GB2093985A - Determining optical transmission of fluid samples - Google Patents

Abstract

A system for determining optical transmission of fluid samples, in particular for determining the concentration of haemoglobin and other parameters in blood by colour reaction, uses a double light-ray technique in which one ray only is passed through the fluid and each ray impinges on one of two photosensitive elements 6, 7, the two resulting signals each being passed to one of two signal converters 8, 9, the outputs of which are sent to a signal processing, storage and control unit 10, 11 which inter alia converts the analogue signals to digital signals and forms a ratio of the two signals. The control unit also monitors the whole system and performs, for example, error estimation, averaging of measurements and linearising; controls a pneumatic sampler 15 and displays results on a digital display unit 13. <IMAGE>

Description

SPECIFICATION Determination of fluid characteristics The present invention relates to a circuit for determining fluid characteristics, for example, the haemoglobin concentration of a blood sample and other parameters based on colour reaction. In the course of tests based on photometry extinction, absorption or transmission may be observed as satisfying the Beer-Lambert law, in which the absorption of light passing through a fluid sample varies exponentially with the molor concentration of the sample i.e. 1 n extinction concentration.

In one known method of determining concentration, the extinction is read directly from a logarithmic scale and can be multiplied by a predetermined factor; in another method a logarithmic amplifier is used to linearise the logadthmic values proportional to the concentration. The absorption of an applied reagent (or reagents) taking part in a colour reaction can be similarly determined in which case the "blind-value", i.e. the absorption of the light at the given wavelength by the applied reagent, is subtracted from each measurement. These methods and the instruments used operate either with direct analog signal or conversion thereof to digital signal.

In order to increase the accuracy, it is important to be able to ignore the disturbances to the signal.

However, the methods previously described do not always achieve this. Errors may arise, for example, to reference signal drifting, fluctuation of signal with time and linearity fluctuation of the zero point. Such errors prevent high-accuracy measurement.

An object of the invention is to eliminate disturbances to the signal which interfere with measurement and thus increase the accuracy of the measurements.

According to the present invention, we propose a circuit for accurately determining the concentration of a fluid, comprising a source of 2 light rays, a receptacle containing a sample of fluid for testing, two photosensitive elements, the first element receiving the first ray via the fluid receptacle and the second element receiving the second ray which by-passes the fluid receptacle.

Preferably, the output of each photosensitive element is connected to a signal converter, the output of which is connected to an A/D converter which is connected with a signal processing storage and control unit Thus, in order to increase the accuracy of measurements, a method using two light rays is employed in which the second ray act as a reference, in particular for compensating instability of the light source.

One of the rays of light emitted from the light source impinges onto a sample to be tested, which has been previously drawn into a measuring cuvette from a receptacle. Having left the cuvette, the ray of light arrives at a light-sensing element. The output of the light-sensing element is connected to the input of an electric signal converter, and the output of this is connected to one of the inputs of an A/D converter.

The other ray of light from the light source by-passes the sample, traverses the interference filter and impinges on another light sensing element, the output of which is connected to a second signal converter, and the output of this converter is connected to a second input of the A/D converter. The digital surface of the A/D converter is connected to the input of a signal processing and/or control unit and outputs of this are each connected to the input of an operating unit, to the light source, to the input of a D/A inverter, to the input of a display unit and to the input of the already mentioned measuring signal converter. The output of the testing D/A inverter is also connected to a further input of the measuring signal converter. The pneumatic sampler is connected to the signal processing and/or control unit and the measuring cuvette.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing, Figure 1, in which a schematic arrangement of a circuit according to the invention is shown.

Two rays of light are emitted from a light source 1. One ray first passes through an interference filter 5, located in front of a measuring cuvette 3, and then through the cuvette 3. The cuvette contains a fluid sample 4, e.g. blood; the fluid is drawn into the cuvette from a receptacle 2. After leaving the cuvette 3 the ray impinges on a light-sensing element 6. The second ray from the light source 1 is a reference ray and passes through the filter 5 to a light-sensing element 7 without traversing the cuvette 3. The resulting electrical signal from the light-sensing element 6 goes to an input of a signal converter 8, and from the output thereof to an input "IN" of an A/D converter 10.The resulting electrical signal from the light-sensing element 7 goes to an input of a signal converter 9 and from the output thereof to a second input "REF" of the aforesaid A/D converter 1 0.

The two signals arriving at the A/D converter are proportional to the actual values of the signal voltages "UXM" and "Uref" resulting at the sensing elements 6 and 7 respectively. The signal at the output of the A/D converter is: D =UXM Uref (1) Thus, obtaining D disturbing factors along the light paths may be ignored, as changes in UXM and Ref will cancel one another out. The output ratio D will thus remain constant providing the "symmetry" of the light sensing elements is ensured with respect to e.g. light and heat i.e. disturbances affect the two signals equally.

Accordingly, under normal conditions asymmetric variations cannot occur to the signals throughout the normal measuring range. The signal converters 8, 9 are operated in a switching mode to maintain stability and are controlled by a signal processing unit 11. To the converters 8 and 9 a signal "Utest" is connected via a D/A inverter 12 or an interpolation is performed before measuring for error estimation.

The unit 11 determines the ratio of the reference voltage Uref and the estimated measured voltage, UXM + Utest, using the following equation:

In applying the method herein four important precautionary checks may be carried out: 1. Testing the detector elements and the converters.

2. Testing linearity of the circuit.

3. Testing the electrical processing unit (without the light-sensing elements).

4. Measuring the reference voltage.

The tests of linearity and of the electrical circuit can be performed throughout the whole measuring range by generating discrete pulse trains by the aid of the unit 11, the D/A inverter 12 and the converters 8, 9.

