for his Realization off.

If the sensor element for detecting the deflection of the

Membrane - as already indicated above - is designed as a piezoresistive full bridge circuit, the bridge voltage is used as a position signal, and also if the value zero of the bridge voltage is used as the predetermined value of the position signal, which is achieved by means of the temperature setting, this has the advantage that also the sensor element is characterized by an excellent long-term stability. For example, if the individual resistors of the bridge circuit experience a drift due to aging effects, this does not affect a shift in the zero position of the bridge voltage, since all measuring resistors of the circuit are exposed to the same influences and thus experience the same drift. In addition, for the realization of the sensor element for detecting the diaphragm deflection no redesign of a conventional membrane is required, as used for example in the pressure transmitter SITRANS P.

In an advantageous manner, the membrane can be in the state of a pressure equilibrium between the two membrane sides at the predetermined value of the position signal substantially. In this case, the membrane is free of mechanical stresses and aging phenomena, for example material fatigue. These then do not lead to a drift of the membrane layer in the equilibrium state. This further improves the long-term stability of the pressure transmitter.

Preferably, the reference pressure chamber is filled with a medium which, at the operating temperature of the pressure measuring partly in the gaseous and partly in the liquid state of aggregation. This has the advantage that the temperature dependence of the media pressure can be described in a particularly long-term stable manner by the vapor pressure curve of the medium. The vapor pressure is the pressure of the vaporous phase of a substance when the liquid and gaseous phases are in equilibrium. The vapor pressure of a pure substance is only dependent on its temperature. When the temperature or volume changes, so much of the substance evaporates or condenses, until at equilibrium the vapor pressure again reaches the saturation vapor pressure. When using such a medium, even small leaks in the reference pressure chamber are therefore tolerable, as long as there is still a two-phase mixture of the medium in the chamber.

In a further embodiment, the pressure sensor with the novel measuring principle can be used as a reference for calibrating a pressure sensor, which performs the pressure measurement according to a conventional principle. For this purpose, both pressure sensors are subjected to the same system pressure and the pressure sensor, which operates according to the conventional principle, is calibrated with the aid of the long-term stable measuring signal of the reference pressure sensor with a new measuring principle, in particular if a constant system pressure is present for a certain period of time. This has the advantage that the merit of the long-term stability of the new pressure sensor with the advantage of responsiveness to pressure fluctuations of the conventional pressure sensor is combined.

With reference to the drawings, in which an embodiment of the invention is shown, the invention and refinements and advantages are explained in more detail below.

Show it:

1 shows a schematic representation of a pressure sensor of a pressure transducer, FIG. 2 is a block diagram of a pressure transmitter;

FIG. 3 shows a block diagram for explaining the temperature setting,

FIG. 4 shows a flowchart of the pressure measurement and

Figure 5 vapor pressure curves of various paraffins.

In the figures, like parts are given the same reference numerals.

A pressure sensor 1 for a pressure transducer comprises, according to FIG. 1, a silicon membrane 2 which is arranged on a substrate 3. On one side of the membrane 2, a system pressure P acts, on the other side a reference pressure, which is generated by a medium 5, 6 in a reference pressure chamber 4. A part 5 of the medium is in the gaseous state, a part 6 in the liquid state. To detect a deflection of the diaphragm 2, measuring resistors 7 doped in the upper side of the diaphragm are used, which are electrically connected to form a Wheatstone full bridge. The reference pressure in the reference pressure chamber 4 corresponds to the saturation vapor pressure of the medium and is therefore almost exclusively dependent on the temperature of the medium. By setting the temperature by means of a heater 8, which is shown schematically in Figure 1 as a heating coil, therefore, the reference pressure can be varied within a wide range. For this purpose, a control signal u is given to the two connections of the heater 8. By means of a temperature sensor 10 as a means for detecting the temperature, a temperature signal T is generated, which corresponds to the temperature of the medium. So that the medium remains as permanently as possible in the reference pressure chamber 4, this is made almost diffusion-tight.

In the block diagram according to FIG. 2, the basic interconnection of the pressure sensor 1 with an evaluation device 20 of a pressure transmitter 24 is shown. On the pressure As already described with reference to FIG. 1, the system pressure P is guided as a physical variable to be measured. The pressure sensor 1 transmits to the evaluation device 20 a displacement signal x representing the deflection of the diaphragm 2 (FIG. 1), which in the illustrated embodiment corresponds to the bridge voltage and the temperature signal T representing the fluid temperature. The evaluation device 20 determines based on these signals x and T in a manner to be described in more detail later, a temperature control signal u, in the pressure sensor 1 for controlling a

Heating and possibly also for controlling a cooling for the medium in the reference pressure chamber is used. Depending on the temperature T of the medium, which is established when the position signal x corresponds to a predetermined value, a value of the system pressure P to be measured is determined in the evaluation device 20 on the basis of a vapor pressure curve stored in a memory 21 and, for example, via a fieldbus or a fourth to 20 mA interface 22 to a programmable logic controller or a control station for further processing in a process plant, in which the pressure transmitter 24 is used issued. In the evaluation device 20, a suitably programmed microcomputer 23 is present, which performs the necessary calculations, inter alia, to specify the temperature control signal u or to determine the pressure measurements.

