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.

 

Wort – How Much Dissolved Air Will It Typically Hold?

I spent some of last week helping brewers troubleshoot issues with their wort aeration, so this seems like a good time to jot a few ideas about the limiting factors of dissolved gasses.

Gasses dissolve into liquids based upon the ability of the liquid to hold a specific gas. For example, there are hydrocarbons that at room temperature can hold more than 200 mg/L of O2. Compare that to pure water, which holds only 45 mg/L of 100% oxygen gas.  Air dissolved in wort will dissolve in about 1/5 the concentration of pure O2.

Barometric pressure can also have an effect. Imagine you’re injecting air into wort and your brewery is in San Francisco at sea level: The air will hold about 16% more oxygen than if your brewery is on the Colorado Front Range at 5,300 feet, even though the relative percentage of oxygen compared to nitrogen is similar.

Cold liquids have a greater ability to hold gas than warm liquids, so liquid temperature is another important factor in determining dO2 content. The graph below shows the relative concentration of air dissolved in water, based on temperature. The solubility of oxygen in wort is slightly less than water, but as far as I’m aware there is no published literature showing dO2 in wort according to temperature, so that’s why we’re using water as our example.

And here’s the way it looks using the raw data used to generate the graph:

Temp (oC)

dO2 (ppm)

N2 (ppm)

0

14.65

23.51

1

14.24

22.92

2

13.85

22.35

3

13.48

21.82

4

13.13

21.30

5

12.79

20.81

6

12.47

20.34

7

12.16

19.89

8

11.86

19.46

9

11.58

19.05

10

11.31

18.66

11

11.05

18.28

12

10.80

17.92

13

10.55

17.58

14

10.32

17.25

15

10.10

16.93

16

9.98

16.63

17

9.68

16.34

18

9.48

16.06

My final thought comes back to the importance of controlling and closely monitoring those process parameters over which you have control. You may not be able to control barometric pressure, but you can be aware of it. And you do have control over temperature, so that’s another help toward meeting your goal of consistent dissolved gas levels.

Dissolved Oxygen Measurements in Wort

Last week I spent a couple of mornings measuring dissolved oxygen in wort at two different breweries. One was injecting oxygen and the other was injecting air. It was easy to get high dO2 concentrations injecting O2, but the air was another story – the O2 in the air simply didn’t want to fully saturate. Here’s a bit about the two setups.

Each gas was injected into a process pipe where the brewery had the ability to control the volume of gas injected into the wort. In both cases the gas was injected at a similar distance from the measurement point. The two measurement points were off the process pipe through a sample valve and not in the fermentation vessel. The target gas concentration for the air injection was to achieve air saturation, which was about 9.5 ppm O2. The target for the oxygen injection was 15 to 17 ppm.

The measured concentration of dO2 from the oxygen injection was as expected: between 15.0 to 17.5 ppm. But the dO2 with air was not the estimated 9.5 ppm, measuring instead around 7.5 ppm. Flow rates were similar, the distance to the measurement point was similar, the method of injection was similar, but not all of the oxygen in the air went into solution.

Let’s explore a few different ideas as to why it’s more difficult to get saturation by injecting air. I think the main reason is that there’s just more total gas volume that needs to go into solution. To inject a given concentration of the O2 in air, you must inject four times more nitrogen than oxygen.

Another other reason is that N2 isn’t as easy to dissolve into a liquid. If you dissolve air into water, about 40% of the gas that dissolves into solution will be O2, while 60% will be nitrogen. To get all of the gas into solution, the overall pressure in the process pipe needs to be high enough to dissolve not just the oxygen, but all the nitrogen. It’s not impossible to achieve, but it probably takes a little more time than if it is pure oxygen.

Once you get the gas you want into solution, you still need to keep it from coming out of the wort in the fermentation vessel. If you’ve injected air, then it is important to keep some back pressure on the tank until the tank is filled and the liquid is quiescent. If you don’t maintain back pressure then the nitrogen can degas and potentially take some O2 with it. Even if you inject oxygen, it’s important to control the flow of wort into the fermentation vessel, keeping the fill as non-turbulent as possible so the O2 will stay in solution.

My final thought is that there is a lot to keep track of, no matter which technique you use to oxygenate or aerate your wort. Be consistent, and make sure you understand your dissolved oxygen levels, so you’ll know if your yeast are getting enough to keep them happy.

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