sight glass (tubes)

Has anyone installed sight glass (tubes) on their diesel fuel tanks?
I don't trust my gauges 100%. Got used to sticking my tanks on my previous boat. Cant do it with this boat. Its a 1997 Mainship 350. Twin Volvos, twin 150 gallon fuel tanks

What material are your tanks? My tanks have a ball valve at the top and bottom and clear tubing between them. Very basic for my steel tanks, but it works.

I have wanted to. The technology is straight forward. Valves on the top and bottom to protect from site tube failure. I have seen glass or plastic tubing used. My old company used to make them for Navy ships using floats with magnets and all kinds of readouts.

Mac2 said:

What material are your tanks? My tanks have a ball valve at the top and bottom and clear tubing between them. Very basic for my steel tanks, but it works.

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looks like aluminum

Look forward to the suggestions. I’ve always wanted something like TJM described. PO has painters tape next to clear tube with marks/gallons

I have them too, both on the main diesel tank and on the day tank.

Greeting,

Pascal.

Our boat had them on the rear two tanks, and I added them to the two forward saddle tanks. There were already sumps on the tanks, and valves to drain off any nasties collected in the sumps periodically. Only there have never been any nasties! I just purchased fuel and caustic chemical rated clear flexible tubing, ran them up the outside of the tanks in an areas protected from damage/risk of brushing against. Installed a ball valve at the top (already had one at the sump).

1. When taking measurements:
2. Open the bottom valve
3. Open the top valve
4. Note fuel level
5. Close both valves when not actively taking fuel quantity readings

When first "calibrated", while refueling:

1. I was below decks while my wife called out every 25 gallons (using marriage savers)
2. Stopped, waited for fuel levels to even out
3. Marked level and quantity
4. Continue filling and marking readings until tank was full
5. Switch to next tank
6. Rinse and repeat

Hint: Pick a time at the fuel dock when business is slow, as it will take longer than standard fueling!

I am a firm believer in either sight gauges, or dipping tanks. I've been led astray in the past by gauges that have either failed, or given erroneous readings.

jclays said:

Has anyone installed sight glass (tubes) on their diesel fuel tanks?
I don't trust my gauges 100%. Got used to sticking my tanks on my previous boat. Cant do it with this boat. Its a 1997 Mainship 350. Twin Volvos, twin 150 gallon fuel tanks

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I use my site tubes before every trip. Allows me to run a 200 gallon tank down to 5 gallons.

I don’t know how you would add valves to an existing tank. It’s not a Case of drilling a hole and tapping. Typically, the aluminum is 1/4 or 3/8 thick. Usually, a 1/2 block is welded to the tank and then drilled and tapped giving the valve a minimum of 1/2 inch of threads for security.

OP could tee off the outbound fitting/hose and then use a thru hull at the top of there is a nearby access port. Would need a second person to capture shavings when drilling the tank.

Mac2 said:

OP could tee off the outbound fitting/hose and then use a thru hull at the top of there is a nearby access port. Would need a second person to capture shavings when drilling the tank.

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Mac2,

There is no need to plumb the top of the sight gauge back into the tank. Just extend the sight gauge tube slightly higher than the top of the fuel tank and install a ball valve there. Must be opened to allow the tank lever to equalize, then close again after measurement. But no need to plumb in the top of the tank.

I put sight tubes on my 120 Gal. day tanks about 12 years ago.

I first put an 18"x 21" access plate on the wall if each tank and then put the Liquid Level Gages on the removable plate.
I gave up having a reading for the top and bottom 1.5" of the tank, but it was easier to drill the holes and mount the gage on the removable plate while it was at the workbench rather than mounted on the tank in the Laz.

The tank's removable wall plate is 3/16" Alum. and the Alum/Glass gage has stayed crystal clear and has never leaked. No thickening of the mounting surface to accept a pipe thread is required as the mounting tubes are threaded 1/2"x 20 and sealed with a washer.

Lots of models to choose from here. Liquid Level Gages On Oil-Rite Corporation

slowgoesit said:

There is no need to plumb the top of the sight gauge back into the tank. Just extend the sight gauge tube slightly higher than the top of the fuel tank and install a ball valve there. Must be opened to allow the tank lever to equalize, then close again after measurement. But no need to plumb in the top of the tank.

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Agreed. Plumbing the top into the vent line may also work. That's how they're done on my boat, and I don't have a top valve.

I'd add that the sight gauge doesn't necessarily have to be mounted on the tank, unless convenient. Anywhere close by where you can mount a clear tube and mark graduations will work. But you need a small valve at/near the bottom of the tank to connect it to.