WO2006067371A1

WO2006067371A1 - A method and apparatus for checking in situ the calibration of a device for measuring the value of a variable property

Info

Publication number: WO2006067371A1
Application number: PCT/GB2005/004666
Authority: WO
Inventors: Andrew J. Coleman
Original assignee: Guy's & St. Thomas' Hospital Nhs Trust
Priority date: 2004-12-21
Filing date: 2005-12-05
Publication date: 2006-06-29
Also published as: GB2437023B; GB0427993D0; GB2437023A; GB0714159D0

Abstract

A method of checking in situ the calibration of a plurality of measuring instruments uses a calibration verification device (20) to measure a trial output of an attached instrument (10). This is repeated for other instruments, and all the information forwarded to a remote server (40) via the Internet. By repeating the calibration of the same instrument at different times, a calibration history record for each of the instruments under test can be built up.

Description

A method and apparatus for checking in situ the calibration of a device for measuring the value of a variable property.

The present invention relates to a method and apparatus for checking in situ the calibration of a device for measuring the value of a variable property. The variable property may be any property that varies with time, but in a particular embodiment the invention relates to a method and apparatus for checking the calibration of a pressure measuring device such, as, for example, a sphygmomanometer.

A sphygmomanometer is an instrument that is widely used in clinical or veterinary practice for measuring the blood pressure of a human or animal patient. A sphygmomanometer typically comprises an inflatable cuff that is configured to be placed around part of a limb of the patient's body, and a bulb for inflating the cuff in order to stop temporarily the flow of blood within the part of the limb that is surrounded by the cuff. The sphygmomanometer further comprises a pressure measuring gauge for measuring the pneumatic pressure in the cuff. By gradually releasing the pressure applied by the cuff after the cessation of blood flow, a physician or veterinary surgeon can measure the patient's systolic and/or diastolic blood pressure as blood flow recommences.

The diagnosis, monitoring and treatment of patients with hypertension, coronary heart disease and cerebrovascular disease are key concerns in primary care, and are reliant on accurate blood pressure (BP) measurement. It is widely recommended that sphygmomanometers, used to measure BP, are maintained and calibrated regularly to ensure that the pressure scale remains accurate to within the European Standard specification of +3mmHg. Such checks are increasingly important as mercury sphygmomanometers, favoured by the medical profession for over a century, are decommissioned and substituted by an increasingly diverse range of automated and aneroid devices. In the UK, for instance, there are now an estimate 40 companies actively involved in the supply of 90 different models of automated oscillometric sphygmomanometers, many of uncertain provenance or quality. The diversity of new devices has raised concern about discrepancies in BP readings from different instruments.

A wide range of studies have demonstrated significant inaccuracies in the pressure scale of mercury and aneroid sphygmomanometers. One study, for example, covering 231 general practices in England, found 10% of all sphygmomanometers reading in error by more than 5mmHg, with some aneroid devices reading in error by as much as 30mmHg. It is relatively simple to check the pressure scale of sphygmomanometers, so that inaccurate devices can be removed from use or recalibrated. In the hospital setting, for example, it has been shown to be possible to achieve adequate accuracy in aneroid devices of 3- 4mmHg over long periods by checking devices annually against a reference device. In primary care, however, such checks are not widely performed.

Conventionally, a sphygmomanometer must be sent back to its manufacturer, or to an approved service engineer for re-calibration. This is inconvenient and, moreover, a sphygmomanometer that has been sent away for re-calibration is unavailable for use. hi inevitably, many sphygmomanometers are sent away to have their calibration re-checked, whilst in fact their calibration remains satisfactory within the pre-determined acceptable tolerance range mentioned above.

Accordingly, there is a need to provide an improved method of checking the calibration of sphygmomanometers and other measuring devices, which avoids the need to return such instruments to their manufacturers or other approved service engineers.

