WO1980002600A1

WO1980002600A1 - A device for detecting and measuring small capacitance variations

Info

Publication number: WO1980002600A1
Application number: PCT/SE1979/000113
Authority: WO
Inventors: S Dahlqvist
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-22
Filing date: 1979-05-22
Publication date: 1980-11-27
Anticipated expiration: 1981-11-22
Also published as: EP0029023A1

Abstract

A device for detecting and measuring capacitance variations which are small in relation to the capacitance of a capacitive measuring body, preferably for measuring the concentration of a gas or liquid. The device includes a measuring cell (1) with a measuring capacitor (3) the capacitance variations of which are to be measured, and a detector unit (2) which converts the capacitance of the measuring capacitor into an electric signal. The device is characterized by an astable multivibrator (5) the output of which is connected, on the one hand, to the input of an inverter (6) and, on the other hand, to one input of an EXCLUSIVE-OR circuit (7), and by a monostable multivibrator (11) connected to the output of the inverter. The components of said multivibrator determining its pulse length includes said measuring capacitor. The output of the multivibrator is connected to a second input of the EXCLUSIVE-OR circuit which thereby at its output produces a pulse signal, called difference pulse signal, which is the difference in pulse length between the output signal of the monostable multivibrator and that of the astable multivibrator.

Description

A DEVICE FOR DETECTING AND MEASURING SMALL CAPACITANCE VARIATION

The present invention refers to a device for measuring and detecting small capacitance variations of a capacitive measuring cell. Such capacitance variations may be conditioned by the circumstances that the dielectric constant of the medium between the plates or foils of the measuring capacitor changes, that the distance between the plates or foils of the measuring capacitor changes or that the size of the plates or foils of the measuring capacitor changes. The invention is particularly directed to a device for measuring the concentration of a liquid or gas. Devices of the last-mentioned kind are known and include a capacitive measuring cell consisting of a measuring capacitor between the capacitor plates or foils of which the medium is disposed the concentration of which is to be measured. If the concentration is changed the capacitance of the measuring cell also changes. The disadvantage of these prior capacitive concentration meters is that they are of a very complicated and delicate construction.

The present invention aims at removing this disadvantage and refers to a device which is of a simple and reliable construction, and which permits measuring capacitance variations which are small in relation to the capacitance of the measuring cell. The characteristic features of the invention appear from the attached claim 1.

Many of the prior capacitive concentration meters are also strongly temperature dependent, which has the disadvantage that the temperature of the gas or liquid must be held constant. The present invention aims at eliminating this disadvantage.

For this purpose, the device according to the invention may optionally be provided with a temperature compensation circuit which eliminates the temperature dependence of the measurement results. In principle, the temperature compensation circuit is constructed in the same way as the device according to claim 1 but uses, instead of a measuring capacitor, a reference capacitor which is subjected to the same temperature variations as the measuring capacitor but which measures a medium of a known concentration. Thereby, it is possible to balance the dependence of the measurement results on temperature variations.

The invention will be described below in connection with a device for measuring the concentration of a liquid or gas, but it is to be understood that the invention is not restricted to this embodiment.

Figure 1 shows a circuit diagram of the device according to the invention, with an optionally connectable temperature compensation circuit. Figure 2 shows pulse shapes obtained at different points of the device according to Figure 1 with the temperature compensation circuit disconnected, while Figure 3 shows pulse shapes obtained at different points of the circuit according to Figure 1 with the temperature compensation circuit operative.

In Figure 1, the device according to the invention is show to include a capacitive measuring cell 1 and a detector unit 2. The measuring cell includes a measuring capacitor between the plates or foils of which the medium is dispose the concentration of which is to be measured. The measuring capacitor is connected with the detector unit through a line 4 of optional length. Since the dielectric constant of the liquid (or gas) is dependent on the concentration of the liquid (or gas), then the capacitance of the measuring capacitor is also dependent on the concentration.

