Heating oil storage tank level gauge accuracy & precision:

How to get an accurate reading of just much oil is in the oil storage tank. How accurate is the oil tank gauge? How should you read an oil tank gauge? Where to get a highly accurate oil tank gauge.

This article series describes how to find, read, and test the oil gauge on a home heating oil tank. If your oil tank is above ground indoors or outside it should have a fuel level gauge installed similar to the one shown in our photo.

How accurate are oil tank gauges

Conventional float type oil storage tank gauges are not precise in reporting the quantity of oil remaining in the tank.

A heating oil tank gauge does a fine job of telling you the oil level in the oil storage tank: oil is near the top, 3/4 full, 1/2 full, or just 1/4 full or nearly empty. Sketch at left showing the location oil tank gauge and of places to watch for oil tank leaks is courtesy of Carson Dunlop Associates Associates.

[Click to enlarge any image]

"Full" in this case just means the level of oil in the tank, not an accurate read of the number of gallons of heating oil remaining.

Oil tank gauges are not accurate in reading the quantity of oil in a tank in gallons or liters. But then neither is the gas gauge in your car - and for the same reason. The tank shape.

What's inaccurate about all oil tank gauges is that because most oil tanks are not square but round or oval, knowing the height of oil in the tank does not tell us very accurately just how much oil is in the tank except at three points:

1. When oil is at the top of the tank we know the tank is full. If we know (or calculate) the tank size or volume we know how much oil we have. Usually the oil company already knows your oil tank size - just ask them.
   
   See OIL TANK SIZES & VOLUMES for help determining the capacity, size or volume of an oil tank
2. When heating oil is at the bottom of the tank we know we don't have a bit - accurate but troublesome.
   
   Watch out: even on an oil tank whose delivery piping is connected to the tank bottom, it is not normal and it's not a good idea to run all of the oil out of an oil storage tank; there are risks of drawing water or sludge into the heating equipment, causing malfunction.
   
   Furthermore, when oil delivery piping is taken out of the top of the oil tank, the pick-up end of that tubing is never extended all the way to the tank bottom, exactly to reduce the chances of picking up water or sludge.
3. When heating oil is exactly in the middle of a square, round, or oval shaped oil tank we indeed have an accurate measurement at this point.

Accurate Oil Tank Float Gauges

Below we offer an example of a high-precision float gauge used for oil tanks.

Scully produces the "Golden Gallon Gauge" that can provide an accurate readout of the level of oil in indoor our outdoor oil storage tanks, including buried oil tanks.

Scully gauges are a double float that communicates to an oil tank level gauge that can be read in inches, gallons, imperial gallons, or liters.

The Scully oil tank float gauge fits oil tank tapped openings of 1 1/2" or 2" diameter (NPT), and can handle oil tanks up to 60" in tank depth (from gauge mount to tank bottom).

Scully's product literature indicates that this is the most accurate oil tank gauge of its type.

Contact the Scully Signal Company.

Reader Question: accuracy of oil tank gauges

When we bought our house, our (oval 275 gal) tank had a reading of about 7/8 (halfway between F and 3/4) and our oil company estimated this to be 190 gals, which would be closer to 2/3 than 7/8 and certainly below 3/4, which it was not.

We used this oil from the time we bought it, through the end of last year's heating season until the end of this September.

At this point, we had about 1/8 of a tank. We received a fill-up, which was for 203 gallons and put us at full. It would reason that with an oval tank, the round parts at the top and bottom (round parts) would hold less oil per vertical inch than the middle (straight parts).

That being said, it seems like the top 1/8 and bottom 1/8 combined hold about 150 gallons, or half a tank's worth.

Other than this, our oil gauge seems to be reading fine, in that it went from full to half over the course of late September to today (middle of December). I just got a fill up and my tank was reading almost exactly at 1/2, but it was only 100 gallons to fill it.

Is this normal? Is the gauge reading relatively accurately? Does a 275 gallon tank actually hold 275 gallons? - RJ

...

