JPH0339620B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a liquid helium level meter that measures the liquid helium level by changing the resistance of a superconducting wire.

<Prior art> Conventionally, a liquid helium level gauge that measures the level of liquid helium using the resistance change of a superconducting wire is configured as shown in Figure 7, and its configuration and measurement principle are as follows. That's right.

That is, in FIG. 7, 1 is liquid helium, 2
is a superconducting wire inserted into this liquid helium 1 from above, and 3 is a constant current source. 4 is a zero, full scale, and measure changeover switch; 5 is a zero adjustment resistor; 6 and 7 are differential amplifiers; 8 is a reference voltage setting device; and 9 is a display. Note that terminals a and b are connected to each other.

When measuring the liquid level of liquid helium 1, when a constant current is applied from both ends of superconducting wire 2 by constant current source 3, the part immersed in liquid helium 1 becomes superconducting, and its resistance value becomes zero. The voltage V at both ends of the superconducting wire 2 is proportional to the length of the normal conducting portion, that is, the length of the portion extending above the liquid helium 1.

Therefore, the resistance when the liquid helium 1 level is at the lower end of the superconducting wire 2, that is, at the zero point position, is Rl,
When the resistance of the superconducting wire 2 when the liquid level is at an arbitrary position x from the lower end of the superconducting wire is Rx, the relationship of equation (1) holds true.

x/l=k(1-Rx/Rl) (1) where l: length of superconducting wire k: proportionality coefficient In equation (1), any liquid level position x can be found by measuring Rx.

<Problems to be solved by the invention> However, in reality, the resistance of the normal conducting part of the superconducting wire 2 is temperature dependent, so the proportionality coefficient k is not constant, but depends on the temperature distribution T of the normal conducting part. (k
=k(T)). Furthermore, this temperature distribution changes depending on the level x of the liquid helium 1 (T=T(x)).
Therefore, the equation (1) becomes the following equation (2), and the liquid level ratio x/l and the resistance Rx/Rl have a nonlinear relationship.

x/l=k(x)(1-Rx/Rl) (2) The error caused by this nonlinearity usually has a maximum error of 10 to 15%. Therefore, the output calibration curve of the conventional liquid helium level gauge shown in FIG. 7 is greatly affected by the estimation error of the superconducting wire resistance at the zero level, as shown in FIG.

In addition, as a method to suppress this error, there is a method to reduce the temperature coefficient of resistance of the superconducting wire itself (Japanese Patent Laid-Open No. 49
-60761, JP-A-52-48996), but the reality is that there is still a resistance change of several percent over a wide range of temperature changes from room temperature to liquid helium temperature.

In view of the above-mentioned problems, an object of the present invention is to correct the influence of the temperature dependence of the resistance of a superconducting wire in any temperature distribution environment so that the liquid level can always be accurately measured.

<Means for Solving the Problem> The technical means of the present invention for solving this technical problem is to measure the liquid level of liquid helium using the resistance change of a superconducting wire inserted into liquid helium. In the helium liquid level gauge, annular parts of the superconducting wire bent in a direction substantially perpendicular to the longitudinal direction are provided at a plurality of reference positions in the longitudinal direction of the superconducting wire, and the liquid helium liquid level passes through these annular parts. By detecting the sudden change in the resistance of the superconducting wire when
The purpose is to calibrate the resistance value of the superconducting wire at this time using its reference position.

<Function> Since the resistance value of the superconducting wire 11 is calibrated with the actual level for each reference position of each annular portion r, highly accurate output can be obtained as shown in FIG. 4. Furthermore, since extrapolation calculations can be performed sequentially using the immediately preceding two measured values, a gradient with a high degree of approximation can be obtained, and therefore a highly accurate output can be obtained.

