Anti-theft alarm for a liquid storage tank

1. An anti-theft alarm for a liquid storage tank, said alarm comprising:

a housing having means for securing the housing to the storage tank, said housing containing a motion and/or vibration sensor for detecting tampering with the liquid storage tank;

an alarm means; and

a control means for activating the alarm means in response to a signal from the motion/vibration sensor;

wherein the control means controls the alarm means to perform a first action, to provide a relatively localized and short audible and/or visual alert, in response to a first detection of tampering by the motion/vibration sensor, the control means controlling the alarm means to perform a further action, to activate a full alarm response, in response to detection of tampering by the motion/vibration sensor one or more further times within one or more predetermined time periods, thus reducing the occurrence of false alarms and enabling the alarm means to provide a deterrent signal prior to full alarm activation.

2. An alarm as claimed in claim 1 , wherein the control means controls the alarm means to issue a brief audible alert in response to the first detection of tampering by the motion/vibration sensor, a longer audible and/or visual warning upon a second detection of tampering by the motion/vibration sensor within a predetermined time interval from said first detection, and the full alarm response upon a third detection of tampering by the motion/vibration sensor within a further predetermined time interval from said second detection.

3. An alarm as claimed in claim 2 , wherein the full alarm response includes a relatively prolonged visual and/or audible warning and/or transmission of an alarm signal to a remote location and/or activation of a further device.

4. An alarm as claimed in claim 1 , wherein alarm means are adapted to communicate with a remote location, for example to a pager or mobile phone, or to an alarm located at a remote location.

5. An alarm as claimed in claim 1 , wherein the housing is securable within an opening in the tank such that the housing extends into the tank to protect the alarm.

6. An alarm as claimed in claim 5 , wherein the housing is mounted on a filling cap for closing a filling opening of the tank such that the housing extends into the tank when the filling cap is fitted to the filling opening, preferably on an inner surface of a filling cap or on an exterior surface of the tank.

7. An alarm as claimed in claim 1 , further comprising a remote sensor for detecting motion and/or vibrations caused by environmental conditions, said control means responding to a signal from the remote sensor to prevent activation of the alarm means due to such environmental conditions, the remote sensor preferably being provided at a location remote from the oil tank closure.

8. An alarm as claimed in claim 7 , wherein said control means increases a threshold at which the alarm means is activated in response to said signal from the motion/vibration sensor, effectively reducing sensitivity of the anti-theft alarm, thereby reducing risk of false alarms due to said environmental conditions.

9. An alarm as claimed in claim 1 , wherein the control means includes a memory for recording operation of the alarm and wherein the control means is programmed to record time and date of each detection of tampering by the motion/vibration sensor.

10. An alarm as claimed in claim 1 , further comprising a level sensing means for monitoring the level of liquid in the storage tank.

11. An alarm as claimed in claim 10 , wherein the control means is programmed to provide a warning when the liquid level reaches a predetermined minimum level to alert a user to refill the tank.

12. An alarm as claimed in claim 11 , wherein such low liquid level warning is provided remotely from the storage tank, for example on a control panel within an adjacent building.

13. An alarm as claimed in claim 10 , wherein the level sensing means comprises a transmitter for transmitting a signal into the tank and a receiver for receiving a reflection of the signal from a surface of the liquid within the tank, and processing means for determining the level in the tank based upon a time delay and/or frequency shift in a received signal.

14. An alarm as claimed in claim 13 , wherein the transmitted signal is a sound and wherein the liquid level is determined by determining a resonant frequency of an air volume in a head space of the tank, which is a function of its volume.

15. An alarm as claimed in claim 1 , further comprising a heat or smoke detection means whereby the alarm means may be activated in an event of a fire and/or excessive temperature in or adjacent a tank to which the alarm is attached.