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Old 27-11-2011, 13:35   #16
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Re: Presure Sensors to Measure Tank Fluid Level?

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Originally Posted by twistedtree View Post
... The Computank senders are proprietary, so I can't just use a Maretron meter that connect to the existing senders ...
Doesn't the Maretron system use ultrasonic (depth) sensors?
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Old 27-11-2011, 13:57   #17
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Re: Presure Sensors to Measure Tank Fluid Level?

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Originally Posted by delmarrey View Post
I'm surprised I'm the first to respond to this. And that the tank was installed this way on such a quality vessel.
Fuel tanks are not to ABYC or USCG code with fittings below the top, with exceptions noted below.

*58.50-10***Diesel fuel tanks. :: PART 58--MAIN AND AUXILIARY MACHINERY AND RELATED SYSTEMS :: CHAPTER I--COAST GUARD, DEPARTMENT OF HOMELAND SECURITY :: Title 46 - Shipping :: Code of Federal Regulations :: Regulations :: Law :: Justia



Fittings coming off the bottom of a fuel tank is asking for trouble. What would happen if it came loose, or even caught fire in the bilge?

As for waste tanks; it's up to you if you want a seeping septic system inside your boat. And seeping may not show up until one is out in the rough seas, exercising the integrity of it's construction.
The section you refer to is pertaining to fuel tanks for auxiliary systems on ships. 58.01-55 is the section for the tanks for main propulsion on ships, I found it interesting that they want if possible the tanks shall be an integral part of the ship in this section.

33 C.F.R part 183 are the rules pertaining to recreational vessels. There are a lot of rules for gasoline tanks, but I believe there aren't any for diesel tanks.

Of course ABYC has rules for diesel tank installation on recreational boats.

John
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Old 27-11-2011, 14:37   #18
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Re: Presure Sensors to Measure Tank Fluid Level?

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Originally Posted by GordMay View Post
Doesn't the Maretron system use ultrasonic (depth) sensors?
Maretron has three ways to implement tank gauges.

1) Combined ultrasonic sender and N2K interface. One is required per tank along with an N2K drop. It's not clear that these will work with my fuel tanks given their shape. The water tanks should be fine.

2) An N2K interface that works with a standard resistive sender unit. If you already have standard sender units in your tanks you can just wire up these adapters and it puts the info on N2K. One is required per tank. My Computank senders are capacitive, not resistive, so I don't think they will work with this interface.

3) An N2K pressure sensor interface and up to 6 pressure sensors. One sensor is required per tank, and up to six can wire to a single interface. Maretron lets you use these pressure sensors to report fluid depth in a tank.

Until I talked to the Maretron guys I didn't realize the pressure sensors could be used as tank gauges - I just assumed they were used as normal pressure gauges.
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Old 27-11-2011, 16:56   #19
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I believe the pressure sensors are not very accurate. I also think the ultrasonic do have a mechanism to calibrate for irregular tank shapes (I'm sure actually). You will need a Maretron display or the USB100 interface to program them I think.

Actually, the owner of Maretron is member here... May be he'll read this and can explain the details

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Old 27-11-2011, 17:29   #20
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Re: Presure Sensors to Measure Tank Fluid Level ?

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Originally Posted by s/v Jedi View Post
I believe the pressure sensors are not very accurate. I also think the ultrasonic do have a mechanism to calibrate for irregular tank shapes (I'm sure actually). You will need a Maretron display or the USB100 interface to program them I think.

Actually, the owner of Maretron is member here... May be he'll read this and can explain the details

ciao!
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For what it's worth, it was the president of Maretron who I talked to at the Ft Lauderdale show and he suggested I use the pressure sensors as fuel gauges. He showed me how you program them, etc.
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Old 27-11-2011, 17:33   #21
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Re: Presure Sensors to Measure Tank Fluid Level ?

The pressure sensor doesn't have to be at the bottom of the tank. You can connect a differential pressure sensor to a tube going from the top to the bottom of the tank. The changing level of the liquid will change the pressure in the tube and the readout from the sensor. The atmospheric pressure changes and temperature changes are compensated by the other input of the differential sensor. Additional advantage of this solution is you can use a cheaper sensor because there is no direct contact of the sensor with the liquid. There is some vapor of the liquid in the tube but most of the sensors can handle this.
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Old 27-11-2011, 17:44   #22
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Re: Presure Sensors to Measure Tank Fluid Level ?

