Thanks for all the responses!
To clear a few thinks up:
My scale showed empty weight as 2.2 lbs. (The tank was completely empty)
The manufacturer lists the weight online as 2.2 lbs
*I was not able to find the tare weight on my bottle I'll keep looking
I was expecting the filled capacity to be 3.7 lbs (2.2 + 1.5 (24 oz)). When I weighed the "filled" bottle it was only 3.0 lbs.
I went back to the store and told them that it was not filled all the way and asked them to fill it up. So they went in back and did what they could. The guy told me he placed 31 oz in the tank. It did feel heavier and when I got back home and weighted it the tank was 3.4 lbs. This was close enough for me.
Additionally, I tested my scale at home. I placed 4, 8, 10, 15 and 20 lbs dumbbells on it and the scale showed the correct weight exactly. This gives me some confidence in the calibration of my scale!
I would appreciate any comments