Method and device for verification and/or calibration of a pressure sensor

A method for calibrating a pressure sensor includes connecting the pressure sensor to first and second fluid storage vessels; providing an initial fluid pressure at the pressure sensor and at the fluid storage vessels; and carrying out a pressure measurement of the initial fluid pressure at a time t0. The method then disconnects the second fluid storage vessel from the pressure sensor and the first fluid storage vessel; provides a first fluid pressure at the second fluid storage vessel; and carries out a pressure measurement of the first fluid pressure at a time t1. The method then connects the second fluid storage vessel with the pressure sensor and the first fluid storage vessel, so that a second fluid pressure between the initial and first fluid pressures is provided at the pressure sensor; and carries out a pressure measurement of the second fluid pressure at a time t2.

BACKGROUND

1. Field of the Invention

The invention relates to a method for verifying and/or calibrating a pressure sensor and a device for automatically carrying out the verification and/or calibration of the pressure sensor. The invention likewise relates to a computer program product for automatically carrying out the method.

2. Description of the Related Art

Various apparatuses for production or testing comprise one or more pressure sensor for reading the pressure of a fluid, such as a gas, a liquid or a mixture thereof, for example to control the pressure in a biochemical process container or to verify filter devices. Since the reading of the pressure sensor may be prone to some time drifts during use of the apparatuses, it is necessary to verify, whether the readings of the pressure sensor are reliable or whether the pressure sensor must be replaced by a new one.

In order to perform a verification of the pressure sensor it is required to compare the reading of the pressure sensor of the apparatus with a reading of a reference pressure sensor. However, high efforts of time and manpower are required to provide a reference sensor and perform the verification test. Thus, this procedure is not suitable to provide a continuous verification whether the pressure sensor is possibly affected by a time drift phenomenon.

It is a problem to provide a more quick and reliable test method and a corresponding test device to perform a verification or calibration test of the pressure sensor.

SUMMARY

One aspect of the invention relates to a method, preferably a microprocessor controlled method, for carrying out a calibration test on a pressure sensor the method comprising the steps:fluidly connecting the pressure sensor to a first fluid storage vessel;fluidly connecting the pressure sensor to a second fluid storage vessel;providing an initial fluid pressure p0at the pressure sensor, the first fluid storage vessel and the second fluid storage vessel;carrying out a pressure measurement of the initial fluid pressure p0at a time t0by means of the pressure sensor;fluidly disconnecting the second fluid storage vessel from the pressure sensor and the first fluid storage vessel;providing a first fluid pressure p1at the second fluid storage vessel;carrying out a pressure measurement of the first fluid pressure p1at a time t1by means of the pressure sensor;fluidly connecting the second fluid storage vessel with the pressure sensor and the first fluid storage vessel, so that a second fluid pressure p2is provided at the pressure sensor, wherein the second fluid pressure is between the initial fluid pressure p0and the first fluid pressure p1;carrying out a pressure measurement of the second fluid pressure p2at a time t2by means of the pressure sensor.

The method can be carried by means of a microprocessor or computer controlling appropriate means for connecting or disconnecting fluid connections between the different elements, such as the first and second fluid storage vessels. Therefore, the result of the method is reproducable.

The term pressure sensor describes all means, which are capable to measure a hydrostatic or pneumatic pressure or a force applied to the sensor. The pressure sensor may use the piezo-electric effect, the measurement of strain or stress of an elastic material or other parameter which vary under the influence of the pressure.

The first and second fluid storage vessels can be any fluid tight container connectable to the pressure sensor in a fluid tight manner. However, it is preferred that the vessels have a constant volume also under pressure conditions, such as pressures of greater than about 200 kPa, preferrably greater than about 500 kPa. It is further desirable that the vessels are chemically and physically inert with respect to the fluid to be stored in the vessels. Thus, the vessels may be made of stainless steel, polymers or glass.

The providing of the initial fluid pressure p0can be performed by a fluid source to provide an initial pressure higher than the atmosphere pressure, a fluid drain to provide an initial pressure lower than the atmosphere pressure or simply by a fluid connection to the atmosphere in order to provide an initial pressure, which is equal to the atmosphere pressure.

The readings or results of the pressure measurements can be stored together with the belonging time of the measurement in order to keep proper records of the verification or calibration. Furthermore, it can be enforced that the measurement of the first pressure p1is performed after a sufficient stabilization time t1−t0has been passed by. Correspondingly, the stabilization time t2−t1might be equal to the stabilization time t1−t0.

This stabilization time might be long enough to establish equilibrium conditions with respect to pressure and/or fluid temperature in all volumes fluidly connected to the pressure sensor. Thus, the stabilization time might be longer than 1 second, preferably longer than 10 second, and even more preferably longer than 30 seconds. In order to reduce the overall time of testing the stabilization time might be less than 5 minutes and preferably less than 2 minutes.

