A tank can be gauged two ways: by measuring up from the bottom to the liquid surface, which is the innage, or by measuring down from a fixed reference point at the top to the surface, which is the outage or ullage. Both describe the same liquid level, just from opposite ends. Innage-outage conversion is the simple arithmetic that turns one into the other using the tank's total reference height. This page shows the conversion itself and the errors that creep in when it is done carelessly, building on the underlying idea of innage versus ullage rather than repeating the definitions.
Innage-outage conversion in one line: Innage-outage conversion is the calculation that relates the two ways of gauging a tank through its fixed reference height: innage equals the reference height minus the outage, and outage equals the reference height minus the innage. Because both measure the same liquid surface from opposite ends of the same total distance, either figure can be derived from the other as long as the reference height is correct.
The whole conversion rests on one fixed quantity: the tank's reference height, sometimes called the total or gauge height. This is the distance from the datum plate at the bottom of the tank up to the reference point at the top, usually the reference gauge point on the gauge hatch. It is a property of the tank, established at calibration and recorded, and it does not change with the level of liquid inside. Every conversion is just the recognition that the reference height is split into two parts by the liquid surface: the liquid below it, which is the innage, and the empty space above it, which is the outage.
From that single idea the two formulas fall out directly. Innage equals reference height minus outage, because the liquid depth is whatever is left after subtracting the empty space. Turned around, outage equals reference height minus innage, because the empty space is whatever is left after subtracting the liquid depth. There is nothing more to it than that: the two numbers always sum to the reference height. If a technician gauges an outage of a certain amount, subtracting it from the known reference height gives the innage, and that innage is what the capacity table needs to return a volume.
The reason the conversion exists at all is that capacity tables and custody procedures are usually built around innage - the height that maps to a contained volume - while the field measurement is sometimes taken as an outage. Converting outage to innage bridges that gap so the same table can be used regardless of how the level was physically observed. Keeping the relationship as a single, memorized equation, innage plus outage equals reference height, is the cleanest way to avoid sign mistakes under field conditions.
If innage is what the table wants, it might seem simplest to always measure innage directly. In practice several conditions make measuring down from the top - taking the outage - the better or only choice. On a floating-roof tank, the roof sits on the liquid and there is no clear surface to touch from below with a bob; measuring the space above a known reference is more workable. When the product is hot, viscous, or crusted, or when there is foam or a heavy bottom layer, dropping a tape to the true liquid surface from the bottom can be unreliable, and an outage taken from the top surface of the liquid avoids the troublesome bottom region entirely.
There are also safety and practical reasons to prefer outage. A near-full tank leaves very little tape to work with for an innage cut, and reading a small remaining space from the top is easier and less error-prone than reading a large innage. Some products or tank configurations make the datum plate hard to reach cleanly, so the reproducible reference at the top becomes the trustworthy datum. In all of these cases the operator gauges what can actually be measured well, records it as an outage, and then converts to the innage the table needs.
The key discipline is to be explicit about which quantity was measured and which was derived. A gauge sheet should make clear whether the number written down is an innage read directly or an innage computed from an outage, because the two carry different failure modes. Being disciplined here prevents the common confusion of treating an outage as though it were an innage, which would send a completely wrong height into the capacity table and produce a volume that is off by nearly the full reference height.
Every conversion is only as good as the reference height it uses, and a wrong reference height poisons every outage-derived innage in the same direction. If the recorded reference height is larger than the true value, every innage computed from an outage will be too large, and every derived volume will be overstated; if it is too small, everything is understated. Because the error is systematic, it does not average out over many gaugings - it biases them all - which is exactly why the reference height is measured and documented carefully at calibration and treated as a controlled value rather than something re-estimated in the field.
Reference height can also drift physically. Tank settlement lowers the bottom relative to the top reference point, changing the true distance; a bent or replaced gauge hatch, a modified datum plate, or a repair can shift it as well. This is why a reference-height check is part of gauging discipline and why any event that could move the datum is a reason to re-verify. A conversion done with an out-of-date reference height looks perfectly correct arithmetically while being quietly wrong, which makes this one of the more insidious measurement errors.
A cloud SCADA platform such as Merobix reduces the room for these mistakes by holding the reference height as a stored property of each tank and doing the conversion automatically. When a field reading comes in - or when a manual outage is entered - the system applies the same reference height every time, so there is no chance of a technician using yesterday's figure or transposing a digit under time pressure. Because the reference height lives with the tank record, it can be updated deliberately after a settlement survey or repair and then applied consistently everywhere, keeping innage, outage, and the resulting volume aligned across every reading and every operator.
Innage equals the tank's reference height minus the outage. The reference height is the fixed distance from the datum plate at the bottom to the reference point at the top, so subtracting the measured empty space above the liquid leaves the liquid depth below it. Turned around, outage equals reference height minus innage.
Outage is preferred when measuring from the bottom is difficult or unreliable - on floating-roof tanks, with hot or viscous product, with foam or heavy bottoms, or on a nearly full tank where little tape remains for an innage cut. In those cases measuring the space above the liquid from a fixed top reference is more workable, and the reading is then converted to the innage the capacity table needs.
Every innage derived from an outage will be off by the same amount, biasing every volume in one direction because the error is systematic rather than random. The conversion still looks arithmetically correct, which makes the mistake easy to miss. Reference height is therefore measured and documented at calibration and re-verified after settlement or any repair that could move the datum.
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