RU2523085C1 - Liquid nitrogen level alarm - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: proposed gadget comprises thermistors arranged at control levels in Dewar to carry heating current. Measuring assembly of every thermistor defines thermistor resistance change, compares measured resistance with reference values and serves as the indicator. It is composed of microcontroller with current output and voltage measurement input connected to thermistor. Said microcontroller generates heating current at current output. Besides, it doubles as the device to update the thermistor reference resistance at generation of signal indicating that liquid nitrogen reaches the controlled level.

EFFECT: fast response, lower non-productive loses of liquid nitrogen, ruled out influence of ageing effects on control functions.

2 cl, 1 dwg

Description

The invention relates to techniques for measuring liquid levels and can be used in automatic automation and alarm systems for measuring liquid nitrogen.

A known indicator of the level of liquid nitrogen (US patent No. 4187723, IPC G01F 23/22, publ. 12.02.1980) containing the first and second resistors located at controlled levels in the dewar. When the resistors are immersed in liquid nitrogen, they cool, their resistance and voltage change, which is recorded by the comparison device and displayed on the indicator as an excess of the controlled level with liquid nitrogen. When the resistors exit the liquid nitrogen, they are heated by the direct current passing through them, their resistance and voltage drop across them change, which is recorded by the comparison device and displayed on the indicator as a decrease in the level of liquid nitrogen below the controlled levels.

The disadvantage of this detector is the effect of the aging of thermistors on the control result, as well as the low, often insufficient, performance of the detector when the level of liquid nitrogen decreases, since a thermistor cooled through liquid nitrogen, flowing through it with a small fixed current, requires a rather long time .

The known regulator of the level of liquid nitrogen (US patent No. 4404809, IPC F17C 13/02, publ. 09/20/1983) (prototype), which includes the first and second carbon resistors, the resistance of which changes when immersed in a low-temperature liquid, the first and second carbon resistors are positioned to detect upper and lower fluid levels; the first and second devices for determining changes in the resistance of the first and second carbon resistors, each of which contains a first and second reference resistor; and the first and second devices for comparing the resistances of the first and second carbon resistors with the resistances of the first and second reference resistors, respectively, and a device that responds to the signals of the first and second comparison devices.

The disadvantage of this indicator is also the possibility of the effect of the “aging” effect of thermistors on the control result, as well as the low, often insufficient, speed of the signaling device when emptying a container with liquid nitrogen, due to slow heating in a gas medium of thermistors cooled by liquid nitrogen with a slight heating current.

The value of the heating current of thermistors in such devices is limited by the maximum dissipated power of the thermistor in the gas environment: its increase leads to overheating of the thermistor in the gas environment and its failure, as well as to increased consumption of liquid nitrogen due to boiling and evaporation on the constantly excessively heated thermistor. In addition, with an increase in the value of the heating current, the performance of the device improves when emptying a container with liquid nitrogen, but the performance decreases when it is filled, since it takes a considerable time to cool an overheated thermistor with liquid nitrogen.

The objective of the invention is to increase the speed of the liquid nitrogen detector during emptying and filling the dewar, as well as to reduce the non-production losses of liquid nitrogen caused by boiling and evaporation of liquid nitrogen on a heated thermosensitive element and eliminating the influence on the result of monitoring changes in the characteristics of the thermistor due to the aging effect.

This problem is solved in that in the liquid nitrogen level switch, which includes thermistors located at controlled levels in the dewar and through which the heating current passes, the measuring unit of each of the thermistors, which performs the function of determining the change in resistance of the thermistor, the function of comparing the measured resistance with the reference one, and an indication function , made in the form of a microcontroller connected by a current output and an input for measuring voltage to a thermistor, and the microcontroller on the current output de heating current generates: the dewar filling - the minimum necessary to determine the change of the thermistor resistance, and when empty - is determined by the formula

$$i_P=i_0\cdot e^{\alpha \cdot |\; |\; |R-R_{\mathrm{uh}t}|\; |\; |},(\mathrm{one})$$

,Where $$\alpha =\frac{\ln \left(\frac{i_{PRed}}{i_0}\right)}{|\; |\; |N|\; |\; |}$$

,

R is the current value of the resistance of the thermistor, Ohm;

R _et - the reference value of the resistance of the thermistor, Ohm;

i _before - the maximum value of the current flowing momentarily through the thermistor, A;

i ₀ is the current value of the current passing through the thermistors, determined by the measuring circuit, A;

N is the threshold value of the difference between the values of the current resistance of the thermistor and its reference value, at which a decision is made on the absence of liquid nitrogen at a controlled level, Ohm.

The microcontroller additionally has the function of updating the reference value of the resistance of the thermistor when generating a signal that liquid nitrogen has reached a controlled level.

An example of a circuit of a signaling device is shown in FIG.

The signaling device consists of thermistors R1, R2, a microcontroller MK with a power circuit (not shown in Fig. 1) and indicator I. Thermistors R1, R2 are simultaneously connected to the corresponding current outputs Out1, Out2 and measuring analog inputs In1, In2 of the microcontroller MK. The output of the microcontroller MK is connected to the indicator I. Thermistors R1, R2 are located at the corresponding controlled levels inside the dewar (not shown in Fig. 1).