The measurements of the pressures p0, p1, and p2can be used for a verification or calibration of the pressure sensor without a calibrated reference pressure sensor. Therefore, there is no need to keep a reference pressure sensor calibrated in a stand-by condition, in case the pressure sensor has to be verified.

The calibration of the pressure sensor is exact, since the differences between the measured pressures p0, p1, and p2are only dependent on the volumes of the vessels and the volume of the fluid connection or fluid lines connecting the vessels and the pressure sensor. These volumes can be determined with a high precision and, thus, the verification or calibration is reliable.

The relation between the volumes and the pressure is given by the ideal gas law under the assumption that the fluid within the volumes can be treated as an ideal gas. The ideal gas law can be written as:
pV/T=nR=constant.  (1)

Assuming that the volumes fluidly connected to the pressure sensor, such as the internal volumes of the vessels and the respective fluid connections, are fluid tight, the number of fluid molecules n remains constant. The parameter R is also a constant factor. In case the changes in temperature T during the testing procedure are insignificant, the ideal gas law can be simplified to read:
pV=constant.  (2)

During the test method the system of the first and second vessels and the pressure sensor are transformed into a first condition, wherein the second vessel with a volume V2is under the initial pressure of p0and the first vessel having a volume V1is under the first pressure of p1. In terms of the ideal gas law and neglecting the volumes of the connecting fluid lines, which can be insignificant small compared to the volumes of the first and second vessels or integrated in the value of each volume, the first condition of the test device can be written in term of the left side of equation (3) below:
p1V1+p0V2=const.=p2(V1+V2)  (3)

After fluidly connecting the first and second vessels, the pressure in these vessels becomes an intermediate second pressure p2within an interval between the initial fluid pressure p0and the first pressure p1, which can be exactly determined using the right side of equation (3) above. Thus, the three pressures (initial fluid pressure p0, first fluid pressure p1, and second fluid pressure p2) measured in the test method are related to each other by an exact relationship dependent on the given volumes of the vessels. Any mismatch of the measured pressures which is beyond an acceptable tolerance can be an indication that the pressure sensor does not provide reliable pressure readings and should, therefore, be replaced or calibrated by external means.

As an option, the method comprises the further step of:determining the difference Δp2between the measured second fluid pressure p2and the predicted second fluid pressure <p2> predicted by means of above formula (2).

Under the assumption that the fluid in the storage vessels and the interconnecting fluid lines is an ideal gas and the changes in temperature during the test procedure are insignificant, the following equation (4) can be used to predict the second fluid pressure:
<p2>=(p1V1+p0V2)/(V1+V2),  (4)
with p0≦<p2>≦p1and wherein <p2> is the predicted fluid pressure belonging to the measured second fluid pressure p2at the time t2, when the fluid pressure in the storage vessels is in an equilibrium.

In the special case that the volumes of the first and second storage vessels are identical, i.e. V1=V2, the predicted fluid pressure <p2> is the mean value of the pressures p0and p1, i.e. <p2>=(p1+p0)/2.

Optionally, the method comprises the further step of determining whether the difference Δp2is within a predefined limit ε.

Particularly, it can be determined, whether the difference between the predicted and the measured value of the second fluid pressure is smaller than 1000 Pa (10 mbar) or smaller than 500 Pa (5 mbar). In case the difference is greater than the given acceptable value, a warning signal can be generated. The warning signal might be an acoustical and/or an optical signal to inform a user about the pressure sensor failing the verification test.

As an option, the method further comprises the steps of:providing a third fluid storage vessel having a volume V3and being initially at the initial fluid pressure p0;fluidly connecting the pressure sensor, the first fluid storage vessel and/or the second fluid storage vessel to the third fluid storage vessel, so that a third fluid pressure p3is provided at the pressure sensor, wherein the third fluid pressure p3is between the initial fluid pressure p0and the second fluid pressure p2;carrying out a pressure measurement of the third fluid pressure p3at a time t3by means of the pressure sensor.

In analogy to formula (4) the third fluid pressure <p3> can be predicted by the equation
<p3>=(p2(V1+V2)+p0V3)/(V1+V2+V3),  (5)
with p0≦p3≦p2. The prediction is valid in case the fluid is an ideal gas and for a time t3, when the fluid pressure in the storage vessels is in equilibrium.

Optionally, the method further comprises the steps of:iteratively performing the following method steps subsequently for all natural numbers n being within the interval 4≦n≦N and N being a natural number of maximum iterations:providing a n-th fluid storage vessel having a volume Vnand being initially at the initial fluid pressure p0;fluidly connecting the pressure sensor and the first to n-th fluid storage vessel to the n-th fluid storage vessel, so that a n-th fluid pressure is provided at the pressure sensor, wherein the n-th fluid pressure pnis between the initial fluid pressure p0and the (n−1)-th fluid pressure p(n−1);carrying out a pressure measurement of the n-th fluid pressure pnat a time t0by means of the pressure sensor.