US-A-6558321 (Burd, et al.) discloses a method of remotely calibrating a receiving device that is adapted to receive subject information from a medical device, by transmitting said subject information to a central monitoring system where it is processed to generate manipulated information, the manipulated information then being returned from the central monitoring system to the receiving device for calibrating said receiving device. In particular, the medical device may comprise a device for measuring the concentration of an analyte such as glucose or prothrombin in biological fluid for monitoring a medical condition such as diabetes. According to US-A-6558321, the calibration of the medical device must be checked periodically, for example at four-week intervals, and to effect calibration the central monitoring system requires calibration information to be provided from an independent calibration device for independently measuring the subject information. The subject information obtained by the medical device is then compared with the subject information obtained by the calibration device to check and, if necessary, re-calibrate the receiving device or the medical device. The method of US-A-6558321 thus requires the provision of independent calibration data at the location of the medical device.

According to the present invention there is provided: a method of checking in situ the calibration of a plurality of measuring instruments, the method comprising:

(a) attaching to a first measuring instrument be checked a calibration verification device;

(b) measuring on the device a trial output of the attached instrument when operated under defined conditions;

(c) repeating steps (a) and (b) for other measuring instruments to be checked and, either one-by-one as the measurements are made, or in consolidated from after all measurements have been made, transmitting the trial outputs as well as corresponding instrument identifiers to a remote location;

(d) at the remote location, storing the transmitted information in a data store; and (e) repeating steps (a) to (d) at a plurality of times thereby building up in the data store a calibration history record for each of the said plurality of measuring instruments.

The method of the present invention not only reduces down time for the instruments being checked, it also provides a series of consolidated auditable records for legal compliance purposes. Records may be collected over an extended period both in respect of each of the individual measuring instruments under test, and also in respect of the calibration verification devices that are in use.

Where the instruments being checked are medical instruments, for example sphygmomanometers, it is expected that all instruments of that type at a particular site, such as for example at an individual doctors' surgery, will be checked together using a single common calibration verification device (e.g. a pre-calibrated pressure sensor). Data from a plurality of sites, such as a large number of individual doctors' surgeries, may be consolidated together in a common data store held at a remote location. Typically, communication between the individual sites and the data store may be via the Internet.

The calibration verification device will include a sensor of a type which may very according to the type of medical or other measuring instruments to be checked. Typical sensors may include (without limitation) pressure sensors, temperature sensors, flow meters, volume measuring devices, weight sensors, distance measuring devices, and sensors for measuring electrical characteristics such as voltage, current, resistance, impedance, capacitance and so on.

The calibration verification device may include a pass/fail indicating system, which advises the user either aurally or visually whether the measuring instrument under test has passed or failed the check. The pass/fail boundary may be pre-programmed into the device according to any required legislative or desired limit.

The calibration verification device may itself need recalibrating or replacing at intervals. In the preferred embodiment, the device is sent off to a laboratory for recalibration once a year. During that time, of course, the measuring instruments under test can continue to be used in the normal way, without interruptions.

Following is a description, by way of example only, with reference to the accompanying drawings of embodiments of the present invention.

In the drawings:

FIG. 1 is a schematic diagram of a system in accordance with an embodiment of the present invention for checking the calibration of a pressure measuring device such, for example, a sphygmomanometer;

FIG. 2 is a schematic diagram of apparatus in accordance with another embodiment of the present invention for checking the calibration of a sphygmomanometer; and

FIGS 3 - 5 are flow diagrams illustrating operation of a preferred method of the present invention.

FIG. 1 shows a system for checking the calibration of a pressure measuring device in accordance with an embodiment of the present invention. In the embodiment shown, the pressure measuring device comprises a sphygmomanometer 10 comprising an inflatable cuff 11, a bulb 12 for inflating the cuff 11, and a pressure gauge 14. The bulb 12 and pressure gauge 14 are connected to the cuff 11 by a pneumatic line 13. The sphygmomanometer 10 shown schematically in FIG. 1 is of the kind in which the bulb 12 and pressure gauge 14 are generally incorporated in integral unit. However, the present invention can be used for checking the calibration of any kind of sphygmomanometer, including those in which the bulb and pressure gauge are connected separately to the cuff.