The signaling device for one monitoring point, for example, at the installation site of the thermistor R1 works as follows. Initially, the dewar is completely free of liquid nitrogen, and the indicator is in the filling control mode. From the output Out1 of the microcontroller MK, a small constant current i ₀ flows through the thermistor R1 located in the gas medium, which slightly heats the thermistor R1 to the equilibrium temperature and creates a voltage drop U _{1 on it} , which is fed to the analog measuring input In1 of the microcontroller MK. In the microcontroller MK, in accordance with a given program, the current value of the resistance of the thermistor R1 is calculated from the measured value of U ₁ and the known current value at the output Out1. In the microcontroller MK, the difference between the measured resistance value R1 and its reference value recorded during the initial calibration is calculated and compared with the program threshold value N. Since the thermistor R1 has a temperature higher than the temperature of liquid nitrogen, its measured resistance differs from the reference value. Their difference exceeds the threshold value N. The microcontroller MK at its information output generates a signal about the absence of liquid nitrogen at a controlled level, which is displayed using the And indicator in any form convenient for staff.

When filling the dewar with liquid nitrogen, it reaches the level at which the thermistor R1 is installed, and cools it to the temperature of liquid nitrogen. In this case, the resistance of the thermistor R1 changes, and the difference in resistance of the thermistor R1 and its reference value measured in the microcontroller MK becomes equal to or less than the threshold value N. The microcontroller MK at its information output generates a signal that liquid nitrogen has reached a controlled level, which is displayed using the And indicator in any form convenient for staff. The signaling device for this monitoring point enters the emptying control mode. In this case, the reference value of the resistance of the thermistor R1 is updated in the microcontroller MK.

The control emptying time from the current output Out1 microcontroller MC flows through the thermistor R1 corresponding constant current i _n1, which is formed in accordance with formula (1), implemented with software. When the thermistor R1 is in liquid nitrogen, its measured resistance is almost equal to the reference value, the current i _p1 in accordance with formula (1) is i ₀ . With a gradual decrease in the level of liquid nitrogen in the dewar at some point in time, the resistor R1 appears in the gaseous medium. The current i _p1 , flowing through the thermistor R1, begins to heat it, and the measured resistance of the thermistor R1 begins to differ from the reference value. In this case, the microcontroller MK in accordance with formula (1) generates a current i _p2, slightly greater than initially equal to i ₀ . Increased current i _n2 is even greater heating of the thermistor R1 and even greater deviation of its resistance measured from the reference value, resulting in the formation on the current output Out1 further current values i _n2. This process has an avalanche-like character and leads to the fact that the difference between the resistance R1 measured in the microcontroller MK and its reference value exceeds the programmed threshold N. In this case, a current equal to i ₀ is formed at the output Out1, the thermistor R1 takes the equilibrium temperature in the gas medium , the microcontroller MK at its information output generates a signal about the absence of liquid nitrogen at a controlled level in the place of installation of the resistor R1, which is displayed using the And indicator in any convenient for Sonala form, and the signaling device for a given control point goes into filling control mode.

The process of controlling the level of liquid nitrogen on which the thermistor R2 is installed is identical to the above. If necessary, the number of control points can be increased.

Due to the fact that the heating current in the mode of emptying control increases like an avalanche almost immediately when the thermistor housing exits from liquid nitrogen, the time required for heating the thermistor and the alarm is significantly reduced compared to using a small fixed heating current.

Reducing the response time in the emptying control mode increases the accuracy of the control, since with a long heating time at the moment the alarm is triggered, the level of liquid nitrogen may differ from the installation level of the thermistor.

Since the heating current in the filling control mode is set small, the thermistor in the gaseous medium does not overheat and its cooling when immersed in liquid nitrogen occurs almost instantly, and the alarm performance when filling the dewar with liquid nitrogen remains high.

Due to the fact that when a signal is generated that liquid nitrogen has reached a controlled level, the reference value of the resistance of the thermistor is updated, the influence of the change in the characteristics of the thermistor due to aging on the control result is prevented.

Claims (2)

1. The liquid nitrogen level switch, including thermistors located at controlled levels in the dewar and through which the heating current passes, the measuring unit of each of the thermistors, which performs the function of determining the change in resistance of the thermistor, the function of comparing the measured resistance with the reference one, and an indication function, characterized in that the measuring node of each of the thermistors is made in the form of a microcontroller connected by a current output and an input for measuring voltage to a thermistor, etc. why does the microcontroller at the current output generate a heating current: when filling the dewar - the minimum necessary to determine the change in resistance of the thermistor, and when emptying - determined by the formula

,
Where

,
R is the current value of the resistance of the thermistor, Ohm;
R _et - the reference value of the resistance of the thermistor, Ohm;
i _before - the maximum value of the current flowing momentarily through the thermistor, A;
i ₀ is the current value of the current passing through the thermistors, determined by the measuring circuit, A;
N is the threshold value of the difference between the values of the current resistance of the thermistor and its reference value, at which a decision is made on the absence of liquid nitrogen at a controlled level, Ohm. 2. The liquid nitrogen level switch according to claim 1, characterized in that the microcontroller additionally has the function of updating the reference value of the resistance of the thermistor when generating a signal that liquid nitrogen has reached a controlled level.