Gas Bro's sensor is gotten from Mopeka LLC, the global leader in gas cylinder monitoring.
From the ones below 5kg used in the kitchen to the 350,000kg tanks used by distributors

Small cylinder? Stick it under. It has magnets. Very big tank? Use the adhesive collar and fix it on top. Done in minutes
Set a particular level for the app to alert you for refill once your gas reaches that level. Never ever run out of gas again.