To get as accurate a measure as possible with the NAU7802 and a strain gauge, you'll need to go through a calibration process. To do this, you weigh something whose weight you are sure of, such as conveniently named calibration weights, and figure out the offset value from the sensor.
Essentially, you want to see what the raw value is from the sensor when the known weight is on top of it. Then, you know that that raw value is equal to a certain weight and can figure out the weight of other objects going forward.
To enter calibration mode, select calibration from the mode selection list. To start, you'll be prompted to clear the scale. Press the blue button when its cleared.
Then, the code will run the
zero_channel() function to zero out the sensor. Press the blue button when it lights up.
You'll be prompted to put your calibration weight item on the scale. When it's on the scale, press the blue button.
The code measures the average raw value with the calibration weight. Then, it calculates the new offset value based on the average zero measure and average calibration weight measure.