Correspondingly, the n-th fluid pressure <pn> can be predicted by the equation
<pn=(p0Vn+p(n−1)Σi=1n−1Vi)/Σi=1nVi(6)
with p0≦pn≦p(n−1). Again the prediction is valid in case the fluid is an ideal gas and for a time tn, when the fluid pressure in the storage vessels is in equilibrium.

Particularly, a norm of the differences between the plurality of predicted and measured values p2, p3, p4and so on can be calculated, such as the L2-norm or the L1-Norm, particularly when considering the pressure measurements and predictions each as contained in a vector.

As an option, the method may comprise the steps of:fluidly disconnecting the pressure sensor and the first fluid storage vessel from the second fluid storage vessel;providing again the initial fluid pressure p0at the pressure sensor and the first fluid storage vessel;fluidly connecting the second fluid storage vessel with the pressure sensor and the first fluid storage vessel, so that a third fluid pressure p3is provided at the pressure sensor, wherein the third fluid pressure p3is between the initial fluid pressure p0and the second fluid pressure p2;carrying out a pressure measurement of the third fluid pressure p3at a time t3by means of the pressure sensor (3).

Using this option the third pressure p3at time t3can be predicted using the equation below:
<p3>=(p2V2+p0V1)/(V1+V2),  (7)
with p0≦p3≦p2and wherein <p3> is the predicted fluid pressure at a time t3, when the fluid pressure in the storage vessels is in an equilibrium.

The above described steps can be reiterated in order to measure a fourth, fifth, sixth or subsequent fluid pressure and calculate the corresponding pressures. The verification, whether the pressure sensor is reliable, can be performed by calculating differences between single pairs of measured and predicted fluid pressures or using a norm, such as the L2-norm or the L1-Norm, applied to a plurality of corresponding pairs of measured and predicted fluid pressures.

Optionally, the method can comprise the calculation of a linear regression of the measured pressure values versus the predicted pressure values in order to determine a calibration function.

Linear regressions are well-known in data analysis. The linear regression results in a linear function of measurable fluid pressure values depending on the real fluid pressure. In case of a perfect match the regression coefficient will be 1.00. However, a regression coefficient lower than a predetermined value, for example 0.95 or 0.9, might indicate that the linear regressions will not work properly. In this case, a warning might be generated that the pressure sensor is not reliable.

Another aspect of the invention relates to a test device for automatically carrying out a calibration test on a pressure sensor, the test apparatus comprising:a fluid line which is fluidly connected to the pressure sensor to be tested via a pressure sensor valve;a first fluid connector configured to fluidly connecting a first fluid storage vessel to the fluid line;a vessel valve configured to fluidly connect or disconnect the second fluid connector from the pressure sensor;a fluid outlet configured to release fluid from the fluid line via an outlet valve;a fluid inlet configured to fluidly connecting a fluid source to the fluid line via an inlet valve;a control unit configured to perform automatically the following control steps:

closing the inlet valve;opening the vessel valve to fluidly connect the pressure sensor to the second fluid connector;opening the outlet valve to provide atmosphere pressure at the pressure sensor, the first fluid connector and the second fluid connector;performing a pressure measurement of the atmosphere pressure as the initial fluid pressure p0at a time t0by means of the pressure sensor;closing the outlet valve and the vessel valve;opening the inlet valve and providing a fluid having a first fluid pressure p1at the first fluid connector and the pressure sensor;closing the inlet valve;carrying out a pressure measurement of the first fluid pressure p1at a time t1by means of the pressure sensor;opening the vessel valve in order to fluidly connecting the second fluid connector with the pressure sensor and the first fluid connector, so that a second fluid pressure p2is provided at the pressure sensor;carrying out a pressure measurement of the second fluid pressure p2at a time t2by means of the pressure sensor.

Preferably, the test device further comprises:a first fluid storage vessel fluidly connected to the first fluid connector and/ora second fluid storage vessel fluidly connected to the second fluid connector.

Preferably, the test device further comprises:

calculation means, which are configured to determine the difference □p2between the measured second fluid pressure p2and a predicted second fluid pressure <p2> predicted using the formula
<p2>=(p1V1+p0V2)/(V1+V2),
wherein V1is the volume of the first fluid storage vessel, which may include the volume of the first fluid lines if not neglectable, and wherein V2is the volume of the second fluid storage vessel, which may include the volume of the second fluid lines if not neglectable.

Preferably, the test device further comprises:a third fluid connector configured to fluidly connecting a third fluid storage vessel to the fluid line;a second vessel valve configured to fluidly connect or disconnect the third fluid connector with the pressure sensor;

Preferably, the test device further comprises:an information system for retrieving values of the atmospheric pressure at the location of the test device and/ora location determining means for determining the location of the location of the test device.

