LT4095B - Capacitance hygrometer - Google Patents

Abstract

Invention belongs to metering devices, operating under the high frequency power field (HFPF) penetration principle and may be used to measure (based on dielectric constant) the humidity volume of various materials. In the sensor unit (1) are installed a couple of identical condensers Cw1, Cw2 with different frequency f and f/n and constant ratio voltage Uf and Uf/n drops. Their electrodes, extended on the surface, operate as ADEL antennas A1, A2, and because of the switch unit (2) can, alternately with standard condensers Co1, Co2, switch on the measuring bridge (8). And for received signal correction according to the type of measured medium, a correction chain unit (10) is installed before the integrator (11).

Description

Capacitive humidity meter belongs to measuring devices working on high frequency electric field (ADEL) penetration and can be used for volumetric measurement of moisture content of materials according to dielectric constant (DK) in agriculture, building materials, light, food and other industries.

Known cereal moisture tester GM200 from Denmark, using a system for measuring changes in resistance of a test substance (article in the newspaper "Farmer's Advisor" 08/07/1995 "How to determine the moisture content of cereals").

Lack of device - long, up to 1 min. the time-consuming preparation of the test material for measurement, and the limited measurement invariance of the device, e.g. it is only used to measure the moisture content of the grain.

Also known is a method and device for measuring soil moisture, which operates by comparing a dielectric constant (DK) [US5341673 G01N27 / 22 A method and device for determining the soil moisture level. Isobretenia stran mira N ^a 22. 1995],

The disadvantage of this device is also the limited invariance of the measurement, since the sensors of the device are mounted on a rod which is inserted into the soil and therefore cannot be used in solid bodies, fig. for measuring the humidity of wood.

Known Grain Humidity Gauge for Capacitive Moisture Gauges for Measuring Humidity of Grain and Similar Bulk Products by DK Constructed with Measuring Unit, Frequency Differential Keys, Inverter, Signal Detector, Summit, Low Frequency Filter, Generator Frequency Control Unit and Temperature Correction block [RU2034288 G01N27 / 22 Grain moisture meter. BI N ² 12 1995],

The disadvantage of this gauge is that the invariance of the gauge is limited to bulk products since all sensors, including the temperature gauge, are mounted in a rod-shaped unit with a separate lifting mechanism, which determines the complexity of the gauge design.

-2EN 4095 B

The nearest analogue to the prototype is the automatic humidity meter for measuring the moisture content of building materials, eliminating the effect of the active resistance on the measurement results. The humidity meter has a generator, a voltage stabilizer, a measuring bridge with a capacitance converter in the first branch, a capacitance converter in the second and identical capacitors in the second, and transformers in the third and fourth bridge branches, and a measuring bridge in the bridge. node [SU1368758 G01N27 / 22. Automatic Hygrometer BI N ^a 3. 1988].

The disadvantage of this humidity meter is that the measuring bridge uses constant capacitors and transformers that can operate only in low frequency electric field conditions, which reduces the sensitivity and accuracy of the device, and the need to place the test material between parallel capacitor electrodes. materials.

The object of the present invention is to extend the technical capabilities of the humidity meter in terms of measuring invariance, to increase the sensitivity and accuracy of the device, and to simplify the device design.

For this purpose, a capacitive humidity meter containing a sensor unit, an AD generator, a measuring bridge, a switching device, a stabilized current source, an optional bridge switching unit, a reference unit, a frequency divider, a detector, a correction circuit block, an integrator, an analog-to-digital converter In addition, all the elements of the device are housed in a common housing, and the switch unit provides the possibility to connect a transducer-probe.

The essence of the invention and its novelty according to the state of the art are determined by the following essential features: a pair of identical capacitors having capacitance dependent on the measured medium DK, having different frequency constant voltage drops; the switching unit has the possibility of switching to the measuring bridge alternating with the reference capacitors, and the correction of the received signals according to the type of the medium to be measured is preceded by an integrating rectifier block consisting of empirically selected resistors and a thermistor resistor mounted in the sensing unit a correction signal evaluating the temperature of the transmission integrator.

-3EN 4095 B

The construction of the humidity meter is explained graphically:

FIG. 1-flow chart of the humidity meter.

FIG. 2-Schematic diagram of the bridge.

FIG. 3-Use Scheme

FIG. 4-diagram of dependence of humidity W and capacitor capacitance C on various media: A for gaseous media, B for bulk materials, C for solids, D for liquids.

According to the flowchart of FIG. Capacitive humidity meter 1 comprises sensor unit 1, switch unit 2, reference unit 3, switches 4 and 5, of which switch 4 is connected to a relaxation generator 6 and this via frequency divider 7 is connected to switch 5, measuring bridge 8, frequency detector 9, a correction circuit block 10, an integrator 11 communicating with a stabilized current source 12, an ASK 13 and an indicator 14.

A schematic of the gauge bridge 8 is illustrated in Figs. 2.

