Baltic Blender

The up and down arrows can be used to select from a set of standard gasses. Up selects a gas which is richer oxygen content than the current selection (up meaning used at shallower depths) and down selects a gas with a leaner oxygen content. If the Trimix feature has been purchased, the helium-containing gasses will always be found deeper than the non-helium gasses regardless of oxygen content. Even when making a non-standard selection, these buttons are a quick way to get close to the desired mix, and then minor adjustments can be made directly with the percentage wheels.

The standard gas presets are oxygen, EAN 80, EAN 50, EAN 36, EAN 32, Air (EAN 20.9), Tx 35/25, Tx 21/35, Tx 18/45, Tx 15/55, Tx 10/70, Tx 7/80. This list of standard gases can be edited in Settings.

As a gas mixture is selected, the Maximum Operating Depth (MOD) is shown for partial-pressures of oxygen of 1.2, 1.4, and 1.6 atmospheres. The Equivalent Narcotic Depth (END) at the MOD is also displayed. The Minimum Operating Depth (MinOD) is shown if the partial-pressure of oxygen is less then 0.16 at the surface. The END is calculated based on the nitrogen exposure; it can be calculated taking into account helium and oxygen exposure if so enabled in the Settings.

As a visual reminder that the mixture is moving from normoxic towards hypoxic, a gas mixture with less than 20.9% oxygen will be shown in orange, and a mixture with less than 18% oxygen will be shown in red.

Pressure

All pressure readings are indicated in gauge (relative to the ambient atmosphere), not absolute (relative to a vacuum). For example, an empty cylinder is at 0 bar gauge, which is 1 bar absolute. If your fills are taking place at high altitude, there will be a minor error introduced (e.g., at 2km above sea level, there will be an error of roughly 0.2 bar). It is recommended that you use a high-quality digital gauge which reads pressure relative to ambient.

Temperatures

All temperature readings are the temperature of the gas inside the cylinder. This is obviously difficult to measure directly, so instead we wait for the temperature of the gas to reach thermal equilibrium with the cylinder wall and measure its temperature. How long do you need to wait? After adding gas to a cylinder, if you continue to observe the temperature you'll notice that it continues to increase and stabilizes. This typically takes a few minutes, but experimentation and experience will quickly teach you how long is long enough. It is recommended that you use a infrared thermometer to measure the cylinder wall temperature. Even a cheap one is pretty accurate. (These are sometimes incorrectly described as "laser" thermometers because all but the cheapest models use a red laser pointer to show you where they are aimed, but the laser is not involved in the temperature measurement.) An infrared thermometer is not accurate when measuring a metallic object (for example, if you point one at a mirror it will report the temperature of the object reflected in the mirror, not the temperature of the mirror surface). The proper procedure for unpainted SCUBA cylinders is to measure the temperature of a sticker or tape attached to the surface, e.g., the annual Evidence of Inspection (VIP) sticker.

There are 3 temperature sliders:

The first is in Starting Mixture and should be set to the temperature of the cylinder when the initial mix and pressure is read. After the mix and pressure is entered, this slider should not be changed until you have completed the blending.

The second slider, at the bottom of the Step-by-Step section, is the "current" temperature of the cylinder. As you go through the steps, you will be changing this slider to calculate the correct fill pressure for each gas. This is the slider you'll be using most.

A third slider appears at the top of the Step-by-Step section if you use a preblend-gas, and have less than the maximum amount available. See Using Preblended Gas.

Tapping on the dark area on the left or right end of the slider moves the slider in small steps for better precision.

Starting Mixture

All blending software requires you to enter a starting gas mixture (the gas that is currently in the cylinder) and a desired mixture (the gas you want to end up with) and presents a step-by-step set of instructions to accomplish the blending. Baltic Blender requires that you enter into the Starting Mixture section the mixture, pressure, and temperature that you are starting with. Remember—even an empty cylinder contains gas at atmospheric pressure, so an empty cylinder must still have the starting pressure (zero), mixture (air, or whatever was last in the cylinder), and temperature entered before Baltic Blender will perform blending calculations.

