[Xringer]

I feel dumb for not noticing this feature before..
1. Turn on scale.
2. Press Zero/Tare. (for primary calibration).
3. Place weight (recovery tank) on scale.
4. Press Zero/Tare again, so the scale will remember that weight as a secondary calibration number.
5. The display will show 00:00 (But, if you lift up a 17.20 lb tank, -17.20 will be displayed).
6. Any weight (like R410a) added or taken away from the weight/tank on the scale will be displayed. (+ or -).

Timeout and the display goes dark. No big deal. You can come back hours later,
and press the Off-On button to wake up the scale and,
it will display the difference between the current weight and the weight saved in memory..
The Zero/Tare (or secondary calibration) weight (empty tank?) you saved in step 4 will stay in memory,
until you press the Z/T button again.. Regardless of the display shutting off.