GB2319614A - Portable apparatus for calibrating a probe - Google Patents

Abstract

The apparatus calibrates a probe which is responsive to a condition of a fluid such as the pH of water. It comprises a housing which supports reservoirs 3,4, 5 for calibration fluid and reservoir 6 for washing fluid. Pump 18 delivers calibration and or washing fluid to the device to be calibrated 27. A CPU 23 stores the response of the device to the calibration. The system is arranged so that the response of the device to the calibration fluid cannot be recorded until the pump has been activated. The fluid may be supplied to the probe in a single pass or in circulation round loop 21.

Description

CALIBRATION APPARATUS The present invention relates to apparatus for calibrating a probe responsive to a certain condition of water or other liquid (such as pH level, or the levels of certain chemical species present). Particularly, but not exclusively, the invention provides apparatus for calibrating pH probes used in water treatment plants.

In the treatment of water for domestic supplies, certain conditions of the water, such as its pH and chlorine content, must be carefully controlled. This involves the use of probes to monitor the condition of the water. For instance, electro-chemical probes are widely used in water treatment plants for monitoring the pH of water.

It is necessary for the accuracy of such probes to be checked regularly, and the probes re-calibrated if necessary. For instance, in the case of pH probes used in water treatments plants, the conventional method of calibration requires a technician to visit the site of the probe with pre-prepared buffer solutions of known pH against which readings of the probe are tested and adjusted if necessary.

The typical procedure is for the technician to first clean the probe and then insert it into a receptacle into which of a first buffer solution has been poured. The output reading of the pH probe is then adjusted until it gives a steady reading acceptably close to that of the known pH of the buffer. The process may then be repeated using a second buffer of different pH.

A problem with this conventional calibration method is that it is not possible to be certain that a calibration has actually been performed or performed properly.

The effectiveness of the method is to a certain extent dependant upon the thoroughness of the technician performing the calibration, particularly in such areas as ensuring the probe is properly cleaned between calibration readings (if more than one reading is taken), that the buffer solutions are not allowed to become contaminated, and that the readings are taken properly (for instance that any given reading is taken for a sufficient length of time to ensure that the reading is steady).

It is an object of the present invention to obviate or mitigate the above disadvantages.

According to the present invention there is provided portable apparatus for calibrating a device which is responsive to a certain condition of a fluid, the apparatus comprising a housing which supports a reservoir of at least one sample fluid for which said condition is known, a pump for delivering said sample fluid to said device, and storage means for recording the response of the device to said known condition of the or each sample fluid, wherein the response of the device to a sample fluid cannot be recorded until the pump has been activated to deliver that fluid to the device.

The apparatus could be used to calibrate a wide variety of devices such as probes used to monitor pH, chlorine content and other conditions of water in a water treatment plant. The apparatus is not, however, limited to such uses.

As mentioned above, one problem with existing calibration methods is that they are not always performed properly or indeed performed at all. The present invention addresses this problem by providing apparatus which includes means for recording calibration readings and moreover ensures that a reading cannot be recorded until sample fluid has been supplied to the device.

In addition, use of a pump to supply fluid to the device being calibrated enables a degree of consistency to be maintained from one calibration reading to the next. For instance, operation of typical eletro-chemical probes is affected by the flow conditions around the probe when a reading is taken as the mass transport of ions within a fluid will vary with changes in the flow conditions. With conventional calibration methods, one technician might take a reading whilst stirring a probe in a container of fluid whilst another might simply sit the probe in the container. The present invention however enables the provision of consistent and repeatable flow conditions.

The apparatus preferably includes at least one reservoir of a cleaning fluid such as water, the pump being operable to deliver cleaning fluid to said device to clean the device prior to delivery of a sample fluid.

Provision of a cleaning fluid which is pumped to the device prior to the supply of sample fluid, i.e. prior to a calibration measurement, helps ensure that the device is properly cleaned before calibration, or inbetween successive calibration measurements.

In a preferred embodiment of the invention an inlet conduit is provided for delivering fluid to the device and an outlet conduit is provided for carrying fluid from said device, and said pump is operable to pump fluid past the device via said inlet and outlet conduits.