Desired Mixture

The Desired Mixture section is where you enter the mixture and pressure that you'd like the full cylinder to contain after the blending process. Note that this pressure is always described at the configured standard temperature (defaults to 21℃); you do not enter a final temperature. Typically, local laws and regulations will state that a cylinder may not exceed the marked service pressure at temperatures between 15℃ to 21℃. In our earlier example of an 10L filled to 216.7 bar at 38℃, the cylinder is not overfilled because if it were cooled to the configured standard temperature (eg. 21℃), it would contain 200 bar. Similarly, cylinders filled and used in very cold environments legally need to contain less than their service pressure. You should check your local laws and regulations regarding high pressure cylinders.

Now we're ready for our examples. Each example below is intended to teach you about a specific set of features of Baltic Blender; they get progressively more complicated.

Blending Examples

Example #1—Blending Nitrox into an empty cylinder

A customer purchases an empty 10L cylinder and requests that you fill it with EAN 32. You install the valve and begin blending. Because the cylinder was previously opened to the atmosphere, enter a Starting Mixture of air at 0 bar and measure and enter the starting temperature, say 24℃ in this example. There is no preblend gas in this example, so we leave this field blank. This field will be discussed in a later example. Next, we enter a Desired Mixture of EAN 32 at 200 bar. Only when all of the required Starting Mixture and Desired Mixture parameters are entered will the Step-by-Step instructions appear. Once these parameters are entered, they should not be changed until the blending operation on this cylinder is completed.

In the Step-by-Step section, we read that the first step is to add oxygen to 27.6 bar at 24℃. However, after adding 27.6 bar of oxygen, we will find that the cylinder is hotter than 24℃, and therefore we have actually added less than the amount required. After waiting a few minutes for the temperature to stabilize, we measure the temperature, and say it is at 30℃. We enter this new temperature into the slider in the Step-by-Step. Baltic Blender now calculates that the correct final pressure of the oxygen at 30℃ is 28.2 bar, so we add slightly more oxygen before proceeding to the next step. It is important to understand that 27.6 bar of oxygen at 24℃ is the same amount of gas as 28.2 bar of oxygen at 30℃. You have not added the correct amount of oxygen unless you reach the indicated pressure when the Step-by-Step temperature slider is set to the measured temperature. Because the second addition of oxygen is small, the further temperature rise that occurs is negligible and can be ignored, and the blender can proceed directly to the next gas without further temperature measurement and correction.


Before adjusting temperature.		After adjusting temperature.

The pressure-temperature relationship is an important concept. For example, suppose we added 27.6 bar of oxygen, and, with the cylinder now at 30℃, we left for lunch knowing that we didn't yet have the full amount of oxygen added and intending to perform the top off later. However, we didn't make it back to the shop after lunch because of the 30cm of snow that fell while we were away, and when we do make it back the next day, the cylinder is now at 7℃. If we measure the pressure, we'll find it at 25.9 bar. If we enter the temperature 7℃ into the Step-by-Step temperature slider in Baltic Blender, it will now show us the correct top-off pressure for the oxygen is 25.8 bar at 7℃.

Back to our original example, now that we've topped off the oxygen to 28.2 bar at 30℃, it is time to add the air. Baltic Blender calculates that we would add air to 208.9 bar at 30℃. We then allow the temperature to stabilize and measure it at, say, 49℃. We enter this number into the Step-by-Step slider and Baltic Blender calculates that we would add air to 227.7 bar at 49℃. We add the additional air, but since this is a pretty big step, and will result in substantial additional heating, it may be necessary to perform a third iteration to reach the desired pressure at the measured temperature.

After the gas is allowed time to homogenize, the gas mixture can now be tested and the cylinder delivered to the customer.

Following this iterative process correctly is vital to achieving a correct final blend. If you have any confusion at this point or have not obtained the same results as shown in this example, please re-read this section and practice again until you can reproduce the results shown.

Sources of error

The blender should resist the temptation to fudge by not following the instruction precisely. This is a practice that all blenders learn when using ideal gas laws, and it will only introduce error into the final mix when using Baltic Blender. As you gain more experience with Baltic Blender, you may slightly "overshoot" on the pressure when you know that the actual temperature is hotter than the temperature slider, especially with the final air fill. For example, in the above example you could have added air to more than 208.9 bar initially, knowing that the heating of the cylinder would require it. A better technique is to measure the temperature more than once. In the above example, as the air pressure approaches 208.9 bar, and before allowing the temperature to stablize, I might measure a temperature of 44℃ and immediately enter this into the Step-by-Step temperature, and continue adding air to the calculated 222.8 bar at 44℃. Only then do I stop, stabilize, measure, and top off.

