KR20010089057A - Thermal conductivity detecting method for fluid and gas - Google Patents

Abstract

PURPOSE: An apparatus for measuring thermal conductivity of a liquid state and a gas state is provided to improve the accuracy of measurement of liquid state and gas state samples having a characteristic that a measurement error increases due to convection phenomenon when high heat capacity is applied in the measurement. CONSTITUTION: An apparatus for measuring thermal conductivity of a liquid state and a gas state includes a Wheatstone bridge circuit formed with a first resistance(R1) and a parallel cell opposite to each other, and a second resistance(R2) and a variable resistance(Rv) opposite to each other; a DMM1 and 2(20,30) connected crossedly to detect potential difference between opposite points of the Wheatstone bridge circuit; and a PC(40) receiving signals from the DMM1 and 2 for obtaining thermal conductivity by an algorism of λ=q/4πm.

Description

Thermal conductivity detecting device for liquid and gas phase

The present invention relates to a thermal conductivity measuring apparatus of liquid and gaseous phase, more specifically, a gaseous and liquid sample having a characteristic that the measurement error is increased due to convection phenomenon when the amount of heat applied during the measurement, unlike the solid phase sample The present invention relates to an apparatus for measuring thermal conductivity in liquid and gaseous phase, which improves measurement accuracy.

1 is a graph for schematically explaining the principle of calculating the thermal conductivity of a solid sample.

When a certain amount of heat is applied to a solid sample such as a metal, the temperature change T ₁ and T ₂ are measured at each time t ₁ and t ₂ , and thus the temperature change with time is linearized to some extent. This is expressed as follows.

As described above, since thermal conductivity of solid samples is less likely to cause errors due to measurement, many commercially available products are released at home and abroad by applying the algorithm represented by the above formula.

On the other hand, in the case of liquid phase or gaseous phase, due to convection phenomenon, it is difficult to determine thermal conductivity by the above algorithm.

Therefore, an object of the present invention is to solve the above-mentioned problems, and unlike the solid phase sample, when the amount of heat applied during measurement is large, the measurement error is increased in the gas phase and the liquid phase sample having the characteristic of increasing the measurement error due to the convection phenomenon. It provides a liquid and gas phase thermal conductivity measuring device to improve the accuracy.

1 is a graph for schematically explaining the principle of calculating the thermal conductivity of a solid sample,

2 is a simplified circuit diagram showing the connection of main parts of the device according to the present invention;

Figure 3 is a block diagram showing the decomposition of the parallel cell used in the present invention.

Explanation of symbols on the main parts of the drawings

10: parallel cell 11: thermocouple

12 copper wire 13 platinum wire

14 tube 20 DMM1

30: DMM2 40: PC

DMM: Digital Multimeter

In order to achieve the above object, the present invention includes a Wheatstone bridge circuit connected such that the first resistor R1 and the parallel cell 10 face each other and the second resistor R2 and the variable resistor Rv face each other; DMM1 & 2 (20) (30), which are crossed and connected respectively to detect the potential difference between opposite points of the Wheatstone bridge circuit; And receives the signal from the DMM1 & 2 (20) (30) PC 40 to obtain thermal conductivity by the algorithm of.

In this case, the balance cell 10 of the Wheatstone bridge circuit includes a platinum wire 13 for detecting a resistance change while supplying a heat source to the fluid and includes a thermocouple 11 for measuring an initial temperature of the measurement.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

Figure 2 is a simplified circuit diagram showing the main part connection state of the device according to the present invention, Figure 3 is a block diagram showing an exploded parallel cell used in the present invention.

When the infinitely long and thin line heat source is submerged in the fluid and heat flux is applied through the line heat source, the temperature change of the metal wire is shown as follows.

Where a is the radius of the metal wire, q is the heat flux applied per unit length, C is the exponential value of the Euler constant, Represents the thermal diffusivity of the fluid. Therefore, in order to measure the thermal conductivity of a fluid, when a certain heat flux q is supplied, the temperature change experienced by a metal heating wire according to time is obtained, and a linear relationship is shown between the temperature change and the logarithmic time. The thermal conductivity of the fluid can be obtained from

After all, in order to obtain the thermal conductivity of the fluid, it is necessary to obtain the temperature change of the metal wire over time. To this end, the Wheatstone bridge is constructed to measure the offset voltage over time, and the voltage is converted into the resistance change. The temperature relationship is used to change the temperature over time.

In FIG. 2, the apparatus according to the present invention includes a parallel cell 10 consisting of platinum wire and resistance, a Wheatstone bridge circuit using the same, and a DMM1 & 2 (20) measuring voltage change over time ( 30) and the part of the PC 40 which calculates the result and displays the result.

As discussed in the theory above, when the heat flux is applied to measure the temperature change of the hot wire over time, the resistance change of the hot wire is measured using a Wheatstone bridge and then converted into a temperature. In order to measure the resistance change of the hot wire, a whistle bridge is constructed. The whistle bridge includes a platinum wire 13 having a diameter of 25 u m included in the parallel cell 10 and a first resistance which is a standard resistance of 10 K ?? R1) and the second resistor R2, and a variable resistor Rv for balancing the circuit.

In FIG. 3, the platinum wire 13 is connected to the copper wire 12 and inserted into the tube 14 using an omega purity product of 99.9% or more. The length of the platinum wire 13 is measured using a precision altimeter (Cathetometer). Since the SUS cell, which is the main body of the parallel cell 10, is in direct contact with the fluid to measure thermal conductivity, proper sealing is performed.