The detector unit 2 converts the capacitance of the measur ing cell into a corresponding analogous voltage signal. The detector unit includes an astable multivibrator 5 the output of which is connected both to an inverter 6 and to one input of an EXCLUSIVE-OR circuit 7. The latter connection is formed through a switch 8 (in the illustrated po sition), a line 9 and a switch 10 (in the illustrated position). The switches 8 and 10 are preferably ganged together as indicated in the drawing by the dot and dash line. The output of the inverter is connected to the trigger input of a monostable multivibrator 11 the output of which is connected to the other input of the EXCLUSIVE-OR- circuit 7. As is well known, in response to an input signal, in the general case a monostable multivibrator delivers an output pulse the length of which is fixedly and unambiguously determined but is freely selectable through a suitable choice of certain RC elements incorporated in the multivibrator. In the present case the components of the multivibrator 11 determining its pulse length consist, on the one hand, of the measuring capacitor 3 and, on the other hand, by a potentiometer 12. Thus, it is clear that in the present case the multivibrator 11 delivers an output pulse the length of which is dependent on the capacitance of the measuring capacitor 3 and thereby on the concentration. The mode of operation of the device is most simply described in connection with Figure 2. The astable multivibrator 5 produces a train of pulses of constant frequency shown in Fig. 2a. The pulse length Tcl is constant. The pulse train is inverted in the inverter at the output of which the pulses according to Figure 2b are produced. These pulses are applied to the trigger input of the monostable multivibrator 11. The multivibrator is triggered by the negative edges of the pulse train 2b and delivers output pulses (Figure 2c) of varying pulse length T_c corresponding to the actually measured concentration. Thus, at the output of the EXCLUSIVE-OR circuit 7 there will occur pulses (Figure 2d), hereinafter called difference pulses, the length T_d of which is the difference between the pulse lengths T_a and T_c. By adjusting the potentiometer 12 T_d may be made very small in relation to T_a. If T_d is small, this means that a small change in T_c is reflected in a change in T_d which is great in relation to T_d. Therefore the device responds with great sensitivity to small changes of concentration. (If only the pulse length T_c should be utilized, which as mentioned above is proportional to the concentration, as a measure of the concentration, this would mean that a small change in the concentration is reflected as a small change in T which change will be small in relation tc T whereby small changes of concentration could not be detected with reliability.) The difference pulses (Figure 2d) are integrated in an integrator 13 which delivers a D.C. voltage the amplitude of which is proportional to the concentration. The integrator 13 also has a low-pass filter connected after the same, so that at the output of the integrator a smoothed D.C. voltage is obtained which is then applied to a buffer amplifier 14 so as to pass thereupon to a display and/or recording instrument.

The following experimental apparatus was used to determine the concentration dependence of the output signal derived from the buffer amplifier:

The astable multivibrator was of the TTL type (555:a connected as a TTL multivibrator) and produced output signals having a frequency of 6 kHz. The invertor 6 was of the 7414 type, the potentiometer 12 was one of 100 kΩ , the measuring capacitor 3 had a nominal capacitance of 3 nF, the multivibrator 11 was of the TTL type (built up of a 555:a), the EXCLUSIVE-OR circuit 7 was of the TTL type (7486), a resistor 15 of 10 kΩ connected the output of circuit 7 with the inverting input of the operational amplifier 16 of the integrator 13 which amplifier was of the 741 type and had a feed-back circuit formed by a parallel link consisting of a capacitor of 0.1 μF and a resistor of 100 kΩ . The non-inverted input of the operational amplifier 16 was connected via a potentiometer of 20 kΩ to a reference voltage source. The potentiometer was utilized for adjusting the D.C. voltage level of the operational amplifier 16 in a well-known way. The output signal of the operational amplifier 16 was filtered in the low-pass filter of the integrator which filter consisted of a capacitor 17 of 1 μF and a resistor 18 of 10 kΩ . The filtered output signal is fed to the buffer amplifier 14 which was of the 741 type. As the measuring capacitor 3 a conductivity cell (Philips PW 9511) with a cell constant of 1.44 was utilized. The measuring cell was immersed into various test tubes having ethanol-water mixtures of known concentration. The following values were measured:

percentage by weight of ethanol output signal, mV

80.5 -540 84.2 -347

86.6 -205 88.2 -120

From the above experiments it is seen that the sensitivity of the device amounts to about 50 mV/percentage by weight of ethanol.

