JP2000180244A - Liquid level detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid level detector capable of accurately detecting the liquid level of a sample and the arrival at the liquid surface with a simple structure where the electrostatic capacity changes rapidly when arriving at the liquid surface, and the reference value cannot be followed, so that there is no period when the difference between the AC output signal of the electrostatic capacity detecting circuit and the reference value held in the reference value holding circuit becomes smaller than a prescribed threshold in one cycle of the AC output signal. SOLUTION: This liquid level detector is equipped with a reference value holding circuit 5, which holds the peak value of AC output signal of an electrostatic capacity detecting circuit 4 as a reference value, a definite time constant longer than one cycle of the AC output signal, and a liquid level detecting circuit 6, which outputs liquid level detection signals on condition that there is no period when the difference between the AC output signal of the electrostatic capacity detecting circuit 4 and the reference value held in the reference value holding circuit 5 becomes smaller than a prescribed threshold in one cycle of the AC output signal, and does not output it if there is a period when the output signal of a differential circuit becomes smaller than the prescribed threshold in one cycle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid level detecting device for detecting whether or not a nozzle has reached a liquid level when sucking a sample in a container.

[0002]

2. Description of the Related Art In order to detect a liquid surface of a sample, a mechanism for detecting a change in capacitance of the detection probe using a nozzle itself as a detection probe is used (Japanese Patent Laid-Open No. 1-1788).
No. 26). According to this configuration, the tube for sucking the sample is used as the detection probe, and when the output of the detection probe exceeds the reference value, it is determined that the sample comes into contact with the sample liquid surface, and the detection signal is output.

In some cases, a sample container is sealed with a film for preventing evaporation. In this case, since the humidity inside the film is high and the detection probe gets wet and outputs even before it reaches the liquid level, the sample liquid level may not be accurately detected. Therefore, according to the technology described in the above publication, the output of the detection probe before the probe comes into contact with the sample liquid surface after the detection probe has penetrated the film can be set as a reference value by a command.

Thus, even if the capacitance value varies due to a water drop or the like, a reference value that absorbs the variation can be set, so that the liquid level can be reliably detected.

[0005]

However, in the technique described in the above-mentioned publication, a switch must be operated to set the timing for setting the reference value, and the detection probe penetrates the film for the operation of the switch. We need a way to know. For example, the operator visually confirms, or the sensor detects the vertical position of the detection probe.

[0006] However, the visual observation of the operator places a burden on the operator, and the installation of the sensor complicates the apparatus and increases the cost. Therefore, according to the present invention, when the sample container is sealed with a film for preventing evaporation, after the detection probe penetrates the film, the change in capacitance before contact with the sample liquid surface is relatively slow. The object of the present invention is to realize a liquid level detection device that can accurately detect a sample liquid level with a simple configuration.

[0007]

According to the present invention, there is provided a liquid level detecting apparatus comprising a constant time constant longer than one cycle of the AC output signal, using a peak value of the AC output signal of the capacitance detecting circuit as a reference value. And a difference between the AC output signal of the capacitance detection circuit and the reference value held by the reference value holding circuit is smaller than a predetermined threshold value during one cycle of the AC output signal. The liquid level detection signal is output on condition that there is no period in which the output level of the differential circuit becomes smaller. If there is a period in which the output signal of the differential circuit becomes smaller than a predetermined threshold value during one cycle, the liquid level detection signal is not output A liquid level detection circuit.

According to this configuration, after the detection probe breaks the film and before it reaches the liquid level, the change in capacitance is slower than the time constant of the reference value holding circuit. Follows changes in capacitance. Therefore, there is always a period in which the difference between the AC output signal of the capacitance detection circuit and the reference value held by the reference value holding circuit is smaller than a predetermined threshold value during one cycle.

On the other hand, at the moment when the liquid reaches the liquid level, the capacitance rapidly changes, so that the reference value cannot be followed. For this reason,
There is no period during which the difference becomes smaller than the predetermined threshold value during one cycle, and it is possible to accurately detect that the difference has reached the sample liquid level.

[0010]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a circuit configuration diagram of the liquid level detection device. The liquid level detecting device includes a mounting table 1 (working as a ground electrode) for a sample container 7, a detection probe 2 having at least a tip made of a metal and inserted into the sample container 7, and a capacitance of the detection probe 2. A capacitance detection circuit 4 for outputting an AC output signal having an amplitude corresponding to the reference value, a reference value holding circuit 5, and a liquid level detection circuit 6. 3 is a suction device.

The capacitance detection circuit 4 includes an AC signal oscillator 4
The AC frequency is set to about several kHz. The reference value holding circuit 5 is a peak hold circuit that holds the peak value of the AC output signal of the capacitance detection circuit 4 with a constant time constant, and the hold time constant is set to about 1 msec here. The retention of the peak value is reset manually or automatically every time measurement of one sample is completed (usually it takes several seconds).

The liquid level detection circuit 6 divides the voltage held by the buffer circuit 6a for inputting the AC output signal of the capacitance detection circuit 4 to perform impedance conversion and the signal held by the reference value holding circuit 5 (the voltage division ratio is 0 in this case). .8) voltage dividing circuit 6b,
A differential amplifier circuit 6 that amplifies the difference between the output signals of the buffer circuit 6a and the voltage divider circuit 6b (the amplification ratio is about 10).
c, a comparison circuit 6d for comparing the output signal of the differential amplifier circuit 6c with a predetermined threshold value, and a one-shot circuit 6e for changing the pulse-like output signal to a continuous signal of a constant level.

