There are two types of regulators: single stage and two stage.  They perform the same function but the two-stage regulator  maintains a more constant outlet pressure and flow as the  cylinder volume and pressure drops. Two-stage regulators  can be identified by a larger, second pressure chamber under  the regulator knob.  [Figures 5-13  and  5-14] A welding hose connects the regulators to the torch. It is  typically a double hose joined together during manufacture.  The acetylene hose is red and has left hand threads indicated  by a groove cut into the connection nut. The oxygen hose is  green and has right hand threads indicated by the absence of  a groove on the connection nut.  Welding hoses are produced in different sizes from ¼-inch to  ½-inch inside diameter (ID). The hose should be marked for  light, standard, and heavy duty service plus a grade indicating  whether it has an oil- and/or flame-resistant cover. The hose  should have the date of manufacture, maximum working  pressure of 200 psi, and indicate that it meets specification  IP-90 of the Rubber Manufacturers Association and the  Compressed Gas Association for rubber welding hoses.  Grade-R hose should only be used with acetylene gas. A  T-grade hose must be used with propane, MAPP$^{®}$, and all  other fuel gases. The check valve stops the reverse flow of the gas and can  be installed either between the regulator and the hose or the  hose and the torch.  [Figure 5-15] Excessive overheating  of cutting, welding, and heating tips can cause flashback  conditions. A flashback can be caused when a tip is  overheated and the gas ignites before passing out of the  tip. The flame is then burning internally rather than on the  outside of the tip and is usually identified by a shrill hissing  or squealing noise.  A flashback arrestor installed on each hose prevents a high  pressure flame or oxygen-fuel mixture from being pushed  back into either cylinder causing an explosion. The flashback  arrestors incorporate a check valve that stops the reverse flow  of gas and the advancement of a flashback fire.  [Figure 5-16] The equal pressure torch is the most commonly used torch  for oxy-acetylene welding. It has a mixing chamber and uses  acetylene fuel at 1–15 psi. The flame is easy to adjust and  there is less chance of flashback with this torch. There are  Figure 5-13. Single-stage acetylene regulator. Note the maximum  15-psi working pressure. The notched groove cylinder connection  nut indicates a left-hand thread. Figure 5-14. Two-stage oxygen regulator. No groove on the cylinder  connection nut indicates a right-hand thread. Figure 5-15. Check valves. Check Valves and Flashback Arrestors Welding Hose Torches Equal Pressure Torch 5-8