RU2494383C1 - Method for pulsed thermal express inspection of process liquids - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves rapid cyclic heating of the liquid with electric current pulses on the surface of a filament heater-probe. On each measuring pulse, starting with the second pulse, maximum temperature of the probe is successively increased, and the exceeding of the threshold duration of the measuring interval indicates a temperature value which violates thermal stability of the liquid on the given time interval. Maximum temperature of the probe is then successively lowered to a value corresponding to thermal stability of the liquid, and the inspection process is repeated in cycles.

EFFECT: high accuracy of long-term measurements and automation of the process of inspecting the quality of process liquids.

5 dwg

Description

The invention relates to analytical technique and can be used for operational thermophysical quality control of process fluids in a production environment without human intervention.

For these purposes, a known method of rapid analysis of liquid media (RU 2221238, publ. 10.01.2002) [1]. According to the known method, the test liquid is quickly heated by an electric current pulse on the surface of the wire heater probe to a temperature below the temperature of its achievable overheating, after which a second electric pulse is applied to the probe, reheated to the same temperature, the time of its achievement is measured, and the value of this time is judged about the value of the thermal resistance index of a liquid medium.

When a liquid medium is heated by the first (heating) electric current pulse to a selected temperature, a controlled temperature perturbation is introduced into it, which is estimated using the second (measuring) pulse, repeatedly reaching the selected temperature and recording the response as a pulse duration. The probe is a platinum wire with a diameter of 20 microns and a length of 1 cm. Included in the bridge circuit, it acts as a heater and a resistance thermometer. The resistance values in the high-resistance branch of the bridge are selected so that the balance of the bridge occurs at a probe temperature equal to the selected value.

However, such an indicator as the thermal resistance of a liquid medium used to evaluate the control physical parameter of a liquid depends on the initial temperature of the sample, which negatively affects the accuracy of long-term measurements. The objective of the present invention is to improve the accuracy of long-term measurements and automation of the process of quality control of process fluids.

To solve this problem, the method of heat-pulse express control of process liquids includes rapid cyclic heating of the liquid by electric current pulses on the surface of the wire heater probe, measuring the duration of the interval between successively reached balance states of the measuring bridge, using this value to estimate the control physical parameter of the liquid, by the value of which judge changes in the composition of the fluid compared to the standard, while as a control the physical parameter of the liquid, the temperature value of violation of its thermal stability is used, for which, at each measuring pulse, starting from the second, the maximum temperature of the probe is sequentially incremented, and when the measurement interval is exceeded, the temperature value violating the thermal stability of the liquid is judged for a given time interval, then the maximum probe temperature is successively reduced to a value corresponding to the thermal stability of the liquid, and cyclically oryayut control process.

The essence of the claimed method is as follows. The temperature of the violation of thermal stability T * characterizes the state of the liquid, but in contrast to the known method, where the thermal resistance of the liquid medium depending on the initial temperature of the sample is used as the control physical parameter of the liquid, the value of T * is determined only by the composition of the liquid and the time of thermal exposure. It has been experimentally established that for a certain class of liquids (for example, technical oils) there is a region of increased sensitivity of the T * value to the content of volatile impurities at a thermal exposure time of 1-10 ms. In case of violation of thermal stability, heat transfer between the liquid and the probe increases and the duration of the interval between successively achieved balance states of the measuring bridge increases (the duration of the measuring pulse Δt). Preliminary experiments determine the threshold value of the measuring pulse duration t _P , the excess of which indicates a violation of the thermal stability of the liquid. The threshold value of the duration of the measuring pulse in the claimed method is used as a criterion for software adjustment of the measuring bridge, which is carried out in accordance with the result obtained in the previous measurement.

The result is the transition of a given threshold for the duration of the measuring pulse. In the stable mode (Δt <t _P ), at each subsequent measurement step, the maximum temperature achieved in the pulse is incremented due to the adjustment of the balance point of the bridge. When a fact Δt> t _{P is} detected, the corresponding value of the probe temperature is remembered, and the bridge is rebuilt in the opposite direction, that is, the maximum probe temperature is reduced until the state Δt> t _P is reached, after which the algorithm is repeated.