Resistors R _b R; are installed in the adjacent branches of the bridge, and in the opposite, constant capacitance reference capacitors Coi, C02, are switched to the bridge alternately with a pair of identical capacitors Cw _b Cw _{2 of} transducer unit 1 having different frequencies f and f / n. voltage Uf and U _t / _n , and their electrodes are at the same time antenna Ai, A ₂ , moreover, the stabilized current source 12 through switch k ₂ may be alternately loaded with resistors Ro or R _t .

The humidity meter works as follows:

A moisture meter 15 is placed on a leveled (if test substance bulk) surface E (Fig. 3), with a pair of capacitors Cw _b Cw _{2 of} the sensor unit 1 flattened in a plane mounted on a dielectric base 16 at distances a to contact the test substance 17 via a dielectric a gap b consisting of an insulating electrode coating and, by receiving from the frequency divider 7 and the AD generator 6 different constant-frequency voltage drops, spreading the ADEL to the volume of the medium to be performed as an antenna; whereas the initial bridge bridge balance set at a constant frequency ratio is affected by humidity at volume 17 of the test environment, resulting in a difference in reactance between the capacitors pair of transducer block 1, Cw _lt Cw ₂ , resulting in a signal change through detector 9 and block circuit 10 comes to integrator 11 also receiving environment

-4LT 4095 B correcting signal from thermistor resistor 1 of sensors unit 1 via stabilized current source 12 and transmits it to ASK 13 and thence to digital indicator

14th

The invariance of the humidity meter measurement shall be proved as follows:

Capacitor capacitance has the following dependence:

S

C - K · - · ε where a

C - capacitor capacity of sensor unit 1;

K is the empirical coefficient;

- - ratio of capacitor electrode area S to electrode gap a;

ε is the dielectric permeability coefficient of the test medium.

Because the values S, a, K depend only on the design parameters and do not change over the course of measurement, only ε remains variable, varying from 1 for air and up to 80 for water, thus measuring various types of media: gaseous, bulk, solid and the liquid body moisture, C changes as shown in the diagram in Figs. 4.

During the initial alignment, a zero reference is entered by reference in block 3 to reflect the readings of indicator 14, then the humidity meter in switch unit 2 is set to operating mode and numerical value in indicator 14 judges the humidity of the test medium with a tenth% reference error.

For measuring the humidity of the liquid medium, a sensor of known design is used, which is connected to the humidity meter via a switch unit 2.

The positive effect of the proposed design humidity meter consists of the following factors:

1. The operation of the humidity meter is based on the principle of interaction between the humidity of the ADEL and the volume of the medium under study, which makes it possible to dispense with special preparation of the material for measurement by mechanical action or probing. This speeds up and lowers the humidity measurement process.

2. Measurement accuracy is ensured by a pair of identical capacitors in the sensor unit, which have voltage drops at constant ratios at different frequencies, since this eliminates side effects and the sensor unit signal is further adjusted to the ambient temperature.

-5LT 4095 B

3. The measuring bridge scheme provides for the possibility of alternating connection of reference or sensor block capacitors via a multi-contact switch, correcting the received signal change according to the measuring medium in the DK correction circuit block, which ensures the measurement invariance, i.e. the same humidity meter can be used to measure the humidity of different types of media: gas, bulk, solid and liquid.

4. Expanding in-plane transducer unit capacitor electrodes perform ADEL radiation-to-volume volume antennas, which allow all elements to be housed in a common instrument housing, and for liquid-medium testing, a removable transducer probe is added to simplify design and facilitate humidity meter operation.

Claims (6)

DEFINITION OF INVENTION 1. Capacitive humidity meter consisting of a sensor unit, a reference unit, a relaxation oscillator, a frequency divider, a pulse switching device, a measuring bridge, a detector, a stabilized current source, an integrator, an ASK and a digital indicator, characterized in that capacitance depends on the measuring medium DK, the pair having constant frequency voltage drops of different frequencies, their electrodes dissipate to the plane, while the ADEL antennas are connected to the measuring bridge alternately with the reference capacitors, and a rectifier circuit is mounted in front of the integrator. 2. Humidity meter according to p. 1., characterized in that the block of correction circuits consists of an empirically selected DK resistor according to the medium to be measured. 3. Humidity meter according to p. 1., characterized in that the thermistor resistor mounted in the sensor unit has a possibility to connect alternately with a reference impedance to a stabilized current source, the latter being further connected to an integrator. 4. Hygrometer according to p. 1. characterized in that all the elements of the humidity meter are mounted in the common housing of the device, and the switch unit is provided with the possibility to connect a portable sensor probe. 5. Hygrometer according to p. 1. characterized in that it is in contact with the capacitance of the capacitor block on the electrode planes of the sensor unit through the dielectric gap. 6. Humidity meter according to p. 5. characterized in that the dielectric gap is formed by an insulating electrode coating.