A further aspect of the invention relates to a computer program product for a computer-controlled verification or calibration test on a test device, wherein the computer program comprises coding segments that when loaded and executed on a suitable system, preferably the control means of the test device according to claims10to14, can execute a method for carrying out a calibration test on a pressure sensor according to any one of claims1to9.

Additional objects, advantages and features of the present invention will now be described in greater detail, by way of example, with reference to preferred embodiments depicted in the drawings.

DETAILED DESCRIPTION

FIGS. 1 to 7show a test device1for carrying out an automatic verification and/or calibration of a pressure sensor3. The test device1can be part of a test apparatus, for example a test apparatus for performing an integrity test on a filter device (not shown) or any other apparatus, which must comprise a pressure sensor3in order to measure a fluid pressure. Therefore, the test device1may be located in a housing5, which further comprises additional electric, electronic, mechanic and/or electromechanic components, which are not intended to perform the verification and/or calibration of the pressure sensor3, but are provided to perform further tasks needing the pressure sensor3.

The test device1comprises a fluid inlet7and a fluid outlet9. The fluid inlet7and/or the fluid outlet9can be formed as a fluid connector in the housing5. The fluid inlet7is preferably configured to be connected to a fluid source (not shown). For example the fluid inlet7can be fluidly connected to an external pressure vessel, which may contain a pressurized fluid. A fluid in the sense of the invention may comprise a gas, a liquid or a mixture thereof. As an alternative, the fluid source may comprise a fluid compressor, which is configured to provide the fluid with a predetermined fluid pressure at the fluid inlet7. Although it may be preferred to provide an external fluid source, which is fluidly connectable to the fluid inlet7, in order to allow an easy maintenance of the fluid source, it has to be understood that the fluid source can also be an internal fluid source, which is located within the housing5of the test device1.

Particularly, the fluid source may provide a gas, preferably a sterile gas, such as compressed air, nitrogen, carbon dioxide and so on.

The pressure sensor3is fluidly connected to the fluid inlet7by a fluid line11. Fluid line11establishes also a fluid connection between the pressure sensor3and the fluid outlet9. The fluid line11further comprises an inlet valve13associated with the fluid inlet7and an outlet valve15associated with the fluid outlet9. By means of the inlet valve13and the outlet valve15the pressure sensor3can be fluidly connected independently with the fluid inlet7and the fluid outlet9.

Furthermore, the test device1comprises a first fluid storage vessel17, which is connected to the fluid line11by means of a first fluid connector19. The first fluid connector19may be formed integrally with the first fluid storage vessel17. A second fluid storage vessel21is connected to the fluid line11by means of a second fluid connector23. Again, the second fluid connector23can be formed integrally with a second fluid storage vessel21. By means of fluid line11the first and second fluid storage vessels17,21are fluidly connectable with the pressure sensor3. The embodiment shown inFIGS. 1 to 7comprises a vessel valve25, which is arranged within the fluid line11so as to fluidly connect or disconnect the second fluid storage vessel21with or from the pressure sensor3and the first fluid storage vessel17. The first fluid storage vessel17is permanently fluidly connected with the pressure sensor3, since there is no valve arranged in the fluid line11between the first fluid connector19and pressure sensor3in the embodiments shown inFIGS. 1 to 7. It has to be understood that a further vessel valve could be placed in fluid line11so as to connect or disconnect the first fluid storage vessel17from pressure sensor3. However, for the method to verify or calibrate pressure sensor3described below with respect to the embodiment of the test device1shown inFIGS. 1 to 7this additional vessel valve is not required, but just an option.

The test device1further comprises a control unit27as preferred control means27, which is configured to control the inlet valve13, the outlet valve15and the vessel valve25. Therefore, the control unit27is connected to the inlet valve13, the outlet valve15as well as the vessel valve25in order to switch the state of each of the valves electrically, pneumatically or hydraulically. The connections between the control unit27and the valves13,15,25, such as electrical wires or additional control fluid lines, are not shown in the figures. Furthermore, control unit27is connected to pressure sensor3in order to initiate a pressure measurement and to read and store the measured pressure value within the control unit27. The connection between the pressure sensor3and a control unit27is also not shown in the figures.

In order to perform the verification and/or calibration of the pressure sensor3automatically, control unit27is provided with a microprocessor29and storage means31. Additionally, control unit27may be provided with a communication means, which is configured to establish a communication connection to an external apparatus outside from the test device1. The external apparatus might be a computer system or a display device. The communication means33may be configured to establish the communication link wireless or by cable. In particular, the communication means33can comprise an USB interface, an ethernet interface, a bluetooth interface, a WLAN interface, any other parallel or serial interface, or an optical interface. The storage means31may comprise a read only memory (ROM), a random access memory (RAM), an erasable programmable read only memory (EPROM), a hard disk, a memory card, such as an SD card, a CD drive, a floppy drive and so on.