In order to check the calibration of said sphygmomanometer 10, a "T" connector 21 is inserted in the pneumatic line 13 for connection to an electronic pressure measuring device 20 having a first port 22. The pressure sensing device 20 is adapted to measure the pressure within the cuff 11 of the sphygmomanometer 10, and to output the measured pressure as a first digital signal via an output port 24. The device may also measure ambient atmospheric pressure via an open port 23, and may output this information via the output port 24. The device may also measure, and report upon, the ambient temperature at the time of measurement.

The output port 24 is connected to a computer 30 by a suitable interface such, for example, as a USB interface or an RS 232 serial port. The computer 30 may comprise for example, a desktop computer, or a notebook computer, as illustrated in FIG. 1.

The computer 30 is thus adapted to receive data from the pressure measuring device 20, which data includes the measured pressure of the cuff 11. The computer 30 is configured to run suitable application software to acquire information about the calibration of the sphygmomanometer 10. The application software may include a graphical user interface including instructions for a user.

In order to check the calibration of the sphygmomanometer 10 therefore the user may operate the bulb 12 to inflate the cuff 1 1 to a predetermined internal pressure according to the pressure gauge 14. The internal pressure in the cuff 11 is then measured accurately by the pressure measuring device 20, and the reading is transmitted from the pressure measuring device 20 to the computer 30, where it may be stored in association with the figure for the predetermined pressure. The error between the predetermined pressure and the measured pressure may be calculated by simple subtraction and, in some embodiments, may be displayed on the screen of the computer. The value of the error may also be stored in association with the predetermined pressure.

Advantageously, the application software may be configured to require the user to inflate the cuff 11 using the bulb 12 to a series of different predetermined pressures as measured according to the pressure gauge 14. At each predetermined pressure, the internal pressure of the cuff 11 may be measured accurately using the pressure measuring device 20 as described above, and the measured pressure value may be transmitted to the computer 30 and stored electronically in association with the respective predetermined pressure value. The error between the predetermined pressure value and the corresponding measured pressure value at each predetermined pressure value may be calculated by a simple subtraction and saved in association with the respective predetermined pressure value, and optionally displayed upon the screen of the computer 30. In this way, a calibration curve for the sphygmomanometer 10 may be generated, which calibration curve may be displayed graphically on the screen of the computer and/or may be printed for future reference.

The sphygmomanometer 10 may then be disconnected from the pressure measuring device 20 and the "T" connector, and if desired further sphygmomanometers may be 5 connected for testing in the manner described above.

After checking pressures measured by the sphygmomanometer 10 on the pressure gauge 14 should be corrected according to the calculated error value or calibration curve obtained as a result of the checking procedure.

As shown in FIG. 1, the computer 30, which may be situated, for example, in a 10 doctor's surgery, is provided with a connection to the Internet (for example via the World Wide Web WWW).

A remote server 40 at a different location is also connected to the Internet and includes a database.

In some embodiments, the application software running on the computer 30 may require the user to input a unique identifier for each sphygmomanometer or other pressure 15 measuring device 10 that is checked. The computer 30 then transmits to the server 40 via the Internet the identifier for each sphygmomanometer 10 tested, together with the date (and optionally the time) of testing, and information relating to the results of the calibration checking procedure. In particular, the computer 30 may transmit to the server 40 the pressure value measured by the pressure measuring device 20 for the or each predetermined pressure value as described above. The computer 30 may also transmit to the database the error value or values between the or each predetermined pressure value and the respective corresponding measured pressure value or values. Where the device 20 measures the ambient temperature, that information is of course also transmitted.

This information is stored in the database on the server 40 for future reference, and may be accessible by the surgery where the sphygmomanometer or other pressure measuring device is used, or to anyone else having an interest in the data, such, for example, as a healthcare trust, via the Internet.

