... only time will tell whether the life time of the built-in lithium battery will be as long as claimed by the manufacturer. This product will hopefully redeem the reputation of the manufacturer who dared to put on the market the previous unreliable product (now withdrawn from the list of their products) which comprised a part closer to a DIY job than to a properly manufactured one: the length of standard 15mm diameter copper pipe pompously called "antenna" contained 3 AAA batteries of very ordinary quality (incidentally this "spare" part costed a hefty £40 repeated too often to the like of unfortunate users!). That had to be said.
By contrast, the "Sonic Watchman Alarm Ultrasonic Oil Level monitor" uses the well proven ultrasonic technology for measuring the distance between the base of the device and the surface of the oil contained in the tank. Provided the installation simple steps - well explained by the manufacturer - are respected, the system will work properly and will display the level in the tank on the plugged unit with an acceptable accuracy. The useful feature of this product is the alarm going off if the oil level drops too quickly (leak or theft) or if the level is low enough to justify a prompt delivery from your usual heating oil supplier.