By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed September 10, 2026
Tank gauging is the measurement of product in a bulk storage tank to the standard needed to book a quantity. That means a level reading and an average product temperature, plus free water and density where the transfer calls for them. The tank’s calibration table turns those readings into a volume both parties will sign for.
An automatic tank gauge, usually shortened to ATG, produces that level reading continuously from a fixed instrument instead of a hand dip. Under OIML R 85 the level indication is held to 1 mm before the gauge goes on the tank. Once it is installed the limit becomes 4 mm.
Those two numbers are worth more than they look. On a 40 m diameter tank, 4 mm of level is 5.03 m³, close to 31.6 barrels. A 1 °C error in the average temperature on that same tank moves nearly three times as much product.
This guide takes one tank from an innage reading through to net standard volume with every step shown. It then sets out what R 85 permits and how that is checked, and puts the gauging technologies on a single accuracy basis.
Contents
- What tank gauging means
- Two gauging worlds
- Innage outage and ullage
- From level to volume
- Accuracy under OIML R85
- What one millimeter costs
- Temperature versus level error
- Gauging technologies compared
- FAQ
What tank gauging means
A tank gauging system on a bulk tank is doing three separate jobs, and they carry different accuracy expectations. Inventory control asks how much is standing in the tank tonight, and a few centimeters of uncertainty across a farm is tolerable. Custody transfer asks how much crossed the fence line, and that number carries legal weight. Safety asks whether the surface is approaching a level that has to raise an alarm, which is a different instrument on a different circuit.
The measuring chain is the same in all three cases. A level instrument reports the position of the liquid surface. A temperature element reports the average temperature of the product, not the temperature at one spot.
The tank’s calibration or strapping table converts a height into a volume, because a real tank is not a perfect cylinder. Corrections then take that observed volume back to a reference temperature. Quantities measured on a cold morning and a hot afternoon can then be added together.
The level instrument is the part that gets specified and argued about, and it is rarely the part that dominates the error. That becomes obvious once the volume chain is written out with numbers, which is what the middle of this guide does.
Two gauging worlds
Search for tank gauging and two unrelated markets come back mixed together. One is the dial on a domestic propane cylinder or the sight gauge on a heating oil tank. There the only question is whether enough is left until the next delivery. The other is the terminal, refinery and depot world this guide covers, where a gauge feeds a transfer ticket and is subject to legal metrology.
Nothing here transfers to the first world, and it should not be forced to. Buried fuel tanks under a forecourt follow release detection rules of their own. Those rules and the equipment that satisfies them are covered in our guide to underground tank level measurement.
If it is an above ground fuel tank on a site rather than at a terminal, start from diesel tank level gauge selection instead. For a broader view of the instrument families before narrowing down, see level transmitter types.
Innage outage and ullage
Three words describe the same tank and are constantly swapped for one another. Getting them straight is the difference between a gauge that agrees with the manual dip and one that does not.
| Term | What is measured | Where it is read from |
|---|---|---|
| Innage | Depth of liquid | Upward from the datum plate on the tank bottom |
| Outage | Empty distance above the liquid | Downward from the reference point at the gauge hatch |
| Ullage | The same quantity as outage | Downward from the reference point, common in European practice |
The vertical distance from the reference point to the datum plate is the reference height. It is a property of the tank, not of the gauge.
Level = reference height − ullage
Non-contact radar and servo gauges measure downward and therefore produce an ullage natively. A hydrostatic transmitter or a magnetostrictive probe produces an innage natively. Either can be converted to the other, but only if the reference height held in the gauge matches the one written on the tank. When a new gauge disagrees with the dip tape by a suspiciously round number, a stale reference height is the first thing to check.
From level to volume
A level reading is not a quantity. Four deductions and one multiplication stand between the gauge and the number that gets booked, and each of them has its own source of error.
Published explanations usually stop after one or two of them, and several skip the free water entirely. That is why a level reading taken against a strapping table is so often called a gross observed volume. It is a total observed volume.
