I found out a long time ago (personal experience) that apparently once you use one even partially, the powder gets under the seals and they bleed down rather rapidly. Be safe, be sure, buy a new one, and don't test them out. As the pilots say, "trust your instruments"... meaning in this case, the gauge with the red and green segment son it..
Excellent advice about using them for training. Beats having to read the instructions while a fire is raging..
But make sure you have a good one handy during this excercise.
Or two.
Terry, 230RN