The output signal of the measuring capacitor described above is dependent on temperature. To eliminate the temperature dependence a temperature compensation circuit 19, illustrated within the large dashed rectangle in Figure 1, can be optionally connected by means of the switches 8, 10. In principle, the circuit 19 is built in the same way as the detector unit 2 and includes an inverter 20 connected to the output of the astable multivibrator 5, the output of said multivibrator being connected to a monostable multivibrator 21 which is identical with the multivibrator 12 and is triggered by the trailing edge of the pulses from the inverter 20. A reference capacitor 22 and a potentiometer 23 are included in the components of the multivibrator determining the pulse length in the same way as the components 3 and 12 of the .multivibrator 11. The reference ca pacitor 22 is in this case located in a fluid of known concentration. If, for example, the measuring capacitor 3 consists of two plates submerged into a liquid, the reference capacitor may consist of a capacitor which is encased in a container (with a substance of known concentration) which together with the measuring capacitor is located in said fluid the concentration of which is to be measured and the temperature of which varies. The output signal from the monostable multivibrator 21 passes to said other input of the EXCLUSIVE-OR circuit 7 via the contact 10. The mode of operation of the device is most simply explained with the aid of Figure 3, where Figure 3a shown the pulse train from the astable multivibrator 45. The pulse train is inverted in the inverter 20 and gets the appearance according to Figure 3b. The trailing edge of the pulses in Figure 3b triggers the monostable multivibrator 11 which delivers an output signal according to Figure 3c where the pulse length T_c thus is dependent on the actually measured concentration. The output pulses (Figure 3a) from the astable multivibrator are also inverted in the inverter 20 and the trailing edge of the inverted pulses (Figure 3b) also triggers the monostable multivibrator 21 which delivers pulses (Figure 3d) the pulse length (T_d) of which is determined by the concentration of the known substance. At its output the EXCLUSIVE-OR circuit 7 delivers a pulse train (Figure 3e) in which the pulse length

T_e is the difference between T_c and T_d. Provided that the temperature coefficient of the capacitor 22 is equal to that of the capacitor 3 the pulse lengths T_c and T_d will change by equal amounts at equal temperature changes. The difference pulse signal T_e will thus be independent of temperature and may be integrated and filtered in the same way as the signal T_d according to Figure 2. The embodiment of the invention described. above may be modified and varied in many ways within the scope of the basic concept of the invention. For example, the measuring capacitor may be utilized for measuring pressure or position. In each of these cases the measuring capacitor is arranged such that the distance between the capacitor plates or foils changes in response to a change of* pressure or position, respectively. In the case of a change of position the device may be utilized as a position indicator.

Claims

1. A device for detecting and measuring capacitance variations which are small in relation to the capacitance of a capacitive measuring body, preferably for measuring the concentration of a gas or liquid, said device including a measuring cell (1) with a measuring capacitor (3) the capacitance variations of which are to be measured, and a detector unit (2) which converts the capacitance of the measuring capacitor into an electric signal, characterize by an astable multivibrator (5) the output of which is connected, on the one hand, to the input of an inverter (6) and, on the other hand, to one input of an EXCLUSIVE-OR circuit (7), and by a monostable multivibrator (11) connected to the output of the inverter, the components of said multivibrator determining its pulse length including said measuring capacitor and the output of said multivibr tor being connected to a second input of the EXCLUSIVE-OR circuit which thereby at its output produces a pulse signal, hereinafter called difference pulse signal, which is the difference in pulse length between the output signal of the monostable multivibrator and that of the astable multivibrator.

2. A device according to claim 1, characterized by the fact that said components determining the pulse length include a potentiometer (12) adjusted so that the difference pulse signal is short in relation to the pulse length of the output signal of the monostable multivibrator (11), whereby a small change in the capacitance of the measuring capacitor produces a great percentage change in the pulse length of the difference pulse signal.