I have 2 275 gallon oil tanks. I had 150 gallons of oil delivered when the gauge showed I had just less than an 1/8th of oil in the tank. After the 150 gallons was put in the gauge went just above 1/8th. How is that possible? 150 gallons represents 1/4th of the tank and the tank already had oil in it? - Carl 6/20/2012

(Oct 2, 2014) jen said:

I have two 275 connected tanks with one gauge. My gauge read 3/8 so I ordered 275 gallons. The driver was only able to fill to 183 gallons and told me my tanks were full. I understand that although my tanks read 275 gallons, it is actually 260 which goes into each of them.

Since the reading was at 3/8, I should have easily been able to hold the 275 (137.5 in each). I believe I may have a blockage but my oil company does not feel as such. Thoughts??

Reply:

RJ, there are a few sources of confusion about just how much oil a particular oil tank holds, how much is in it at any given moment, and about just how near to capacity the oil company fills the oil storage tank during delivery.

• Often the oil tank delivery driver fills an oil tank until the tank whistle indicates that oil is really at the top of the oil storage tank. But not always. Depending on the oil tank age and type, as well as the condition of the tank whistle or alarm (audible or not), the heating oil delivery driver may stop filling the tank before it is completely to the tank top.
  
  When this happens, trying to relate the size of the oil delivery in gallons to the tank size can be a little confusing. In other words, "Full" on the gauge, or "full" when the delivery person stopped pumping oil in the tank may not be a completely full tank.
• Because oil tanks are often oval or round in cross section, a conventional float type oil tank gauge is accurate only when the level of oil in the storage tank is exactly at the mid point. Given the radius of the tank top and bottom curve, it is of course possible to calculate tank contents more accurately, but that's beyond what's needed for most situations.
• Oil tank gauges on residential oil tanks are not precisely accurate. Please take a look at "How accurate are oil tank gauges" for details.
• Oil tank sizes used indoors (and sometimes outside) at residential properties are often referred to generically as a "275 gallon tank" but actual oil tank sizes vary. Some typical capacities include 200 gallons, 225 gallons, 250 gallons, 275 gallons, and 300 gallons of oil.
  
  Take a look at the label that should be welded or glued onto your oil tank - the actual tank capacity may be indicated there.
  
  See OIL TANK SIZES & VOLUMES for help determining the capacity
  
  It's reasonable to ask your oil company what size tank they think you have installed. They might be correct.
  
  Otherwise if you want to get a pretty accurate estimate of the tank size [excluding the volume occupied by the thickness of the oil tank steel or plastic shell] you'll need to measure the tank height, width, length, and curve radius and do a spread sheet calculation. [Or ask our friend Paul Galow who has prepared an Excel spreadsheet that provides this function.]

Maybe so, Carl. Consider that especially when filling the lower section (or top section) of an ovate oil tank, the gauge, which moves linearly, can't accurately reflect the actual tank amount. The gauge is accurate only at 3 points: empty, half empty, and full.

When filling two 275g oil tanks simultaneously, you got 75 gallons into each tank. If the tanks were actually nearly empty, you've filled 1/4 of one 275-g oil tank (75/275 = 0.27) but since there are 2 tanks, you've got to divide that again by 2, so you've filled just about 1/8 of each tank - which is what the oil gauge is reflecting.

How to Read Oil Storage Tank Gauges Accurately

In this photo of an oil tank gauge (above) the red disk is sitting above 3/4 - showing that the heating oil level in this oil tank is more than 3/4 full.

For readers who have asked "Do I read the oil level at the top, center, or bottom of the red disc?"

I have to say that the gauge is nowhere near accurate enough to be read with that precision, particularly with round or oval oil storage tanks.

Unless your oil tank gauge has been calibrated with care and is measuring oil in a rectangular oil tank, the gauge is only close to accurate at empty, half-full, and completely full levels.

That griping done:

• If your oil tank gauge has a thin indicator disc such as the red disc shown in the oil tank gauge above, read the oil level at the center of the disc. It's not that precise anyhow.
• If your oil tank gauge has a thick indicator disc like the more-recent Scully oil tank gauge shown below, our experiments to date (January 2020) suggest you should read the oil level at the top of the yellow disc.