FIG. 1shows the test device in an initial state, wherein the inlet valve13is closed and the outlet valve15and the vessel valve25are opened. Closed valves are indicated in the figures by a black filling of a contour. In the initial state, as shown inFIG. 1, the first fluid storage vessel17, the second fluid storage vessel21, fluid line11and pressure sensor3are fluidly connected via the outlet valve15with the atmosphere, such that atmosphere pressure conditions are provided in the fluid storage vessels17,21and the pressure sensor3. The pressure in fluid line11can be measured by means of the pressure sensor3. The reading of pressure sensor3can be stored in storage means31of control unit27. Furthermore, the time of the reading to can also be stored together with pressure reading p0in control unit27.

Additionally the storage vessel21can be equipped with a pressure sensor (not shown in the figures) which is used to verify that the storage vessel21is at atmospheric pressure i.e. that the vessel valve25works correctly and that there is no residual pressure. The additional pressure sensor is typically of lower accuracy and does not interact in the verification of the pressure sensor3. It is only provided to verify that the valves are working correctly and that no residual pressure is in the system, which would give incorrect pressure verification and calibration results.

After the initial pressure measurement has been carried out, the test device1is brought into the configuration as shown inFIG. 2. As shown inFIG. 2, the vessel valve25is closed in order to fluidly disconnecting the second fluid storage vessel21from the pressure sensor3and first fluid storage vessel17. The outlet valve15is also closed in order to disconnect fluid line11from the outside. After the outlet valve15is closed the inlet valve13is opened so that fluid line11is fluidly connected to the fluid source connected to fluid inlet7. By means of the external fluid source a first fluid pressure p1is provided at pressure sensor3and the first fluid storage vessel17. After the pressure conditions in fluid line11, first fluid storage vessel17and pressure sensor3are brought into an equilibrium with the pressure provided by the external pressure source connected to fluid inlet7the inlet valve13is closed to disconnect fluid line11from the external fluid source.

After closing off the inlet valve13the test device1is in a configuration as shown inFIG. 3. In this configuration the volumes of the first fluid storage vessel17, a part11aof the fluid line11, which is in this configuration fluidly connected to the first fluid storage vessel17, and pressure sensor3connected thereto are filled with the fluid at a pressure p1. The pressure p1can be measured by means of pressure sensor3, wherein the reading of pressure sensor3and the time t1at which the pressure measurement is carried out can be stored in storage means31of control inlet27. After the pressure measurement has been carried out the valve vessel25is opened so that the test device1is in a configuration as shown inFIG. 4.

After the opening of vessel valve25the pressure conditions in the first fluid storage vessel17and the second fluid storage vessel21will equalize. Since the pressure p1and the first fluid storage vessel17is generally higher than the pressure p0, i.e. the atmospheric pressure, in the second fluid storage vessel21, fluid will flow from the first fluid storage vessel17via vessel valve25to the second fluid storage vessel21. After the stabilization time the fluid pressure within the first and second fluid storage vessels17,21and the fluid line11will be at an equilibrium. This pressure p2can be measured by means of pressure sensor3and stored together with the time t2of the pressure measurement in storage means31of control unit27. Pressure p2is generally larger than the initial pressure p0and smaller than the pressure p1.

It is also understood that the possible additional pressure sensor (not shown) in vessel21can be used to verify that the valve vessel25was correctly opened. The pressure measured by this additional sensor (not shown) is not used in order to calibrate the pressure sensor3.

Under the assumption that the fluid is an ideal gas and the temperature remains constant during the test, the pressure p2can be predicted using the ideal gas law. In case the temperature of a fluid is not constant during the test procedure, a temperature sensor can be provided additionally to the pressure sensor3or in any of the vessels or in all vessels in order to measure the fluid temperature. The influence of the temperature can also be considered by using the ideal gas law.

In order to enhance the result of the test procedure further pressure measurements can be carried out. To do so the vessel valve25is closed and the outlet valve15is opened so that the test device is in a configuration as shown inFIG. 5. While the fluid in the second fluid storage vessel21remains at a pressure p2, fluid can be discharged from the first fluid storage vessel17and the fluid line11through the fluid outlet9until atmospheric pressure conditions are present in the first fluid vessel17and fluid line11. After a predetermined time of discharging fluid outlet valve15is closed. Optionally a pressure measurement can be carried out by means of pressure sensor3in order to verify that atmospheric pressure conditions are present in the volumes of the first fluid storage vessel17and the part of fluid line11connected thereto. The reading of pressure sensor3should correspond to the pressure p0measured at the time t0as measured in the initial state of test device1, as shown inFIG. 1.

By opening the vessel valve25the test device is brought into a configuration as shown inFIG. 7. Again, the fluid pressure in the first and second fluid storage vessels17,21and fluid line11will equalize to a pressure p3, which is larger than the atmospheric pressure and smaller than pressure p2. After a predetermined time of stabilization a further pressure measurement can be carried out by means of pressure sensor3. The pressure p3measured at the pressure sensor3can be stored together with a time of measurement t3in the storage means31of control unit27.