Work one tank all the way through. Take a vertical cylindrical tank 40 m in diameter, cross sectional area 1256.64 m². The innage is 12.500 m and the water bottom is 120 mm.
| Step | Operation | Result |
|---|---|---|
| TOV | Innage 12.500 m against the strapping table | 15,708 m³ |
| GOV | Subtract free water, 120 mm equals 151 m³ | 15,557 m³ |
| GSV | Multiply by a volume correction factor of 0.991 | 15,417 m³ |
| NSV | Subtract sediment and water at 0.2 percent | 15,386 m³ |
The volume correction factor comes from the average temperature and the density at reference temperature through the petroleum measurement tables. The 0.991 used here is an illustrative value for a light product a little above its reference temperature.
Accuracy under OIML R85
OIML R 85, in its 2008 edition, is the international recommendation for automatic level gauges on storage tanks. It separates what the instrument can do from what the installation is allowed to do, which the vendor literature mostly does not. There is no accuracy class, only two maximum permissible errors.
| Condition | MPE | Applies to |
|---|---|---|
| Prior to installation | 1 mm | The gauge on its own, for type approval and initial verification |
| After installation | 4 mm | The gauge on the tank, for initial and subsequent verification |
Both limits apply to the indication of a dip or an ullage. R 85 also holds hysteresis, resolution and the spread between multiple indicating devices to 1 mm each. It suggests a subsequent verification interval of two years or less.
Two consequences follow, and both are missed on pages that quote a bare millimeter figure. First, the after installation limit is checked by comparing the gauge reading against the manual reference measurement. A working dip tape and a documented gauge hatch are therefore part of a modern tank gauging system, not a leftover from one.
Second, the 3 mm difference between the two limits is the budget for everything the tank itself does to the gauge: shell expansion, bottom movement, roof loading, a gauge platform that flexes, a stilling well that is not plumb. If that budget is ignored, the vendor figure is gone before the gauge is commissioned.
Cited standards drift, so write the edition down. R 85 of 2008 supersedes the 1998 text.
On the API side, MPMS Chapter 3.1B is now in a fourth edition dated 2021. At least one United States federal rule still points at the earlier text and sets its own check. It requires an automatic tank gauge to be verified to within one quarter of an inch by the method in that chapter. A specification that says only “per API 3.1B” has not said enough.
What one millimeter costs
Accuracy arguments get shorter once the millimeters are converted into product. For a vertical cylindrical tank the volume represented by one millimeter of level is fixed by the diameter alone.
Volume per mm = π D² / 4 × 0.001
| Tank diameter | Per 1 mm of level | Over the 4 mm MPE |
|---|---|---|
| 10 m | 0.079 m³, 0.49 bbl | 0.31 m³ |
| 20 m | 0.314 m³, 1.98 bbl | 1.26 m³ |
| 30 m | 0.707 m³, 4.45 bbl | 2.83 m³ |
| 40 m | 1.257 m³, 7.90 bbl | 5.03 m³ |
| 60 m | 2.827 m³, 17.78 bbl | 11.31 m³ |
Barrels converted at 6.2898 bbl per m³. Values are for a plain vertical cylinder and ignore bottom shape and internal steel, which the strapping table accounts for.
On the 40 m tank worked above, the full 4 mm allowance is 5.03 m³ against a standing 15,557 m³, or 0.032 percent. That is a small share of the total. It is also why arguing about the last half millimeter of gauge specification is usually the wrong argument.
Temperature versus level error
Put a temperature error on the same tank and the comparison changes character. Petroleum volume correction uses a coefficient of thermal expansion near 0.00095 per °C for gasoline. A 1 °C error in the average product temperature therefore moves the corrected volume by 0.095 percent.