The calculation of the default values for the temperature control signal u is carried out with the aid of a control, which is explained in more detail with reference to FIG. As control variable x, the position signal repre- senting the deflection is used, which corresponds to the bridge voltage of the bridge circuit located on diaphragm 2 (FIG. 1). With the aid of a comparator, on which in addition to the position signal x a setpoint 0 (zero) is performed, a control deviation is determined, which is on a controller 30, such as a PID controller, out. This provides a default value of the temperature control signal u, with which a temperature actuator 31 is driven for heating or cooling. With the temperature change of the medium is the reference pressure in the Referenzdruckkaituner 4 (Figure 1) which is compared using the membrane 2 (Figure 1) with the system pressure P. A member 32 in the control loop according to FIG. 3 represents means for detecting the temperature T of the medium and the sensor element for detecting the deflection of the diaphragm and for generating the displacement signal representing the position signal x, which, as already explained above, corresponds to the bridge voltage in the embodiment shown , With the aid of the actuator 31, the temperature of the medium in the reference pressure chamber is thus adjusted such that the position signal x assumes the predetermined value zero, at which the membrane of the pressure sensor just does not deform. With changing system pressure P, the temperature T of the medium is always tracked by the controller 30 so that the position signal x, and thus the bridge voltage of the piezoresistive bridge circuit and the material stresses in the membrane, are regulated to zero. In the regulated state, a pressure equilibrium prevails between the two sides of the membrane.

FIG. 4 shows the basic sequence of a cycle for measuring the system pressure P. In a step S1, first the position signal x is adjusted, as described with reference to FIG. When the position signal x has reached the predetermined value, it goes to a step S2, in which the temperature T of the medium is measured accurately. In a step S3, the reference pressure is subsequently determined on the basis of the stored vapor pressure curve, which reflects the previously known temperature dependence of the media pressure. With knowledge of the reference pressure, a value of the system pressure P is then calculated in a step S4 and output, for example, to a higher-level control station. Thus, the value of the system pressure P is determined and output via a physically fundamental, substance-dependent and constant relationship, namely via the vapor pressure curve of the medium. If the reference pressure chamber is designed to be diffusion-tight, no medium is consumed over the entire service life of the pressure sensor, and there is also no mistaking cleaning the medium instead. Even small leaks are tolerable as long as there is still a two-phase mixture in the reference pressure chamber.

When filling the reference pressure chamber with a two-phase mixture, the measuring range of the pressure transducer is essentially determined by the vapor pressure curve of the medium, which in this case can also be referred to as liquefied gas. When selecting the liquefied gas, care must be taken that the vapor pressure range of the liquefied gas lies within the microtechnically manageable temperature range for doped silicon, which is usually chosen as the material for the membrane. The lower and upper limits of the pressure transmitter should also be overlapped if possible. A suitable liquid gas must therefore be selected according to the absolute size and position of the measuring range. For high-pressure sensors, for example, low-order paraffins with a steep vapor pressure curve and for low-pressure sensors higher-order paraffins with a flatter vapor pressure curve are recommended.

FIG. 5 shows vapor pressure values for various paraffins in a diagram. The abscissa shows the temperature in ⁰ C, the ordinate the pressure P in megapascals. Values of the same paraffin are marked with the same symbols. The following assignment of symbols provided with reference signs applies:

50 - methane,

51 - ethane, 52 - propane,

53 - butane,

54 - pentane,

55 - hexane,

56 - heptane, 57 - octane,

58 - Nonane and

59 - Dean. Of course, the graph points associated with the individual paraffins may be linked together by interpolation into a vapor pressure curve, as shown by curve 60 for the example propane.

The accuracy advantage and the advantage in terms of long-term stability compared to the conventional pressure transmitter SITRANS P will be briefly explained by the following numerical example. A conventional SITRANS P pressure transmitter with a measuring range of 1 bar to a maximum of 30 bar, a measurement uncertainty of 0.075%, ie a maximum of 22.5 mbar, and a long-term stability of 0.25% over five years, which is a measuring error of maximum 75 mbar. If propane is selected for the new pressure transmitter to fill the reference pressure chamber, then the

Vapor pressure curve 60 are derived according to Figure 5 comparative values. The uncertainty of the vapor pressure curve 60 is 0.02% of the absolute value. This corresponds to a pressure of 8364 mbar at 20 ° C corresponding to the vapor pressure curve 60 with a maximum deviation of +/- 1.7 mbar. A temperature measurement with a maximum error of +/- 0.02 K thus means an uncertainty of the measured pressure value of a total of +/- 5 mbar. The measurement error is thus significantly lower than with the conventional pressure transmitter. It follows that the pressure can be measured much more accurately if the temperature measurement is accurate. The long-term stability of the new pressure transmitter is also significantly better than that of the conventional pressure transmitter SITRANS P, as the reference pressure chamber can be made diffusion-tight and the temperature measurement can be designed for a long-term stability.

