# FAQ

We've organized information in response to questions frequently asked by our customers. Information shown here, as well as through links to other pages throughout our website, makes it easy to take the guess work out of purchasing gas handling systems and gas handling equipments from EXCEL Gas.

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How do I find how much volume of compressed gas is in a gas cylinder?

Using the Ideal Gas Law (PV=nRT) as the starting point, it is fairly simple to calculate the cylinder content. An example of this calculation is given:

Volume of Gas in Cylinder

To find the volume of gas available from a compressed gas cylinder, we apply the Ideal Gas Law (PV = nRT). In a high-pressure cylinder, the volume will be affected by the content's compressibility factor Z (PV = ZnRT). For example, an AL cylinder of pure helium may contain 134 cu. ft. of gas while the same cylinder of pure air may contain 144 cu. ft. under the same conditions. For these practical calculations, however, we assume ideal gas behavior for simplicity.

The Ideal Gas Law PV = nRT

Where :

- P is pressure
- V is volume
- n is the number of moles

- R is the gas constant
- T is the absolute temperature

- When the temperature is kept constant, we can derive the equation

P (1) x V (1) = P (2) x V (2)

Where :

- P (1) is the pressure of the compressed gas in the cylinder (psi)
- V (1) is the internal volume of the cylinder, often referred to as water volume (liter)*
- P (2) is the atmospheric pressure (1 atm - 14.7 psi)
- V (2) is the volume of gas at pressure P (2) (liter)

For example, an AL sized cylinder is filled with nitrogen at 2000 psi. What is the gas volume of nitrogen from the cylinder?

- P (1) is 2000 psi
- V (1) is the internal volume of AL cylinder 29.5 liter*

- P (2) is 14.7 psi
- V (2) is the unknown volume of gas

Solving the equation above for V (2) gives:

V (2) = [p (1) x V (1)]/P (2) = (2000 psi x 29.5 liters)/14.7 psi = 4013 liters (approximately 140 cu. ft.)

What is a flam-ox gas mixture?

A flam-ox gas mixture is one that contains a flammable component such as methane and an oxidizer component such as air. Depending on the concentrations involved, there may be pressure restrictions placed on the mixture so that an explosion would be contained by the gas cylinder.

What is the difference between a single-stage gas pressure regulator and a two-stage gas pressure regulator?

Single-stage gas pressure regulators reduce the cylinder pressure to the delivery pressure in one step, where a two-stage gas pressure regulator accomplishes the same thing with two stages of pressure reduction. This means that a single-stage regulator has to be adjusted to maintain a constant pressure as the gas in the cylinder is used, but a two-stage regulator does not. Two-stage regulators are typically used when more stability of operation is required and are often used in specialty gas applications, like gas chromatography.

On the other hand, if gas is being used for a short duration instrument calibration, a single-stage gas regulator with a wide accuracy envelope (supply pressure effect) but a comparatively flat droop should be chosen. This will eliminate the need to allow the gas to flow at a constant rate before the calibration can be done.

Can I regulate the gas flow rate using a gas pressure regulator?

Gas pressure regulators are used to regulate the gas pressure (pounds per square inch) and are not appropriate for measuring flow rate (liters per minute). Flowmeters, Rotometers or Mass Flow Controllers should be used to regulate gas flow rates.

Flowmeters measure liquid and gas flow rates for general plant and laboratory applications. Rotometers are special flowmeters named for the rotating float that functions as the indicating element. Mass flow controllers precisely measure, monitor and control gases on the basis of mass flow, eliminating the need for continuous monitoring and readjustment of gas pressures to provide a stable gas flow.

How do you connect more than one compressed gas cylinder to each side of an automatic changeover gas manifold?

In order to connect more than one cylinder to each side of an automatic changeover gas manifold, you will need to use header manifolds. Header manifolds are attached to the changeover manifold with flexible pigtails equipped with CGA fittings and check valves. Header manifolds are usually used in pairs - a left header and a right header. The left header has the outlet on the left end and the right header has the outlet on the right end. Header manifolds are typically available with two stations, but can be constructed to accommodate many more cylinders. Depending on the space available, they can be single rows or multiple rows. Each header station also comes with a station valve and a 24" flexible stainless steel pigtail equipped with a check valve and CGA fitting to attach to a cylinder.

Can gas manifolds be fastened to the cylinder rack?

Gas manifolds can easily be attached to special types of cylinder racks called process cylinder racks. Process cylinder racks differ from standard cylinder storage racks by having a cross beam above the tops of the cylinders. The manifolds are attached to the cross beam using the mounting brackets included with the manifolds.