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.

 

Diagnosing Dissolved Oxygen Loss During Filling

 

Dissolved oxygen can increase or decrease at many points during packaging, and sometimes we see values that differ pretty wildly, depending upon where they were taken in the process and the specific parameters of the measurement.

I have a customer who was getting some odd package oxygen results and she wanted to understand if the data were real, and if so, why. Here are the numbers:

Base of Filler dO2:  110 ppb
Unshaken dO2: 93 ppb
Headspace O2:  38 ppb
Shaken dO2:  60 ppb
Total Package Oxygen:  131 ppb
Package Volume:  355 mL
Headspace Volume:  17 mL
Temperature:  9 oC

It’s unusual to see an unshaken dO2 value that’s lower than the value at the base of the filler, so that was our main focus. After establishing that this was an occasional trend for her filler, but not something she was seeing all the time, and after validating the instrumentation to be sure the readings were correct, logic dictated the rest:

1.    The flow into her fill bowl was very turbulent and the beer was losing oxygen in the bowl because the O2 content of the bowl’s headspace CO2 was lower than the O2 content of her beer.

2.    The shaken dO2 was lower than the unshaken dO2 due to the efficiency of the jetter. Since the package dO2 was a bit on the high side, the beer lost oxygen into the headspace during shaking.

This particular case was interesting because beer usually picks up dissolved oxygen between the base of the filler and the unshaken dO2 in the package, but every once in a while the opposite can happen. In this particular case I suggested that the flow of CO2 into the bowl might be excessive and that lowering it would probably result in a more expected set of numbers.

My final thought is that when your results don’t make sense and you know your instrumentation is working correctly, pay attention to whether it’s a trend you see repeated, or if it happens only under specific circumstances. If it happens during specific situations, then there’s usually an easy explanation.

Package Headspace Volume and its Effect on TPO

 

I’m at the CBC this week, so come by and say “hi” if you’re there. I’ll be in the Hach booth at the Brew EXPO.

Most total package oxygen measurements are determined by the dissolved oxygen concentration in a shaken package. A straight shaken dissolved oxygen measurement can be a great part of your brewing quality toolkit, and it never hurts to have it. But it doesn’t reflect the oxygen in your headspace and its potential effect on your package. For that you need to calculate the TPO.

Headspace oxygen concentration can actually be more than twice your dissolved concentration before shaking. So I thought I’d show an example of how much the TPO can vary on two packages with the same dO2, especially if their volumes are slightly different.

Let’s start with two cans of beer, each with 100 ppb of dO2. (That means we’re looking at dO2 in the liquid only, before shaking.) The first has a 15 mL headspace due to a slightly high fill and the second has 25 mL because of a slightly low fill. While both packages have 100 ppb of dO2, the first has a TPO of 227 ppb and the second has a TPO of 312 ppb. For some this may be considered above their total package threshold.

Headspace Example

My final thought is that you should always consider the headspace volume when measuring dissolved oxygen in packages. You don’t need to know the exact TPO of every package, but by being aware that low fill measurements have less latitude than high fill measurements, you can help keep your package concentrations as low as possible.

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