• If you are using an electronic sensor type oil tank level gauge like the Smart Oil Gauge shown below, then the sensor's electrical readout or wi-fi connected readout will give an accurate percentage fill of the oil tank provided the sensor has been properly installed and if the sensor is properly set-up and adjusted it might also be accurate in actual gallons quantity.
  
  See details at OIL TANK GAUGE SMART WIFI INSTALLATION

Reader Question: how do I read the oil tank gauge against the scale: from the top, center, or bottom of the plastic disc?

I have an oil gauge just like the one in the 5th image above. Has a checker like indicator. Do you read the bottom/middle/top edge of indicator as to the amount of oil in tank? - Cliff 11/13/2012

The following photo is from your site and is just like mine. It has a disc in a tube like the one below and red marks on tube as shown.

The disc is like a checker. And I was wondering if you read the top edge or bottom of checker. - Cliff by email 12/5/2012

Reply: read from the bottom of the plastic disc.

Clifford,

Now that you sent us our own photo and an email query I understand the question better.

I apologize that my answer may not be very satisfying, but to be safest, I would read from the bottom of the plastic disc.

That's because the object is to avoid running out of oil and suffering the consequences of lost heat. In general I call the oil company immediately  if the tank shows 1/4 full or less.

On a 250g oil storage tank on most residential heating installations in typically cold winter weather I figure if the oil company cannot make an oil delivery a day or two we're still safe.

Having taken a few of these gauges apart I can assure you that they are certainly not precise. Not only is there play (or even bends) in the float mechanism that drives the oil tank gauge, but worse, because the oil storage tank is typically ovate in cross section, the ONLY chances that the gauge even MIGHT be precisely accurate are in three positions: Full, half-full, and Empty.

Watch out: as we get near the bottom of the oil tank, because of that ovate shape, the oil level falls more rapidly than while we're using oil from a tank filled around mid-way.

Watch out:  for oil storage tanks whose fuel lines are taken off of the top of the oil tank, the installer almost never places the oil pick-up line all the way at the bottom of the tank. Typically the line is kept a several inches or even more above the tank bottom in order to avoid picking up sludge or water that might be sent to (and foul) the oil burner.

For this reason, we never use the very last bit of oil in the storage tank. This means that for practical purposes (actually being able to draw oil from the tank) that last 1/4 or 1/8 of an oil tank level showing on the oil tank gauge may in fact be very close to TANK EMPTY.

Reader Question: how do I read the oil tank gauge?

Does the gauge in the attached photo read 1/4 tank or 5/16? I guess the main question is, "Do I read from the top of the marker or the bottom?" - T.G. 1/6/2014

[Click to enlarge any image]

Reply:

Generally we read the center of the plastic disc against the scale. I know this sounds like begging the question, but oil gauges are just not that precise.

Watch out. You are risking a more fundamental error. The oil tank gauge is not accurate anywhere near the range of precision of your question unless your oil tank is a cube or rectangle rather than the oval cross section most often installed.

Search InspectAPedia for Oil tank gauge accuracy for details and for the math used to calculate oil tank volume for oval tanks.

At 1/4 tank remaining and especially if the oil lines exit at the top of the tank (meaning the tank is never fully emptied anyway) its time to call for an oil delivery.

...

Continue reading at OIL TANK GAUGES - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see OIL TANK GAUGE ACCURACY FAQs - questions & answers about oil tank gauges posted originally on this page.

Or see these

Recommended Articles

• GAUGES ON HEATING EQUIPMENT
• HEATING OIL USAGE RATE to compare oil tank gauge readings with oil consumption rates.
• OIL TANK GAUGES - home
  • OIL TANK GAUGE ACCURACY
  • OIL TANK GAUGE & VENT ALARM INSTALL / REPLACE
  • OIL TANK GAUGE LEAK REPAIR
  • OIL TANK GAUGE SMART WIFI INSTALLATION

Suggested citation for this web page

OIL TANK GAUGE ACCURACY at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING OIL, OIL BURNERS, OIL FIRED HEATERS, OIL TANKS

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