Note that we're doing calculations based on delivering precise amounts of gas into the cylinder. Some fill station configurations have long fill whips or manifolds that hold large volumes of gas. If you precisely finish adding oxygen and then pump air into the cylinder without bleeding the whip, you'll be driving the remaining oxygen in that hose into the cylinder. As Baltic Blender can't predict this volume of gas, the most accurate technique calls for bleeding the fill whips and other dead space whenever changing gasses. This is especially important when blending hypoxic Trimix, as the low oxygen content will be significantly affected by any excess oxygen added to the cylinder.

An apparent source of error is measurement error in the gas sensors. Oxygen sensors are only accurate when calibrated on dry gas using a mixture close to the mixture being tested. If an oxygen sensor is calibrated on air, it may not accurately measure high oxygen gasses like EAN 50 or EAN 80. Helium sensors can be even less accurate, with most being accurate to only 2-3% of the total mixture. If your first attempt to mix Trimix 21/35 reports 20.8% oxygen and 37.5% helium, resist the temptation to correct this mix or add a fudge in future blending to "get this right". Chances are that the discrepency is sensor error!

With patience, experience, and proper technique, Baltic Blender should be able to reliably mix the oxygen content within .2% of the desired mix and the helium content well within the 2% accuracy of most analyzers.

Example #2—Blending Nitrox into a partially full cylinder

Your customer is amazed with your incredible blending prowess and returns the next day with the same cylinder asking for a fill of EAN 36. She tells you that she's diving a wreck tomorrow and the propeller is at 29m. She wants to be sure that you won't be rich on your mix.

You analyze the mixture and find it is EAN 32.3 at 36.3 bar and 24℃ and enter this into the Starting Mixture fields in Baltic Blender. In the Desired Mixture field you enter EAN 36 and 200 bar.

As in the first example, you add oxygen, measure and enter the temperature, top off the oxygen and add air, measure and enter the temperature, and top off the air. In this example, blending into a partially full cylinder is no different than an empty cylinder.

Example #3—Blending requiring a bleed

The customer returns the next day with tales of woe about the bad conditions and the aborted dive on the wreck. Her next dive is deeper, and she wants a fill of EAN 32. You analyze the mixture and find EAN 36.1 at 172 bar and 24℃ and enter this into the Starting Mixture. In the Desired Mixture fields you enter EAN 32 and 200 bar.

Well, surprise, the cylinder already has too much oxygen. Topping off the cylinder to 200 bar with air would result in a mixture of approximately EAN 33.9. Accomplishing the required blending will require bleeding off some of the gas already in the cylinder. Baltic Blender will calculate the required bleed and tell you to bleed the cylinder down to 143.8 bar at 24℃. As you do so, the cylinder will cool, so, just like adding gas, you will need to wait for the temperature to stabilize, enter the new temperature (for example, 18℃), and do a final bleed to the corrected pressure, 139.8 bar at 18℃.

Note—a blending operation that requires a bleed off requires one because there is already too much of one of the components in the cylinder, oxygen, helium, or nitrogen (i.e., air). One of the characteristics of a blend that requires a bleed is that at least one of these three will not need to be added. As a consequence, a bleed done because a cylinder has too much nitrogen (air) will require adding oxygen or helium at high pressure (200-242 bar or more, depending on the temperature). Doing so will require a source of oxygen at a higher pressure than the typical supply cylinder, or an oxygen-safe gas booster. High pressure oxygen is dangerous; even more so if the fill station equipment, cylinder, or cylinder valves are not 100% clean of any hydrocarbons. A blending operation like this is a tradeoff between economy and safety; safety can be improved by bleeding more gas than the minimum, which then permits you to add the oxygen at a lower pressure. Simply enter the new pressure and temperature in the Starting Mixture field, then add the oxygen (now at a lower pressure) and air.

Example #4—Blending Trimix

The following examples require purchasing the "Trimix" upgrade. See Upgrades for more.

