GB2067292A - Ice warning indicator - Google Patents

Abstract

The formation of ice on the surface of an embedded device 10 is detected by the rise in resistance between electrodes 12 and 13 detected by 26. The electrode 12 is cooled by the Peltier effect in response to current pulses from an amplifier 23 applied to a thermo- electric cooler 15. Therefore, incipient icing will lead to a detected rise in resistance regardless of the freezing point of the moisture on the ground. The cooling current pulses preferably alternate with heating pulses so that there is no long term disturbance of the heat balance at the device. <IMAGE>

Description

SPECIFICATION Ice warning indicator The present invention relates to an ice warning indicator which will provide a warning of imminent icing on a ground surface such as an airport runway or road surface. Proper safety precautions at an airport, for example, can only be based upon an advance warning when icing is about to occur.

De-icing chemicals can then be sprayed on the runway before icing actually occurs.

It is not possible to rely on a measurement of ground temperature for this purpose because the local freezing point of moisture is an unknown quantity (and is almost always below OOC if only because of the presence of previously applied deicing chemicals, typically common salt on roads and proprietary chemicals on runways).

We have previously described a system in our British Patent Specification 1 320 251 which uses a refrigerator controlled by an arrangement of temperature sensors to keep a first conductivity probe cooler than a second probe by a predetermined amount. An alarm circuit is responsive differentially to the signals for the two probes.

The object of the present invention is to provide an indicator which warns correctly of imminent icing, regardless of the local freezing point, but which is simpler than the indicator of specification 1320251.

According to the present invention, there is provided an ice warning indicator comprising two electrodes in a device which is, in use, embedded in the ground with the electrodes exposed, a circuit arranged to measure the resistance between the electrodes and to provide a warning signal when the resistance rises above a preset level, a thermoelectric cooler in the said device adjacent to the electrodes and a current source arranged to pass current through the cooler in the direction causing cooling.

The invention operates on the principle that the cooler will cause moisture to freeze on the device before it freezes generally. The conductivity of ice is substantially less than that of water and the measured resistance rises quite abruptly when moisture on the device freezes. The rise is large enough to discriminate the change caused by freezing from varying conductivities of the unfrozen moisture. (Some de-icing chemicals increase conductivity, other reduce conductivity.) However, the preset resistance level can be adjustable to optimise the response of the indicator in a given location.

The cooling current is also preferably adjustable to set the amount of warning given. The larger the current, the greater the differential between the uncooled ground temperature at which the warning is given and the actual freezing ground temperature. The current can be a continuous current of such magnitude that the balance of heat flowing into the cooled region against the thermoelectric removal of heat keeps moisture unfrozen until the said differential reduces to a predetermined value. The adiustment of this balance may be delicate and a large thermal mass may be needed to provide a steady inflow of heat to the cooled region. For this reason it is preferred to use a pulsed current. The amount of warning given can then be adjusted by adjusting either pulse amplitude o duration.The benefit of using current pulses is that sufficient time can be left between pulses to allow heat to flow into the cooled region, without any need for a larger thermal mass. If the said differential reduces to the predetermined value, freezing will occur temporarily after a current pulse. The warning signal then given can be latched to establish a continuous warning signal.

A further improvement is achieved by using pairs of pulses, a cooling pulse followed by an opposite, heating pulse, whereby the net heat balance is maintained undisturbed over the long term. If freezing occurs, it occurs between the two pulses. An embodiment of the invention which uses this technique will now be described, by way of example, with reference to the drawings, in which FIGURE 1 is a combined schematic and block circuit diagram, and FIGURE 2 shows waveforms at points denoted A, B, etc. in Figure 1.

The indicator shown in figure 1 comprises a device 10 which is embedded in the ground and connected by a four wire cable to a remote unit comprising the circuit shown in block diagram form. The device 10 comprises a canister 11 filled with an epoxy resin 19 in which are embedded a control electrode 12 and a ring electrode 13 extending round the central electrode. These electrodes are exposed at the top surface of the device and the resistance therebetween is determined by the conductivity of any moisture or ice which may be present on the device.