Thus, the claimed method allows you to automatically track the temperature boundary of the region of thermal stability of the liquid, use the effect of the manifestation of violation of thermal stability of the liquid through an increase in the duration of the measuring pulse, and through the use of this effect, programmatically rebuild the measuring bridge during the measurement process.

A new technical result achieved by the claimed invention is to use the effect of violation of the thermal stability of the liquid, manifested through an increase in the duration of the measuring pulse, to control the composition of the process fluid and the possibility, based on the criterion of the threshold duration of the measuring pulse, to programmatically rebuild the measuring bridge during the measurement process.

The invention is illustrated by drawings, in which figure 1 shows a diagram of a measuring bridge; figure 2 - characteristic trajectories of the heating of the probe in the liquid under the conditions Δt <t _P ; figure 3 - characteristic trajectories of the heating of the probe in the liquid under the conditions Δt> t _P , here t * is the moment of violation of the thermal stability of the liquid. The durations of the heating sections (t ₀ ÷ t ₁ ) and the measurements Δt = t ₂ -t ₁ are in order of magnitude 10 ^-5 s and 10 ^-3 s, respectively. Figure 4 - calibration of the device with a stepwise increase in the temperature of the probe in transformer oil samples with a given moisture content, expressed in ppm or grams of moisture per ton of oil (g / t). Curves of the change in the voltage of the unbalance of the bridge in time over the measurement interval are presented. Curve 7 shows the process of losing the thermostability of the sample with hazardous oil moisture level c ₇ - 50 ppm (g / t) for heating the probe to a temperature _T1 = 530 K. Figure 5 - an example of the calibration curve for the turbine oils.

The measuring bridge circuit includes R1, R2, R3, R4, including a probe - a resistive temperature sensor R1, designed for immersion in a liquid. The probe is a platinum wire with a diameter of 20 microns and a length of 3 mm, enclosed between two electrical leads. Thus, the circuit (FIG. 1) includes R1 — probe resistance (resistance thermometer); R2, R4 - resistance of the shoulders of the bridge; R3 - digital potentiometer for tuning the bridge; IP - power source; KP - power switch; СБН - fast heating circuit; US - differential amplifier; K is a comparator; Tr - trigger; MP is a microprocessor.

An increase in the probe temperature is accompanied by a change in the potential difference between the terminals of the measuring diagonal of the bridge. This potential difference, which has a value of about tens of millivolts, is adopted by the differential signal amplifier US, which transmits the increased value to the comparator K. The signal value is compared with a zero value, that is, the balance condition of the bridge R1R3 = R2R4 is checked. At the moment of balance, a control signal for the MP and KP is formed at the output of the Tr trigger. The measuring time interval is determined between two points of the bridge balance, which are achieved in the process of unsteady heat transfer of a wire probe with a liquid. The duration of the measuring interval is determined by the intensity of heat transfer in the given conditions. The bridge is tuned to the test temperature using a digital potentiometer (CPU) R3, which is programmatically controlled by a microprocessor control (MP). MP together with the CPU carries out software restructuring of the measuring bridge for each measurement cycle.

начинает выполняться условие Δt>t_П, что свидетельствует о достижении температуры нарушения термоустойчивости жидкости T* за время t*<t_П. Поскольку температура T* монотонно зависит от концентрации летучей примеси, то, определив близкое к этой температуре значение $$T_1^{*}(c)$$

