Dissolved Oxygen in Beer: How It Compares to Total Package Oxygen

When it comes to questions about oxygen in beer, I think the one I’m asked most often is, “What is the difference between dissolved oxygen and total package oxygen (TPO)?”  The main source of this confusion is that when measuring O2 in packages, the O2 in the headspace is often overlooked. If you don’t take headspace oxygen into account, then you are measuring a partial concentration, period. So let’s talk about the differences and what each one tells you.

A significant number of craft brewers have a dissolved oxygen (dO2) analyzer they use to measure the dO2 content of their beer in process. The most common point of measurement is the finished beer tank. The beer in a finishing tank will have O2 pickup from the empty vessel and from the filtration process, plus it will pickup more O2 as it goes through packaging.

Once the beer is packaged, however (assuming good packaging,) rapid O2 pickup from outside sources all but stops. So what can we tell about how much oxygen actually made it into the package?  It is not a simple matter of measuring the O2 in the beer.  The package must be shaken to equilibrate the oxygen in the beer and the headspace before the 02 in the beer is measured, and that number must then be used to calculate your TPO. Let’s think about what it is possible to measure and what each thing tells you.

Package dO2 –

The easiest measurement to take on packaged beer is the dO2 of a package just off the filler without shaking the beer. It is important to measure as quickly as possible, so the product does not “consume” the oxygen in the beer. (Residual or live yeast may be hungry, plus oxidation by trace metals, etc.) In some packages there is a measurable difference within five minutes and in other packages the rate of oxygen consumption takes significantly longer, sometimes hours. It is always best to measure as quickly as possible.

This unshaken package measurement represents the combination of the dO2 of the beer at the base of the filler and the oxygen pickup of the filler. Oxygen picked up at the filler can be quite variable. Most fillers run at about 25 to 50 percent deviation, but in some cases it can be up to 100 percent deviation. The best way to measure the percent deviation is to determine the dO2 at the base of the filler and then measure six to ten packages and determine the variation of each package as compared to the average of all the containers. But remember: this measurement only tells you what is in the liquid. When measuring unshaken packages, any gas in the headspace is left uncounted.

Shaken Package dO2 –

When you shake a package of beer so that the partial pressure of the oxygen in the liquid is equal to the partial pressure in the headspace, it changes the characteristics of the oxygen partitioning in the package. If most of the oxygen in the package is locked in the liquid, then shaking the container will move the O­2 from the liquid to the headspace until equilibrium is reached.

So, you have measured the dO2 and then shaken the package. Now what do you do with the data? If you really want to quantify the TPO of the package you have to take into account the headspace oxygen. To do this accurately you need to know the headspace volume and the package temperature.

Total Package Oxygen –

When using the dissolved oxygen measurement, the TPO can only be calculated from a shaken package. To do this calculation you also need to know the headspace volume, liquid volume and the package temperature. The temperature and the headspace volume are critical values and small inaccuracies can alter the results significantly, but the liquid volume may be estimated by using the average fill volume. Once you have your figures, then you can use a TPO calculator to determine the concentration from your initial DO2 measurements.

My final thought is to not skimp on how much you shake the packages. Cold containers should be shaken for five minutes and room temperature cans or bottles need about three minutes. If you’d like a copy of a TPO calculator built into an Excel spreadsheet, then please click here to request one.

 

 

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Measuring in the Gas Phase – Absolute vs. Gauge Pressure

 

Last week I wrote a piece about gas phase measurement units, but before I uploaded it to the blogosphere I realized there was a part that needed to come first, so here goes.

Different units — whether they are pressure, volume, length, or something as obscure as kinematic viscosity — can be daunting. This is especially true if you are trying to communicate with someone who’s used to using certain kinds of units and needs to switch gears and use a different unit. I want to focus on pressure units, but before we do that let’s define a simple but confusing concept, which is the difference between gauge pressure and absolute pressure.

Absolute pressure is defined as force per unit area that a fluid or gas exerts on the walls of its container. If you take a bottle filled with air from sea level to 10,00 feet high, the pressure in the container will be the same, but when you open the lid, gas will escape until the internal pressure is the same as the atmospheric pressure outside the bottle. Absolute pressure is the pressure exerted upon us by the pressure of the atmosphere on earth.

In a perfect vacuum, absolute pressure would indicate an absence of gas molecules and a pressure of 0.000, regardless of the units used to express the vacuum. For instance, 0.000 pounds/per/sq/inch absolute (psia) is equal to 0.000 in all absolute pressure units.

Since we don’t live in a vacuum, we mainly use gauge pressure, in which atmospheric pressure has already been taken into account, so the units used are 0.000 gauge pressure. In other words, gauge pressure is “an absolute pressure,” minus atmospheric pressure. Unless you are measuring in a vacuum, gauge pressure always starts at zero and is not concerned with the pressure of the earth’s atmosphere.

It’s a lot easier to be precise about the amount of pressure we’re using if we don’t have to account for atmospheric pressure. Here’s an example: Say you need 32.0 pounds/per/sq/inch gauge (psig) in your forklift tire. In absolute pressure at sea level that would be 47.5 psia and at 10,000 feet elevation it would be 42.7 psia. Since we want to specify the same amount of pressure in the tire regardless of the atmospheric pressure, it’s easier to work in gauge pressure.

When does gauge pressure not work? When you are calibrating an instrument, measuring a gas at an elevated pressure or need to know your altitude or weather conditions, you need to distinguish the absolute pressure to get precise results. Most gas analyzers either measure the atmospheric pressure or assume the pressure is at sea level and give you tables to compensate for differences at higher altitudes.

If we want to measure the gas concentration in tanks or vessels that are above atmosphere pressure we have two ways of doing so. The first is to have a remote pressure sensor to take in account the extra molecules due the increase in pressure of the compressed sample. She second is to bring the sample back to atmospheric pressure and measure at ambient pressure.

The next blog post will talk about measuring in “volume barometric” and other obscure units. VBar is used to measure a gas phase concentration while taking into account absolute pressure and then properly compensating for altitude or weather conditions. The VBar barometric units assume that the measurement sensor is at ambient pressure.

My final thought is to understand absolute vs. gauge pressure. There is a time when it matters, so knowing so will help avoid confusion.

 

Frivolous Friday – Tutankhamen Beer Fetches a Royal Sum

I’m sometimes amazed at what connoisseurs are willing to do for a unique experience. Would you pay $7,686 for a bottle of beer? This one happened to be the first filled with someone’s best interpretation of what might have been cooked up in King Tut’s kitchen. Subsequent bottles didn’t fetch nearly as much, but it still sounds like it was an interesting experiment. Here’s the full story:

Happy Friday!

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