RU2186375C2 - Method and device for measurement of dew-point temperature - Google Patents

Abstract

FIELD: measurement of gas moisture content. SUBSTANCE: method consists in measurement of signal parameters at output of measurement circuit and temperature with the help of moisture-sensitive member included into measuring circuit. The present invention is based on development of device measuring dew-point temperature by measurement of amount of precipitated condensate and temperature on moisture-sensitive member by one means of measurement. EFFECT: higher accuracy and quick action of device. 3 cl, 2 dwg

Description

 The invention relates to the field of measuring gas humidity, in particular to hygrometers that measure humidity by dew point temperature.

 A known optical hygrometer (AS USSR 1714478, G 01 N 25/68, publ. 1992, BI 7) containing a thermoelectric battery, a thermal sensor located on it and a condensation mirror, which is a dielectric substrate with two electrodes. The surface of the dielectric substrate between the electrodes is covered with a conductive island film of critical thickness, which is designed to reduce the effect of contamination.

 Also known is a highly sensitive dew point hygrometer (PCT WO 98/12580, G 01 W 1/00), comprising a refrigeration-heating device, on the surface of which there is a surface acoustic resonator (SAW) and an integrated temperature sensor. The resonator is included in the measuring circuit and is used as a moisture-sensitive element for measuring the amount of condensed moisture. The processor means and the cooling-heating device provide control of the temperature of the moisture-sensitive element so that the amount of condensate on its surface remains constant, at the same time, the temperature on the cooled surface is measured with the integrated temperature sensor, thus the dew point temperature is constantly measured by the device.

 Also known is a miniature hygrometer and a method for measuring dew point temperature (US patent 5364185, G 01 N 25/68), which is the closest technical solution to the claimed, taken as a prototype, containing a moisture sensitive element, a cooling and heating device, a temperature sensor integrated on the surface refrigeration-heating device, processor means. The moisture-sensitive element is the interdigital transducer (IDT). The processor means sends a signal to the cooling / heating device for cooling. As the temperature decreases, the processor means processes the signals received from the moisture-sensing element and the temperature sensor, and the second derivative of the signal with respect to temperature is calculated. With an abrupt change in the value of the second derivative, a conclusion is drawn about the beginning of moisture condensation on the moisture-sensitive element and the temperature at the moment is fixed, which is the temperature of the dew point. After that, a signal is issued to increase the temperature and in the future the cycle repeats. With this method of calculating the dew point temperature with respect to the second derivative of the signal with respect to temperature, the influence of surface contamination of the moisture sensitive element is eliminated. The device as a whole is small in size and quite fast. The disadvantage of this invention, as well as others related to condensation hygrometers, is that the determination of the onset of moisture condensation on the cooled surface and the temperature measurement on it are made by two different measuring instruments. As a result of this, even with a large degree of integration of the temperature sensor with the condensate sensor, an error inevitably arises in measuring the temperature in the field of moisture condensation due to the spatial diversity of the sensors.

 The basis of the present invention is the creation of a device that measures the temperature of the dew point with high accuracy and speed by measuring the amount of precipitated condensate and the temperature on the sensitive element with one measuring tool.

 The problem is solved in that in the method for measuring the temperature of the dew point, measure the temperature of the cooled moisture-sensitive element, measure the signal parameters at the output of the measuring circuit including the moisture-sensitive element, determine the temperature of the dew point from a sharp change in the signal parameters, at the output of the measuring circuit, and then signal to increase the temperature, measure the temperature using a moisture-sensitive element included in the measuring circuit.

 In addition, isolated from the signal of the measuring circuit, including a moisture-sensitive element, components containing information about the temperature and about the beginning of condensation on its surface.

 In addition, the temperature characteristic of the moisture-sensitive element included in the measuring circuit by the reference temperature sensor is adjusted.

 The problem is also solved by the fact that in the device for measuring the dew point temperature containing a moisture sensitive element included in the measuring circuit, a cooling and heating means, processor means, while the output of the measuring circuit, the input of the cooling and heating means are connected to the corresponding inputs of the processor means, the moisture sensitive element is made with a hydrophobic coating and is temperature sensitive.

