A DEVICE FOR DETECTING AND MEASURING SMALL CAPACITANCE VARIATIONS

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 capaci¬ tive measuring cell. Such capacitance variations may be conditioned by the circumstances that the dielectric σon- stant 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 in¬ clude a capacitive measuring cell consisting of a measur¬ ing 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 capa¬ citance of the measuring cell also changes. The disadvan¬ tage 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 va¬ riations which are small in relation to the capacitance of the measuring cell. The characteristic features of the in¬ vention 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 compensa¬ tion 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 sub¬ jected to the same temperature variations as the measur¬ ing capacitor but which measures a medium of a known con¬ centration. Thereby, it is possible to balance the depen- dence 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 tempera- ' ture compensation circuit. Figure 2 shows pulse shapes ob¬ tained at different points of the device according to Fi¬ gure 1 with the temperature compensation circuit discon- nected, while Figure 3 shows pulse shapes obtained at dif¬ ferent 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 measur¬ ing 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 meas¬ uring 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 connec¬ tion 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 trig- ger 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 sig¬ nal, in the general case a monostable multivibrator deli¬ vers 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 out¬ put pulse the length of which is dependent on the capaci¬ tance of the measuring capacitor 3 and thereby on the con¬ centration. 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 con- stant. The pulse train is inverted in the inverter at the output of which the pulses according to Figure 2b are pro- duced. 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 corresponding to the actually measured concentration. Thus, at the output of the EXCLUSIVE-OR circuit 7 there will occur pulses (Figure 2d) , hereinafter called diffe¬ rence pulses, the length T, of which is the difference bet¬ ween the pulse lengths T and T . By adjusting the poten¬ tiometer 12 T, may be made very small in relation to T . If Tj is small, this means that a small change in T is reflected in a change in , 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 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 relia bility.) The difference pulses (Figure 2d) are integrated in an integrator 13 which delivers a D.C. voltage the am- plitude of which is proportional to the concentration. Th 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 dis play and/or recording instrument. The following experimental apparatus was used to determin the concentration dependence of the output signal derived from the buffer amplifier: The astable multivibrator was of the TTL type (555:a con- nected 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 k i. , 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 ϋ connected the output of circuit 7 with the inverting input of the operational am¬ plifier 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 pF and a resistor of 100 k . The non-inverted input of the operational amplifier 16 was connected via a potentiometer of 20 k-O. to a reference voltage source. The potentiometer was uti¬ lized for adjusting the D.C. voltage level of the opera- tional- amplifier 16 in a well-known way. The output signal O.V. of the operational amplifier 16 was filtered in the low- -pass filter of the integrator which filter consisted of a capacitor 17 of 1 uF and a resistor 18 of 10 Λ . The filtered out- put signal is fed to the buffer amplifier 14 which was of the 741 type. As the measuring capacitor 3 a conductivi¬ ty cell (Philips P 9511) with a cell constant of 1.44 was utilized. The measuring cell was immersed into va¬ rious test tubes having ethanol-water mixtures of known concentration. The following values were measured: percentage by weight of ethanol . output signal, V 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 eight of ethanol. The output signal of the measuring capacitor described above is dependent on temperature. To eliminate the tempe¬ rature 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 mul¬ tivibrator 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 de¬ termining the pulse length in the same way as the compo¬ nents 3 and 12 of the. ultivibrator 11. The reference ca- 0MP1 A T -* lPθ -mi 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 refe¬ rence capacitor may consist of a capacitor which is en- cased in a container (with a substance of known concen¬ tration) 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 out¬ put 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 multi¬ vibrator 11 which delivers an output signal according to Figure 3c where the pulse length T thus is dependent on the actually measured concentration. The output pulses (Fi gure 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 multivibra tor 21 which delivers pulses (Figure 3d) the pulse length (T,) 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 leng Te is the difference between Tc„ and Td,. Provided that the temperature coefficient of the capacitor 22 is equal to that of the capacitor 3 the pulse lengths T. and T, will change by equal amounts at equal temperature changes. The difference pulse signal T will thus be independent of tern perature and may be integrated and filtered in the same way as the, signal , according to Figure 2. The embodiment of the invention described. bove may be mo¬ dified and varied in many ways within the scope of the ba¬ sic concept of the invention. For example, the measuring ■ capacitor may be utilized for measuring pressure or posi¬ tion. In each of these cases the measuring capacitor is arranged such that the distance between the capacitor pla¬ tes or foils changes in response to change of* pressure or position, respectively. In the case of a change of po¬ sition the device may be utilized as a position indicator,

Claims: Claims 1. A device for detecting and measuring capacitance varia tions 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 cap citance variations of which are to be measured, and a de¬ tector 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 co nected, 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) con¬ nected 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 sig¬ nal, 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 fac that said components determining the pulse length include a potentiometer (12) adjusted so that the difference pul¬ se signal is short in relation to the pulse length of the output signal of the monostable multivibrator (11) , where¬ by a small change in the capacitance of the measuring capa citor produces a great percentage change in the pulse length of the difference pulse signal. _________ , WiFO

Inventors: DAHLQVIST S
Applicants: DAHLQVIST S
Application year: 1979
Main classification: G01R27
IPC codes: G01N27, G01R27