GB2157221A - Improvements in or relating to liquid level sensing systems - Google Patents

Abstract

A liquid level sensing system for a double-walled viewing window enclosing a body of transparent shielding liquid (14) comprises a thermistor (29) whose tip is immersed in a portion of the liquid within an upward extension (24) of the liquid filled chamber between inner and outer window panes (12, 13). The extension (24) is above the top edges of the windows (12, 13). The thermistor (29) is wired up to alarm equipment so that, in the event of a temperature change corresponding to interruption of contact between the liquid and the thermistor, alarm signals are generated. The window is intended particularly for a cave in which radioactive materials are to be handled remotely. <IMAGE>

Description

SPECIFICATION Improvements in or relating to liquid level sensing systems.

This invention relates to liquid level sensing systems.

According to this invention a liquid level sensing system comprises a temperature-responsive device for engaging the liquid, alarm means connected to the device and operable in response to temperature change of the device, the alarm means comprising audible means and visible means.

The device may comprise a thermistor. The thermistor may be connected in the input to an operational amplifier whose output is connected to a light emitting diode providing a visible alarm and a zener diode providing an output connected to control means for an audible alarm.

The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic section through a liquid -containing window with a liquid level probe; Figures 2 and 2a are part of an alarm circuit; Figure 3 is a further part of the circuit; Figure 4 is another part of the circuit; and Figure 5 is a circuit of a suitable voltage source for the alarm circuit.

A double-walled or double-paned window 10 Figure 1 has a frame 11 and the space 9 between the walls 12, 13 filled with liquid 14. In one example the window 10 forms part of the shield of a space within which radioactive material is handled from outside the shield by an operator using a manipulating device. The shield has a concrete wall 15 with windows 12, 13. Inner window 13 comprises cerium stabilised glass plates 16, 17 with gaskets 18. The window 12 comprises sodalime glass plates 19, 20. The space between the windows 12, 13 is filled with a transparent shielding liquid 14, for example zinc bromide solution. The frame 11 comprises spaced annular plates 21 embracing the glass panes and having clamping means 22 for effecting a liquid-tight seal.

The inner window panes may have different thicknesses, for example as at 16a, 17a. An annular epoxy seal 23 is provided. The space or chamber 29 has an upward extension 24 near the outer window provided with a filling spout 25 extending upwardly and outwardly through wall 15 to a mouth 26 outside the shield or cave.

It is desirable to monitor the level of the liquid 14 in the window 10 in case a leak should occur leading to a radiation hazard for the operator. The space or tank between the walls 12, 13 is filled by removing a filler plug 27 in mouth 26 and inserting liquid.

In the present case a bore through the plug 27 slidably and sealingly receives a polythene tube 28 which itself sealingly houses a thermistor 29 extending into the liquid 14 and having wires 30 extending to external alarm equipment illustrated in Figures 2 to 5, which in the case shown is for use with twenty two windows each having a thermistor 29.

Each thermistor 29 (Figure 4) is in series with resistor 29a and connected to operational amplifier 29b whose output is connected through resistor 29c and zener diode 29d to the zero rail, giving a logic output at 29e. The amplifier output also goes via resistor 29f to light emitting diode 299 connected to the negative rail.

Figure 2 shows the overall circuit including the zener diodes Z1 to Z22 of the twenty two circuits similar to Figure 4. The LED's are at 28 to 23B and the outputs of the thermistors are connected at 2 to 23. Integrated circuits A to F are quadrapole operational amplifiers acting as comparators. The logic output associated with diode Z1 and the first window goes to terminal H1 of the integrated circuit H and so on, the logic output of diode Z22 going to terminal 14 of integrated circuit J (Figure 3).

The circuits H and J act as Nand gates and are in circuit with integrated circuits K, L, M, N and 0 and relay 30. Alarms 31, 32, 33, 34, 35 are also in the circuit.

When the tip of the thermistor 29 is immersed in the liquid 14 of the respective window heat is rapidly conducted away and resistance is high, so that the output of the associated amplifier 29b is near the voltage of the positive rail and LED 29g is on, the logic output 29e is 5.1 volts as determined by zener diode 29d.

When all the thermistors 29 are thus immersed, the outputs of Nand gates H and J are low; these outputs are inversed by circuit K and this makes circuit L energise the dual-in-line reed relay 30 so that its contacts 36, 37 close but the alarm 31 does not sound because the voltage on pin 11 of circuit K is low.

If now the liquid level in one window falls below the tip of the respective thermistor 29, the temperature of the tip will increase and thus the thermistor resistance will decrease, causing the associated amplifier 29b to flip, extinguishing the diode 299 and the level at logic output 29e falls to -0.7 volts.

This makes one pin of circuit L so low that the relay contacts 36, 37 open. These contacts can be connected in circuit with a computer to record the time of this occurrence. The voltage on pin 11 of circuit K goes high, and this enables the other half of circuit L, whose other input is from a slow square-wave oscillator (pins 8, 9 of K), and five audible alarms 31-35 are energised in succession by circuits M, N and 0 with about a 0.5 sec interval between starting the successive alarms. The five alarms then continue sounding until the liquid level is restored.

A suitable voltage for the circuit is shown in Fig 5 in which mains supply is rectified and voltage regulators 40, 41, 42 are included.

A control box may be provided for all the LED 299 so that a defective window can be identified when the alarms sound. When a particular LED goes out this may send a signal to a data recording device to record the event.

Claims (5)

1. A viewing window comprising a body of transparent liquid enclosed within the space between inner and outer window panes, the window being provided with a liquid level sensing system which comprises a temperature-responsive device extending into the body of the liquid and providing an electrical output indicative of the temperature sensed and alarm means operable in response to any change in said electrical output corresponding to interruption of contact between the liquid and said device.

2. A viewing window as claimed in Claim 1 in which said alarm means is operable to provide audible and visible outputs.

3. A viewing window as claimed in Claim 1 or 2 in which said device comprises a thermistor.

4. A viewing window as claimed in Claim 1, 2 or 3 in which a portion of the body of liquid is located above the top edges of the inner and outer panes and said temperature-responsive device extends into said portion whereby the alarm means is actuated prior to the liquid level falling below said top edges.

5. A viewing window provided with a liquid level senising system substantially as hereinbefore described with reference to, sand as shown in, the accompanying drawings.