The customer brings in a set of double cylinders containing 34 bar at 24℃ of Trimix 21/35 (Tx 21/35) and requests that they be filled to 248 bar of Tx 10/60. After counseling him on the evils of diving with non-standard gas mixes and overfilling his low-pressure cylinders, you enter this information into the Starting Mixture and Desired Mixture fields. In this case, the Step-by-Step will show the helium, oxygen, and air to be added in that order. After determining that your helium cylinder is almost full and your oxygen cylinder is nearly empty, you decide that you would prefer to add the oxygen first and then the helium. You can touch the "Edit" button and slide the individual blending steps up or down using the gray parallel bars on the right side of the steps to change the order of the gasses, in this case, to move oxygen before the helium. However, do not change the order once you've started the blending operations.


Before rearranging the gases.		After rearranging the gases.

Example #5—Using preblended gas

After completing the blending in Example #4, the same customer calls to tell you that his teammates decided not to dive with a "stroke" and he won't be needing that mixture after all. You would hate to bleed out $100 worth of helium, but don't have any other use for this weird Tx 10/60 gas. This introduces the next concept—"preblend" gas. In the first four examples, we ignored the "Preblend" option in the Starting Mixture field. The Preblend field serves one of two purposes—you can use it to consume an unneeded gas which is already mixed, such as the Tx 10/60 left over from the previous mix (or a gas which needs to be bled from another mix), or you can use it to calculate Step-by-Step using a non-air top-off gas (such as EAN 32 from a continuous-blend-fed compressor). Let's examine the first case in Examples #5 and #6.

Enter a Starting Mixture of Tx 21/35 at 34 bar and 24℃ , and a Preblend of Tx 10/60. The Desired Mixture should be Tx 18/45 at 242 bar. The Step-by-Step instructions have you add preblended Tx 10/60 to 195.2 bar at 24℃. Do so, and iterate topping off the Tx 10/60 until you reach the pressure and temperature called out. Then iterate through the oxygen and air as required. Note in this example, no pure helium was required. What limited the amount of Tx 10/60 that could be used in mixing this gas was the helium content of the Tx 10/60. Preblend used in this manner will always completely replace one of the other three gasses.

You can clear the Preblend field by left-swiping it and touching "Clear".

Example #6—Using preblended gas that runs out

Using the same starting point as Example #5, consider what would happen if you ran out of Tx 10/60 during this blend and were unable to reach the called-for pressure and temperature. You'll notice at the top of the Step-by-Step a field labeled Fill preblend to. Enter the actual pressure that you managed to reach using Tx 10/60, say, 140 bar, and a third temperature slider will pop-up below it. Enter the temperature at which you reached 140 bar, say, 30℃.

At this point, the amount of gas added by the Tx 10/60 is now "locked" in, and the remaining Step-by-Step instructions will tell you how much of helium, oxygen, and air to add to complete the blending. Do not change the preblend pressure and temperature once set. These values, like the Starting Mixture, should not be changed until the blending operation on this cylinder is completed.

You can clear the Fill preblend to field by left-swiping it and touching "Clear".

Example #7—Using non-air top-off mixtures

You're nearly out of oxygen, but have a membrane system feeding your compressor that produces unlimited volumes of EAN 32. Noting that most standard Trimix gasses can be made from helium and EAN 32, you want to use preblended EAN 32 as a top-off gas. Enter a Starting Mixture of 0 bar of air at 21℃, and a Preblend gas of EAN 32.

Enter a Desired Mixture of Tx 21/35 and a final pressure of 242 bar. The Step-by-Step instructions indicate first adding EAN 32, then helium, then oxygen. Because the EAN 32 comes from your compressor at high pressure, you'll want to add it last, so you use "Edit" mode as in Example #4 to change the order of the gasses to put EAN 32 last.

When you do this, you'll notice that the field that allows you to enter how much preblend gas that you added in Example #6 disappears. If the preblend gas is not the first gas, it is assumed that you have enough gas to complete the mix. Baltic Blender will not let you add a second helping of the primary gasses. If you ran out of preblend gas in this case, you'll need to analyze the gas in the cylinder and enter this information into Baltic Blender as a new Starting Mixture.


Before moving the preblend.		After moving the preblend.

When using this technique, you may also notice that a very small amount of oxygen (in this example) or air is called for to get precisely the mix you want. If the amount is very small, you could ignore it and tolerate the inaccuracy in the mix. If you have control over the preblend EAN 32's exact oxygen content, you could try adjusting it until the Step-by-Step additional air or oxygen is minimized.