The accuracy of the linearity depends on the accuracy of the signal converters 8, 9. The results of the test are digitally converted and stored. The results must be within an acceptable, preselected tolerance and are shown on a display unit 1 3. Thus, by monitoring, continuous control of the stability of the system is possible.

With the circuit according to the invention a logarithmic amplifier, which is liable to considerably increase drift in the measured signal, is not necessary and this cause of instability is obviated.

Linearity of the ratio D is checked by logarithmic series expansion, e.g. as follows:

where n and k are either fixed or variables accompanying the choice of the maximum measuring error, which can be determined and/or adjusted by cut-off point interpolating; the number of cut-off points is either fixed or varying.

By increasing n and k, the error of approximation of the function can be reduced.

A further advantage of this method of measurement is that the error is approximately constant throughout the whole signal converting range, whereas, in the cases of logarithmic amplifiers and photometers with pointers, error varies throughout the signal range, being lowest for the highest measurement and increasing as the value of the measurements decreases.

Therefore in comparison with conventional methods, by using the circuit according to the invention, the measurement error may be monitored and kept constant, so that for low concentrations errors are considerably reduced. The series expansion (2), the cut-off point interpolation and logarithmic routine may be carried out by special circuit units e.g. "CPU, ROM, RAM" incorporated in the control unit 11.

Accuracy may be further increased as the unit 11 has an averaging facility by which the mean value of m measurements is calculated, where m is either fixed or selectable.

The equation below is used to ascertain the mean value of m measurements:

where X the mean value of m measurements, XMI = each measurement from the "i"th to the "m"th.

Due to fast conversion times the total cycle time is very fast indeed and by increasing the number of measurements, the statistical accuracy is improved.

When measuring samples of unknown substances of the "blind-value" D1 as previously defined is important. To measure the blind value of the reagent, it is filled into the receptacle 2. A manipulating unit 14 actuates a pneumatic sampler 15 via the unit 1 the sampler sucks up the reagent into the measuring cuvette 3. The unit 11 also actuates the lamp comprising the light-source 1.D1 is taken as the average of a series of measurements and shown on the display unit 1 3. The required modification of D1 - or the zero value - can be performed by the manipulating unit 14 via the unit 11, by modifying the transfer function of the signal converter 8 or the transfer function is produced in the signal converter 8 by means of a potentiometer.

In order to further calibrate the system a substance with a known concentration is tested and filled into the receptacle 2, which may also contain the reagent. The measurements taken are proportional to the absorption of light by the substance of known concentration at given wavelength i.e. D3. A constant characterizing the system and the concentration of the given substance may be defined as follows: in D11 in D3 K, constant = (5) XH where XH stands for the known concentration of the calibrating substance and D1 is the blind-value of the reagent.

The constant K is stored by unit 11 and accounts for all the conventional characteristics to be considered in the photometry.

Having performed the calibration, measurements, D2, of unknown samples may be made, and are calculated in the unit 1 1 as follows: ~ 1 nD1-1nD2 XM = K (6) by substituting K and D1 as determined above. By inserting equation (4) for K: ~ in Dt1 1n D2 XM= .XH in Dt1 in D3 (7) By modifying the units, the value XM can be displayed in the required concentration e.g. g/dl or mmol/l. This adjustment may be carried out in the manipulating unit 14, but it may instead be built-in.

Measuring the light absorption by different fluids at a given wavelength, the suitable choice of the characteristics of the interference filter 5 and the light sensing elements 6, 7, the spectral characteristics and the intensity of the light source 1, and the eigenvalue of light absorption by the measuring cuvette 3 is necessary. Sansitivity and amplification can each be adjusted by modifying the signal converters 8, 9.

Claims (14)

1. A circuit for accurately determining the concentration of a fluid, comprising a source of two light rays, a receptacle containing a sample of fluid for testing, a first photosensitive element for receiving the first ray when transmitted via the fluid receptacle and a second photosensitive element for receiving the second ray which when transmitted by-passes the fluid receptacle.

2. A circuit according to claim 1, including a filter disposed in the paths of both light rays and located between the light source and the fluid receptacle.

3. A circuit according to claims 1 and 2, wherein both the light rays are arranged so as to be substantially identically affected by external factors including light and heat.

4. A circuit according to claims 1, 2 or 3 wherein the output of each photosensitive element is for connection to a signal converter, the output of which is for connection to an A/D converter which is connected with a signal processing, storage and control unit.

5. A circuit according to claim 4, wherein the control unit includes means for converting the two resulting measurements into a ratio.

6. A circuit according to claim 4, wherein the control unit has a measurement averaging facility.

7. A circuit according to claim 4, wherein the unit has an error estimation facility.

8. A circuit according to claim 4, including a digital display connected with the control unit for showing the results of measurements processed in the unit.

9. A circuit according to claim 4, including a manipulation unit connected with the control unit for actuating a pneumatic sampler to draw a fluid sample into the receptacle.

10. A circuit according to claim 4, including a D/A inverter connected between the control unit and the signal converters for adding an estimated error to the signal at the first photosensitive element.

1 1. A circuit according to claim 4, wherein the control unit is for connection with the light source.

1 2. A circuit according to claim 4, including means for monitoring linearity of the signal converters.

1 3. A method of accurately determining the concentration of a fluid comprising sending a first ray of light through a fluid sample so that it impinges a first photosensitive element and sending a second rayon light to a second photosensitive element so that it does not pass through the fluid sample.

14. A method according to claim 1, wherein the two analogue signals obtained are digitally converted and processed in a control unit and shown on a display.

1 5. A circuit constructed and arranged as herein described with reference to and as illustrated in the drawing.