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The pressure sensor doesn't have to be at the bottom of the tank. You can connect a differential pressure sensor to a tube going from the top to the bottom of the tank. The changing level of the liquid will change the pressure in the tube and the readout from the sensor. The atmospheric pressure changes and temperature changes are compensated by the other input of the differential sensor. Additional advantage of this solution is you can use a cheaper sensor because there is no direct contact of the sensor with the liquid. There is some vapor of the liquid in the tube but most of the sensors can handle this.
T.
Isn't their an issue long term with the air in the tube getting absorbed into the fluid? I know it's an issue with water, but I'm not sure about diesel.
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Old 27-11-2011, 17:57   #23
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Re: Presure Sensors to Measure Tank Fluid Level ?

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Isn't their an issue long term with the air in the tube getting absorbed into the fluid? I know it's an issue with water, but I'm not sure about diesel.
I think you are correct but diffusion is a pretty slow process and it might work in practical frame of time. Anyway there is a simple solution for it to end the tube with a flexible bulb. No more vapor problem even.
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Old 27-11-2011, 18:14   #24
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Re: Presure Sensors to Measure Tank Fluid Level ?

The type of level measurement you are discribing has been used for a long time in Industry. The accuracy is determined by the range and quality of the equipment. With newer smart meters a tank can be calibrated regardless of shape etc. Alot of this type of Equipment is available through Industrial Instrumentation Companies. ( My Past field) However the reason you don't see a wide usage in the recreational marine industry is the price.
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Old 27-11-2011, 19:40   #25
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Okay, let's do the math; heaviest liquid to measure aboard is some form of water which gives say 0.44 psi per vertical foot of liquid. The "smallest" sensor is 0-3 psi so that would do for tanks up to more than 6' high. The sensor accuracy is 2% at full scale, which is 0.06 psi.

My tanks are about 3' high, 36". Each inch gives 0.037 psi so I operate between 0 and 1.33 psi. The maximum error is 1.62" which is 4.5% for my full tank.

I can live with that error, but find it significant. I mean, with my measuring stick I easily measure with 1/8" accuracy so much better than 1,62".

Other fluids are lighter so error goes up. Diesel is 0.84x the weight of water so error will be close to 2" so 5.6% error on a 36" high tank.

If your tank is 24" high, error becomes 8.3% but with a 60" high tank it goes down to 3.3%.

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Old 27-11-2011, 20:01   #26
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Oh, the TLM100 ultrasonic version. This one is for max. 40" tanks with 2% accuracy full scale so 0.8". On my 36" tank that translates to 2.2% accuracy. Much better than pressure based.

If Maretron would offer a 0-1.5 or 0-2 psi sensor with 2% accuracy, it would match the ultrasonic version. There's some potential trouble with the ultrasonic sensor too, like when heeled, or baffles in the tank etc. That makes the pressure sensor more interresting again.

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Old 26-12-2011, 16:46   #27
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Re: Presure Sensors to Measure Tank Fluid Level ?

I'm back on the case with this little project....

I wasn't too happy about the accuracy of the pressure sensor measurement approach. For my tanks it ends up being +/-6%. So I decided to use the Maretron ultrasonic senders that direct connect to N2K. I ordered up everything, including focus tubes, cables, Tees, etc.

Today I went to start installing and hit one road block after another. The bottom line is that this approach is not going to work.

The first issue is that although my tanks have standard 5 bolt sensor fittings, the tank hole is much smaller than the standard size. The standard is around a 2" hole, and mine are more like 1". This means the focus tube won't fit and even just the sounder without the focus tube won't fit either. The only solution would be to remove the tank covers from all 4 tanks including all the associated hoses and vents, drill/cut the hole larger, then reassemble. It's certainly possible, but MUCH more work and complexity than I was planning on.

But the next issue was the final nail in the coffin. My fuel tanks are trapazoidal with the outboard side sloping inward with a Vee bottom. The sounders don't like a sloped bottom, which is part of why I got focus tubes, but I thought the location of the existing sender would reach well into the Vee to give an acceptable reading. After all, the existing sender manages it somehow, right?

Not so. The fitting is very much over the sloped part of the tank, and looking straight down the sounder would be reading empty with only about 1/2 a tank. Not good. The existing sender actually deals with this is a clever way. It's a rod with a sense wire barber-polled around it, and some smarts in the probe to take some sort of capacitive measurement according to the manufacturer. What Grand Banks did is bend the tube so that it reaches to the side and all the way to the bottom Vee in the tank. With proper calibration, it works fine.

Since there is no way to get the sounder to measure at an angle like that, it kills them as viable replacements. Rats.