Error Conditions - when you can't blend that exact mixture

Mixtures with too much nitrogen

If you try to blend a mixture which contains a ratio of nitrogen to oxygen greater than that found in air (79:21), another source of purer nitrogen would be required. These gasses are not normally used in SCUBA and Baltic Blender will alert you to this unusual condition.

Consider a Starting Mixture of air at 0 bar and 21℃. A Desired Mixture of EAN 18 at 200 bar. Baltic Blender will express its displeasure in the situation by calling for you to Add N₂ in the Step-by-Step instructions. Your typical SCUBA shop will not be equipped to blend the nitrogen required. Check the Desired Mixture when this happens, as you've probably make an error in your entry.

Problems caused by the inability to completely empty a cylinder

Unless you use a vacuum pump, you can't bleed a cylinder completely empty. Consider a Starting Mixture of 7 bar of air at 21℃ and a Desired Mixture of 100% oxygen at 200 bar. Even if you bleed the cylinder completely, there is still 1 bar of air in the cylinder containing nitrogen. The Step-by-Step instructions will tell you to bleed the cylinder to 0 bar, add oxygen to 200 bar, but it will warn you that it Cannot compute exact blend and tell you that the result would contain EAN 99.6. Should it be necessary to fill a cylinder with oxygen purer than 99.6%, fill and purge the cylinder with oxygen first, and then enter a Starting Mixture of 0 bar of 100% oxygen.

Similarly, if a cylinder starts with a helium-containing mixture and the Desired Mixture does not contain helium, the Step-by-Step will call for completely emptying the cylinder of the helium-containing gas and, as above, it will warn you that it Cannot compute exact blend because of the amount of residual helium. In practice, divers do not want to waste helium and it makes no sense to bleed off a small amount of, say, Tx 18/45 because your next dive requires EAN 32.

In this circumstance, to avoid wasting helium, set your Desired Mixture to account for the helium already in the cylinder. Specifically, experimentally find a value for the helium content of the Desired Mixture in which there's a negligible bleed or a negligible helium addition. For example, if your Starting Mixture is Tx 18/45 at 100 bar and you would like EAN 32, you'll find that if you instead set your Desired Mixture to Tx 32/20.2 that you would be required to bleed off only 0.2 bar, which could be safely ignored. Before performing this procedure, be sure to discuss with the diver the option to keep, rather than bleed, the residual helium in the cylinder. If he chooses to keep the helium, inform him how much residual helium will be in the resulting mixture.

Settings

The Settings screen can be accessed by tapping the wrench icon on the bottom of the screen.

Pressure & Volume picks the unit system used for pressure and volume values.
Temperature picks the unit system used for temperatures values.
Standard Temperature picks the standard temperature for the final pressure.
Default Preblend picks a default preblended gas to use on fills. Eg. if you wish to always use a preblended bank of EAN 32, enter it here. If you do not use a preblend, leave it blank.
Default Blending Sequence lets you control the order that gasses are added.
- Preblend first, adds the preblended gas (if any) before anything else.
- O₂ before He, toggles whether to add oxygen or helium first.
Price allows you to set a price per unit volume of gas used, plus a fixed price for labor per cylinder (only available in upgraded version).
Bill Header allows you to set a multi-line header that will be added on bills when emailed or printed (only available in upgraded version).
END selects whether the theoretical narcotic effects of O₂ or He are included in the END calculation shown in the mixture picker field. The He is narcotic option is only shown if the Trimix feature has been purchased. You can also select which ppO₂ values to show ENDs for in the mixture selector.
Default Gases lets you edit the list of default gases the mixture selector jumps through.
Features lists the purchased upgrades, or, if no upgrades have been purchased, the current price of each. See Upgrades for more information.

Calculating Gas Volumes and Cost

If you've purchased the "Volumes & Pricing" upgrade, you can enter the information about the size of the cylinder filled, and Baltic Blender will calculate the volume of gas added. All gas volumes are corrected to standard temperature and pressure (1 atm at 21℃). If you've configured the price of the gas (per volume unit) in the Settings screen, Baltic Blender will also calculate the amount to bill the customer.

The size of the cylinder is described as the water volume of the cylinder, or the actual interior volume. Eg, a 15L cylinder should be entered as 15 litres and an USA/DOT LP85 would be entered as 13.2 litres.

When technical doubles are filled, the volume of a single cylinder should be doubled. A double set of 15L tanks should be entered as 30 litres.