This feature allows for a constant flow of fluid to be maintained past the device as a calibration reading is taken. In many cases the device in situ will be within a region of fluid flow and thus by pumping fluid past the device during calibration the conditions of the calibration will be closer to the operating conditions of the device than if the device is simply dipped in a container of fluid. This can result in a more accurate calibration bearing in mind that the mass transport of ions within the fluid is affected by the fluid flow conditions. The apparatus may further be provided with means, such as a conventional flow meter, for controlling the rate of fluid flow. This would enable the rate of fluid to be accurately matched to the in situ flow conditions in which a particular device normally operates further increasing the accuracy of calibration.

The outlet conduit is preferably provided with a valve selectably operable to direct fluid flow from the device to either a drain conduit or to a conduit which communicates with the inlet conduit to define a circulation loop around which fluid may be re-circulated past the device.

Provision of a circulation loop allows sample fluid to be pumped past the device whilst minimising the volume of fluid used. However, the provision of a drain conduit allows sample fluid, or cleaning fluid, to be drained after use rather than returned to the respective reservoir in the apparatus. This prevents the reservoirs of sample fluid and/or cleaning fluid from becoming contaminated which can be a problem with conventional calibration techniques. The drain conduit could connect to a drain reservoir housed within the apparatus, but it would generally be preferred that the drain conduit simply drains the fluid off to an external drain.

The apparatus can be adapted for performing a calibration on a device in situ, by providing sample and/or cleaning fluid to a chamber in which the device is mounted in operation. Alternatively, or in addition, the apparatus can be provided with a receptacle for receiving the device. fluids being pumped from respective reservoirs to the receptacle for performance of a calibration operation.

The apparatus may include means, such as a key-pad, for manually inputting the response of said device into the storage means and/or means for automatically recording the response of the device from the signal output by the device. In other words, in one embodiment of the invention the apparatus may be operated in one of two ways (both alternatives may be provided by a single apparatus); one mode in which calibration readings are manually input to the storage means by the operator and a second mode in which calibration readings are automatically recorded by the apparatus. The latter mode of operation ensures that a calibration must be properly performed before a reading can be recorded.

The apparatus is preferably adapted so that a response of the device will only be automatically recorded provided said response lies within a predetermined range of the expected response to the known condition of the sample fluid for a predetermined period of time.

The storage means preferably comprises a processor with memory. Such a processor could also prompt/control operation of the device, for instance prompting entry of calibration readings or progressing through the various stages of a calibration operation automatically (in the case where calibration readings are automatically recorded by the storage means).

The storage means may also be provided with the facility to store other parameters. For instance, operational sensitivity/accuracy of typical pH probes is generally characterised by a property of the probe known as its "slope". For any given probe, the slope should be a particular value but over time the probe may loose sensitivity/accuracy which is reflected in a change of the slope of the probe. It would be advantageous if when calibrating any particular probe, or similar device, the slope of the probe could also be recorded as an indication of the condition of that probe.

In one preferred embodiment of the invention the apparatus is operable to: a) pump cleaning fluid to said device for a predetermined period of time to clean the device; b) pump a volume of a sample liquid to said device for a predetermined period of time sufficient to enable a calibration operation to be performed.

In apparatus including more than one reservoir of sample fluid the apparatus may be operable to reiterate steps (a) and (b) using different sample fluids for each step (b) to enable a number of calibration readings to be taken.

For instance, apparatus including means for automatically recording calibration readings may be operable such that following a first iteration of steps (a) and (b) and recordal of a first calibration reading, the apparatus automatically proceeds to perform at least one subsequent iteration of operations (a) and (b) and to automatically record at least one further calibration reading.

The or each iteration step (a) preferably comprises two pumping operations, a first pumping operation in which cleaning fluid is pumped to a drain outlet via a chamber containing said device, and a second operation in which cleaning fluid is circulated through the pump and chamber containing the device.

Similarly, the or each iteration of step (b) preferably comprises two pumping operations, a first pumping operation in which sample fluid is pumped to a drain outlet via a chamber containing said device, and a second pumping operation in which sample fluid is circulated through the pump and chamber containing the device.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig 1 is a diagrammatic side elevation of apparatus in accordance with the present invention; Fig 2 is a diagrammatic plan view of a base part of the apparatus of Fig 1; Fig 3 is a diagrammatic end view of the base component illustrated in Fig 2; Fig 4 is a diagrammatic plan view from below of a lid component of the apparatus of Fig 1; Fig 5 is a schematic diagram illustrating the operational relationship of various components of the apparatus of Fig 1; Fig 6 is a flow chart illustrating two different modes of operation of the apparatus of Fig 1; Figs 7a, 7b and 7c illustrate one example of a pH probe holder for inclusion in the apparatus of Fig 1; Figs 8a, 8b and 8c illustrate part of an alternative pH holder for inclusion in the apparatus of Fig 1; and Figs 9a and 9b illustrate further components of the alternative pH holder part of which is illustrated in Figs 8a, 8b and 8c.

