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
I will appreciate any information.
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
Why not consider a float? It's simple and would basically be maintenance free
you have a clean process medium.
You could also vary easily characterise the output for volume.
Sometimes simple is better
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.
gasoline is normally
recommendation over other
technology's sake is better
Sorry, I was referring to the mechanical servo gauges as a "float on a
string", not the magnetostrictive device you suggested. They are VERY
different. Here is a web site:
have a look at the pictures you'll see that they obviously have a number of
moving parts, pulleys, gears, springs, etc.
gasoline is normally
recommendation over other
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
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
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...
But this is a *gasoline* tank we are talking about here - it's Zone 0
inside! Not a nice place to put any sort of sensor if you can help it.
Too expensive, too big and not suited to underground tanks with trucks
driving over the top.
You *are* kidding, right?
Servo-gauges suffer from the same problems as conventional radar in this
Ahah! Finally!! :-)
There are also RF Admittance probes and "radar-over-wire" probes that will
do the job too... but capacitive are the norm for this application.