LOW POWER AMPEROMETRIC PROBE

Abstract

An amperometric probe suitable for monitoring chlorine levels in water is described. The probe has low power consumption and maintenance requirements rendering it particularly suitable for long periods of operation in remote locations with portable power supplies.

Description

The practice of adding chlorine to water to act as disinfectant has been well established for many years. There is an associated need to analyse water for chlorine content which has traditionally been met using techniques that involve chemical reagents and buffering. These techniques are time consuming and expensive and do not readily lend themselves to rapid, in-field testing by mobile personnel.

There exists a long standing desire for a method of chlorine analysis that does not involve chemical reagents and can be conveniently carried out in the field, for example, by agents of the water supply industries.

The current trend in the water supply industries is an increasing demand for remote systems that are able to monitor levels of species such as chlorine remotely, with little and infrequent human intervention. Ideally a system is able to perform measurements automatically at pre-determined locations and transmit data so obtained to a central processing site.

The determination of analyte concentration in a solution by amperometry is well known. In such analyses, the electric current generated in a suitable chemical reaction involving the analyte is measured and used as an indication of analyte concentration.

The determination of chlorine concentration in a solution by amperometry is possible but a system which meets the water industries' requirements, of remote, automated operation with long periods between maintenance needs to be robust and reliable over such periods. Such systems are typically powered by stand alone power cells or batteries and it is essential that the systems place low demands on these power supplies.

An object of the invention is to provide a system meeting these requirements.

According to the invention, apparatus for determining the level of chlorine in water comprises the features set out in claim 1 attached hereto.

The invention will now be described by non-limiting example, with reference to the following figures in which:

FIG. 1 shows a schematic block diagram of apparatus according to the invention;

FIG. 2 shows a detailed circuit diagram of the electrical components of the invention;

FIGS. 3 and 4 show transfer characteristics for a temperature compensation module forming part of the invention.

Referring to FIG. 1, apparatus according to the invention comprises an amperometric probe 1, having a reference electrode 2, a counter electrode 3 and a working electrode 4. The probe 1 also includes a thermistor 5.

The apparatus further includes a bias voltage module 6 which drives the counter electrode 3 and maintains the bias voltage at the reference electrode 2.

The probe is rendered selective to species of interest by an species-selective barrier (e.g. an ion selective membrane, not shown) between the working electrode and the sample solution.

The current flowing in the working electrode 4 is converted to a voltage by a current to voltage conversion module 7. Said voltage is then adjusted for temperature of the sample by temperature correction module 8 with reference to thermistor 5.

Finally, a gain adjustment module 9 scales the temperature corrected voltage to a range suitable for further processing, for example to the appropriate range of an analogue to digital converter being used to convert the measurement to a digital form.

Some of the electronic components of the apparatus require a positive and negative voltage. However, a preferred embodiment of the invention is powered by a simple (positive) D.C. power supply (not shown). In such an embodiment, a voltage inverter module 10 provides a negative voltage equal in magnitude to the supplied positive voltage.

Referring to FIG. 2, a −2.5V power supply is generated from a +2.5V system supply using a charge pump inverter U6 (TPS60400). This device requires three external capacitors to operate (C17, C16 & C20) and has been chosen for its low quiescent current requirement. Capacitors C18 and C19 provide additional decoupling. L1 is a ferrite bead, and is intended to give additional filtering of the output ripple voltage.

A micro-power shunt voltage reference, U1, produces a voltage of −1.25V. This device has a capacitor, C2, across the output to maintain stability. The reference voltage is then divided by a potential divider to provide the bias voltage for the probe. The divider for the probe bias voltage is switchable as the circuit will operate with two types of probe as follows:

Free Chlorine Probe (FC1)  Bias −373 mV
Total Chlorine Probe (TC1) Bias −408 mV

The selected bias voltage is applied to the non-inverting input of amplifier U2. The output of this amplifier drives the Counter electrode, and the inverting input is connected to the Reference electrode. This amplifier will drive the Counter electrode voltage to a level required to maintain the Reference electrode at the selected bias potential. The amplifier used (OPA336) has very low quiescent current, and low offset voltage (±125 μV max).

The measurement current flows into the Working electrode and is filtered by R10 and C11. An op-amp configured as a transimpedance amplifier (U3) then converts this current to a voltage, the transfer characteristic is as follows:

V=−Iin×R7

For an input current of 3 nA (highest full scale probe current) the output of this stage will be 19 mV.

The amplifier used for U3 is an OPA336, specifically chosen for low input bias current (Ib), low voltage offset (Vos) and low quiescent current. Capacitors C8 and C12 provide local decoupling for the amplifier, and C6 maintains amplifier stability.

The temperature compensation is performed by an inverting amplifier (U4) with a thermistor in its feedback loop. The thermistor is physically part of the probe and its resistance (R_T) will change with the water temperature. The compensation is different for the TC1 and FC1 probes so links are provided to change resistor values. The two possible transfer characteristics for this stage are given by the following equations, and are also shown graphically in FIG. 3 and FIG. 4:

TC1:

Vout=−Vin((R5+RT)/R8)

FC1:

Vout=−Vin((R6+RT)/(R12+R13))

This is an inverting stage and the input is a negative measurement voltage; the output from this stage is therefore a positive voltage. A single pole filter consisting of R9 and C14 is applied to the output. Capacitors C9 and C13 provide local decoupling, and C7 ensures amplifier stability. The amplifier used in this stage (OPA333) has low voltage offset (Vos) and low quiescent current.

The final stage (U5) is a non-inverting amplifier that is used to scale the voltage to the ADC input range. This stage has a fixed gain of 16 (1+R14/R15), and uses an OPA333 amplifier (U5) with low voltage offset and quiescent current. The amplifier has a single +2.5V power supply so the output cannot be driven below ground which may damage the ADC.

The component values used in the output filter (R11 and C15) are designed to meet the requirements of the ADC input stage.

This circuit is designed for the lowest possible power consumption while still meeting the very demanding measurement requirements. An estimate of the typical operating current is shown in Table 4.1. At 2.5V the total current of 171 μA gives a power consumption of 428 μW. This low power consumption means that the circuit could run continuously for long periods in a battery operated instrument.

TABLE 1
Estimated Operating Current
Circuit section                  I (μA)
Charge pump inverter (U6)        65 μA
Voltage reference (U1)           16 μA
Counter electrode drive amplifier (U2) 20 μA
Transimpedance amplifier (U3)    20 μA
Temperature compensation amplifier (U4) 17 μA
Fixed gain amplifier (U5)        33 μA
Total                            171 μA

Claims

1. Apparatus for determining the concentration of a species in a sample comprising: a probe having a reference electrode, a counter electrode, a working electrode and a thermistor;a bias voltage module arranged to provide a constant voltage between the reference electrode and the working electrode;a current to voltage conversion module arranged to produce a voltage as a measure of current flowing between the counter electrode and the working electrode;a temperature correction module arranged to adjust said voltage responsive to changes in the resistance of the thermistor; anda gain adjustment module arranged to re-scale the temperature corrected voltage.

2. Apparatus according to claim 1, further comprising a voltage inverter module arranged to produce a negative voltage equal in magnitude to a supplied positive voltage.

3. Apparatus according to claim 1, further comprising a species selective barrier arranged between the working electrode and a sample solution.

4. Apparatus according to claim 3, wherein the bias voltage module is switchable between a first mode providing a constant voltage of about −408 mV and a second mode providing a constant voltage of about −373 mV.