Recommended Sensor / Type of the sensors to be used

This may be possible using quantum tunneling composites (qtc's) which use tunneling effects to drastically change in conductivity with applied pressure. You could use a grid of multiplexed electrodes on the back to "address" a particular spacial location of the qtc mat and measure its resistance. This would give you a grid of force data that could then be 'blob' tracked similar to image recognition algorithms. Once the 'blob' that is the object has been identified you could sum the forces contained within its boundaries to find the total weight. Similar things have been done in industry (http://www.nanotech-now.com/news.cgi?story_id=47663)

You can acquire qtc mats and pills from various sources around the internet. If you can not find a source of mats, you could use many qtc pills to form a mat.

Here are some sources of qtc pills http://www.mindsetsonline.co.uk/product_info.php?products_id=1144

Accuracy will depend on the dynamic range of your ADC circuitry as their resistance can vary on the order of 10 orders of magnitude depending on the pressure. I can imagine a digital capture circuit that uses feedback that selects the optimal reference voltage or voltage divider resistor. However, I believe your 1% accuracy requirement is far to high most technologies.

A simple sensor used in such an application is a magnetic reed switch.

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The simplest normally-open reed switch has two contacts, which are separated when there is no strong magnetic field incident on the switch. When the switch is exposed to a strong enough magnetic field, the contacts close and a circuit can be established through it. In your case, the float would have a small magnet on it, which passes close to the reed switch when the float moves.

The advantage of this approach is that no current flows, and hence no power is consumed, while the switch is open. Power requirement of the sensor is marginal even when the switch is off - just sufficient to overcome a pull-up resistor at the microcontroller sense pin, for instance.

^{simulate this circuit – Schematic created using CircuitLab}