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We get many questions about setting MIG gas pressure. Unlike with oxyfuel welding with MIG (or TIG) “flow” is set NOT pressure. Here is why:
Having been in the welding business for over 40 years and having managed an R&D lab for a major industrial gas company, setting the proper gas “flow” is not well documented. Even the 6 volumes of American Welding Society Handbooks with over 4000 pages have little useful information!
Having spent the last 12 years helping folks save wasted shielding gas, we have made hundreds of pressure measurements in fabrication shops (and home shops as well-my own!)
There is actually a pressure you could set if you used a true pressure gauge, but don't use that approach, it will result in significant flow variations! The confusion occurs because many welders use a device that does in fact adjust pressure, it is called a regulator/flowgauge. It has two gauges, one measures cylinder pressure the other is labeled (or should be) CFH for Cubic Feet per Hour, which is flow. That device sets pressure above a very small orifice, usually 0.025 to 0.030 in diameter. That sets the flow. It is a very consistent and accurate way to set flow because it is a “critical orifice” and the flow in the orifice will reach the speed of sound! It cannot exceed that flow rate. (That is why you see lightening before you hear thunder!) Flow will remain the same even when restrictions occur in the system (like bent hoses, spatter in the nozzle, and gas diffuser, the use of a TIG gas lense, etc.)
Whether you are using a regulator/flowmeter (one with a flowtube and ball) or the two gauge device, if you’re using typical MIG flow rates, from our field measurements the pressure in the gas delivery hose, when welding, will vary from 3 to 7 psi. Don’t try to set that low pressure because the flow will vary when you bend a gun cable, spatter builds etc. The pressure above the needle valve or small orifice must be above 25 psi to have what is called “automatic flow compensation.” Quality flow control systems have been built with this feature since MIG and TIG processes were introduced in the 1940's and '50's!
We have a free flow chart which gives suggestions and the reasons for setting minimum and maximum flows depending on nozzle size. Check it out: http://www.netwelding.com/MIG_Flow%20Rate-Chart.htm
Our web site (as well as an article we wrote for AWS Welding journal) shows what happens when a low pressure “Gas Guard” device was tested. Flow varied by over 35% because it used 9 psi, not 25 psi above an orifice. That reduced starting gas surge but caused other, less obvious problems! Be sure you use a quality flowmeter with a minimum pressure of 25 psi, which includes all products sold by Miller, Smith, Victor and ESAB and most we have found coming from overseas.
Last edited by Jerry U; 07-24-2014, 06:41 AM.
Reason: Answering recent "setting pressure" questions