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Old 29-11-2022, 06:37   #1
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Fuel level indicators

Trying to understand better how to measure diesel fuel level (and consumption) as accurately as possible. It turns out there is no perfect approach for older, mechanical diesels without ECUs. Engines are Cummins 450C Diamonds. Fuel tank is 1,600 liters.

First, I tried to measure tank fuel level accurately. A typical resistive sender will give 0.5-1.0% accuracy if we measure the resistance with a multimeter. This sounds much better than a sight gauge, but there are two additional errors. One is that most dials apply the battery voltage across and measure the voltage drop instead. As the boat voltage varies from 12 to 14V, this introduces an error of 10-15%. Some gauges compensate partially for this, others do not. The second error changes in fuel volume with temperature, where the temperature of the tank increases due to the hotter return fuel. In my case, I calculated max error of around 3% based on fuel temperature increase of 30C and a diesel expansion coefficient of .85 ml/liter/degree C. And lastly, there is the measurement error, when the fuel moves around the tank while the boat is in motion, although this can be averaged out.

Basically, these errors seem small if displayed on a small 2" gauge but if we try to calculate how much fuel the boat would use over say 1 hour (100 liters out of a half full tank of 800 liters), the overall error could be as high as 20-30%. One way to compensate for all of this is to use an averaging ohm meter to measure the level, then measure the temperature of the fuel of the tank and do some calculations to find the true amount (weight) of fuel used. Seems like a mini project but it could all be done with an Arduino.

The alternative approach is to install flow meters, although these are expensive, restrict fuel flow and introduce errors on their own (for example, at slow speed, the forward flow would measure 150 liters (with 1% error), the return will measure 120 liters (with 1% error), which will give max error of 9%. Plus, temperature compensation should also be introduced, although the temperature compensation error will be much smaller.

What are your thoughts on this? The options seems to be:

1) Measure tank levels and temperature compensate, where the error in fuel consumption will be around 10% for short periods (10-15 min) but will decrease over a long period of time, may be to 3-5% over 1 hour. Extensive fuel tank shape calibration required but no engine work. $200

2) Build a fuel flow meter with Arduino, four positive displacement flow meters for the two engines, the error would be 10% at slow speeds and 5% at cruising speeds. $400

3) Buy two NMEA2000 flow meters with temperature compensation, such as Eurosens Delta 500, stated error is 3%. $2,500 installed.

All of this is mostly for fun, I do not need to know fuel level/consumption that accurately but it is an interesting project.

Thank you,
MV Ithaka
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Old 29-11-2022, 07:04   #2
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Re: Fuel level indicators

It seems you may have answered your own question.

If this is just a "fun" project, don't thing I would go out of my way to spend $$$ for high accuracy, if a ballpark consumption rate would be good enough.

If you want to go through the mental exercise, would say a gravimetric approach would eliminate the temperature issues that would be associated when attempting to measure volume/flow.

I'm all about accuracy, but with all the potential errors, are you really getting a better answer?
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Old 30-11-2022, 00:48   #3
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Fuel level indicators

In my last boat I built a simpl 6 point gauge using stainless rods of fixed lengths. Each was fed by an isolated low current 2v AC supply.

I then Calibrated The tank by filling it with diesel fuel and noting the pump figures as each sensor tripped

No moving parts. Very accurate , more rods could have been added to increase the measurement range

In my current boat Iím using external capacitive sensors stick to the outside tank wall
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Old 30-11-2022, 04:57   #4
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Re: Fuel level indicators

Quote:
Originally Posted by Pizzazz View Post
First, I tried to measure tank fuel level accurately.

CruzPro makes tank level gauges that can be calibrated relatively accurately. One, simpler and less expensive, works well enough for regular rectangular tanks.

The other, not horrendously costly, will "learn" your tank starting with your first fill, and then works pretty well from there. Their first fill routine isn't really easily done with large diesel tanks, but probably isn't onerous for smaller tanks.

Their customer service was excellent.

-Chris
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Old 30-11-2022, 07:15   #5
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Re: Fuel level indicators

Install an ultrasonic device to measure the air space in the tank. The pitch of the echo will change with the increasing air volume as the fuel is used.
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Old 30-11-2022, 08:04   #6
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Re: Fuel level indicators

I note the hours when I fill up. I use 1.25 gallons per hour.
I've got a wooden dowel to dip the tank.
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Old 30-11-2022, 09:58   #7
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Re: Fuel level indicators

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I note the hours when I fill up. I use 1.25 gallons per hour.
I've got a wooden dowel to dip the tank.


How does it go around the filler pipe corners !
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Old 30-11-2022, 10:31   #8
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Re: Fuel level indicators

what brand of external sensors are you using?
are you happy with there performance?

Quote:
Originally Posted by goboatingnow View Post
In my last boat I built a simpl 6 point gauge using stainless rods of fixed lengths. Each was fed by an isolated low current 2v AC supply.

I then Calibrated The tank by filling it with diesel fuel and noting the pump figures as each sensor tripped

No moving parts. Very accurate , more rods could have been added to increase the measurement range

In my current boat Iím using external capacitive sensors stick to the outside tank wall
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Old 30-11-2022, 10:32   #9
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Fuel level indicators

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what brand of external sensors are you using?

are you happy with there performance?


I made my own

Yes the rod single point of contact produce very accurate results albeit they have to be calibrated

The are also extremely reliable and easy to fix

What I did was set 6 stainless rods in a 3D printed holder. Each rod is trimmed to a suitable length

The electronics produces a AC waveform of very low power and as the fuel touches the corresponding rod the amplifier flexibles thd standard gauge

Iím upgrading the gauge board this winter to make it nmea 2000 compatible

I have the same system in my water tanks also.
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Old 30-11-2022, 14:04   #10
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Re: Fuel level indicators

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How does it go around the filler pipe corners !
pick up the floorboard, unscrew the metal rod, insert dowel.
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Old 01-12-2022, 03:40   #11
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Re: Fuel level indicators

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pick up the floorboard, unscrew the metal rod, insert dowel.


Hmm that would sink my boat !!
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