EP0029023A1 - A device for detecting and measuring small capacitance variations - Google Patents

A device for detecting and measuring small capacitance variations

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Publication number
EP0029023A1
EP0029023A1 EP19790900533 EP79900533A EP0029023A1 EP 0029023 A1 EP0029023 A1 EP 0029023A1 EP 19790900533 EP19790900533 EP 19790900533 EP 79900533 A EP79900533 A EP 79900533A EP 0029023 A1 EP0029023 A1 EP 0029023A1
Authority
EP
European Patent Office
Prior art keywords
measuring
output
capacitance
capacitor
multivibrator
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19790900533
Other languages
German (de)
French (fr)
Inventor
Sten Axel Dahlqvist
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Individual
Original Assignee
Individual
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Individual filed Critical Individual
Publication of EP0029023A1 publication Critical patent/EP0029023A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
    • G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
    • G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
    • G01R27/2605—Measuring capacitance
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
    • G01N27/228—Circuits therefor

Definitions

  • 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.
  • the device according to the invention may optionally be provided with a temperature compensation circuit which eliminates the temperature dependence of the measurement results.
  • 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.
  • 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.
  • 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.
  • a monostable multivibrator 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.
  • 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.
  • 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.
  • Figure 2d hereinafter called difference pulses, the length T d of which is the difference between the pulse lengths T a and T c .
  • T d 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.
  • 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 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 k ⁇
  • 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 k ⁇ 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 ⁇ F and a resistor of 100 k ⁇ .
  • the non-inverted input of the operational amplifier 16 was connected via a potentiometer of 20 k ⁇ 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 ⁇ F and a resistor 18 of 10 k ⁇ .
  • 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:
  • the sensitivity of the device amounts to about 50 mV/percentage by weight of ethanol.
  • 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.
  • 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.
  • 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.
  • 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 .
  • 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 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.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

Abstract

Dispositif de detection et de mesure de variations de capacitances qui sont petites par rapport a la capacitance d'un corps de mesure capacitif, de preference pour mesurer la concentration d'un gaz ou d'un liquide. Le dispositif comprend une cellule de mesure (1) avec un condensateur de mesure (3) dont les variations de capacitance doivent etre mesurees, et un detecteur (2) qui convertit la capacitance du condensateur de mesure en un signal electrique. Le dispositif se caracterise par un multi-vibrateur non stable (5) dont la sortie est connectee, d'une part, a l'entree d'un inverseur (6) et, d'autre part, a une entree d'un circuit EXCLUSIF - OU (7), et par un multi-vibrateur monostable (11) connecte a la sortie de l'inverseur. Les composants dudit multi-vibrateur determinant sa longueur d'impulsion comprennent ledit condensateur de mesure. La sortie du multi-vibrateur est connectee a une seconde entree du circuit EXCLUSIF - OU qui par consequent produit un signal d'impulsion a sa sortie, que l'on appelle signal d'impulsion differentielle, representant la difference de longueur d'impulsion entre le signal de sortie du multi-vibrateur monostable et celui du multi-vibrateur non stable.Device for detecting and measuring variations in capacitances which are small compared to the capacitance of a capacitive measuring body, preferably for measuring the concentration of a gas or a liquid. The device comprises a measuring cell (1) with a measuring capacitor (3) whose variations in capacitance are to be measured, and a detector (2) which converts the capacitance of the measuring capacitor into an electrical signal. The device is characterized by an unstable multi-vibrator (5) whose output is connected, on the one hand, to the input of an inverter (6) and, on the other hand, to an input of a circuit EXCLUSIVE - OR (7), and by a monostable multi-vibrator (11) connected to the output of the inverter. The components of said multi-vibrator determining its pulse length include said measurement capacitor. The output of the multi-vibrator is connected to a second input of the EXCLUSIVE circuit - OR which consequently produces a pulse signal at its output, which is called differential pulse signal, representing the difference in pulse length between the output signal of the monostable multi-vibrator and that of the non-stable multi-vibrator.

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 Tc 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 Td of which is the difference between the pulse lengths Ta and Tc. By adjusting the potentiometer 12 Td may be made very small in relation to Ta. If Td is small, this means that a small change in Tc is reflected in a change in Td which is great in relation to Td. Therefore the device responds with great sensitivity to small changes of concentration. (If only the pulse length Tc 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 Tc 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 (Td) 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
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 Tc and Td will change by equal amounts at equal temperature changes. The difference pulse signal Te will thus be independent of temperature and may be integrated and filtered in the same way as the signal Td 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

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.
EP19790900533 1979-05-22 1980-12-01 A device for detecting and measuring small capacitance variations Withdrawn EP0029023A1 (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/SE1979/000113 WO1980002600A1 (en) 1979-05-22 1979-05-22 A device for detecting and measuring small capacitance variations

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EP0029023A1 true EP0029023A1 (en) 1981-05-27

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Cited By (1)

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Publication number Priority date Publication date Assignee Title
KR101256328B1 (en) * 2012-02-08 2013-04-18 박재덕 Portable chair

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Publication number Priority date Publication date Assignee Title
US4524088A (en) * 1983-01-31 1985-06-18 Magnetic Technologies Corp. Method and apparatus for controlling the thickness of developer on an applicator, such as a magnetic brush, in electrostatic reproduction
CN103592342A (en) * 2013-11-20 2014-02-19 四川研成通信科技有限公司 Soil humidity detecting device
EP2905901B1 (en) * 2014-02-06 2019-12-18 Brose Fahrzeugteile GmbH & Co. Kommanditgesellschaft, Bamberg Circuit assembly and method for detecting a capacity and/or a change in capacity of a capacitive component

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DE2101591A1 (en) * 1971-01-14 1972-07-27 Basf Ag Measuring circuit for capacitive hygrometers
IT938824B (en) * 1971-09-16 1973-02-10 Dasco Spa EXTERNAL REVEALS OF THE PRESENCE OF A FLUID IN PIPES OR PIENT VESSELS
US4099118A (en) * 1977-07-25 1978-07-04 Franklin Robert C Electronic wall stud sensor
SE409147B (en) * 1977-11-24 1979-07-30 Dahlqvist Sten Axel DEVICE FOR SATURATION AND DETECTION OF SMALL CAPACITY VARIATIONS

Non-Patent Citations (1)

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See references of WO8002600A1 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR101256328B1 (en) * 2012-02-08 2013-04-18 박재덕 Portable chair

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