Referring particularly to Figs 1 to 5, the illustrated embodiment of the invention is portable apparatus designed for use in calibrating pH probes used at water treatment plants. The various components of the apparatus are housed in a case which comprises a base 1 and a lid 2.

The base 1 houses three bottles 3, 4 and 5 of buffer solutions of various pH (typically pH4, pH9 and pH7 respectively), a bottle 6 of distilled water, and a chamber/holder 7 for receiving a pH probe to be tested. Pipes 8 to 11 feed from bottles 3-6 respectively to a common supply pipe 12 via respective two-way valves 13-16. Supply line 12 is connected to a fluid circuit (generally indicated by reference 17) comprising a pump 18, a flow meter 19 (which facilitates adjustment of the flow rate as mentioned above), the pH probe holder 7, and a three-way valve 20, which are connected in that order in the circuit 17 by a pipeline 21. A separate drain pipeline 22 is also connected to the three-way valve 20 for draining fluid to an external drain on site (i.e. not housed within the case). When the three-way valve 20 is open fluid from the probe holder 7 is flushed to drain via the pipeline 22, but when the valve 20 is closed, the drain pipeline 22 is closed and fluid circulates through the circuit 17. The operational arrangement of the various components relative to one another is best appreciated from Fig 5.

The lid 2 which is hinged to the base 1 houses a c.p.u. 23 which is used to control operation of the apparatus as is described in more detail below. Interface with the c.p.u. 23 is by way of a small keypad 24 and LCD display screen 25 which are also housed within the lid X.

The apparatus is provided with a battery 26, which is preferably rechargeable, but may also be powered from a 240V mains supply.

Operation of the illustrated apparatus in testing/calibrating a pH probe 27 will now be described with particular reference to Fig 5. The basic scheme of operation is as follows: (i) The probe 27 is first cleaned by the operator and then fitted into the probe holder 7.

(ii) The pump 18 is then activated and valve 11 opened so that water is pumped from the bottle 6 to the drain line 22 via the pump 18, the flow valve 19, the probe holder 7, and the three-way valve 20 (which is set appropriately). This is performed for a limited period of time sufficient to ensure that the probe 27 is adequately cleaned and that any fluid from previous calibrations is flushed from the apparatus.

(iii) Valve 11 is then closed and valve 20 reset to close the drain but allow circulation of water in the circuit 17. This washes any fluid from previous calibrations out of that part of the pipeline 21 between valve 20 and supply line 12.

(iv) Valve 8 is then opened and valve 20 is re-opened to drain for a limited period of time so that the first buffer is pumped from the bottle 3, through the pH probe holder 7 to the drain 22. This flushes out most of the water remaining in the circuit 17.

(v) Valve 20 is then closed and valve 8 is closed so that the buffer circulates within the circuit 17 through the pH probe holder 7. Circulation of the buffer is continued for a period of time sufficient to enable any required adjustment of the probe output reading until the probe gives a steady reading acceptably close to the known pH of the buffer.

(vi) Steps (ii) and (iii) (which are referred to below as the wash operation) are then repeated to wash the pH holder and pH probe.

(vii) Steps (iv) and (v) are then repeated using the second buffer from bottle 4.

(viii) Finally, accuracy of the re-calibrated probe is validated by repeating steps (ii) to (v) with the neutral buffer from bottle 5. This final validation reading is then recorded.

(ix) The calibration/validation process is now complete and the probe may be removed from the apparatus. A further washing operation may be carried out to flush buffer from the circulation circuit, but this is not strictly necessary as a washing operation will in any event be carried out before measurements are taken in the next calibration as outlined in paragraphs (ii) and (iii) above.

The procedure outlined above is a basic calibration procedure which may be modified. For instance after the initial washing operation, an initial validation reading may be taken (using the neutral buffer from bottle 5) prior to calibration. If the validation reading is accurate the calibration procedure may be skipped.