<Example> The present invention will be described below according to the illustrated embodiment. FIG. 1 shows an example of the circuit configuration of the present invention, and in the figure, 10 is a sensor having a superconducting wire 11; As shown in Fig. 2, there are a plurality of reference positions provided at regular intervals in the longitudinal direction.
An annular portion r of the superconducting wire 11 is provided which is bent in a direction perpendicular to the longitudinal direction. sensor 10
is installed in the dewar, and liquid helium 12 is injected from above. A current lead wire 13 and a voltage lead wire 14 are connected to the upper and lower ends of the superconducting wire 11. 16 is a constant current power supply, 17 is a differential amplifier, 18 is an A/D converter, 19 is a CPU, and 20 is a
RAM, 21 ROM, and 22 are indicators.

Next, the principle of operation of the circuit will be explained with reference to the relationship between the liquid helium level and the voltage V _IN applied to the superconducting wire 11 as shown in FIG.

The level of liquid helium 12 increases and the superconducting wire 1
1, _the voltage V _IN suddenly drops by a certain width. Since the CPU 19 measures the voltage V _IN continuously, the voltage V _IN
It is possible to recognize the point at which the value changes rapidly. The voltage V _IN at this time, that is, V ₀ is stored in the RAM 20 . As the level of liquid helium 12 increases and reaches the next annulus r ₁ , the CPU 19 detects a similar voltage drop in the voltage V _IN and changes the current voltage V _IN , that is, V ₁ ' and
V ₁ is stored in the RAM 20, and the slope k ₁ is calculated using the following equation (3), and the level value l shown in the following equation (4) is displayed on the display 22 as a level change until l ₂ is recognized. Output against.

K ₁ =l ₁ −l ₀ /V ₁ −V ₀ (3) However, k ₁ : Gradient l in the interval [l ₀ , l ₁ ]=l ₁ +k ₁ (V _IN −V ₁ )
(4) However, l: Level output value at any time in the interval [l ₁ , l ₂ ] When the CPU 19 recognizes _{the n+1th} annular part r using the same procedure, the voltage V _IN at that time, that is,
While storing Vm and Vm in RAM20,
Calculate the change gradient km using the following formula (5), and
2, the level shown in equation (6) below is output for the level change until l _n+1 is recognized.

Km＝lm−l _n−1 /Vm−V _n−1 (5) However, km: slope l in section [l _n−1 , lm] = lm＋km
(V _IN −Vm) (6) However, l: Level output value at any time in the section [lm, l _n+1 ] As described above, the resistance value of the superconducting wire 11 for each reference position of each annular part r Since it is calibrated using the actual level, highly accurate output can be obtained as shown in FIG. Furthermore, since extrapolation calculations are performed sequentially using the immediately preceding two measured values, a gradient with a high degree of approximation can be obtained, and therefore a highly accurate output can be obtained.

By the above method, even when using this liquid level meter for the first time, the liquid level can be measured while automatically calibrating. Note that the normal calibration method involves inserting the sensor 10 at a certain length into liquid helium 12 at a certain level accumulated in the dewar and comparing the output. This is different from the actual case where the sensor 10 is installed in the dewar and the liquid accumulates one after another, so an error occurs. This method is also characterized by the absence of such errors.

By the way, once each calibration value is memorized, the internal circuit is switched from calibration mode to measurement mode.
It is possible to output by interpolation using the stored calibration values. In this case, accuracy is further improved.

Note that in the first calibration mode, the interval [l ₀ , l ₁ ]
The output will be made by using the design value as the gradient.

FIG. 5 shows a block diagram of a measurement circuit to which the present invention is applied. In the same figure, 24 is an input circuit, and the resistance of the superconducting wire 11 similar to that of the previous embodiment is provided with a plurality of annular portions r in the longitudinal direction. Measure. Reference numeral 25 denotes a calibration value input device into which a level value of the liquid helium 12 corresponding to the resistance value currently measured by the input circuit 24 is input. Reference numeral 26 denotes a mode changeover switch, which allows switching between two modes: calibration mode and measurement mode. 27 is a set input circuit,
In the calibration mode, the resistance value R _c,o of the input circuit 24 and the level value l _c,o of the calibration value input circuit 25 are read simultaneously. Reference numeral 28 denotes a memory circuit, which stores the above-mentioned set of multiple read values (R _c,o・l _c,o ) [where n=1, 2, 3, . . .
..., m, m: number of read items] is stored. Reference numeral 29 denotes an arithmetic circuit that calculates the level of the liquid helium 12 using the resistance value R measured by the input circuit and the stored value (R _c,o ·l _c,o ) in the measurement mode. 30 is an arithmetic expression storage circuit which stores arithmetic expressions necessary for calculation by the arithmetic circuit 29, that is, the expressions in the above embodiments.
Memorize (5) and (6). 31 is a display circuit that displays the level of liquid helium 12.

