Hey Steve-
This is a multifacet challenge, which means a multi-facet solution.
Yes, propane distributors already do this kind of thing, but they generally don't do it for the CUSTOMER, they do it for THEMSELVES. They do it basically to automate the demand collection/monitoring process for 'keep full' contracts, and usually it's on tanks THEY own. they use a for-purpose tank reader sensor that clips over the top of the tank's regular 'swing needle' indicator... but one must recognize how that needle works...
The needle isn't physically connected to any mechanism THROUGH the tank... the tank is a high-pressure vessel, and doing so would place the gauge in situation of being a rupture or leak point, so rather, that needle is usually just a magnetic compass. The float level mechanism is connected to a rotating shaft that ends just UNDER that needle, and has a magnet bar on it. The needle you see, matches the position of the magnetic bar beneath...
...as long as the needle isn't sticky from being 300yrs old... and the magnets are still good...
In the case of MY tanks, about half of 'em have a piece of steel sitting on top, whose function is to serve as a whacker... when I go to LOOK at the gauges, I tap on 'em with the whacker a couple times to 'wake up' the needle, which will swing to a 'more suggestive' suggestor of content level...
Now, the 'clip on sensor' thingies the supply company uses, consist of a fluxgate sensor that measures the magnetic field under that needle (directly from the magnet inside, right?), and once it figures out what the angle is, it converts it to digital data, and sends it over what is usually a cellphone-like data connection. The propane supplier pays a fee for a low-volume data connection for each tank, and they integrate all those readings into their management software to figure out when they'll send more your way.
Now, I own my tanks, and I have MY contracts set up for 'will call' delivery. I never wind up placing a 'small' order, so don't worry about 'short loads'... but I DO like to see what my fuel levels are without trudging through three feet of snow in high wind, and my gauges are always sticky, So I use a more reliable method:
I weigh my tanks.
So your question has several steps for the answer: First of all, distance...
You're far from the tanks, and don't want to go there (like me, snow, and wind, just... farther.
but distance isn't the whole story... and it's not necessarily a problem... vector and obstruction is an important part... CAN YOU ESTABLISH A SIGHT-LINE betwixt the two points?
IF you have a pole, a tall building, a hillside, or some other situation at each end, that can establish a line-of-sight, then you can use ordinary off-the-shelf wifi routers (especially older, obsolete units) with a removable antenna, set to 'BRIDGE' mode, will provide a data connection from one to the other. IF you're in a situation where there's no utilty power available, a rechargable battery pack (or old lead-acid car battery) with a solar charger will do. You'll need a pair of directional antennas pointing towards eachother, and some suitable feedline (RF cabling) to connect the antennas to the routers.
Now... the INFORMATION...
build a pallet or platform to place under the tank. Install two load cells (strain gauges.. like single-ended shear-beam 2k capacity) to mount on one side of the pallet, and two support feet on the other. Half the weight of the tank will thence be borne atop the load cells.
Connect the load cells' excitation and signal wires together, into an inexpensive scale indicator. Use one that has a network connection, which you'll set to some static address that works in conjunction with your bridges and your home network.
Get a scale-tech friend to help get the initial calibration of the tank scale set up, and at some point, get the tank down to about 20% to get a good zero capture. Then fill the tank, use the fill receipt to calculate the filled weight (in a 1000gal tank, 80% from 20% is 600gal, so for a 500gal that's 300...) at propane's 4.24lbs/gal, you'll be around 4.24*300=1260lbs.
Set up the scale indicator to stream those weights. Find a code-kid to write a little program that'll display those weights on an HTML page (many scale indicators can include text strings inot the weights stream that can act as HTML, so a web browser will actually just show it like a web page).
If your code-kid is really good, he can whip up a little script that'll take regular readings of your tank, put it in a spreadsheet, and make it so you can see how fast you're using how much propane over a given timeframe.
Edited by DaveKamp - 8 minutes ago at 11:05am
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