What Is Dissolved Acetylene (DA)?

The short answer

DA stands for dissolved acetylene. Acetylene gas (C₂H₂) cannot be safely compressed and stored on its own above about 1.5 bar — under pressure it can decompose violently, even without any air present. To store it safely, an acetylene cylinder is packed with a solid porous mass and charged with a solvent — usually acetone, or DMF (dimethylformamide) for higher pressures. The acetylene dissolves into the solvent held in the pores, so the cylinder holds a large quantity of gas at a safe, usable pressure.

That is why the fuel-gas cylinder in oxy-acetylene welding and cutting is stamped “DA”: it is not free acetylene, it is dissolved acetylene.

Why acetylene has to be dissolved

Acetylene is a fuel gas with an unusually high flame temperature, which is exactly why it is prized for cutting and welding. But the same energy that makes it useful makes free, compressed acetylene unstable. Dissolving the gas in a solvent breaks up the conditions needed for decomposition and lets a cylinder store far more acetylene than compression alone ever could — safely.

This is the final stage of every acetylene plant: the purified, dried gas is charged into prepared DA cylinders under a cooling-water spray, dissolving into the solvent rather than simply being compressed like oxygen or nitrogen.

How a DA cylinder is built

A dissolved-acetylene cylinder is a very different object from an ordinary high-pressure bottle. Three things work together:

1. 1
   Porous mass

   The cylinder is filled solid with a monolithic porous mass (calcium silicate or similar). Its fine, uniform pores subdivide the gas space so decomposition cannot propagate, and hold the solvent by capillary action.
2. 2
   Solvent

   The porous mass is charged with a measured quantity of acetone or DMF. Acetylene dissolves into this solvent — roughly in proportion to pressure — which is how the cylinder stores its charge.
3. 3
   Acetylene charge

   Purified acetylene is filled slowly under cooling so it dissolves evenly into the solvent rather than pooling as free gas. The cylinder settles before dispatch and is used and stored upright to keep solvent in the mass.

Using DA cylinders safely

• ✓Always store and use DA cylinders upright — lying them down lets solvent reach the valve.
• ✓Never draw gas too fast; excessive withdrawal carries solvent out with the acetylene.
• ✓Keep cylinders away from heat and use an approved flashback arrestor on the regulator.
• ✓Let a cylinder that has been horizontal stand upright before use so the solvent settles back.