Good day

can you give us please more detail about the tank ?
how you are planning to connect your level instrument
- on the top of the tank ?
- from the side (piping connection ?)
- to the bottom ?

do you have some nozzle available on this tank ?
do you have air instrument available ?
do you need a visible and permanent reading from the level instrument ?

I am thinking about some solution but it depends of your hardware configuration
- tubular level gauge (glass) : total range is 12-3=9 meters, you can use 5 x 2 meters tubes
- a pressure gauge connected to the bottom of the tank : it will give the pressure proportional to the height of liquid
(problem is that you have to estimate the average density of the liquid in the tank due to the relation P= q x g x h)
- Bubble tube level system if you have air pressurized available...with the same issue for the evaluation of the average density
you need a tube sense inserted inside the tank from the top or with some adaptation from the side

I believe that the average density of a mixture of water/oily water is very close to 1, especially if you have a TOG < 20ppm
it depends if you need precision and an accurate measure

Sorry, I thought you were looking for tank volume, not water flow volume through the drain. Disregard.
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I'd advocate a pressure gauge too.

It requires no power, needs to be mounted low in order to read the head pressure above the tap port which is where your tap is now and will be quite accurate when water is NOT draining from the tank. The 20ppm oil will not bother the gauge and is insignificant in affecting the density (and the head pressure) of the water.

There are distributors who will disassemble a standard mechanical bourdon tube pressure gauge and have its dial silk-screened with custom units and put it back together for you. Custom units can be meters or volume (liters). Obviously you have to come up with the volume-to-level relationship for your sized tank. Call your supplier for details.

12m of water head pressure is 118 kPa (1.2 bar) that will use 75% of the span of a standard pressure gauge range of 160 kPa or 1.6 Bar.

I assume winter time freezing is not a concern.

Flomec has battery powered turbine meters with a digital display for rate and total, but these have no hazardous rating:
Link

There are a number of companies that make nutating disc, or similar, mechanical flow meters with mechanical counters (registers).

Here is a link to one that has useful information. http://accurateflowmeters.com/a/pdf/Nutating%20Dis...

I am more familiar with Badger Meter but I could not quickly find useful literature from them (that is a bad sign).

You can also make an over-center dumper bucket. They're a counter-balanced pail that self dumps every time it reaches a specific fill level. You use a simple mechanical counter that mechanically increments on each dump.


Youtube small but what I'm describing.

You need none of the electronics. The lever part is where you'd put a counter. I'm also expecting something MUCH larger for your application.

Keith Cress
kcress - http://www.flaminsystems.com

Thanks guys for all inputs
I have selected TechnipFMC ( Formerly Smith meters ) Rotary Wane meters with mechanical register.

this is tough job, mechncial flow meter like oval gear flow meter cna work, it is a kind of mechanical flow meter ,has no request for the straight pipe lines , perfect choice to measure water, but it contains water, the material should be cast steel or stainless steel, it is much more expensive than iron ,click here ,you can find the flow meter here.

Silver automation Instruments
www.silverinstruments.com