Next, the operation will be explained.

First, switch 26 to calibration mode and apply liquid helium 12 to the dewar at a constant level.
Inject every l _c,o [where n = 1, 2, 3, ..., m, m: number of calibration points]. At that time, the resistance value R _c,o [n=1, 2, 3, ..., m] of the superconducting wire 11 is passed through the input circuit 24, and the level value l _c,o [n=1, 2,
3 _{, .}
It is stored as [n=1, 2, 3, ..., m].

Next, the switch 26 is switched to the measurement mode, and the resistance R of the superconducting wire 11 at the time of liquid helium measurement is read through the input circuit 24. In order to convert the resistance R to the level l of the liquid helium 11 at that time, data stored in the memory circuit 28 (R _c,o l _c,o ) [n=1, 2, 3, ..., m ] and resistance R
is calculated according to the arithmetic expression stored in the arithmetic expression storage circuit 30. This calculation formula is determined by the material of the superconducting wire 11. The level l obtained above is displayed on the display 31.

Note that the data stored in the memory circuit 28 is retained as long as the level of liquid helium 12 is continued to be measured in the same dewar. In addition, in order to perform measurement with another dewar, when the switch 26 is set to the calibration mode, the previous calibration memory value is automatically cleared, so recalibration can be performed.

By configuring the measurement circuit with the circuits described above, it can be used universally in various environments without being affected by the temperature distribution that differs depending on the dewar.
What's more, it can measure liquid helium levels with extremely high precision. In other words, the temperature distribution inside the dewar differs depending on the shape and structure of the dewar that stores liquid helium, and therefore the resistance value of the superconducting wire 11 differs even at the same liquid level, so the liquid helium level gauge must be calibrated for each dewar used. Although necessary, the liquid helium level can be measured with extremely high accuracy in a general-purpose manner even for various dewars having different shapes and structures. Incidentally, the operation of this measuring circuit is shown in a flowchart as shown in FIG.

<Effects of the Invention> According to the present invention, the annular portion r of the superconducting wire 11 bent in a direction substantially orthogonal to the longitudinal direction is provided at a plurality of reference positions in the longitudinal direction of the superconducting wire 11, and detects a sudden change in the resistance of the superconducting wire 11 when passing through those annular portions r, and calibrates the resistance value of the superconducting wire 11 at this time using the reference position. This non-linearity with respect to level changes in the resistance due to temperature dependence of the resistance can be corrected very accurately, and highly accurate output can be obtained.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure is a side view of the sensor part, Figure 3 is a graph showing the relationship between the liquid helium level and the voltage V _IN applied to the superconducting wire, and Figure 4 is the relationship between the liquid helium level and the level gauge output. 5 is a block circuit diagram showing another embodiment, FIG. 6 is a flowchart showing the operation of the same circuit, FIG. 7 is a configuration diagram showing a conventional example, and FIG. 8 is a diagram showing the liquid helium level and liquid in the conventional example. It is a graph showing the relationship with surface meter output. 11...Superconducting wire, 12...Liquid helium, r
...Annular part.

Claims (1)

[Claims] 1. In a liquid helium level meter that measures the liquid level of liquid helium 12 using the resistance change of superconducting wire 11 inserted into liquid helium 12,
At a plurality of reference positions in the longitudinal direction of the superconducting wire 11,
Annular portions r of the superconducting wire 11 bent in a direction substantially perpendicular to the longitudinal direction are provided, and a sudden change in resistance of the superconducting wire 11 when the liquid helium level passes through these annular portions r is detected. A liquid helium level gauge characterized in that the resistance value of the superconducting wire 11 at this time is calibrated by its reference position.