In the particular embodiment of the invention illustrated operation of the apparatus is controlled/prompted by the c.p.u. which in the preferred example of the invention is programmed to operate in one of two alternative ways. These are referred to below as "open loop" and closed loop' modes of operation respectively. As with any c.p.u., the c.p.u. 23 may be programmed with a number of features including many optional features. However, for simplicity, only the basics of the operation of the c.p.u. in relation to the open and closed loop modes of operation of the particular calibration apparatus described above will be discussed in detail. Examples of other possible features of the programming will be mentioned later. Operation of the apparatus is described with particular reference to Fig 6 which is a flow chart summarising the procedural steps in both open and closed loop operating modes.

Having first cleaned the probe 27, the operator inserts the probe 27 into the probe holder 7, switches on the apparatus and initiates operation with appropriate input via the keypad 24. The operator is then presented with a message on the LCD display screen 25 prompting him to enter both the probe identification number and the employee's own identification number. Having done so, the operator is then prompted to select either open or closed loop operation.

If the open loop mode is selected (again by appropriate input to the keypad 24) the operator is prompted to either perform an initial validation reading or skip straight to the calibration. If an initial validation reading is required, this option is chosen and the device automatically cycles through the washing and buffer circulation steps described above using buffer from bottle 5. The cycle times are predetermined and programmed into the c.p.u. 23 prior to the calibration operation (this is mentioned further below). After the buffer circulation cycle is completed the operator is prompted to enter the initial validation reading output by the probe 27. Having done so, the operator is then given the choice to continue with the calibration (if for instance the first validation reading is inaccurate) or to quit the procedure (if for instance the first validation reading is accurate).

If calibration is not required, the apparatus will cycle through a final wash operation. If calibration is required and selected, steps (ii) to (v) above will be performed using buffer from bottle 3 (typically of pH4). During circulation of the buffer the operator is first prompted to enter the initial pH reading given by the probe (a first calibration reading) following which the operator adjusts the probe output reading to that of the known buffer pH.

After a set circulation time the cleaning, drain and circulation steps are then repeated with the buffer from bottle 4 (which may typically be pH9) and a second calibration is performed as above.

The slope of the probe (which is a function of the probes operational condition, as mentioned above) is then entered into the c.p.u. 23 following which a final validation (as outlined in step (viii) above) is performed to check the calibrated accuracy of the probe. The validation reading is then entered into the c.p.u. 23 which terminates the procedure.

Closed loop operation is essentially the same as open loop operation except that the output of the probe is directly linked to the c.p.u. 23 so that the c.p.u. takes its readings direct from the probe 27 rather than requiring the operator to manually input the values. Thus, if closed loop operation is selected, the operator is required to select either a first validation or to skip straight to the calibration procedure. If initial validation is selected, the wash, drain and circulation steps are performed and the probe value automatically recorded. Following the first validation, the operator is then prompted to select calibration or to quit the procedure. If calibration is selected, first and second calibrations are performed as described above in relation to open loop operation, except that initial calibration values of the probe are automatically recorded by the c.p.u. 23.

During calibration, the pH meter 27a is adjusted by the operator and the c.p.u.

23 will only advance to the next step if the steady output value of the probe falls within a predetermined error margin for a predetermined period of time. If a valid reading is not obtained an error message is displayed and the process continued until a valid reading is obtained. The apparatus then cycles through the cleaning, drain and circulation steps of the second calibration. Once the second calibration step is complete, the operator is prompted to enter the slope of the probe following which a final validation is performed.

Whether open or closed loop operation is selected. the final result of a full calibration procedure (including an initial validation) is that the first validation reading, the calibration readings (i.e. values of the probe 27 prior to adjustment), slope, and final validation reading of the probe are stored in the c.p.u. 23 in a file identified by the probe identification number, along with the operators identification number and the pre-set values of the buffer solutions etc. (see below). This information is stored in the c.p.u. 23. together with files created from calibration of other probes, until the operator has an opportunity to down load the information to a central computer.

It will thus be appreciated that the invention provides careful control, and a certain degree of automation, of the calibration procedure which eliminates many of the opportunities for inaccuracies that can occur with conventional calibration procedures outlined in the introduction to this specification. Furthermore, for every calibration, all relevant calibration details are accurately recorded with the data in a form that can be readily manipulated and transferred to a central control computer.

In addition to the features mentioned above, the system can be provided with additional features to enhance its flexibility. In particular, the c.p.u. can be programmed to provide for variation of such things as the buffer pH values and the cycle times (i.e. the time durations of the wash, drain and circulation cycles).

For instance, although the buffers used may typically have the pH values of 4, 9 and 7 respectively (which may be input into the system as default values) the system may be provided with the facility to change these values if desired.

