GB2051371A - Calibration of liquid analyser - Google Patents

Abstract

Unknown sample 11 (buffered by 12) and standard 13 solutions are fed alternately to a flow cell and measuring transducer 15 before being exhausted to waste. The transducer derives an electrical output dependent on a parameter of the measured solution and passes it by way of impedance matching circuit 18 and linearising circuit 19 to display 20. A servo system comprising calibration circuit 21 and motor 22 adjusts the response of the system when the standard solution is flowing so that the output is brought within predetermined limits. <IMAGE>

Description

SPECIFICATiON Calibration of measuring apparatus DESCRIPTI0191 This invention relates to measuring systems and, in particular, to methods of calibration of apparatus for analysis of liquids.

For the measurement of a parameter such as the concentration of ions in a liquid solution, a potentiometer is connected to a reference electrode and to an electrode immersed in the liquid under test. A tube containing an electrolyte connects the two electrodes and an electromotive force, the magnitude of which depends on the ion concentration in the tese liquid is generated.

Measurement of this e.m.f. provides an indication of the ion concentration which may then be displayed on a suitable readout device or fed to an appropriate data processing apparatus. One problem associated with electrochemical systems is that the voltage generated by the measuring electrodes tends to drift over a period of time. It is therefore necessary to incorporate some form of calibration to compensate for this drift. Usually the calibration is provided by immersing the measuring electrode in a solution with known characteristics (a so-called standard solution) and adjusting the response of the apparatus until a desired readout is obtained.Theoretically, a number of standard solutions giving different readouts should be employed, but, in practice, provided a constant temperature is maintained, use of a single standard will be satisfactory since the shape of the response curve of the apparatus does not change significantly over a long period of time. With an electrochemical measuring system it is therefore necessary to have a calibrating arrangement which adjusts the response curve offset.

According to the present invention there is provided a liquid sample measuring system having a measuring station, means for separately supplying an unknown solution and a standard solution to said station at different timers, electrical transducer means at said station to produce an electromotive force dependent on the instantaneous measurmenet of a given parameter of the respective one of said solutions, electrical detector means connected to said transducer feeding indicator means to provide an indication dependent on the magnitude of said electromotive force and cyclically operable calibration means also connected to said electrical detector means to adjust the response thereof, wherein said calibration means comprises a reference voltage course, comparator means for comparing the electrical output from said electrical detector means with the reference voltage to produce a first output when the output from the elecrical detector means is within a predetermined range of values and a second output when it is outside that range, and adjusting means connected to said electrical detector means and responsive to said second output to bring the output of said electrical detector means within said predetermined range.

An ernbodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a known measuring system having measuring and reference electrodes for measuring the concentration of hydrogen ions in solutions Figure 2 shows schematically the liquid flow paths and electrical circuit block diagram for an ion concentration measuring system in accordance with the invention, and Figure 3 is the circuit diagram of an autocalibrating system suitable for incorporation in the apparatus of Figure 2.

Referring to Figure 1 of the drawings, this shows a known hydrogen ion concentration measuring system. In this the e.m.f. from a standardised potentiometer circuit 1 is applied to an electrometer valve 2 in series opposition to the e.mf. from a measuring cell comprising a reference electrode 3 and e.g. a glass, electrode 4 - the electrode may, however, be a membrane or solid state electrode. Two valve operates a moving coil balance indicator 5 and the controlling dial is calibrated to give a direct reading of hydrogen ion concentration. Standardisaíion is carried out by adjusting a poientiometer 6 in the cathode circuit of the electrometer valve when the electrodes are immersed in a standard solution.

Whilst the method of standardisation used in this instrument has the advantage of simplicity, it is not suitable for on-line measuring systems such as are used for process control, since it is not automatic in operation. In order to provide an automatic system, some form of servo control must be employed.