The operation of the above circuit will be described with reference to FIGS. FIG. 2 is a waveform diagram of each part when the detection probe does not reach the liquid surface and is dry. FIG. 2A shows an output signal of the buffer circuit 6a,
That is, the AC output signal a of the capacitance detection circuit 4 is represented. Let the amplitude be A. FIG. 2B shows a holding signal (the level is A) b of the reference value holding circuit 5 and a divided voltage signal c of the voltage dividing circuit 6b.

FIG. 2C shows a waveform d = ca obtained by subtracting the AC output signal a from the divided voltage signal c, and FIG.
, Ie, the output signal e of the differential amplifier circuit 6c
Represents In FIG. 2 (4), a threshold value is also entered. FIG. 2 (5) shows a signal f obtained by comparing the output signal e of the differential amplifier circuit 6c with the threshold value in the comparison circuit 6d. The signal f has a high level when the output signal e is lower than the threshold value, and has a low level 0 when the output signal e is higher than the threshold value.

FIG. 2 (6) shows a signal that holds the high level of the output signal e. From the above waveforms, when the AC output signal a is not 0 and exists and the holding signal b of the reference value holding circuit 5 follows it, the output signal e of the differential amplifier circuit 6c always becomes the threshold value. There is a period that is smaller,
If this period is detected, it can be determined that the detection probe has not reached the liquid surface.

The reason why the voltage dividing circuit 6b is provided is that there is a period in which the output signal e of the differential amplifier circuit 6c becomes negative as shown in FIG. If the voltage divider 6b is not provided, the waveform d in FIG. 2 (3) does not fall below 0, and the output signal e of the differential amplifier 6c does not fall below the threshold value due to some noise. Erroneous detection may occur.

FIG. 3 is a waveform diagram of each part when the detection probe does not reach the liquid surface and is wet. Fig. 3 (1)
Represents the AC output signal a of the capacitance detection circuit 4, the amplitude A of which is small. FIG. 3 (2) shows a holding signal (the level is A) b of the reference value holding circuit 5 and a divided voltage signal c of the voltage dividing circuit 6b, and FIG. FIG. 3 shows a waveform d = c−a obtained by subtracting the output signal “a”.
(4) represents the output signal e of the differential amplifier circuit 6c.

When comparing FIG. 3 (4) with FIG. 2 (4), FIG.
In (4), the amplitude of the waveform itself is small,
There is a period that is less than the threshold because it crosses the level. Therefore, as shown in FIG.
The output signal f of d shows a high level in one cycle,
The output signal of the one-shot circuit 6e holds a high level.

FIG. 4 shows waveforms at various points when a time t has elapsed since the detection probe reached the liquid surface. That time t
Is shorter than the hold time constant of the reference value holding circuit 5. Since the detection probe is on the liquid surface, the AC output signal a of the capacitance detection circuit 4 is 0 (FIG. 4).
(1)). However, the holding signal (the level is A) of the reference value holding circuit 5 holds the previous value (FIG. 4).
(2)), a waveform d obtained by subtracting the AC output signal a from the divided voltage signal c
= Ca remains and has a non-zero positive value. Therefore, since the amplified signal e always exceeds the threshold value, the output signal f of the comparison circuit 6d becomes 0, and the output signal of the one-shot circuit 6e holds a low level.

As described above, the output signal of the one-shot circuit 6e has a low level immediately after the detection probe has reached the liquid level, and since the output signal holds the value thereafter, it can be distinguished from a state in which the detection probe is wet.

[0021]

As described above, according to the liquid level detecting device of the present invention, even when the sample container is sealed with a film for preventing evaporation, the sample liquid level can be accurately detected with a simple structure. Can do.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a detection circuit configuration of a liquid level detection device according to the present invention.

FIG. 2 is a waveform diagram showing signal waveforms at various parts of a detection circuit when a detection probe is dried.

FIG. 3 is a waveform chart showing signal waveforms of various parts of the detection circuit when the detection probe is wet.

FIG. 4 is a waveform diagram showing signal waveforms of various parts of a detection circuit at the moment when a detection probe comes into contact with a liquid.

[Explanation of symbols]

 Reference Signs List 1 installation table 2 detection probe 3 suction device 4 capacitance detection circuit 5 reference value holding circuit 6 liquid level detection circuit 7 sample container 6a buffer circuit 6b voltage dividing circuit 6c differential amplifier circuit 6d comparison circuit 6e one-shot circuit

Claims (1)

[Claims] 1. A grounding electrode for grounding a sample container, a detection probe inserted into the sample container, and a capacitance detection circuit for outputting an AC output signal having an amplitude corresponding to the capacitance of the detection probe. A reference value holding circuit that holds a peak value of the AC output signal of the capacitance detection circuit as a reference value with a constant time constant longer than one cycle of the AC output signal, The condition that the difference between the AC output signal of the capacitance detection circuit and the reference value held by the reference value holding circuit does not have a period smaller than a predetermined threshold value during one cycle of the AC output signal. A liquid level detection circuit that outputs a liquid level detection signal and does not output a liquid level detection signal if there is a period during which the output signal of the differential circuit becomes smaller than a predetermined threshold value during one cycle. Liquid level inspection characterized by Apparatus.