Calculated value between to gas measurements

I have a table with gas measurements from different gas meters. Just a flat table with three column:

1. Date/Time

2. GasMeter

3. GasMeasure

This table is filled with gas measurements, recorded each minute (with a risk of skipping minutes now and then) for various gas meters. I would like to calculate gas usage per row: basically the difference between a gas measurement f and the previous gas measurement for the same meter.

Does anybody now how to do that?