In some embodiments, the application software running on the computer 30 may also require the user to input information identifying the make or model of the sphygmomanometer 10, or such information may be collected automatically from a suitable electronic record (not shown) held within the sphygmomanometer under test. This information may also be transmitted to the server 40. The database may store information regarding the acceptable tolerance limits for a plurality of different kinds of sphygmomanometer, and thus by identifying the make and model of sphygmomanometer associated with each set of data received by the server 40, the user can determine whether or not the error exhibited by a sphygmomanometer under test is within said acceptable tolerance limits or not.

The server 40 is adapted to receive test data from a plurality of different sites where pressure measuring devices or sphygmomanometers are used. By collecting data from such a plurality of different sites, a useful collection of data can be established within the database relating to the comparative performance of the different kinds of pressure measuring device. Thus, for example, once sufficient data has been acquired by the database, the information might show which particular pressure measuring devices remain most accurate for the longest periods of time. This information would be useful to manufacturers and purchasers of pressure measuring devices when deciding respectively how to improve their manufacture or which devices to purchase.

It will be appreciated that with the cuff 11 and pneumatic line 13 connected to the pressure measuring device 20, the cuff 11 and pressure device 20 may be used as a sphygmomanometer without the pressure gauge 14. In such an arrangement, which would provide a substantially more accurate sphygmomanometer, the application software may be configured to display on the screen of the computer 30 the instantaneous pressure measured by the pressure measuring device 20. In such an arrangement, the pressure gauge 14 of the sphygmomanometer 10 could therefore be omitted.

The system may also be used to detect leaks in the sphygmomanometer 10. The user inflates the cuff 11 using the bulb 12 to a predetermined pressure, and the pressure measuring device 20 connected to the cuff 11 then measures the pressure in the cuff 11 over a predetermined period of time. The pressure measurements obtained by the pressure measuring device 20 are stored by the computer 30, and at the end of a data acquisition period, the rate of pressure drop with time is calculated by the application software.

If desired, this data could also be transmitted to the remote server 40 for comparison with a predetermined value corresponding to an acceptable rate of pressure loss associated with the particular make and model of the sphygmomanometer 10. The user could then determine whether or not any leakage of the sphygmomanometer 10 was within the acceptable limits, or whether the sphygmomanometer required repair.

A single pressure measuring device 20 is used for checking the calibration of all of the sphygmomanometers at a given site. The results of each test may either be transmitted immediately to the server 40 as soon as the test is complete, or alternatively where the device 20 has its own memory, the individual test results may all be stored in memory, and transmitted across the Internet all together once testing is complete.

The device 20 used to calibrate the sphygmomanometer 10 should also have its calibration checked regularly. To some extent this can be done by virtue of the fact that the test results are automatically fed back to the central server 40, where they can be compared with earlier test results from the same calibration device. By comparing aggregate trends over a time, it is possible to estimate any long term drift and, where required, to recall the device for calibration. The device may in any event automatically be recalled for calibration at regular intervals, for example once a year. Since the device 20 is small, it can easily be returned to a central laboratory by mail, as and when necessary. Of course, the fact that the device 20 may be unavailable for use for a few days each year does not put any of the sphygmomanometers out of commission. They may continue to be used in the normal way, safe in the knowledge that they have recently been calibrated, and that the device 20 will be returned in good time before the next deadline for sphygmomanometer calibration. Alternative procedures may of course be envisaged, such as the device 20 being regularly recalibrated on site by an engineer carrying a known reference against which the calibration can be performed. Alternatively, instead of the device 20 being recalibrated and returned, an identical replacement could be sent automatically by mail, for example annually. The replacement would automatically report its identity to the central server, on first use. By having the device 20 automatically report its identification to the server, the central database can automatically be kept up to date with information on which individual device is in use at each particular site.

