Home Search

Up Purchase Gas Saver Training Purchase TIP of the MONTH Improve Quality How GSS Works Flow Rate Limiter Sales Literature Video Overview About Company Weld Safely PRODUCT INDEX Production Results Orifice Flow Control Lean Manufacturing Weld School Payback Estimates Go Green Other Weld Info More GSS Info Services Overview Car Buffs Specials Contact Us Site Index

 

Tip of the Month

MIG Shielding Gas Flow Control is Much More Complex Then it Appears 

One of our first video's discusses how  MIG welding gas flow control functions. It sites two published references documenting that the average MIG welder wastes over 2/3 of the shielding gas used!   In the opening scene, my ’34 Pro Street Rod sound system is playing "Roll Over Beethoven."  That song referred to Beethoven rolling over in his grave if he heard Rock and Roll!  A comment is made that the famous fluid mechanics scientist, Daniel Bernoulli would roll over in his grave because of our lack of understand of MIG shielding gas control and the cause of very high gas waste.

MIG gas flow control is much more complex than it first appears. We’ll provide a quick overview and link to other pages in this web site for more details if you want to understand it fully.

First, MIG gas flow is controlled and measured in CFH (cubic feet per hour) not psi Don’t confuse CFH with the flow of an air compressor which is given in CF per minute-CFM.  The flow rate through a MIG gun at 30 CFH is only 0.5 CFM.  It is very low, about a human breathing rate when sitting!  There is a maximum flow rate, that if exceeded, will cause turbulent flow, which mixes air into the shielding gas stream causing weld quality problems.  For a typical 5/8 inch ID industrial MIG nozzle size, that velocity is about 50 CFH.  Setting flows higher than about 50 CFH is counter productive! (See reference 4 from TWI)

For a smaller nozzle size, like a 1/2 inch ID often used where better access is needed, 40 CFH is the maximum before creating a turbulent shield.  Low current MIG welders, often use a 3/8 inch ID nozzle when welding sheet metal, and with this size nozzle 30 CFH should not be exceeded.  Who would use more than these levels?  The answer is very MIG welder at the weld start - this issue is covered below!  In addition, some folks believe if some is good, more must be better when it comes to shielding gas flow-they are wrong!

The pressure needed in the gas delivery hose to flow gas through a welding machine (or wire feeder,) MIG gun cable, gun and nozzle will vary from about 3 to 7 psi.  The amount needed will change as flow restrictions change when welding.  These flow restriction changes are caused by:

a) bends of the small gas passage in the gun cable,

b) spatter blocking one or more of the small gas diffuser holes,

c) spatter build-up in the  nozzle, and

d) since, in most MIG gun cables, the gas hose doubles as the holder the spiral wire liner, the wire drawing compound, debris and copper flakes from a steel wire that build up in that area.

With these flow restriction changes the flow set on a flowmeter needle valve or an orifice in a regulator/flowgauge could cause the flow to vary significantly.  The engineers who designed the MIG flow control  system when the process was invented knew this was a problem and employed a flow control approach that would automatically compensate for restriction downstream of the flow control device.  The approach used relies on the fact that the speed of the a gas through a small opening in a needle valve or orifice can not exceed the speed of sound When the speed of sound is reached, that speed in a small opening is what controls flow and it does not depend on the downstream pressure.  To achieve what is called "choked flow" that provides this Automatic Flow Compensation, the absolute pressure (gauge pressure + atmospheric pressure)  upstream of the flow control device must be 2.1 times the  absolute downstream pressure.  Using 2:1 for an approximate calculation, see figure below for the pressure required:

The above shows the pressure needed to achieve "choked flow" is 25 psi.  It is no coincidence that quality flow controls use pressures from 25 to 80 psi.