A thermo-electric cooler 1 5 is sandwiched between, but insulated from, the elctrode 12 and a block 1 7 of brass. The electrode 12 is also a block of brass and the two brass blocks provide a substantial thermal mass or heat sink for the cooler 1 5. The cooler 1 5 is a commercially available multi-junction Peltier effect device with two leads 25. The Peltier effect is reversible and current through the cooler 1 5 of one sense causes cooling while current of the opposite sense causes heating.

A pulse generator 20 generates pulses A (see Figure 2) whose rate can be adjusted by a control 21. Conventional pulse logic 22 provides a pulse train B consisting of pulses of alternating polarity and gating pulses C which are at 1 level between each positive pulse in pulse train B and the ensuing negative pulse. the pulse train B is applied to a current amplifier 23 with a control 24 for setting the amplitude. The output of the amplifier 23 is applied to the leads 25.

The resistance between the electrodes 12 and 13 measured by a resistance detector 26 connected to the electrodes by wires 27. A low frequency oscillator 28 running at say 13.5 Hz is connected across the electrodes. The voltage at the input to the resistance detector is proportional to the resistance between the electrodes since the oscillator 28 has a high output impedance. The use of alternating current avoids polarization problems. The detector provides a 0 level signal so long as the resistance is below a level present by a control 29. When the resistance rises above the preset level, the output switches to 1 level.

A three-input AND gate 30 provides an alarm signal when the resistance detector output is at 1 level and a pulse C is present and an enable signal is present on a line 31. This signal is provided by a conventional moisture detector such as that sold by Findlay, Irvine Ltd. under the designation ICELERT 168. When no moisture is present the enable signal is at 0 level which prevents a high resistance measurement arising from the absence of moisture giving rise to an alarm signal.

The alarm signal at the output of the gate is necessarily of brief duration. It is employed as a trigger signal to set a latch circuit 32 which is a retriggerable one-shut multivibrator which resets after a delay period preset by a control 33. The delay period is set to exceed the duration between cooling pulses, whereby a permanent latch signal is given so long as each cooling pulse leads to an output from the gate 30. The latch circuit can be interfaced to a buzzer or any other form of warning device.

Although pulse rates, etc. are not believed to be at all critical, suitable valves may be a pulse period of about 2 minutes for the pulses A with a pulse duration of about 30 seconds. The pulses C will then have a period of about minutes (and a duration of about 90 seconds). the amplitude of the pulses B at the output of the amplifier 23 may be in the range up to 1 amp. As described, the pulses B are uniformly spaced. This is not essential; the interval between a positive pulse and an ensuing negative pulse may be different from the interval between a negative pulse and an ensuing positive pulse.

If desired the ring electrode 13 can be omitted and its function be performed by the canister 11.

An additional advantage of the preferred embodiment is that, when moisture is frozen on the device, it is promptly re-melted and allowed to be displaced or dispersed normally along with the other surrounding moisture. Again, the advance warning depends on the amount of cooling applied, which can be varied (by control 24). If the ambient temperature is high enough, the thermoelectric cooling will be unable to cause any freezing but, when the ambient temperature is low enough nearly to cause freezing, the moisture on the device 10 will start to freeze and give the alarm, regardless of the ambient temperature at which this arises. The indicator can be supplemented by a device which measures and displays the actual ground temperature. This is important as some chemicals no longer have a deicing effect below a certain temperature.

Claims (4)

1. An ice warning indicator comprising two electrodes in a device which is, in use, embedded in the ground with the electrodes exposed, a circuit arranged to measure the resistance between the electrodes and to provide a warning signal when the resistance rises above a preset level, a thermoelectric cooler in the said device adjacent to the electrodes and a current source arranged to pass current through the cooler in the direction causing cooling.

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2. An ice warning indicator according to claim 1, wherein the current source passes current pulses in the direction causing cooling.

3. An ice warning indicator according to claim 2, wherein the current source passes current pulses in the direction causing heating alternately with the current pulses in the direction causing cooling.

4. An ice warning indicator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.