FIG. 2 show schematically alternative apparatus for checking the calibration of the sphygmomanometer 10. In this embodiment, the computer 30 is connected to a pressure measuring device 200 comprising an output port 204 that is connected to a USB interface or RS 232 serial port on the computer 30, and a pressure sensor port 202 that is connected to an inflatable arm A. The inflatable arm A comprises an inflatable hollow body of generally cylindrical shape when inflated. The body has a closed outer wall formed of a suitably strong, flexible material which is capable of withstanding internal pressures at least as high as the normal operating pressures of a sphygmomanometer. For example, the hollow body of the inflatable arm A should be capable of withstanding internal pressures of at least 150 mmHg, and preferably at least 200 mmHg. As shown in FIG. 2, the inflatable arm A is also connected to a manually or automatically operable pump P for inflating the inflatable arm A.

The hollow body has a diameter substantially equal to the diameter of a part of a limb of a human or animal body to which a sphygmomanometer is normally applied for measuring the bloody pressure of the human or animal. For example, the inflatable arm A may have a diameter substantially equal to the diameter of a human upper arm.

In order to check the calibration of the sphygmomanometer 10, the inflatable arm A is inflated using the pump P to an internal pressure that is approximately equal to normal, human or animal bloody pressure, for example about 100-120 mmHg. The cuff 1 1 of the sphygmomanometer 10 is then placed around the inflatable arm A as shown in FIG. 3 and secured in the usual manner. The bulb 12 of the sphygmomanometer 10 is operated to inflate the cuff 11 until the pressure in the cuff 11 is substantially equal to the pneumatic pressure in the inflatable arm A. At this point, the pressure measured by the sphygmomanometer 10 by the pressure gauge 14 can be compared with the pressure measured by the pressure measuring device 200.

In some embodiments, the sphygmomanometer 10 may have an electronic output for outputting in digital form the pressure measured by the sphygmomanometer. In such case, the sphygmomanometer reading may also be inputted to the computer 10 via a suitable interface for automatic comparison by application software running on the computer 30.

The embodiment of FIG 2 has the advantage that a plurality of different sphygmomanometers can be tested using the inflatable arm A₁ without having to connect and disconnect repeatedly the electronic reference pressure measuring device 200, thereby allowing the reference device 200 to remain permanently connected to the inflatable arm A.

This embodiment therefore provides a convenient method of checking the calibration of a plurality of different sphygmomanometers.

In a practical embodiment, the device 20 of FIG 1 or the device 200 of FIG 2 may be contained within a small, robust plastic housing (not shown). In addition to the pressure sensor and suitable interfaces to connect to the computer and to the device under test, the housing may in addition contain a microprocessor, a keypad for the entry of user data and/or instructions, a display, a memory, such as for example, a FLASH memory, a source of power for example a battery, a battery voltage detector and an Analogue to Digital Converter (ADC) for converting an analogue pressure measurement into digital form. In some embodiments, the device may, as previously mentioned, also include a temperature sensor. It may also be desirable in some embodiments to include a means for measuring and recording ambient atmospheric pressure, either by way of an additional pressure sensor or, alternatively, by taking separate atmospheric pressure measurements using the main pressure sensor either before or after measurement of the sphygmomanometer pressure.

Software running on the device itself, or alternatively on the computer 30, instructs the user what to do and at least partially automates the process. User instructions and feedback may be displayed either on an LCD screen of the device, or alternatively on a display of the computer 30.

The operation of the software in a preferred embodiment is shown schematically by the flow diagrams of Figures 3 to 5.

As may be seen in Figure 3, when the device has been switched on and an identifier for the medical device under test has been entered, the system first carries out a leakage rate measurement and, once that has been completed, a spot pressure measurement. The results are then saved, as required, on the local computer 30 and appropriate information is automatically passed on via the Internet to the remote server 40 (FIG 1).

FIG 4 shows details of the automated leakage rate measurement, and FIG 5 details of the process for the spot pressure measurement.

