Agilent Technologies 7890A GC Safety Manual page 13

Measuring hydrogen gas flows
Do not measure hydrogen together with air or oxygen. This can create explosive
WA RN ING
mixtures that may be ignited by the automatic ignitor.
To avoid this hazard:
Turn the automatic ignitor off before you begin.
Always measure gases separately.
When measuring gas flows on a detector using hydrogen for the detector flame
or carrier gas, measure the hydrogen flow separately. Never allow an air
stream to enter when hydrogen is present in the flow meter.
7890A GC Safety Manual 13