LPG content indicator

[carlandrewg]

Can someone help me? I'm trying to build a device that would display the contents of an LPG tank.

 

[Nigel Goodwin]

As far as I'm aware you can either measure it's weight, or (more commonly for a fixed tank) the internal pressure.

 

[stevez]

The pressure would tell you little until all of the liquid is gone - if the "L" in LPG is for liquified. The same would be true if 100% of the volume were liquid - and only then would you know that you've got a very dangerous situation.

If some LPG is being drawn off then the liquid is being cooled as the heat in the liquid is part of the heat of vaporization. At the place were the liquid (below) and gas (above) meet gets very cold - at times you'll see frost. You might try to use temperature - possibly an array of sensors on the side of the tank.

As Nigel suggests, weight might be a good choice too. You could also use two pressure sensors - one at the very top of the tank and one at the very bottom. The pressure at the top will be relatively constant and is related to temperature - the pressure at the bottom is the pressure at the top plus the result of the liquid level - so the difference can be used to determine the level. In terms of safety, I'd avoid this approach unless you truly understand what you are doing.

 

[stevez]

As I clicked the button I realized I neglected the simple float mechanism that is on the large tank on my cottage.

 

[Nigel Goodwin]

The pressure would tell you little until all of the liquid is gone - if the "L" in LPG is for liquified.

I thought it was 'Low Pressure Gas'? - although I could be imagining it?.

 

[stevez]

The "L" could certainly be for Low - therefore my comment on it. Here in the US Propane and Liquified Petroleum Gas seem to be interchangable. I understand that LPG is largely propane anyway. Common uses are for home heating and the backyard barbeque grill. The advantage is that a moderate pressures it exists as a liquid so a lot of heat energy can be stored in a smaller space than a gas.

 

[Hero999]

The L stands for liquified.
https://en.wikipedia.org/wiki/Liquified_petroleum_gas

 

[carlandrewg]

Thanks guys. I would probably consider Nigel's suggestion about the weight thing. My other problem is that how would I convert weight into electrical pulses so that I can interface it to a PIC circuit? My idea would be to have two LCD displays. One for the Original weight of the LPG tank and the other for the percent of its original weight remaining. Another problem is, I'm new to PIC's and I really don't have any idea how to use one. so can you guys pls help?

 

[Nigel Goodwin]

First thing is the sensor - you can't really start designing until you have that. If it gives a voltage output then you need a PIC with analogue inputs to read that voltage.

As for displays, you could use a two line display, one line showing the original weight and the other showing the percentage left.

 

[stevez]

"Load cell" might be something to google on - look at the Omega site - good for lots of measurement info. The Omega site starts with strain gages and bridges so no need for me to repeat that.

An alternative to a load cell would be a spring that would deflect significantly when the tank is full - and show a measurable change in deflection when the tank is empty. Keep in mind that the tank has to move freely for this to provide any kind of reliability. You might also have a variable resistor that will be attached - the variation in resistance indicates the variation in position - from that you might get the weight change. A slide pot could be used - they do make LVDTs - more or less a linear position indicator. A rack and pinion could drive a rotary pot thru 180 deg or more of rotation. Just some thoughts on how to improvise at home - if that's what you are doing.

There are some other ways but this gives you a start.

 

[bobledoux]

I don't know where you are located so my comments refer to tanks produced in the USA. Tanks have a tare weight (empty weight) stamped on the side. They also have a weight assuming the tank is filled with water.

Propane weighs about 4.2 pounds per gallon. AT -44 degrees F it has a pressure of 0 PSI. At 60 degrees its about 100 PSI. Pressure is not a good fuel quantity measure. The liquid has a high coefficient of expansion. The volume of liquid propane goes up as the temperature goes up. This is why tanks are never filled beyond about 85% of their capacity.

The most reliable measure is weight.

One third party device employs a temperature stick that is applyed up the side of the tank. It works on the principle that the temperature will be different for the metal surface in contact with gas versus liquid.