15,557 m³ × 0.00095 per °C × 1 °C = 14.8 m³
Divide that by the 1.257 m³ that one millimeter is worth. The answer is 11.8 mm. One degree of average temperature error costs what roughly twelve millimeters of level error costs, on a gauge whose whole legal allowance is four. Anyone who specifies a millimeter class radar and then averages the product with a single spot sensor has spent the money in the wrong place.
Average is the operative word. A tall tank stratifies, particularly on products that are heated or that sit through a temperature swing. The normal answer is a multipoint averaging element with sensors spaced along the height. Only the submerged elements go into the average.
Our temperature sensor range covers the multipoint assemblies used for this, and the underlying element characteristics are set out under RTD temperature sensors.
Application example
Vegetable oil storage, Southeast Asia. A palm oil tank operator needed average product temperature down the full depth of tall tanks rather than a reading from one elevation. The arrangement proposed was multipoint PT100 assemblies on flexible probes, at 15 m and 18 m insertion depth. Elements were spaced every 1 m across a 0 to 150 °C range, entering through a VTA thread.
Gauging technologies compared
Level accuracy claims are quoted on incompatible bases across the industry, which makes vendor tables hard to read side by side. The table below keeps to what each technology measures directly. That is the quantity the verification will be run against.
| Technology | Reads | Notes |
|---|---|---|
| Non-contact radar | Ullage | Data sheets in this class commonly state 1 mm. Nothing enters the product |
| Guided wave radar | Ullage | Probe in the product, tolerant of low reflectivity, needs clearance |
| Servo gauge | Ullage | Also profiles density and finds interfaces. Moving parts, more maintenance |
| Magnetostrictive | Innage | Float on a rod, second float gives the water bottom directly |
| Hydrostatic | Head pressure | Good at mass, poor as a level device. Level equivalence in the tens of millimeters |
| Manual dip tape | Innage or ullage | The reference measurement used for after installation verification |
Figures quoted for radar and servo gauges are manufacturer statements for the instrument alone. R 85 would place them in the 1 mm prior to installation column, not the 4 mm installed column.
The hydrostatic row needs a word of explanation. A hydrostatic system infers mass from head pressure and does that well. Converting head back into a height needs the density, and small density uncertainty turns into large level uncertainty.
It is a good mass instrument and a weak level instrument. Hybrid arrangements therefore pair it with a level gauge rather than replace one. The instrument side is covered under hydrostatic level sensors.
Instranova does not supply OIML certified custody transfer gauging systems or servo gauges, and this guide is not an attempt to sell one. What we do supply for tanks in this class is radar level instrumentation, including a version built for oil storage tanks. The range also covers multipoint averaging temperature assemblies and hydrostatic transmitters for the mass side.
For corrosive stored products, material selection is handled in our note on sulfuric acid tank level measurement. The full instrument range sits under level instruments.
Pressurized product changes the instrument list and adds a fill limit that is set by volume rather than height. For propane and LPG specifically, see propane and LPG tank level sensors.
FAQ
What does tank gauging mean?
Tank gauging means measuring the contents of a bulk storage tank well enough to book a quantity. In practice that is a level reading, an average product temperature and often free water and density. The tank’s calibration table turns those into a volume that both parties to a transfer will accept.
What does a tank gauge do?
A tank gauge reports the position of the liquid surface continuously, and on most tanks it also carries or reads the averaging temperature element. The gauge itself does not produce a booked quantity. The strapping table and the volume corrections that follow it do that.
What is manual tank gauging?
Manual tank gauging is hand measurement with a graduated steel tape and a weight. The tape goes through the gauge hatch to the datum plate for an innage, or is read at the surface for an ullage. It is still the reference measurement used to verify an automatic gauge after installation.
What is the oldest method of tank gauging?
Hand dipping with a tape or dip stick is the oldest method still in service and it predates every electronic technique on the tank. It survives in the standards because it is the independent check that an automatic gauge is verified against, not because nothing better exists.
Request a quote
Tell us the tank diameter, the stored product and the reference height. Say whether the reading feeds inventory control or a transfer ticket, and we will size the level and temperature instrumentation around that. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.