Pressure canning: ran out of propane

[Sandcastle]

Question: If you're pressure canning and run out of propane and the pressure drops below the recommend amount while you're changing out the tank do you have to start from the beginning or just subtract the time you were below pressure and proceed?

 

[Rockergirl]

I'm new to canning, but what I've read you are supposed to start over. But I don't recall where I've read it....so I could be wrong.