Flowmeters and Flow-gauges

“The shop where I work has only a valve where I turn on the welding shielding gas from a manifold system, but no way to control the flowrate.  At other companies that I’ve worked at we had gas cylinders with a flowmeter.  How does the manifold system work?”

There are three common ways that we control the flowrate for welding shielding gases (e.g., argon or CO2 or mixed gases).  

Most welders are familiar with the traditional flowmeter-regulator device with the graduated tube and the little ball inside.  This tube is called a ‘Thorpe-tube’ and the inside is slightly tapered so as the rate that the gas flow increases, the ball blows up higher in the tube.  To determine the flowrate in cubic feet per hour (cfh) or liters per minute (LPM), the welder adjusts the needle-valve at the outlet and reads the top of the ball (see Figure 1).  These Thorpe tubes are calibrated for a particular gas at a fixed inlet pressure (e.g., argon at 50 psig – pounds per square inch gauge).  Just like a water tap, as the needle valve is opened, the flowrate will increase.

Over the past few years another type of control device called a flow-gauge has become more common.  In this device, illustrated in Figure 2, the outlet hole (or orifice) size is fixed, and the pressure is adjusted by the operator.  As the pressure in the gauge increases, the flowrate will increase.  These gauges are also calibrated for a particular gas type, since if the density of the gas (specific gravity) changes the flowrate reading will not be entirely correct.    

Many larger shops prefer to distribute their shielding gases to the various workstations using a manifold system.  Most of these systems use a heavy-duty two-stage regulator to feed the manifold system at a pressure of 50 psig (or 345 kpa).  This is enough pressure to move the gas around the shop.  At the various welding stations there are two options to control or limit the flowrate:  If flowrate adjustments are required then a Thorpe-tube flowmeter will be used, however for stations where a standard preset flowrate is acceptable, a flow restrictor (orifice meter) will be installed.  Since the line pressure is fixed at the cylinders (or gas mixer) both devices will work if they are calibrated for that line pressure and gas mix, typically 50 psig and argon for a baseline.  This system is illustrated in Figure 3.

In certain critical or automated welding operations there are also digital control and display devices such as the mass-flow meter or the gas turbine meter, but for most manual or semi-automatic welding operations one of the devices described above will be suitable.