The invention is illustrated by specific examples of its implementation and the accompanying drawings and graphs, in which:
FIG. 1 depicts a general diagram of a device for measuring dew point temperature, where a moisture-sensitive element 2 with a hydrophobic coating 3 included in the measuring circuit 1 and a reference temperature sensor 5 located on the refrigeration-heating means 4, and the outputs of the measuring circuit 1, the reference temperature sensor 5 and the input of the refrigeration-heating means 4 is connected to the corresponding inputs of the processor means 6, the output of which is connected to the indicator means 7;
FIG. 2 is a graph of the dependence of lnρ $\genfrac{}{}{0ex}{}{\text{t}}{\text{new}}$ relative humidity φ for various materials.

 The device operates as follows.

 The main source of error when measuring dew point temperature using condensation hygrometers is due to the fact that in the known technical solutions, the temperature is measured and the amount of condensate deposited on the moisture-sensitive element is measured by two different measuring means, between which, due to their spacing in space, a temperature difference occurs leading to measurement error. To reduce it, the inventive device uses a moisture-sensitive element capable of measuring temperature and moisture condensed on its surface. Such a moisture-sensitive element can be, for example, a resonator on surface acoustic waves (SAW), a volume quartz resonator, etc.

Studies have shown that the process of moisture condensation on the cooled surface of quartz begins 9 degrees before reaching the dew point, which is confirmed by the data cited in the literature [Spensor-Gregory and Rourke. Hygrometry - M.: Metallurgizdat, 1963], this fact makes it difficult to isolate a component corresponding to temperature from a signal of a moisture-sensitive element. However, the process of deposition of water molecules on a cooled surface at temperatures close to the dew point temperature depends on the nature of the surface: the material and the degree of processing. Figure 2 below shows the experimental dependence for paraffin-based coatings, as well as dependencies taken from a literature source [Spensor-Gregory and Rourke. Hygrometry - M.: Metallurgizdat, 1963, p. 84], the value of lnρ $\genfrac{}{}{0ex}{}{\text{t}}{\text{new}}$ , which is included in the expression of resistance between parallel electrodes deposited on a cooled surface at a distance L and a width d, R = ρ $\genfrac{}{}{0ex}{}{\text{t}}{\text{pov}}$ L / d from humidity. When analyzing the dependences, it is seen that on the surface of hydrophobic materials, condensation at a relative humidity of less than 100% is practically absent, and at 100%, intense condensation begins. Therefore, the use of a hydrophobic coating deposited on the surface of the moisture-sensitive element, allows to prevent condensation from falling out until the temperature approaches the dew point temperature. Thus, a cooled moisture-sensitive element with a hydrophobic coating applied to its surface can be used to accurately measure the temperature before condensation begins, and the beginning of the condensation process is associated with the dew point temperature being reached.

 The application of this invention allows to increase the accuracy of measuring the temperature of the dew point, since the measurement of temperature and the moment of the beginning of condensation is carried out by one measuring tool, and thereby reduces the error in measuring the temperature of the dew point due to the fact that the temperature sensor and the sensing element are in different areas temperature field.

 In addition, measuring the temperature and the beginning of condensation with a single sensitive element makes it possible to use volumetric structures as a moisture sensitive element, for example a volume quartz resonator, since integration on the same surface of the temperature sensor and the moisture sensitive element to place them in one temperature field is not required, as well as makes it possible to increase the cooling rate of the surface of the moisture-sensitive element, since the creation of a uniform temperature field in which condensate must be a temperature sensor and, due to slow cooling.

 The technology of applying a hydrophobic coating is similar to the technology of applying sorbents to quartz resonators, used, for example, in gas analysis problems [V. Malov Piezoresonance sensors. - M .: Energoatomizdat, 1989].

