I need a sensor to measure the level of a fuel tank in a gas station. I don't need something "fancy". Any resolution and accuracy will be OK. Of course, it must comply with the safety requirement of a gas station. A 4-20ma output will be perfect, but something else is also OK.
How about an ultrasonic transducer? They can measure distance from the top of the tank to the gas level. An alternative is a float on the gasoline, or a pressure sensor at the bottom of the tank. Would any of these work?
The bubbler would work, too. It's simple enough to implement with an air source, a precision low pressure regulator, and a P/I transducer located outside the tank.
But I can see a few potential problems with it.
Left on constantly, the air flow, however small, may flush fuel vapor from the tank. That in turn may cause an actual or perceived fire hazard, odor complaints, and some product loss. All of which could be minimized, depending on the actual application, by only 'querying' the gage, i.e., turning on the air source, when the tank level data is actually needed.
Druck makes a nice submersible probe with a 4 to 20ma output. I just bought one for dam level and the rep showed me one for petroleum applications.. They have two accuracies with 2 price ranges... $600 to $900
Yes, a flange-mounted magnetostrictive animal like the one referred to below is probably the best value-for-money for 4-20mA or digital level transmission - but for this application you *will* need something flameproof (the LTM-100) or IS (the LTM-300) or you could be fined severely.
Radar and ultrasonic are generally overkill for underground fuel tanks (and there is no easy way to check that they are working correctly), and servo-gauges are too expensive for small tanks.
The vast majority of service stations world-wide use capacitive probes (eg. the old Enraf "Stic" gauges) for this job. Do you need Custody Transfer Approval? If so, an approved capacitive probe will be your best option.
If you want to get fancy and have $2000 US the company called VEGA have a microwave radar sensor that will give you a 4-20mA via an accurate radar measurement. You can even opperate via a plastic/glass/ceramic lid in the roof of the tank. Problem solved.
A pressure sensor (basically a diaphragme with a strain guage mounted could be submerged and give you a presssure reading. This will give you the mass of the fuel above the sensor and as the density of gasoline is 0.72L/kg the depth.
The pressure sensor will needed to be connected to a strainguage/pressure sensor via 'intrinsically safe barriers' or better still 'intrinsicaly safe isolation amplifiers'. Thes barriers use a combination of resisotors, zeners and fuses to ensure that there is no way there will ever be enough current of voltage to cuase an ignition. (zener clamps the voltage, resister limit current and fuse blows to prevent the zener of resistor overloading)
Use the isolation amplifiers as they do not need an 'earth' that will need to conform to all sorts of onerous earthing requirments. (they are essentialy the same price anyway)
Contact a company called "MTL" who will have the ability to advise you further. Measurement Technology Limited do this sort of stuff all the time for petrochemicals industries.
Any stainguyage should do so long as it is all staineless steel (not aluminium).
Or you can use two pressure sensors separated vertically by some known distance. Then you can calculate the density (if you have wide temperature variations) and get an accurate indication of the weight of gasoline in the tank, interesting if that's what you pay for. The possibilities are endless...
I'm assuming this is an underground tank. How do you install the lower pressure transmitter? Actually the better method, and by far the most common, is to connect a differential pressure transmitter at the top and bottom of the vessel. Unfortunately this is equally awkward with a U/G tank. Remember you have to have some controlling the content of the lower leg. Things get very much more complicated than one might think. That is why the various forms of bubblers were suggested.
Using any pressure measuring device automatically gives you weight, not volume. By nature it compensates for density.
Now that you have brought it up (grin), just where would you mount any sensors on the tanks? The tanks in our neck of the woods and I assume yours have one largish flange where they fill the tanks and possibly a smaller breather. Is there a spare flange that I can't see? Not only that but the tanks tend to be under the forecourt and not too deeply buried, would there be enough space for any of the instruments we have been discussing?
I would think that any mods to the tank would be substantial in terms of the logistics. The last time I worked on anything petrochem is a while back and we used to have to go to extreme lengths to avoid explosions when welding or cutting a vessel that had been used for flammable liquids or gasses.
Yep. Under the API regs there has to be a man-way hatch in the top of the tank for tank cleaning, inspections etc. The man-way hatches are usually covered with bitumen or a reinforced plastic cover ('cause they're not often used), so you may not notice them unless you know where to look..
We have used the man-ways for flange-mounted float-style instruments, but the capacitive probes usually go in through a 3/4" or 1" weldolet in a convenient spot on the tank top..
Maybe! ;-) I can only speak for Oil Company installations (eg. Shell, BP, ExxonMobil, etc.) as the Independents and installations all over the countryside can do what they like pretty much - but usually the top of the tank will be at least 150mm below ground level.
It needs to be about this to allow a fully-laden semi to drive over the top without pushing the tank in (very embarrasing!). Having said that, it is usually 6" of reinforced concrete!
The trick is to carve away a small square of the concrete (about a foot square) over the tank top, bung a hole in the roof, mount the probe and cover the top with a steel or (more usually) a reinforced plastic lid.
Yes - the tank has to be gas-freed first. Usually it's just a matter of emptying the tank and connecting a nitrogen purge whilst the welding is in progress.. but you need to find a welder who knows what he's doing and is
*very* good at his job! But in practice it will take longer to run the cabling to the probe than it will to install it in the tank...