In the preferred embodiment, the results from the leakage and spot measurement tests contain the following information: • Identifier

• Date last results set

• Measured leakage rate

• Pass/fail for leakage rate • Flag indicating spot pressures valid (test may not have been performed)

• Measured spot pressures

It will be understood, of course, that provided the device is capable of storing the test results, it is not essential for it to be connected to the computer 30 during the testing process. In particular, the device 20, 200 may be taken from one sphygmomanometer to the next, recording and storing in internal memory each of the individual test results. Only once all of the results have been collected will device need to be coupled to the computer 30 to allow the results to be uploaded to the computer and transferred across the Internet to the central server. During the upload process, the device may be powered via the USB or other communication cable, reserving battery power for carrying out the actual tests only.

It will of course be understood that in its broadest form the invention may be used to check the in situ calibration of a variety of different measuring instruments, not only the sphygmomanometers already described by way of example in Figures 1 and 2. In other specific embodiments (not shown) the device 20 may include one or more types of senor, depending upon the type of instrument that is to be checked. Examples include the following:

• Pressure sensor (blood pressure monitors)

• Temperature sensors (thermometers, including thermometers within vaccine fridges and sterilizers, tympanic thermometers) • Weight monitors (weighing scales)

• Length measuring devices (height scales) • Devices measuring electrical characteristics such as voltage, current, resistance, impedance, capacitance (electrical devices)

• Flow rate or volume measuring sensors (spirometers)

Claims

1. A method of checking in situ the calibration of a plurality of measuring instruments, the method comprising: (a) attaching to a first measuring instrument be checked a calibration verification device;

(c) repeating steps (a) and (b) for other measuring instruments to be checked and, either one-by-one as the measurements are made, or in consolidated form after all measurements have been made, transmitting the trial outputs as well as corresponding instrument identifiers to a remote location;

(d) at the remote location, storing the transmitted information in a data store; and

(e) repeating steps (a) to (d) at a plurality of times thereby building up in the data store a calibration history record for each of the said plurality of measuring instruments.

2. A method as claimed in claim 1 including, after step (b), determining from the trial output whether the attached instrument calibration is acceptable and, if not, providing an indicating to a user that the instrument should be withdrawn from service.

3. A method as claimed in claim 1 or claim 2 including estimating any drift of the calibration verification device from the calibration history record, and indicating to a user that the device should be re-calibrated or replaced if the measured drift exceeds an allowable amount.

4. A method as claimed in any one of claims 1 to 3 including checking the calibration of the calibration verification device at intervals, for example annually.

5. A method as claimed in claim 4 in which the calibration verification device is returned to a central calibration centre for checking, for example by mail.

6. A method as claimed in any one of the preceding claims in which at step (c) an identifier for the calibration measuring device in use is also transmitted to the remote site.

7. A method as claimed in any one of the preceding claims in which the instruments to be checked are sphygmomanometers, and in which the device measures a trial output pressure.

8. A method as claimed in claim 7 in which the trial output pressure is indicative of a spot pressure.

9. A method as claimed in claim 7 or claim 8 in which the trial output pressure is measured over a time period to check a leakage rate of the sphygmomanometer.

10. A method as claimed in any one of claims 7 to 9 in which the device in addition measures an ambient temperature, temperature data being transmitted to the remote site in step (c).

1 1. A method as claimed in any one of claims 1 to 6 in which the instruments to be checked are pressure sensors, and in which the device measures a trial output pressure.

12. A method as claimed in any one of claims 1 to 6 in which the instruments to be checked are spirometers, and in which the device measures a trial output volume of air or a trial airflow speed.

13. A method as claimed in any one of claims 1 to 6 in which the instruments to be checked are thermometers, and in which the device measures a trial temperature.

14. A method as claimed in any one of claims 1 to 6 in which the instruments to be checked are weighing scales, and in which the device measures a trial weight.

15. A method as claimed in any one of claims 1 to 6 in which the instruments to be checked are height scales, and in which the device measures a trial distance.