 In FIG. 1 shows a general diagram of a device where a moisture-sensitive element 2 with a hydrophobic coating 3 included in the measuring circuit 1 is used as a moisture-sensitive element to obtain information about the temperature and the onset of condensation on its surface. The cooling-heating means 4 is intended to change the temperature of the moisture-sensitive element 2. The processor means 6 issues a command to the cooling-heating means 4 to lower the temperature. The signal generated by the measuring circuit 1, which simultaneously contains information about the temperature and the amount of condensate deposited on its surface, is processed by processor means 6. The hydrophobic coating 3 prevents condensation from falling out on the surface of the moisture-sensitive element 2 at a temperature higher than the dew point temperature. In the process of lowering the temperature, the signal parameters of the measuring circuit are analyzed, for example, the frequency, signal amplitude, etc., depending on the type and circuit of the inclusion of the moisture-sensitive element 2, which, for example, are given in the literature [V.V. Malov Piezoresonant sensors. - M .: Energoatomizdat, 1989, p. 84-90, 224-228]. The processor means 6 calculates the first and second derivatives of the indicated parameters, and if these parameters have undergone a sharp change, then the judgment is made that the dew has fallen, otherwise the temperature is determined using the same signal using the moisture-sensitive element 2 included in the measuring circuit. When the dew point temperature is reached, condensation forms on the surface of the moisture sensitive element. When analyzing the signal by means of a sharp change in the first or second derivatives of the signal with respect to time or temperature, the processor means 6 makes a judgment about the dew point temperature and displays the current temperature as the dew point temperature on the indicator means 7, after which it signals a temperature increase and then the cycle repeats.

To eliminate the effect of contamination of the surface of the moisture-sensitive element and, as a result, its temperature characteristic, after several heating-cooling cycles, an adjustment procedure is performed consisting of the following steps:
a) turn off the refrigeration-heating means 4 or it is transferred to a stationary mode;
b) using the reference temperature sensor 5, located in the same temperature region as the moisture-sensitive element 2, monitor temperature changes;
c) wait cycles are performed until the temperature drift is less than a predetermined value, which depends on the accuracy of measuring the dew point temperature, after which it is concluded that the temperature field in the measurement area has become uniform;
g) read the readings of the reference temperature sensor 5 and the moisture-sensitive element 2, operating in the temperature measurement mode;
d) the processor means 6 calculate the corrections to the temperature characteristic of the moisture-sensitive element 2, which will then be taken into account when measuring temperature using the moisture-sensitive element 2.

 Thus, in the claimed invention, in comparison with the prototype, the measurement of temperature and the moment of the beginning of condensation is made by one measuring means, i.e. there are no errors in measuring the dew point temperature, which arise due to the fact that the temperature sensor and the sensing element are in different areas of the temperature field, which allows to increase the accuracy of measuring the dew point temperature. In addition, measuring the onset of condensation and temperature with one tool allows you to increase the cooling rate, since no additional time is required to create a highly uniform temperature field and thereby increases the speed of the entire device. In addition, the combination of temperature measurement and the onset of condensation with one measuring device makes it possible to use volumetric structures, for example, a quartz resonator, as a moisture sensitive element, since integration of a temperature sensor and a moisture sensitive element on one surface for more accurate temperature measurement is not required.

 The invention can be used in those industries in which the humidity of gases is measured at the dew point.

 Moreover, all measuring circuits can be performed using integrated circuits, including using microprocessors. As a device for controlling temperature, a thermoelectric refrigerator can be used.

Claims (3)

 1. The method of measuring the temperature of the dew point, which consists in measuring the temperature of the cooled moisture-sensitive element, measuring the signal parameters at the output of the measuring circuit and determining the dew point from a sharp change in the signal parameters at the output of the measuring circuit, characterized in that the temperature is measured using the moisture sensitive element to the surface which hydrophobic coating is applied, and the temperature characteristic of the moisture-sensitive element included in the measuring circuit is adjusted according to Porn temperature sensor.  2. The method of measuring dew point temperature according to claim 1, characterized in that it is isolated from the signal of the measuring circuit, including a moisture-sensitive element, components containing information about the temperature and the beginning of condensation on its surface.  3. A device for measuring dew point temperature, including a moisture sensitive element, a refrigeration-heating means and processor means, while the output of the measuring circuit and the input of the refrigeration-heating means are connected to the corresponding inputs of the processor means, characterized in that it is equipped with a reference temperature sensor, moisture-sensitive the element included in the measuring circuit is temperature sensitive, and a hydrophobic coating is applied to its surface.

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