Similarly, although the system may, for instance, be set with default values for the cycle times of 30 secs for draining waterlbuffer, 60 secs for circulating water, and 120 secs for circulating buffer, it may be desirable to alter these values to suit particular circumstances and allow for different conditions. In addition, the system may include the facility to allow the operator to override the pre-set cycle times during a calibration cycle by terminating any particular wash/drain/circulation cycle short of its set time.

Also, as mentioned above, in closed loop operation the system needs to be preset only to accept a calibration once the probe reading falls within an accepted error margin for a set period of time (this may be referred to as the "deviation"). Thus the system preferably allows for the default values of pH error range and the time period over which the probe must output a steady reading within this range, to be changed as desired. Typical default values for the "deviation" may, for instance, be plus or minus 0.15 pH for a period of 6 secs.

It may also be desirable to include the facility to vary the pH scale over which pH readings are taken. For instance, the scale need not be a standard scale of 1-14 but could be changed to anything up to, say, 1-100.

Examples of other features which may be programmed into the system are file management and review features, date and time stamping of calibration readings, and the facility to calibrate the response of the unit to match the probe output and ensure accurate probe readings are recorded in the closed loop mode. For instance, it is standard for such probes to output a current between 4mA and 20mA dependant upon the pH value. However, the probe can be calibrated so that this output current range can correspond to any desired pH range. not necessarily extending across the entire pH scale. It is therefore desirable to enable the c.p.u. to be calibrated accordingly.

It will be appreciated that details of the various components of the illustrated embodiment of the invention could be varied widely. For instance, standard laboratory bottles could be used for the bottles 3-6; for example 500mL bottles might be used for the buffer bottles 3-5 whereas a 1 Litre bottle might be used for the water bottle 6. The valves 8-11 could be simple solenoid operated valves. Examples of such valves that are readily available have brass bodies and around 1.5mm orifices and are powered by 2 watt coils.

The circulation pump 18 could similarly be of any suitable type such as a single head diaphragm pump (which may for instance have a diaphragm fabricated from PTFE). For instance apparatus as illustrated has been constructed using such a pump with a flow rate of 0.3L/min and a suction head of 6m Wg. Pumps with higher flow rates could of course be used if desired. Similarly, other forms of pump, such as peristaltic pumps could be used although such pumps may require more regular maintenance than a simple diaphragm pump.

The flow meter 19 could also take any desired form and could for example comprise a needle valve and allow flow to be adjusted between, say, 50mL/min and 300mL/min. For the various pipelines, six millimetre outside diameter nylon tube, or similar tubing, could for example be used. All fittings for connecting the tubing valves etc. may for example be nickel plated brass bodied fittings with re-usable compression type connections.

For convenience, the battery is preferably of a re-chargeable type, for instance a sealed lead acid unit producing 6.5amp/Hr. This should be sufficient to power apparatus comprising components mentioned above, in which running power consumption would be of the order of 13 Watts (2 X 2 Watts for two valves operated at once, approximately 7 Watts for operation of the pump and approximately 1 Watt for operation of the c.p.u.) requiring a current of about 1.02amp so that a battery of 6.5ampMr will give approximately six hours running time.

The pH probe holder 7 may be constructed to a variety of designs depending on the types of probe the holder is intended for use with. A single apparatus may accommodate interchanging of a variety of different probe holders if the apparatus is to be used with a variety of different probes. However, individual probe holders may themselves be constructed to accommodate different probes, and particularly probes of different diameters. In this regard, typical probe diameters vary between about 29mm and 25mm. In addition, it will be preferable for the pH probe holder to have a relatively small volume, of about 50cc, to reduce the volume of fluid required during the various circulation stages.

Details of one example of a pH probe holder for inclusion in the apparatus described above, are illustrated in Figs. 7a to 7c. The holder comprises a generally cylindrical body 28 with a supporting annular flange 29. The body 28 is provided with apertures 30 and 31 to allow circulation of fluid therethrough. Soft neoprene annular seals 32 and 33 are positioned around the base and neck of the body 28 respectively. The holder has a screw on lid 34 provided with an aperture 35 for insertion of a probe. The soft neoprene seals can accommodate a variety of probe diameters.

An alternative probe holder is illustrated in Figures 8a to 8c (Fig. 8b and 8c are plan views of 8a) and 9a and 9b. The holder again comprises a generally cylindrical body portion 35 (illustrated in Figs. 8a to 8c) and a lid portion 36 (illustrated in Figs.