The brings me back to the pressure sensors since they couldn't care less what the shape of the tank is, and what twists and turns it takes along the way. It just measures fluid head, and with proper calibration (which Maretron supports), can handle any shape tank. I also could use the existing sender wiring to bring the signals all back to a common point at the helm for the N2K interface. I still don't like the +/-6% accuracy issue, and might investigate a 0-1.5 psi sender which would bring the accuracy up to match other methods.

I've also stumbled on another possible approach and have an inquiry off to Maretron to see if it will work. Their TLM100 is an N2K adapter designed to connect to a standard 240-33 ohm resistive sender. Sounds good, except the Centroid Computank senders are not resistive. They take a 12V input and output a voltage signal of 0V to 4V according to tank level. But it turns out the TLM100 has something called Analog Gauge Mode where it can be connected in parallel to an existing gauge. My presumption is that rather than putting out a constant current and measuring the resulting sender voltage (which I think is how these gauges work), in the Analog Gauge mode the TLM100 just monitors the sender voltage produced by the original gauge and puts out no sense current of it's own. Assuming the expected voltage is between 0V and 4V, then I should be able to use this mode of sensing in conjunction with the existing Centroid senders. I've got my fingers crossed.

By the way, does anyone know for sure how these gauges work? I know there are a couple of "standards" for the senders which are just variable resistance depending on fluid level. But how so the gauges work with the sender? As I said earlier, my best guess is that the gauge sources a constant current and measures the voltage across the sender. I'm further guessing that it sources 20ma which gives a voltage range of .66V to 4.8V across a 33-240 ohm sender. I've got my fingers crossed.

Anyone know for sure?
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Old 31-12-2011, 11:08   #28
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Re: Presure Sensors to Measure Tank Fluid Level ?

Gentlemen,

Sorry for not jumping in sooner, but I was just made aware of the thread. With regards to some of the questions regarding Maretron tank monitors, I'll try to answer any questions.

As twistedtree suggest the TLA100 (Tank Level Adapter) operates in one of two modes. Check out paragraph 3.3 of the user manual to understand how to operate the TLA100 with a stand alone resistive sensor ("NMEA 2000" mode) , or using the TLA100 together with an analog gauge and resistive sender ("NMEA 2000 & Analog Gauge" mode). The stand alone or NMEA 2000 mode is straight forward given a resistive sender in the tank, but the gauge mode gets a little more complex. In order for the TLA100 to work correctly in the gauge mode, a couple of resistive measurements need to be performed on the gauge when it is disconnected from everything, the manual goes into detail how this is done.

http://www.maretron.com/support/manu...A100UM_1.2.pdf

With regards to using pressure transducers, it looks like it is well understood that although the full scale accuracy of the Maretron supplied sensors is good (2%), it can have a significant impact on overall accuracy when one isn't using the entire range of a given sensor. Check out the graphs in section 1.6.2 of the user manual for an example of how this works.

http://www.maretron.com/support/manu...M100UM_1.1.pdf

As of right now, the lowest pressure transducer Maretron offers is 0-3 PSI, and from the graph, the tank needs to be around 3' deep to be within 5% overall accuracy. We are looking into offering lower PSI transducers to accommodate shallower tanks, but until then check out the 0-1 PSI transducer at the following link.

HVACQuick - Setra 209 Liquid Pressure Transducers

The trick is to try and maximize the range of the transducer for the corresponding tank (0-1 PSI transducer is perfect for 28" water tank, anything shallower and the accuracy begins to erode).

Ok, don't kill me but here comes the sales pitch. All three tank monitoring technologies Maretron offers can be calibrated for odd shaped tanks plus you can define the tank capacity, which is handy if you want to display how much fuel is remaining. On our displays, you can combine two or more tanks together as a single gauge. If you want to go nuts, you can put two tank monitors in a tank, one on both sides of the tank and the display will average them to give you accurate readings even while heeled over. If you add fuel flow monitoring and a GPS to the network, the display will compute miles/gal, time and distance to empty.

I hope this answers some questions, let me know if you have more.
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Old 31-12-2011, 12:57   #29
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Re: Presure Sensors to Measure Tank Fluid Level ?

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Gentlemen,
Welcome to the forum Richard

and happy new year!

cheers,
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Old 31-12-2011, 15:35   #30
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Re: Presure Sensors to Measure Tank Fluid Level ?

Greetings and welcome Aboard the CF, Maretron.

Thanks for you expert input!
Stick with us - you may find we're worth it.
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