Gas Phase Measurement Units

In the past couple of weeks I’ve received several questions about measurement units and how they differ from one another. Have you ever tried to keep bar, mbar, atm, Kpa, %Vbar, %, torr, ppm and ppb straight? If you’re listening to someone in speed mode (I plead guilty) it can be a challenge to follow.

So let’s start by looking at the way different units present at 1 bar, the unit of pressure sometimes also referred to as “atmosphere.”

1 bar =

  • 1000 mbar
  • 750.1 Torr
  • 750.1 mm Hg
  • 29.53 inches Hg
  • 0.987 Atm
  • 14.50 psia
  • 100 kPa

As a brewer you probably won’t see much of units like Torr or mm of mercury (mm Hg), but there’s a unit called %Vbar or ppmVbar that may be helpful. I use them a lot and they can easily be interchanged with percent, but there is a specific distinction in that it is tied to atmospheric pressure and thus stands for “Percent Volume Barometric and “PPM Volume Barometric”. “

So why use Vbar instead of just percent? If you’re at a high elevation and want to specify that that the percent of the gas you are measuring is being measured at atmospheric pressure, then Vbar is your unit. For example, Denver Colorado is roughly 5280 feet. At that elevation there are about 15 percent fewer atmospheric gas molecules —  855 mbar – versus the 1013 mbar you would find at sea level in San Francisco. The Vbar units confirm that the instrument is at atmospheric pressure while the sample is being measured.

This table compares different gas percentages using some of the most common units you may encounter:

Unit

mbar

Bar

Atm

Percent (absolute)

%Vbar

PPM

100% gas

(at sea level)

1013

1.013

1.000

100.0

100.0

1,000,000

100% gas  (atmospheric at 5280 feet)

855

0.855

0.844

84.4%

100

1,000,000

1.000 % gas

10

0.010

0.010

1

1

10,000

0.100 % gas

1

0.001

0.001

0.1

0.1

1,000

0.010 % gas

0.1

0.0001

0.0001

0.01

0.01

100

0.001% gas

0.01

0.00001

0.00001

0.001

0.001

10

0.0001 % gas

0.001

0.000001

0.000001

0.0001

0.0001

1

My final thought is to understand the units available to you. If you are purging down a tank with CO2 and want a specific percentage of CO2 purity, use the units that will equate back to what could dissolve in your beer if the purge didn’t exhaust all of the contaminating gas in the tank.

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.

 

The Units of Gas Phase Measurements Demystified!

I don’t think there is anything more confusing than the units used when measuring gases dissolved in liquid compared to the units used when measuring in the gas phase.  Here is one more post on CO2 before I completely exhaust the topic – pun intended. Let’s see if I can make it clear.

Dissolved gas is always expressed in terms of weight per volume. When someone says that they have 10 ppb or 0.01 ppm of dissolved gas in their beer, what they really mean is that they have 10 micro grams per liter or 0.01 milligrams per liter dO2 content.  Again: this is gas expressed as weight of gas per liter.

Gas phase measurements, on the other hand, are always expressed as a volume of gas per volume of total gas. This can be in units of percent, bar (volume barometric,) mbar, atmosphere, or — the two most confusing units of all – ppm and “%CO2 purity.” Lets talk about those two:

  • Gas phase ppm is a comparison of the gas being measured to all of the gasses in the sample. If you have 100% O2, then you will have 1,000,000 ppm O2. Since it is cumbersome to talk about 1% or more as ten thousand parts per million, we use units of percent. But when we get to trace levels below 0.1%, then we start throwing around units of ppm.
  • Percent CO2 purity is all of the gas being measured that is only CO2. Here’s a table to help you sort it out.
Unit of Measurement O2 Content

 

O2 Content

 

O2 Content

 

O2 Content

 

O2 Content

 

Percent 100 1.0 0.1 0.01 0.001
bar 1.013 0.010 0.001 0.0001 0.00001
mbar 1013 10.13 1.01 0.10 0.01
ppm 1,000,000 10,000 1000 100 10
Atmosphere (ATM) 1.0 0.01 0.001 0.0001 0.00001
% CO2 Purity 0 95.0 99.5 99.95 99.995

Did you follow? Just in case I lost you, %CO2 purity assumes that all of the impurity in CO2 is air.  Since nitrogen comprises 4/5th of air, you have to take the oxygen content, add back in the nitrogen, and subtract all the air from the 100%.

My final thought is to add just a tad more data to compare ppm dissolved to ppm gas phase. Here’s a quick test: if you measure CO2 gas with an instrument reading, in the liquid phase, 0.001 ppm or 1 ppb dO2 at 20 deg C, what would be the equivalent gas phase reading?  At 20 deg C, the O2 content in the gas would be about 22 ppm. In terms of %CO2 purity, that would be about 99.99%.

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