Electricity: measuring and testing – Electrolyte properties – Using a ph determining device
Reexamination Certificate
2001-09-13
2003-09-23
Hirshfeld, Andrew H. (Department: 2858)
Electricity: measuring and testing
Electrolyte properties
Using a ph determining device
C324S071600, C324S071200
Reexamination Certificate
active
06624637
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a device for measuring the concentration of ions, especially H
+
ions, in a measurement liquid. Measurement takes place by means of at least one ion sensitive field effect transistor (ISFET) which is located in the measurement liquid and which is connected together with the resistors in a bridge circuit. There is a bridge feed voltage at the feed points of the bridge circuit. The device delivers an output signal which is a measure of the ion concentration in the measurement liquid. The device has a reference electrode which is likewise located in the measurement liquid.
These devices are conventionally used to measure the pH of a measurement liquid. The pH is determined by the host of constituents dissolved in the measurement liquid, for example, by the concentration of H
+
or OH
−
ions.
In community and industrial waste water treatment, chemical reactions are used to precipitate certain constituents, to neutralize and detoxify the water. The pH plays a key role for the correct progression of these reactions. The mechanical settling processes in the clarification plants can be adversely affected by acid or alkali waste water. Biological processes in self-purification of water or in the aerobic and anaerobic biological stages of clarification plants are likewise linked to certain pH values and their progression is disrupted when there are deviations therefrom. To automate these operations continuous measurement of the pH is essential.
The concentration of H
+
ions in a measurement liquid can be measured for example by means of an ion-sensitive field effect transistor (pH-ISFET), especially by means of a hydrogen ISFET. Compared to conventional glass electrode measurement chains, pH ISFETS have the advantage that they are not sensitive to aging and therefore have a much longer service life. The resistance of the channel of the pH-ISFET and thus the gate potential change linearly with respect to the concentration of H
+
ions in the measurement liquid. So that the pH-ISFET delivers an output signal which is proportional to the input voltage on the pH-ISFET, there is a constant drain current on the pH-ISFET. The pH-ISFET delivers an output voltage as the output signal, for example. This output voltage is measured against a reference electrode which cannot be influenced by the concentration of H
+
ions and which is likewise located in the measurement liquid.
EP 0 155 725 discloses a device of the initially mentioned type in which a pH-ISFET and a reference ISFET are connected together with two resistors in a bridge circuit. U.S. Pat. No. 4,334,880 discloses another device of the initially mentioned type, but without a reference electrode. The circuit described in the patent includes a pH-ISFET with three resistors being connected in a bridge circuit. The diagonal voltage of the bridge circuit is used as the output signal of the device. The conductivity of the pH-ISFET changes during the measurement process depending on the ion concentration in the measurement liquid, by which the measurement bridge is detuned and a diagonal voltage is formed.
Devices known in the prior art differ among one another especially in how the pH-ISFET is integrated into the circuitry of the device. In
Analytical and Biomedical Applications of Ion-Selective Field Effect Transistors
, P. Bergveld, A. Sibbald, Elsevier Science Publishers B.V. Amsterdam 1988, Chapter 8, ISFET instrumentation, pp. 101-107, different types of interconnection of a pH-ISFET in a device of the initially mentioned type are disclosed. FIG. 8.3 illustrates a circuit diagram which is detailed in section 8.5. Accordingly the pH-ISFET is integrated into an electrometer subtractor (with operational amplifiers A
1
, A
2
, A
3
) such that it is located in the circuit at the site of that resistor of the electrometer subtractor by which the gain of the electrometer subtractor can be adjusted. The electrometer subtractor has a power supply with a current source which delivers a constant current I and an adjustable reference voltage V
ref
. At the input of the electrometer subtractor on a resistor R
1
there is a constant voltage I×R
1
. The output signal of the electrometer subtractor delivers an output voltage which is inversely proportional to the resistance of the channel of the pH-ISFET. The output voltage is inverted by means of an inverter (A
4
). Finally, the difference between the inverted output voltage and the reference voltage V
ref
is amplified by means of an operational amplifier (A
5
). The output of the final operational amplifier (A
5
) is fed back to the input of the electrometer subtractor so that a feedback current flows via the resistor (R
2
). In this way the source and drain voltages of the pH-ISFET can be controlled. Based on this control the drain current (I
D
) and the drain source voltage (V
DS
) can be kept constant.
The connection of the pH-ISFET in an electrometer subtractor however has the disadvantage that this circuit is very complex to build. In particular, it requires a large number of components, for example three operational amplifiers (A
1
, A
2
, A
3
) and six ohmic resistances (R
3
, R
4
, R
5
, R
6
, R
7
, R
8
). Due to the large number of components the production of the circuit is time-consuming and complex. The known connection of the pH-ISFET is moreover extremely susceptible to temperature drift.
SUMMARY OF THE INVENTION
The object of this invention is therefore to embody and develop a device of the initially mentioned type such that a pH-ISFET is integrated in a circuit which is as simple as possible, which has a limited number of components, and which at the same time is not very susceptible to temperature drift and has high measurement accuracy.
To achieve this object the present invention proposes that at least one pH-ISFET with at least three resistors be connected in a bridge circuit, the diagonal voltage of the bridge circuit is between the p-input and the n-input of an operational amplifier, with an output which is fed back via two resistors of the bridge circuit to the inputs of the operational amplifier and the output signal of the device is formed as an output voltage which is formed from the difference of the drain potential of the pH-ISFET and the reference potential of the reference electrode.
In this way the pH-ISFET is integrated in a circuit of especially simple structure. The circuit consists of an extremely small number of components and can thus be produced economically. In spite of the simple structure of the circuit, the device of the present invention has all the features necessary to ensure proper operation of the pH-ISFET. Thus, the pH-ISFET operated in the circuit delivers, for example, an output signal which is proportional to the input voltage on the pH-ISFET. Moreover, the output signal is a reliable measure of the ion concentration in the measurement liquid and thus of the pH of the measurement liquid. There is preferably a linear relationship between the common logarithm of the H
+
ion concentration in the measurement liquid and the output signal of the device.
The drain source voltage U
DS
on the pH-ISFET is set using the bridge feed voltage U
BSS
and the resistors R
2
and R
3
via which the output of the first operational amplifier is fed back to the inputs of the first operational amplifier. Here the following relationship applies:
U
DS
=U
BSS
×[R
3
/(
R
2
+R
3
)]
The drain source current I
DS
can then be set by means of the drain source voltage U
DS
and of the resistor R
1
via which the output of the first operational amplifier is fed back to the inputs of the first operational amplifier. Here the following relationship applies:
I
DS
=(
U
BSS
−U
DS
)/
R
1
By means of these relationships the working point of the pH-ISFET can be easily set without changing the bridge feed voltage U
BSS
.
The drain potential linearly follows the change of the gate potential caused by the change of the pH, since the operational amplifi
Endress + Hauser Conducta Gesellschaft für Mess - und Regel
Hamdan Wasseem H.
Hirshfeld Andrew H.
Hoffman Wasson & Gitler
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