в серии опытов, можно формально соотнести это значение с содержанием примеси в образце.The probe heating at the command of the MP through the RLS and KP is performed in two stages with a significantly different speed due to a change in the heating power (see Fig. 2 and 3). At the first stage, the liquid is transferred to an area of increased sensitivity to the content of volatile impurities. Over a period of time t ₀ ÷ t _{1 the} temperature of the probe, and, consequently, the temperature of the wall layer of the fluid, increase to a predetermined temperature T ₁ . The second stage consists of the process "cooling - reheating to a value of T ₁ " (see figure 2). The deviation of the probe temperature T (t ₁ ÷ t ₂ ) from the value of T ₁ is units of degrees. This stage is measuring. The length of the segment Δt = t ₂ -t ₁ for a given heating function, selected as the measured parameter, depends on the state of the liquid. With an increase in the concentration of volatile impurities with a time t * of the existence of a liquid until a violation of thermal stability decreases, the value of T * decreases accordingly. Upon reaching a certain value of c, a violation of thermal stability is observed for a time t * less than the value of the threshold t _P. The phenomenon is accompanied by a sharp change in the intensity of heat transfer between the probe and the medium, which leads to a significant increase in Δt, i.e. the condition Δt> t _P begins to be satisfied (see Fig. 3). The same effect is observed with a stepwise increase in the temperature of the experiment T ₁ at a given value of c. At a certain value $$T_{\mathrm{one}}^{*}(c)$$

the condition Δt> t _P begins to be fulfilled, which indicates the achievement of the temperature of violation of the thermal stability of the liquid T * during the time t * <t _P. Since the temperature T * monotonically depends on the concentration of the volatile impurity, then, having determined a value close to this temperature $$T_{\mathrm{one}}^{*}(c)$$

in a series of experiments, we can formally correlate this value with the impurity content in the sample.

, полученные при нарастании значений температуры зонда. В результате циклического повторения алгоритма накапливается массив значений температуры $$T_1^{*}(c)$$

, отражающий изменения содержания примеси в процессе контроля.Automatic monitoring of the state of the liquid is ensured by cyclically increasing and decreasing the temperature of the probe near the desired boundary. To this end, the MP periodically initiates test pulses and reconfigures the bridge by reprogramming the CPU. In each experiment, the value of the measuring interval Δt is compared with a predetermined threshold t _P and the next experiment is prepared with a higher temperature if Δt <t _P or less if Δt> t _P. To exclude hysteresis, counts of values are taken into account $$T_{\mathrm{one}}^{*}(c)$$

obtained by increasing the probe temperature. As a result of cyclic repetition of the algorithm, an array of temperature values accumulates $$T_{\mathrm{one}}^{*}(c)$$

reflecting changes in the impurity content in the control process.

The indirect nature of the method involves carrying out calibration measurements on pure samples of oil and samples of oils certified for the content of known impurities. For the preparation of samples, a system of controlled saturation of oil with a volatile component from the vapor phase was used. Based on the operating conditions of turbine units and transformers, water was chosen as an impurity. A characteristic calibration curve for samples with a known moisture content is shown in FIG. 5. The resolution of the device in this case is 2 grams of moisture per ton of oil.

Thus, the claimed method of express control of process liquids allows you to automatically monitor the temperature limit of thermal stability of a liquid, to judge changes in its composition by changing the temperature value of violation of thermal stability, and by using the threshold phenomenon of increasing the duration of the measuring pulse in case of violation of thermal stability of the liquid, to perform program adjustment measuring bridge in the measurement process, which increases the accuracy of long-term measurements and omatiziruet procedure for rapid quality control of the process fluid.

Claims (1)

A method of pulsed thermal express control of process fluids, including rapid cyclic heating of a fluid by electric current pulses on the surface of a wire heater probe, measuring the duration of an interval between successively reached balance states of a measuring bridge, using this quantity to estimate a control physical parameter of a fluid, from which it is judged changes in the composition of the liquid compared to the standard, characterized in that as a control physical the temperature parameter of the violation of its thermal stability is used about the liquid parameter, for which each probe pulse, starting from the second, incrementally increases the maximum temperature of the probe, and judging by exceeding the threshold for the duration of the measuring interval, one judges the temperature value that violates the thermal stability of the liquid for a given time interval, then the maximum probe temperature is successively reduced to a value corresponding to the thermal stability of the liquid, and the process is cyclically repeated ss control.

Images