3.1 Outage (Ullage) & Thermal Expansion
Key Takeaways
- The AAMVA manual says never load a cargo tank totally full, because liquids expand as they warm.
- Outage is the room left for the expanding liquid, and different liquids expand by different amounts, so they need different outage.
- Under 49 CFR 173.24b, hazardous liquids in cargo tanks need at least 1% outage, or 5% for poison-inhalation-hazard materials.
- That outage must exist at a reference temperature of 115°F for a non-insulated tank or 105°F for an insulated tank.
- Gasoline expands several times more per degree than water, so a fuel load warming 50°F can grow by roughly 3%.
Outage looks like wasted space until you see what a liquid does as it warms. A tank loaded to the top on a cool morning has nowhere to put the extra volume by afternoon. The AAMVA manual (8.2.6) keeps the rule short, and the test expects you to know it word for word.
The Rule
Never load a cargo tank totally full. Liquids expand as they warm and you must leave room for the expanding liquid. This is called outage. Since different liquids expand by different amounts, they require different amounts of outage. You must know the outage requirement when hauling liquids in bulk.
Ullage is another name for the same space. The hazardous materials regulations define outage or ullage as the amount by which a packaging falls short of being liquid full, usually expressed as a percentage of volume.
Why Liquids Need Room
When a liquid warms, it takes up more volume. The change can be estimated with:
Change in volume = starting volume x expansion coefficient x temperature rise
Liquids resist being squeezed. If a completely full, closed tank warms, the liquid has nowhere to go, so pressure rises quickly. Cargo tanks are not built for that kind of pressure. The maximum allowable working pressure (MAWP) of a DOT-406 petroleum tank is only 2.65 to 4 psig, and a DOT-407 chemical tank has an MAWP of at least 25 psig (49 CFR 178.346-1 and 178.347-1). Once pressure reaches the relief setting, the pressure relief valves open, and a tank that is liquid full can push liquid rather than vapor out through them. Gaskets and fittings are also stressed.
Different Liquids, Different Expansion
The coefficients below are approximate values near normal temperatures and are for learning only. Shippers and terminals use product-specific tables.
| Liquid | Approx. expansion per °F | Approx. growth for a 50°F rise |
|---|---|---|
| Gasoline | 0.0006 to 0.0007 | about 3 to 3.5% |
| Diesel fuel | 0.00045 | about 2.3% |
| Ethanol | 0.0006 | about 3% |
| Water | about 0.0001 to 0.0002 (varies with temperature) | under 1% |
The pattern is what matters for the test: petroleum products and many solvents expand several times more than water, so the same temperature rise calls for more outage.
Worked example: a cool load on a hot day
A driver loads 9,000 gallons of gasoline from terminal storage at 50°F. On a hot afternoon, the fuel warms to 105°F.
- Temperature rise: 105 - 50 = 55°F
- Expansion: 9,000 x 0.00069 x 55 = about 342 gallons
That load needs at least about 342 gallons of empty space, roughly 3.8% of the loaded volume, before any regulatory minimum or safety margin is added. A driver who thinks "it's cold, so I can top it off" has the risk backwards: cold product loaded full is exactly the load that will expand.
The Federal Minimum for Hazardous Materials (49 CFR 173.24b)
For hazardous materials in bulk packagings, 49 CFR 173.24b(a)(1) requires liquids to be loaded so the outage is at least:
- 5% for materials that are poisonous by inhalation
- 1% for all other hazardous materials
of the total capacity of the cargo tank or compartment, measured at these reference temperatures:
| Tank type | Reference temperature |
|---|---|
| Non-insulated cargo tank | 115°F (46°C) |
| Insulated cargo tank | 105°F (41°C) |
In plain terms, if the product warmed to 115°F in a non-insulated tank, at least 1% of the tank (5% for inhalation hazards) would still be empty. Specific commodities can have stricter limits elsewhere in the regulations, and shippers may set more outage. Non-hazardous products such as milk or water are not covered by 173.24b, but they expand too, so carriers and shippers set fill limits for them.
Controlling the Fill Level
| Tool | How it works |
|---|---|
| Loading instructions and bills of lading | The shipper or terminal sets the gallons per compartment |
| Meters at the loading rack | Stop the load at the preset volume |
| Overfill sensors | Probes in each compartment signal the rack to stop flow if liquid reaches the sensor |
| Compartment level markers | Fixed indicators inside the dome or manhole show the fill line |
| Gauging (dip or outage measurement) | Measuring the liquid depth or the empty space from a reference point |
The driver still has to check. Know the planned gallons for each compartment, watch the load, and do not override shutoffs to squeeze in more.
Outage and Weight Work Together
Outage is one of the three factors in how much to load (Section 1.3). For a light product like gasoline, the expansion limit may control. For a dense product like acid, the weight limit usually controls first, leaving far more empty space than the outage rule requires. Load to whichever limit you reach first.
Outage and Surge
Outage is required, but more empty space means more room for surge.
- A tank loaded to its proper fill level has little room for the liquid to build a wave.
- A tank with a lot of empty space, whether from a weight-limited dense product or a partial delivery, surges more (Section 2.1).
Leave the outage the product and rules require, and drive every partial load as a surge load.
Common Test Traps
- "What is outage?" The room left in a tank so the liquid can expand as it warms.
- "All liquids need the same outage." Different liquids expand by different amounts, so they need different outage.
- "Winter loads can be filled to the top." Cold loads expand as they warm, in transit or at delivery.
- "Outage is for weight." Outage is for expansion; weight limits are a separate factor.
What is outage?
Under 49 CFR 173.24b, what minimum outage is required for a hazardous liquid that is not poisonous by inhalation in a non-insulated cargo tank?
Why might a load of gasoline need more outage than the same volume of water for the same temperature rise?