Parallel cell 10 comprising platinum wire 13 is immersed in a water bath and maintained at a temperature within ± 0.1 K by an external bath circulator. The temperature of the fluid is measured within a range of ± 0.1 K using a K-type thermocouple and a Cole Parmer thermometer. The first resistor R1 and the second resistor R2 are manufactured by Yokogawa Co., Ltd. and have an accuracy of 0.01%. The variable resistor Rv uses the same resistance value as the platinum wire 13 within the measurement temperature range.

The connection of each resistor is soldered to minimize contact resistance and all wires are shielded to minimize external disturbances. Although not shown, in order to supply a constant power supply to the circuit, a switch that can be connected by changing a dummy circuit and a dummy circuit required for the measurement is attached. 100 for stable power supply when the switch is open in a circuit and connected to a dummy circuit Connect the resistance of.

The measurement unit uses DMM1 20 for measuring the voltage V AB between _AB and DMM2 30 for measuring the voltage supplied to the circuit. V ₀ represents the voltage of the power supply for supplying current to the circuit. Also included is a function generator for triggering to DMM1 & 2 (20) (30). DMM1 & 2 (20) (30) can use Hewlett-Packard's HP34401A and can measure the signal in uV unit up to 50 times / second rate when receiving data triggered from the outside. The received data is stored in the PC 40 through a visual basic program.

It looks at the action according to the configuration of the present invention through a specific experimental method.

After constructing the circuit as shown in FIG. 2, the parallel cell 10 is kept at a constant temperature in a tank filled with a fluid to be measured. When the parallel cell 10 is maintained at a constant temperature, a weak voltage of about 0.1V is applied to the circuit using a power supply. In this state, using the variable resistor (Rv) to maintain the circuit in balance so that the number of u V is V _AB. Turn off the thermostat to balance the circuit. When V _AB is almost 0, record the variable resistance (Rv) value, connect the switch to a dummy circuit and apply a voltage of 2.5 to 3 V.

Thereafter, the program of the PC 40 is operated to read data from the two DMM1 & 2 (20) 30 and the switch is connected to the Wheatstone bridge in this state to supply a stable voltage to the circuit. The magnitude of this voltage can vary depending on the experimental conditions. If the voltage is too small, the noise will be severe. If the voltage is too high, the heat generation will be large, so convection can occur. Therefore, supply a voltage of about 1-2 K during the measurement. When voltage is applied, heat is generated in the metal wires, thereby increasing the temperature of the metal wires, which causes a change in the resistance of the metal wires. This breaks the circuit's equilibrium and creates a voltage between the ABs. The time-dependent values of the offset voltage V _AB and the supplied voltage V ₀ are simultaneously recorded by the computer. V _AB and V _{0 over} time receive and store about 200 points.

Since the data obtained through the measurement are V _AB , V ₀ , the resistance change of the platinum wire and the amount of heat supplied are to be used. When a constant voltage is supplied to the circuit, using R ₁ = R ₂ , the potential difference between the resulting ABs is given by

Therefore, the resistance value of the metal wire over time can be expressed as follows.

That is, the resistance value of the metal wire with time can be obtained using the calculated V _AB , V ₀ . From this, it is necessary to find out the temperature change of the metal wire, which can be obtained by correcting the relationship between the temperature of the metal wire and the resistance as follows.

Therefore, the temperature change of the metal wire with time is as follows.

If the temperature T obtained through the given equations is plotted against ln t, there is a section with a straight line, and if the start and the end of this section are t ₁ and t ₂ , respectively, the slope m can be known. The heat flux generated in the metal wire does not have a constant value because the resistance of the metal wire changes as the power is supplied and the current flowing also changes. Therefore, the average value of heat flux is used during the measurement period. The following formula can be used for calculation.

At this time, t ₁ and t ₂ are the start time and end time of the measurement target section as mentioned above. Dividing this value by the length of the metal wire gives the heat flux per unit length, so the thermal conductivity of the fluid

Can be obtained. Since the temperature of the metal wire changes during the measurement process, use the value calculated as follows.

According to the above configuration and operation, the present invention has an effect of improving the accuracy of the measurement of the gas phase and liquid samples having a characteristic that the measurement error is increased due to the convection phenomenon when the amount of heat applied during the measurement, unlike the solid phase sample have.

Claims (3)

A Wheatstone bridge circuit connected such that the first resistor R1 and the parallel cell 10 face each other and the second resistor R2 and the variable resistor Rv face each other; DMM1 & 2 (20) (30), which are crossed and connected respectively to detect the potential difference between opposite points of the Wheatstone bridge circuit; And Receive a signal from the DMM1 & 2 (20) (30) Liquid crystal and gas phase thermal conductivity measurement apparatus comprising a PC (40) for obtaining the thermal conductivity by the algorithm of. The method of claim 1, The balance cell 10 of the Wheatstone bridge circuit includes a platinum wire 13 for detecting a resistance change while supplying a heat source to the fluid and has a thermocouple 11 for measuring an initial temperature of measurement. Liquid conductivity and gas phase thermal conductivity measuring device. The method of claim 1, The algorithm of the PC 40 is And the slope (m) Apparatus for measuring the thermal conductivity of liquid and gas phase is calculated from the relationship of.