Gas level guage for LPG cylinders

  I have a hard time deciding when to order for LPG gas cylinders which is used for cooking since I can't know how much more remains. Since LPG containers are not going to be replaced with hi-fi stuff in the near future (the gas companies must be having millions of such containers), I was thinking it would be good to make a small gadget which can be placed at the stove end, just before the burner, which could measure the amount of LPG left indirectly by measuring the pressure of the gas in the tube, and giving a visual indication of the amount of gas left. Could come precalibrated for different sizes of cylinders.