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Tip of the Month
MIG Shielding Gas Flow Control is Much
More Complex Then it Appears
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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. |
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 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! |
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 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. |
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 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: |
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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. |
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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.
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 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 |
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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! |
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 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! |
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Robert Boyle
1627-1691 |
Herman Helmholtz
1821-1894 |
Ernest Mach
1838-1916 |
Osborne Reynolds
1842-1912 |
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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."
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Free Technical Paper,
"MIG SHIELDING GAS CONTROL"
CLICK ICON for FREE PDF DOWNLOAD |
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