I'm not trying to discourage you, just want to point out that a tank of propane has propane in two phases, liquid and gas. As the gas phase propane is used the liquid propane undergoes an endothermic phase change from liquid to gas. This is why the tank gets cool as you use it. As the gas is used and the liquid changes phase and turns into gas the pressure will rollercoaster a little but remain relatively level until all the liquid has phase changed to gas, then you will get a pressure drop. 

A way around this would be to track flow and keep a running total from when you start out with a full tank. So, 5 lbs of propane equals 5x(gas expansion for propane) equals Y liters of gas. Differential Flow can be measured with your pressure transducer across a fixed orifice. 

Just trying to help. 

So, you are saying that as long as the flow is continuous the amount of gas is proportional to pressure? I could see that being the case, but it won't be linear and as soon as the tank temperature equilibrates then the pressure will come up again. A way to verify that would be to chart pressure over time at a fixed flow and measure tank temperature, then stop and let the temperature equilibrate then test again. I predict the pressure will be more temperature dependant than indicative of remaining propane in the tank. Tracking flow would be your best bet. I dont usually use a whole tank when I grill. 

I wonder if the rate of the pressure drop would be proportional to the amount of liquid in the tank?

Thanks I appreciate it. And to be honest, I had not thought about using my transducer to measure differential flow. I was kicking around some ideas with a hall effect sensor as a flow meter, but never thought of using pressure differential to measure flow. Wouldn't I need a second transducer for that? My only issue with that would be cost as the transducer already represents the majority of the cost of the prototype. 

Well, a quadrature encoder on the flow control knob might work instead but would have to be calibrated. Another thought would be adding a temperature sensor as well. Then you could play around with estimating the volume of liquid remaining with the rate of pressure drop (related to heat capacity of the liquid propane).

To use the hall effect sensor were you thinking of using a metal ball in a flow tube? Those are calibrated gas dependent. A CO2 flow tube may be close, but those things are expensive.

Arrrgh, you would need two thermisters. One for the tank and one for the ambient temperature.

Haha nice! I am using a temperature sensor as well. 

Also, I am only using temp sensor(s) to measure ambient temp. I decided that measuring tank temp would be redundant because pressure is effectively temp (r^2 of .99 based on many days of just letting the ProReg sit and collect pressure and ambient temp data). I am using multiple sensors to log ambient temp though...this is being done to reduce sensor error as much as possible. 

why not use a system like the a/c recharge systems use and just measure the mass of the tank?

 since the gas likes to stay at a constant pressure (at a given temp) mass is the only way to go with r134a

not to belittle the project, it looks pretty cool.

Andres, thanks for the compliment. There are many solutions out there that use a scale to measure the weight of the tank so I wanted to take a different approach with this project. The problem I had with the commercially sold scales was that they only provided about 10% accuracy. So with 26 hours of grilling time in a tank of propane, there was a 2.6 hour window that was unknown. The purpose of the ProReg is to try to achieve much higher accuracy using back-end data.

can it compensate for the gas cooling during high flow rates or is that a non-issue for this application? I'm not very familiar with the flow rates of a grill.

also, have you thought about using piezo elements? http://www.gemssensors.com/en/CustomerSupport/Literature-pdfs/Operating-Principle-Installation-and-Maintenance/ExOsense-Operating-Principle

andres, 

A Piezo would be great, but when I first started this project it was based on the principal that I could find a less expensive, and more accurate way to measure gas consumption in a residential barbecue. I tried to seek out all forms of sensors and their price points before building the prototype. My recollection is that the Piezo was too expensive to justify its own utility in the first application. If you can locate a piezo with +/- 1.5% accuracy for anything under $20, please let me know.  

For all those wondering the meaning of "ProReg", there are two words for each part of the syntax:

"Pro" = Propane and/or Professional

"Reg" = Regulator and/or Register 

Its a quadruple entendre.

Awesome project.

Thank You. I appreciate the comment!

I need this!