One such system is that illustrated in Figures 2 and 3. With this an unknown solution 11, a buffer solution 12 and a standard solution are fed by a peristaltic pump 1 4 to a flow cell 1 5 containing measuring and reference electrodes, before being exhuasted to waste.The unknown and standard solutions are supplied at different times by means of a three-way controi valve 1 6 under the control of a process time 1 7. The e.m.f. generated by the flow call electrodes is fed to an electrical detector comprising an electrometer amplifier 1 8 which has a high impedance input and a low impedance output, a linearising module 1 9 which converts an exponentially varying input signal to a linear output signal and a readout device 20 which provides a linear display. Connected to the linearising module is a calibration circuit 21 which is also controlled by the process timer 17.The catibration circuit in turn controls a DC motor 22 and gearbox 23 which adjusts an offset potentiometer in the electrometer amplifier. The sequence of operations is that periodically the process timer actuates the three-way valve, cutting off the flow of sample liquid and connecting a supply of standard solution to the pump and flow cell. After a delay in which the electrometer output stabilises, the calibration circuit is activated. If the detector output is not within a predetermined range of values, the setting of the offset potentiometer is adjusted by the DC motor until the output falls within this range. Subsequentiy the calibration circuit is deactivated and, after a further delay, the threeway valve operates to disconnect the standard and reconnect the sample flow.

A calibration circuit which operates over a signal range of 2.7 to 1 O.OV and is suitable for operation with a system giving an output voltage of 0--1 OV is shown in Figure 3. The output voltage from a programmable zener diode 31 can be set to a value in the range 2.7 to 1 .2V for convenience, a value of 1 O.OV is chosen and this is tapped by means of potentiometers 32, 33 to give reference voltages of typically 4.995 and 5.01 5V respectively at the inverting and non-inverting inputs of two comparator circuits 34, 35. The reference voltages could be set further apart to give a wider 'dead band'. The other inputs of the comparators are fed from an output of the linearising module. The comparators are coupled to solid state relay circuits 36, 37 which are normally open.The first relay will therefore be closed when the linearising module output is below 4.995V and the second relay will be closed when the module output is,above 5.01 5V. The relay contacts are connected to a power supply by way of a contact on the process timer 17 which is closed only during the calibration sequence. When closed, the relay contacts permit power to flow to a transformer 38 which actuates a servo-motor 22 and controls a continuous rotary potentiometer 40 in the electrometer amplifier circuit 1 8 by way of a gearbox 23. The potentiometer is driven continuously in one direction so long as the solid state relay contacts are closed. This arrangement has the advantage that the calibration point is always the same, either 4.995V or 5.01 5V dependent on the direction of rotation of the potentiometer, since the relay contacts will open as soon as the output voltage from the linearising modeul enters the predetermined calibration range.

A calibration system has been specifically described in relation to its use in a continuous online system with ion selective electrodes for monitoring unknown liquid solutions. It is, however, suitable for other on-line systems, such as calorimetric analysis, where instantaneous calibration is not required and the slow response of the continuously driven servo is acceptable.

Claims (7)

1. A liquid sample measuring system having a measuring station, means for separately supplying an unknown solution and a standard solution to said station at different times, electrical transducer means at said station to produce an electromotive force dependent on the instantaeous measurement of a given parameter of the respective one of said solutions, electrical detector means connected to said transducer feeding indicator means to provide an indication dependent on the magnitude of said electromotive force and cyclicaily operable calibration means also connected to said electrical detector means to adjust the response thereof, wherein said calibration means comprises a reference voltage source, comparator means for comparing the electrical output from said electrical detector means with the reference voltage to produce a first output when the output from the electrical detector means is within a predetermined range of values and a second output when it is outside that range, and adjusting means connected to said electrical detector means and responsive to said second output to bring the output of said electrical detector means within said predetermined range.

2. A liquid sample measuring system as claimed in claim 1 wherein said given parameter is a optical property of said solution.

3. A liquid sample measuring system as claimed in claim 1 wherein said given parameter is the concentration of ions is said solution.

4. A liquid sample measuring system as claimed in any one of the preceding claims wherein said adjusting means is a servo-controlled potentiometer.

5. A liquid sample measuring system as claimed in claim 4 wherein said potentiometer is driven by a motor.

6. A liquid sample measuring system as claimed in either claim 4 or claim 5 wherein said potentiometer is continuously adjustable through a full circle of revolution and is driven in a single direction.

7. A method of measuring a parameter of a liquid sample comprising alternately feeding an unknown solution and a standard solution to a measuring station, measuring said parameter at said station, converting said measurement to an electrical signal, comparing the electrical signal derived from measurement of said standard solution with a preset electrical signal and adjusting the response of the electrical circuits processing the electrical signal derived from measurement of said parameter until the magnitude of said signal is within a predetermined range of values.