Alternatively or additionally vessel21can also be equipped with a vent valve (not shown in the figures). This is preferable when the vessel21is greatly smaller than the vessel17. Maintaining p2within vessel17and bringing vessel21to atmospheric pressure, followed by closing the vent valve (not shown) followed by opening the vessel valve25to create p3would generate a pressure value p3which is closer to the pressure value p2, thus allowing for more pressure verification points with smaller pressure differences: p0<<p3<p2.

FIGS. 8 to 12show a further embodiment of a test device1for carrying out an automatic verification and/or calibration of pressure sensor3. As described with regard to the embodiment shows inFIGS. 1 to 7, the test device1can be part of a test apparatus, for example a test apparatus for performing an integrity test on a filter device (not shown) or any other apparatus, which comprises a pressure sensor3in order to measure a fluid pressure. The elements of the test device1shown inFIGS. 8 to 12, which are identical to the elements of the embodiment shown inFIGS. 1 to 7, are labelled with identical reference signs and the description of these elements with regard toFIGS. 1 to 7applies mutatis mutandis to the elements shown inFIGS. 8 to 12.

Particularly, the test device1comprises a housing5, a fluid inlet7and a fluid outlet9, wherein the fluid inlet7and/or the fluid outlet9can be formed as a fluid connector. As an option the fluid inlet7and/or the fluid outlet9can be formed integrally with the housing5. The fluid inlet7can be configured to be connected to an external fluid source or to an internal fluid source. In case the fluid source is an internal fluid source, the fluid inlet7may be formed as an internal fluid connection or fluid connector between fluid line11and the fluid source. The fluid source may configured to provide a gas, preferably a sterile gas, such as compressed air, nitrogen, carbon dioxide and so on, a liquid, such as water, sterile water, alcohol and so on or a mixture thereof.

Pressure sensor3is fluidly connected to the fluid inlet7and the fluid outlet9by fluid line11. An inlet valve13is associated with the fluid inlet7, i.e. placed within the fluid line11between fluid inlet7and pressure sensor3, and an outlet valve15is associated with the fluid outlet9, i.e. placed within the fluid line11between fluid outlet9and pressure sensor3. By means of the inlet valve13and the outlet valve15the pressure sensor3can be independently fluidly connected with or disconnected from the fluid inlet7and the fluid outlet9.

The embodiment of the test device1shown inFIGS. 8 to 12comprises a first fluid connector19, a second fluid connector23, and a third fluid connector35. It has to be understood that the test device1may also comprise one or more further fluid connector(s). The fluid connectors19,23, and35are configured to be connectable to a corresponding fluid storage vessel. The embodiment shown inFIGS. 8 to 12comprises three fluid storage vessels17,21, and37. As an option at least one of these fluid storage vessels can be an external fluid storage vessel. In this case the corresponding one of the fluid connectors19,23, and35may be configured to connect the external fluid storage vessel outside housing5with the fluid line11inside the housing5. Preferably, one or more of the fluid connectors19,23, and35may be formed integrally with the housing5.

As an alternative any one of the fluid connectors19,23,35may be formed integrally with the corresponding one of the fluid storage vessels17,21,37. Particularly, test device1may comprise any one of the fluid storage vessels17,21,37as an internal fluid storage vessel. In other words, each of the plurality of fluid storage vessels, particularly all fluid storage vessels may be contained within the housing5of the test device1.

By means of fluid line11the each of the fluid storage vessels17,21,37is fluidly connectable to the pressure sensor3. The embodiment shown inFIGS. 8 to 12comprises a vessel valve25, which is arranged within the fluid line11so as to fluidly connect or disconnect the second fluid storage vessel21with or from the pressure sensor3and the first fluid storage vessel17. Furthermore, a second vessel valve39is arranged and configured within the fluid line11so as to fluidly connect or disconnect the third fluid storage vessel37with or from pressure sensor3, first fluid storage vessel17, and second fluid storage vessel21. In contrast, the first fluid storage vessel17is permanently fluidly connected with pressure sensor3.

The control unit27of test device1is configured to control inlet valve13, outlet valve15, vessel valve25, and second vessel valve39. As described with respect to the embodiment shown inFIGS. 1 to 7, the control unit27is connected to inlet valve13, outlet valve15, vessel valve25as well as the second vessel valve39in order to switch the state of each of the valves. Control unit27is also connected to pressure sensor3in order to initiate a pressure measurement and to read and store the measured pressure value.

FIG. 8shows the test device1in an initial state, wherein the inlet valve13is closed and outlet valve15, vessel valve25, and second vessel valve39are opened. Closed valves are indicated in the figures by a black filling of the contour of the respective valve. In the initial state, as shown inFIG. 8, the all fluid storage vessels17,21,37, fluid line11and pressure sensor3are fluidly connected via the outlet valve15with the atmosphere, such that atmosphere pressure conditions are provided in these elements. The pressure in fluid line11, and therefore in the fluid storage vessels17,21,37fluidly connected thereto, can be measured by means of pressure sensor3. The reading of pressure sensor3can be stored in storage means31of control unit27. Furthermore, the time of the reading t0can also be stored together with pressure reading p0in control unit27.

