How to Calculate Ullage and Fill Volume
Last updated: 6/2021
Applies to: Winemakers and beverage producers bottling with corks. Ullage is the space between the cork and the liquid level and can be correlated with fill height and fill volume.
Why Monitor Ullage?
Ullage refers to the void between the wine and the closure, and monitoring it is one of the most useful tools in ensuring bottling success. Ullage is closely related to fill height and fill volume, both of which are affected by wine temperature. Maintaining consistent ullage, fill height, and fill volumes are important for two main reasons:
Maintain legal fill volumes:
As a part of the TTB Regulations on bottling or packing wine (27 § 24.255), the volume of wine in a 750 mL bottle must be ± 2% of 750 mL. Maintaining consistent fill volumes is therefore critical to compliance. Later in this article, we will discuss our suggested method for calculating fill volume.
Inconsistent fill volume is a common culprit for leaking wine bottles. Internal bottling pressure also plays a role here. It is very unlikely that bottles will leak if they are filled at legal fill heights with adequate vacuum (targeting no more than two pounds relative pressure in the bottle at 68°F); However, wines may develop excess bottle pressure if they are overfilled. With excess bottle pressure, leaking may become inevitable as wine warms during shipping or storage and expands.
What is the Proper Ullage?
Proper fill height depends on the glass you are working with. The fill height is measured as the distance from the top of the bottle to the correct wine level in the bottle. The manufacturer should provide bottle drawings to indicate this for your bottles, and the drawing should indicate for what bottling temperature it is referencing (it is commonly 68°F). Generally, the fill point on the 750 mL bottle at 68°F will be approximately 64 mm from the top. It is always best, however, to consult the drawing as a +/-3 mm variance is possible.
The throat diameter of a standard American 750 mL bottle will vary slightly in the ullage area. On average, with a 64 mm fill height, a 49 mm cork will give about 4.8 mL ullage and a 45 mm cork will give about 6.5 mL ullage.
Temperature effects on ullage:
Wine, like water, expands and contracts based on temperature. At higher temperatures, wine will take up more volume than at lower temperatures. Based upon figures from "Principles and Practice of Winemaking" by Boulton et al, the thermal expansion of wine between 20°C (68°F) and 40°C (104°F) is 0.08% or 0.166mL per degree Fahrenheit. Thus, if a winery bottles at 58°F with 4.5 ml in ullage, that ullage will be reduced to under 3 ml at 68°F and internal bottle pressure will have risen significantly.
There are a few ways to take temperature into account and ensure proper ullage and fill height:
- Bottle wine at 68°F and fill to the level designated by the bottle manufacturer and confirmed by the winery.
- Adjust the fill level to compensate for temperature differences. A good rule of thumb is to adjust the fill level by 0.55 mm for every degree Fahrenheit above or below 68°F.
- Adjust vacuum levels to compensate for temperature differences. This method seems less reliable and more complex than adjusting fill levels. Internal bottle pressure should be equivalent to less than 2 psi (relative) at 68°F.
Monitoring for internal quality control:
Bottling managers can chart out target fill heights and internal bottle pressures by bottle type in advance of bottling. Though this will not eliminate their responsibility for a “legal fill”, it will provide an excellent guideline for good bottling. It is also critical that wineries keep good ongoing records during the bottling day.
At a minimum, the following protocols should be observed:
- Internal Pressure/Vaccum: Freshly corked wines from each corker head should be checked for internal pressure (every 30 min, and AT LEAST once an hour).
- Fill Height: Freshly corked wines from each corker head should be checked for fill height (every 30 min, and AT LEAST once an hour).
- Temperature: Quality control should not rely on the temperature gauge at the filler. A thermometer should be dropped into one bottle after the filler every half hour.
If any of these parameters are out of spec, the product bottled since the last in spec. check should be quarantined, flipped upright, and inspected.
Monitoring for compliance:
Depending on your bottling speed and lot size, an appropriate number of bottles should be measured for fill volume (recommended 3-12) at least once an hour. We recommend using the following protocol to calculate fill volume:
- If not done already, consult the bottle drawing to find the manufacturer-specified ullage, fill height, and fill temperature.
- Calculate the approximate fill height based upon the actual temperature of the wine (see the above section on the effects of temperature).
- Individually weigh 3-12 empty bottles and number them or record it's mold number to identify them after being filled. Run them through the filler.
- Weigh each bottle again and calculate the net difference for each bottle (full vs. empty).
- Divide by the specific gravity of the given wine at the bottling temperature to find the volume in mLs.
- Adjust fill heights as required.
If legal requirements force the ullage to be smaller than indicated by the internal pressure table, increasing the bottling vacuum can be used to compensate.
For more information, the Cork Quality Council (CQC) has put together a bottling handbook as a complete resource to bottling best practices. Click below to see the full guide. A part of this guide is a headspace calculator which may be helpful.Read CQC Guide