9a and 9b). The lid 36 which may preferably be fabricated from PTFE, is provided with a neoprene seal suitable for accommodating various sizes of probe.

It will be understood by the skilled reader that in addition to the specific alternatives mentioned above, many modifications can be made to the detail of the apparatus described. For instance, the number of buffers used might be altered, and the apparatus adapted accordingly; details of the c.p.u. programming may be changed; details of the operation procedure may be varied (for instance during calibration readings of the probe both before and after adjustment may be recorded); and details of specifications of individual components could vary widely.

In addition, it will be appreciated that the invention is not limited to provision of apparatus for the calibration of pH probes, but by suitable selection of buffers or similar test solutions, the apparatus may be used to calibrate probes used in determination of other conditions of water (such as chlorine content) or for that matter of any liquid. Other possible applications of the invention will be readily apparent to the appropriately skilled person.

Claims (17)

1. Portable apparatus for calibrating a device which is responsive to a certain condition of a fluid, the apparatus comprising a housing which supports a reservoir of at least one sample fluid for which said condition is known, a pump for delivering said sample fluid to said device, and storage means for recording the response of the device to said known condition of the or each sample fluid, wherein the response of the device to a sample fluid cannot be recorded until the pump has been activated to deliver that fluid to the device.

2. Apparatus according to claim 1 further comprising a reservoir of cleaning fluid and wherein said pump is operable to deliver said cleaning fluid to said device to clean the device prior to delivery of a sample fluid to said device.

3. Apparatus according to any preceding claim, wherein an inlet conduit is provided for delivering fluid to the device and an outlet conduit is provided for carrying fluid from said device, and wherein said pump is operable to pump fluid past the device via said inlet and outlet conduits.

4. Apparatus according to claim 3, wherein the outlet conduit is provided with a valve selectably operable to direct fluid flow from said device to either a drain conduit or to a conduit which communicates with the inlet conduit to define a circulation loop around which fluid may be re-circulated past the device.

5. Apparatus according to claim 3 or claim 4, wherein means are provided for controlling the rate of fluid flow past the device.

6. Apparatus according to any preceding claim, wherein the apparatus is adapted for performing a calibration on a device housed in a chamber external to the apparatus.

7. Apparatus according to any preceding claim, wherein the housing is provided with a receptacle for receiving said device and fluid is delivered to the pump to said receptacle for performance of a calibration operation.

8. Apparatus according to any preceding claim, including means for manually inputting the response of said device into said storage means.

9. Apparatus according to any preceding claim, wherein said storage means is adapted for automatically recording the response of the device from the signal output by the device.

10. Apparatus according to claim 9, wherein a response of the device will only be automatically recorded provided said response lies within a predetermined range of the expected response to the known condition of the sample fluid for a predetermined period of time.

11. Apparatus according to any preceding claim, wherein said storage means comprises a processor with memory.

12. Apparatus according claim 2 and any claim dependant thereon, wherein the apparatus is operable to: a) pump cleaning fluid to said device for a predetermined period of time to clean the device; b) pump a volume of a sample liquid to said device for a predetermined period of time sufficient to enable a calibration operation to be performed.

13. Apparatus according to claim 12, wherein the apparatus includes more than one reservoir of sample fluid and wherein the apparatus is operable to reiterate steps (a) and (b) using different sample fluids for each step (b) to enable a number of calibration readings to be taken.

14. Apparatus according to claim 13 when dependant from claim 9 or claim 10, wherein the apparatus is operable such that following a first iteration of steps (a) and (b) and recordal of a first calibration reading, the apparatus automatically proceeds to perform at least one subsequent iteration of operations (a) and (b) and to automatically record at least one further calibration reading.

15. Apparatus according to any one of claims 11 to 14, wherein the or each iteration step (a) comprises two pumping operations, a first pumping operation in which cleaning fluid is pumped to a drain outlet via a chamber containing said device, and a second operation in which cleaning fluid is circulated through the pump and chamber containing the device.

16. Apparatus according to any one of claims 11 to 15, wherein the or each iteration of step (b) comprises two pumping operations, a first pumping operation in which sample fluid is pumped to a drain outlet via a chamber containing said device, and a second pumping operation in which sample fluid is circulated through the pump and chamber containing the device.

17. Apparatus for calibrating a device which is responsive to a certain condition of a fluid, substantially as hereinbefore described with reference to the accompanying drawings.