Additionally or additionally the storage vessels21and37can be equipped with pressure sensors (not shown in the figures) which are used to verify that the storage vessel21and37are at atmospheric pressure i.e. that the vessel valve25and39work correctly and that there is no residual pressure. The additional pressure sensors of typically lower accuracy do not interact in the verification of the pressure sensor3. They are only there to verify that the valves are working correctly and that no residual pressure is in the system which would give incorrect pressure verification and calibration results.

After the initial pressure measurement has been carried out, the test device1is brought into the configuration as shown inFIG. 9by closing vessel valve25, second vessel valve39and outlet valve15. Thus, the second and third fluid storage vessels21,37are fluidly disconnected from pressure sensor3and first fluid storage vessel17. Further, fluid line11is fluidly disconnected from the outside. After outlet valve15is closed inlet valve13is opened to fluidly connect fluid line11to the fluid source (not shown) connected to fluid inlet7. By means of the fluid source a first fluid pressure p1is provided at pressure sensor3and the first fluid storage vessel17. After the pressure conditions in fluid line11, first fluid storage vessel17and pressure sensor3are brought into an equilibrium with the pressure provided by the pressure source, i.e. after a stabilization time, inlet valve13is closed to disconnect fluid line11from the external fluid source, such that test device1is in a configuration as shown inFIG. 10.

In the configuration shown inFIG. 10a pressure p1is applied to the volumes of the first fluid storage vessel17, a part11aof the fluid line11which is in this configuration fluidly connected to the first fluid storage vessel17and pressure sensor3. The pressure p1can be measured by means of pressure sensor3, wherein the reading of pressure sensor3and the time t1at which the pressure measurement is carried out can be stored by storage means31of control inlet27. After the pressure measurement has been carried out the valve vessel25is opened so that the test device1is in a configuration as shown inFIG. 11.

After the opening of vessel valve25the pressure conditions in the first and second fluid storage vessel17,21will equalize. Since the pressure p1and the first fluid storage vessel17is generally higher than the pressure p0, i.e. the atmospheric pressure, in the second fluid storage vessel21, fluid will flow from the first fluid storage vessel17via vessel valve25to the second fluid storage vessel21. After a predetermined stabilization time the fluid pressure within the first and second fluid storage vessels17,21and parts11a,11bof the fluid line11fluidly connected thereto in this configuration will be at an equilibrium. This pressure p2can be measured by means of pressure sensor3and stored together with the time t2of the pressure measurement by storage means31of control unit27. Pressure p2is generally larger than the initial pressure p0and smaller than the pressure p1.

It is also understood that the possible additional pressure sensor (not shown) in vessel21can be used to verify that the valve vessel25was correctly opened. The pressure measured by this additional sensor (not shown) is not used in order to calibrate the pressure sensor3.

Additional pressure measurements can be carried out for fluid pressures which are lower than pressure p2by performing the following steps. Second vessel valve39is opened transforming test device1into the configuration shown inFIG. 12. Fluid will flow from the first and second fluid storage vessels17,21via the second vessel valve39to the third fluid storage vessel37. After a predetermined stabilization time the fluid pressure within the first, second, and third fluid storage vessels17,21,37and fluid line11will be at an equilibrium. This pressure p3can be measured by means of pressure sensor3and stored together with the time t3of the pressure measurement by storage means31of control unit27. Pressure p3is generally larger than the initial pressure p0and smaller than intermediate pressure p2.

It is also understood that the possible additional pressure sensor (not shown) in vessel37can be used to verify that the valve vessel39was correctly opened. The pressure measured by this additional sensor is not used in order to calibrate the pressure sensor3.

In order to perform a verification or a calibration of pressure sensor3, the readings of pressure sensor3can be compared to predicted or computed pressure value, which are used as a reference. As described with regard to the embodiments shown inFIGS. 1 to 12, the atmospheric pressure can be used as a reference. Generally, the atmospheric pressure can be assumed to be in a range of about 950 hPa to about 1050 hPa depending on the actual weather conditions and the elevation over sea level of the location where the measurements are carried out. An exact verification of the initial pressure p0is possible in case the real atmospheric pressure <p0> at the time of the initial pressure measurement to is known, for example by a measurement by means of a reference pressure sensor. However, the atmospheric pressure <p0> used as the reference can also be estimated with a sufficient precision using other informations so that a second reference pressure sensor can be omitted.

