i reli need your help

[allan11262k]

hi! i would like to ask if it possible to create a a device that determines the amount of LPG inside a tank by using a pressure sensor...
i mean, it is possible that the pressure correspond to a certain amount of gas inside the tank...

pls...can you also give me ideas on how can to make a circuit that will send SMS to my celfone if for instance, the level of gas inside the tank is almost empty...

pls help me asap!!! Thank you!

 

[stevez]

The pressure will not indicate the amount of gas. The gas that you are most interested in is a liquid at the temperatures and pressures that exist in a normal cylinder. As you open the valve and let some gas out (turn it over and liquid will come out) the pressure drops in the cylinder and the liquid boils to evaporate enough for the pressure to rise and stop the boiling.

The "boiling" process requires heat - that will come from the walls of the cylinder. With sufficient boiling the walls become cold to the touch - cold enough at times for frost to form where liquid is in contact with the cylinder walls. If you attempt to withdraw gas at a high rate there is not sufficient heat transfer and the cylinder pressure will drop - all depends on temperature outside the cylinder.

The simplest sensors look at temperature in a vertical line along the cylinder walls from top to bottom - where there is a radical difference (while gas is being withdrawn) will indicate the line between liquid and gas. The problem is that with no gas flow there is no temperature difference.

I've got to wonder if some kind of sonic or ultrasonic system wouldn't work. Liquid level measurements of this kind of thing is a normal everyday thing in industry but is also quite expensive because of the technology and safety issues. Weight is often used to indicate how full or not full a cylinder is - that requires some means to suspend the cylinder and a strain gage to indicate weight - this isn't difficult but does require some mechanical design work.

Be careful - the gas is quite flammable and if not poisonous it can displace oxygen - so if you aren't poisoned you'll suffocate under the right circumstances. Also note that the "cold" from boiling liquid can be harmful as well - to people and materials.