If you need accuracy, you probably want a "load cell". Google it.
You might be able to calibrate your way to success with a piezo, but there are a lot of variables, like temperature that will have to be calibrated out. Better to start with the right sensor.
Probably your biggest problem is that 30 lbs of grams is more than the 10 bit (cough) accuracy you can get from the ADC on the Arduino. A lb is 453 grams, 30 lbs is 13,000+ grams. That means you will need a more accurate ADC, specifically, 14 bits, and you will need to be very careful about noise.
I'd say that you will need a pretty good quality load cell, and you need a good analog engineer to design you a PCB that will get you a real 14 bits from the load cell. Shielding will also be an issue.
It's definitely doable, just not easy.