Once the cylinder size information is entered, and the pricing information is configured under Settings, line items will appear by each gas volume showing the amount used and the cost. The preblend gas costs, if any, will be accounted for separately. If the preblend was a customer-provided gas, you can uncheck the boxes to remove them from the total.

The reader may find it amusing that a 10L cylinder holds 1920 litres of air, but holds 2104 litres of oxygen This is because oxygen compresses more readily than air.

To print or email a bill, tap the Total cost cell in the list of tanks. This will present the various options for exporting the bill. Remember, to print, you need an AirPrint enabled printer. In Settings, you can specify a header to put on bills.

Groups and Cylinders

Baltic Blender lets you work on more than one cylinder at a time. When blending many cylinders, you can use this to save time and reduce the gas lost to purging the blending station lines. For example, if you have many Trimix fills to do, add the helium to all the cylinders at one time, rather than switching back and forth between adding helium and oxygen. By the time you get to the last cylinder, the temperature of the first one will have stabilized and be ready for the helium top off.

You can name a cylinder by tapping the cylinder name. The default name will contain the desired mixture and cylinder volume, if set.

Baltic Blender also lets you group cylinders together for price calculations and billing purposes (a feature available as an upgrade). To use this feature, you need to configure prices in Settings, and for each cylinder you need to specifiy the cylinder size information. If some of the prices or cylinder sizes are not entered, a partial total will be shown in gray. When all the information is entered, the total group cost will be shown.

Cylinders can be rearranged between and within groups. Tap the Edit button at the top of the cylinder list, and you can drag cylinders between and within groups.

While in Edit mode, you can move cylinders to a new group by dragging them to the bottom, where there'll be an empty group when editing; and you can also name a group by tapping the group name.

The trashcan icon at the bottom of the Cylinders page can be used to delete all cylinders and groups.


Cylinder groups.		Editing and moving cylinders.

Topoff

Sometimes it may be necessary to top off a cylinder with another gas in the field, when oxygen or helium is not available. When you do this, you aren't targetting a particular mixture, but rather asking Baltic Blender to calculate what the resulting mixture will be. If the Desired Mixture is left blank, Baltic Blender will calculate the resulting mixture. If you are using a top-off mixture other than air, enter this into the Preblend field.

To compute a topoff mixture, fill in:

Starting Mixture
Preblend (for non-air topoff)
Desired Mixture (leave the blend blank, but fill in the pressure)

The Step-by-Step cell will now show the topoff mixture. As in a regular blend, moving the temperature slider in the Step-by-Step will change the displayed pressure to account for the temperature rise.

Upgrades

Some features discussed here are available only through paid upgrades.

Trimix fills—Allows blending of mixtures containing helium. Preblended gases can also be helium-based.

Volumes & Pricing—Computes the volumes of gasses added to the clyinder in the blending process, separating oxygen and helium in both base and preblend sources. Configurable prices and price calculation.

To purchase these upgrades, go into Settings. At the bottom you'll find a button for each feature.

If you've bought a new iOS device or otherwise lost the purchases, you can restore the purchases from this screen using the Restore Purchases button.

Release Notes

1.3 2013
iOS7 UI
Configurable ppO₂ END display for mixture selector.
1.2 January 6th 2013
Fixes hang when editing cylinder name and tapping pressure/mix (Issue #61).
Minor UI fixes/improvements..
1.1 July 4th 2012
Support setting the standard temperature
Better internationalisation support.
Show MinOD.
1.0 May 2012
Initial release

About

Baltic Blender version 1.3

Gasblending using GERG-2004.

More Baltic products at www.balticdecoplanner.com

Support at www.balticdecoplanner.com

Thanks to Kenneth Adelman for ideas, testing and help.

Thanks to Gabrielle Adelman for writing documentation.

GERG-2004; Kunz, O.; Klimeck, R.; Wagner, W.; Jaeschke, M. (2007): The GERG-2004 Wide-Range Equation of State for Natural Gases and Other Mixtures: GERG Technical Monograph 15 (2007) and Fortschr.-Ber. VDI, Reihe 6, Nr. 557, VDI Verlag, Düsseldorf, 2007.

The partial GERG-2004 implementation used to calculate the compressibility factors of oxygen, helium, nitrogen and their mixtures in the gas phase was inspired by open source code written by David de Marneffe in December 2007. David can be contacted at daviddemarneffe[at]y a h o o . c o m.