Due to the measurement principle, the pressure transmitter does not require a factory calibration or recalibration of the pressure, only the temperature measuring point has to be calibrated at the factory. When using a long-term stable temperature measuring element, a recalibration can thus be completely eliminated. The temperature dependence of the vapor pressure is stored in the memory 21 of the evaluation device 20 according to FIG. The mechanical Demands on the diaphragm 2 of the pressure sensor shown in Figure 1 are relatively low and can therefore be realized with inexpensive components. Likewise, the setting of the temperature is such that the position signal of the pressure sensor corresponds to a predetermined value, with standard components without much effort feasible. A temperature measuring point with the required accuracy is already integrated in a conventional SITRANS P pressure transmitter because the temperature on the diaphragm of the pressure sensor is recorded and its influence on the measured value compensated. In the condition of a temperature compensation within the sensor, this corresponds to the temperature of the medium, nevertheless it is advantageous to provide a temperature sensor directly for the medium temperature.

Advantageously, the new pressure transmitter is thus a pressure gauge with a pressure sensor which has a long-term stable pressure reference due to the dependence of the vapor pressure on the temperature. The pressure measurement can be traced back to a temperature measurement and by a control pressure in the pressure sensor is built up a back pressure, which corresponds to the system pressure. Thus, a pressure sensor is available in which the sensor membrane is not deformed during the measurement and is therefore free from mechanical stresses. The use of a vapor pressure curve as a pressure reference has the advantage that it represents a fundamental, long-term stable material property that is highly accurately referenced for many technically relevant substances and known for example for propane with an uncertainty of 0.02%. This eliminates the need for pressure calibration at the factory and no recalibration in the field. Another advantage is that the mechanical requirements on the pressure sensor are rather low and thus the use of inexpensive sensors is made possible. The realization of a pressure sensor with a pressure reference on the basis of the vapor pressure curve of a medium in the reference pressure chamber is also advantageously already carried out with comparatively small deviations. Conversions of the established pressure transmitter SITRANS P possible.

Claims

claims

1. Pressure transmitter for process instrumentation with a pressure sensor (1) with a membrane (2), on whose one side of the pressure to be measured (P) and on the other side a reference pressure are guided, with at least one sensor element (7) for detecting a deflection the membrane (2) and for generating a deflection signal representing the position signal (x) and with an evaluation device (20) on which the position signal (x) is guided, characterized in that for generating the reference pressure, a reference pressure chamber (4) with a medium (5, 6) is filled, whose pressure has a known temperature dependence, that means (10) for detecting the temperature of the medium (5, 6) and for generating a temperature signal (T) are provided, which led to the evaluation device (20) in that means (8) are provided for adjusting the temperature of the medium as a function of a temperature control signal (u) supplied by the evaluation device (20), and in that the evaluation device (20) is designed to set the temperature of the medium by suitable specification of the temperature control signal (u) such that the position signal (x) corresponds to a predetermined value, and in that the evaluation device (20) is further adapted to the value of to be measured pressure (P) as a function of the thus set temperature (T) of the medium.

2. Pressure transmitter according to claim 1, characterized in that as a sensor element (7) for detecting the deflection of the membrane (2) a piezoresistive full bridge circuit is present, the bridge voltage corresponds to the position signal (x) and that the value (0) zero of the bridge voltage of predetermined value of the position signal (x) is.

3. Pressure transmitter according to claim 1 or 2, characterized in that the membrane (2) at the predetermined value of the position signal (x) is substantially in the state of a pressure equilibrium between the two sides of the membrane.

4. Pressure transmitter according to one of the preceding claims, characterized in that the medium (5, 6) in the reference pressure chamber (4) at operating temperature to part (5) in gaseous and partly (6) is present in the liquid state.

5. A method for pressure measurement with a pressure transducer (24) for process instrumentation, with a pressure sensor (1) with a membrane (2), on one side of the pressure to be measured (P) and on the other side a reference pressure are performed, with at least a sensor element (7) for detecting a deflection of the membrane (2) and for generating a position signal (x) representing the deflection, with an evaluation device (20) to which the position signal (x) is guided, with a reference pressure chamber (4 ), which is filled to generate the reference pressure with a medium (5, 6) whose pressure has a known temperature dependence, with means (10) for detecting the temperature of the medium (5, 6) and for generating a temperature signal (T), which is guided to the evaluation device (20), and with means (8) for adjusting the temperature of the medium (5, 6) in dependence of a temperature control signal (u) supplied by the evaluation device (20), with de n steps:

- Specifying the temperature control signal (u) by the evaluation device (20) for adjusting the temperature of the medium

(5, 6) such that the position signal (x) corresponds to a predetermined value, and

- Determining and outputting the value of the pressure to be measured

(P) as a function of the thus set temperature (T) of the medium.