As shown inFIG. 13the test device1may be connected via communication means33with an information system41, which is capable to provide actual weather data comprising the actual atmospheric pressure data at locations around the location of the test device1. The information system41can comprise means for establishing an internet connection for connecting to one or more servers providing weather information. Furthermore, the information system41can comprise means for connecting to one or more external devices for measuring the atmospheric pressure at or near the location of test device1or at sea level. If required the information system41can also be connected to means for gathering information about actual overpressure or underpressure conditions at the location of the test device1, for example overpressure or underpressure conditions caused by air conditioning, vacuum devices, fans, and so on. Additionally, the test device1can comprise or be connected to location determining means43, such as a GPS or Galileo receiver, in order to determine its own location in terms of latitude, longitude and/or altitude in order to predict the actual atmospheric pressure. For example can the latitude and longitude determined by means of the location determining means43be used to retrieve the corresponding atmospheric pressure value <p0> via the information system41. Furthermore, the altitude determined by means of the location determining means43can be used to compute the atmospheric pressure value <p0> using the barometric formula based on a known atmospheric pressure at sea level.

As an option the information system41may comprise a database containing the atmospheric pressure values at a specific location at a specific time. As shown inFIG. 14the atmospheric pressure values could be collected from pressure measurement devices42around the world. A test device1could connect to the database of the information system41in order to retrieve the recent atmospheric pressure reading of the closest pressure measurement device42. The interface between the database and the test device1may be realized by means of a website. Particularly, the interface may provide the data as human readable output. The test device1may retrieve the recent atmospheric pressure values as well as the time of measurement. In case the time of measurement of the most recent atmospheric pressure values is longer than 10 minutes, 1 hour, 6 hours or longer ago, the test device1may forward a notice to the user that there is no actual atmospheric pressure data available so that the calibration of the test device1is carried out at a later time, when an actual value of the atmospheric pressure is available.

Furthermore, the test device1may retrieve multiple recent atmospheric pressure values of the nearest pressure measurement location and/or the most recent atmospheric pressure values from different locations in an area around the location of the test device1. The test device1may calculate whether the atmospheric pressure is changing in short time at one measurement location and/or whether the atmospheric pressure is changing within the area around the location of the test device1. In each case the test device1may generate a warning notice to the user that the calibration of the test device1might be unreliable due to the uncertainty of the atmospheric pressure value used for calibration. E.g. such a warning notice might be generated in case the atmospheric pressure is changing for more than 5 mbar per hour at a specific location of the pressure measurement device. Moreover, a warning notice might be generated in case the atmospheric pressure is changing for more than 5 mbar within an area having a radius of 5 kilometers from the location of the test device1.

As an option the warning message might alternatively or additionally be generated by the information system41. The information system41may determine, whether the atmospheric pressure conditions are too inhomogeneous (high variation depending on location) or too instable (high variation depending on time) to perform a calibration of a test device1is a specific area. This warning might be transmitted to the test device1.

The information system41might also provide a time reference for the test device1. In other words, the test device1might synchronize its internal clock with the information system41.

FIGS. 15 and 16show diagrams, wherein the horizontal axis represents the real fluid pressure applied to the pressure sensor3, as predicted using the above discussed formulas, and the vertical axis represents the readings of the pressure sensor3. The 45 degree axis drawn as a solid line in the diagram represents the ideal line, wherein the measured pressures exactly match the real pressures. However, a more or less larger deviation from the ideal line has to expected in a real test device caused by a plurality of different measurement errors.

InFIG. 15the measured first and second fluid pressures p1, p2and the corresponding predicted first and second fluid pressures <p1>, <p2> exactly define a regression line, which is drawn as dashed line. The regression line intersects the junction of the horizontal axis and vertical axis, i.e. the point with the coordinates (p0, <p0>). In other words, the pressure sensor3in this case is capable to exactly measure the initial fluid pressure p0. However, the slope of the regression line deviates from the ideal line indicating that pressure differences are not well measured by the pressure sensor. In fact, as shown inFIG. 15, the difference between the measured fluid pressures p1, p2are larger than the real first and second fluid pressures <p1>, <p2>, and thus, the regression line is located above the ideal line.

As the regression line is defined by only two pairs of measured and predicted fluid pressure values p1, <p1>, and p2, <p2> the correlation coefficient of the regression is exactly 1.0 and cannot be used to quantify the quality of the correlation or matching between measured and predicted fluid pressures, and thus the quality of the pressure sensor. However, the regression can also be based on three, four or more pairs of measured and predicted fluid pressure values.

FIG. 16shows the case, wherein an offset error occurs in the measurement of the pressure sensor3. While the slope of the regression line matches the slope of the ideal line, the regression line insects the junction of the and vertical axis at a value greater than zero. In the case shown inFIG. 16, the pressure sensor will provide correct readings of pressure differences, but all readings of the fluid pressure will be too high by a constant amount.

The regressions lines shown inFIGS. 15 and 16can be used as a calibration function in order to correct measurement of the pressure sensor3and, thus, to obtain more precise pressure measurements.

LIST OF REFERENCE SIGNS