If high pressure solves the consistent flow problem, is there a downside?  Yes, when welding stops and the solenoid closes in the welder or wire feeder, gas continues to flow though the needle valve or orifice and quickly fills the gas delivery hose with more gas which raises the pressure.  It reaches the pressure of the regulator or pipeline. This stores a lot of excess gas equal to, in the case above (80+15)/(4+15) = 5 times the hose volume of excess gas.  We find a typical 1/4 inch ID hose also expands another 13% at the high pressure.  Therefore,  the excess gas in the hose is 5 x 1.13 = 5.7 the physical hose volume!  However using pressure less than 25 psi, as some have tried with devices like "Gas Guards" creates bigger, less obvious problems than what they are trying to solve! This eliminates Automatic Flow Compensation causing wide variations in flow.  See test result comparisons of a proper flow control with a commercial low pressure device.

That excess gas stored in the gas delivery hose when welding stops, "blasts out" of the MIG nozzle at each weld start!  This high flow not only wastes gas but the high velocity (we have measured peak flows exceeding 200 CFH) creates a high degree of turbulence that causes air to be pulled into the shielding gas stream.  The air causes problems with weld quality and produces excess spatter.  This gas waste can be over half the total gas used. 

The solution to the gas waste and high peak gas surge is our patented Gas Saver System.  It is a simple, custom extruded, small ID, heavy wall gas delivery hose with a peak flow limiting orifice built into in the welder/feeder end brass fitting.  It maintains system pressure and therefore Automatic Flow Compensation.  The controlled small amount of extra gas it retains, quickly purges air from the weld  start zone and MIG gun nozzle.  Purging air from the weld start area and gun nozzle is essential for quality weld starts. With the Gas Saver System the maximum gas velocity at the weld start is limited by the peak flow control orifice and avoids excess turbulence.  Note, it does not control the steady state flow while welding - that is set by the existing conventional gas flow control device.  For cylinder gas supply that can be a regulator/flowmeter or a regulator/flowgauge. On pipeline supply it can be a flowmeter or simple orifice located at the pipeline drop.

Click PDF Icon Right to Download the Full  Details in a Technical Paper

Click to See Video Reduce MIG Shielding Gas Waste

 

Click to See Video Science of MIG Shielding Gas Flow Control

You can Google these names to see their contribution to the understanding of gas flow.  They may not have used the words "choked flow" but all would understand what it accomplished and why it would be needed in MIG welding flow controls!

Galileo Galile

1563-1642

Was one of the early contributors to understanding sound

Daniel Bernoulli

1700-1782

Pioneer in fluid mechanics.  We use his work to define how airplanes fly!

Robert Boyle

1627-1691

Herman Helmholtz

1821-1894

Ernest Mach

1838-1916

Osborne Reynolds

1842-1912

What do you think of the TIP of the MONTH?  Please Email.

This positive feedback regarding a Tip of the Month discussing the effects of leaks in a gas delivery system was received:

"I have been in Metal manufacturing for over 25 years...  Recently, I have been assigned to a department manufacturing centrifugal compressor impellers where a cover is TIG and MIG welded to the top of blades...Your Tip of the Month is the first plausible explanation (for occasional defects) I have come across that could explain this phenomenon and why it may be more prevalent in springtime or high humidity periods."

Free Technical Paper,

"MIG SHIELDING GAS CONTROL"

CLICK ICON for FREE PDF DOWNLOAD

Purchase

Gas Saving

Products  

 

Purchase Training

Products

 

Save over $1000 per year per welder and improve weld starts with Payback measured in weeks!!
* US Patent  # 6,610,957;  "Welding Shielding Gas Saver Device" August 26, 2003;  Canadian Patent # 2,455,644.  
 The "Flow Rate Limiter" device is covered by 2008 US patent # 7,462,709.  Other site material may be covered under Patents # 7,015,412; # 7,019,248 or # 8,104,094
Questions?   Send E-mail TechSupport@NetWelding.com    Or See Contact Us
Last modified: 03/23/13

Copyright 1999-2012 WA Technology TM; GSS TM;  All rights reserved.