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 Tip of the Month
MIG Shielding Gas; Setting Flow Versus
Pressure
  Thought
the Flow versus Pressure question was only one asked by our “Home Shop” customers.
However found the confusion also exists with some industrial fabricators for good
reasons!
This
months tip relates to setting MIG shielding gas flow rates. For a general
understanding of what Pressure and Flow mean,
SEE BELOW, and then return. |
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 For
MIG welding gas flow rate settings; one reason for confusion of the
terms Flow and Pressure is some flow control devices are in fact setting
pressure! The figure right shows what is called a Flowgauge/Regulator
that installs on a shielding gas cylinder. It is often used to set
shielding gas Flow by setting the Pressure upstream of a very small orifice.
For example, some Flowgauge/Regulators have a very small hole in the outlet
fitting of only 0.026 inches (some are even smaller.) That’s about the
size of the 0.023 inch diameter MIG wire you’re using to weld sheet metal!
With typical MIG shielding gases it will require a pressure upstream of the
0.026 inch orifice of 40 psi to flow 25 CFH. Therefore when adjusting
the regulator you’re setting an outlet pressure of 40 psi but the gauge is
calibrated to read 25 cubic feet of gas per hour (CFH.) The gauge is
 actually
measuring 40 psi but the gauge face is calibrated in CFH for the size
orifice in the outlet fitting and the gas being used! Therefore you must see what the units are on the gauge.
The gauge face
left
is from a flowgauge system and shows the units as CFH. Note it shows
two scales, one for Argon (inner scale) and one for Carbon Dioxide (outer
scale.) Since these gases have different densities the flow through the
orifice will be slightly different. Mixtures of the two gases
will be between these readings.
Note the scales are close in the range where you should be setting your MIG
welder (25 to 35 CFH) so many gauges will only show one scale.
The specific gauge calibration is based on the orifice size so do not
replace a gauge unless it is identical to what is removed.
If the regulator being used has an outlet pressure gauge calibrated in psi then
little doubt it does
not have a small orifice designed to control flow. Then what pressure would
you need to set to get 25 CFH? The answer
depends on your welding system, torch size, torch length, spatter
build-up in the nozzle and gas diffuser, bends in the torch cable etc! In
fact that pressure is probably only 4 to 5 psi and when torch nozzle spatter
accumulates, torch bends occur etc, the flow will change significantly!
The answer really is: "You should not be using a standard pressure regulator
to set flow!" If you purchased a MIG welder and it came
with a cylinder regulator it is probably a
Flowgauge/Regulator. Look carefully and you’ll probably see the output gauge
is labeled CFH not psi. |
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Laboratory Tests
We conducted tests in our Lab to demonstrate what happens when a
 pressure
regulator is used on a MIG wire feeder system. The test system
had a
low current
10 foot long torch attached. The torch has the largest flow restriction in the
system. Testing
was conducted
with a new nozzle, spatter free gas diffuser and with the torch straight,
no bends. The output gauge on the Inert Gas Regulator was a maximum of 100
psi so to get a more accurate reading at low pressures a 30 psi
maximum gauge was installed in the gas hose line (red faced gauge in photo.)
A
rotameter portable gauge was used at the output of the torch to
measure flow
(Our
part number WAT-PFM.)
Here is what was found:
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At
a 3 psi setting the flow was 18 CFH.
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At 4 psi the flow was 22 CFH.
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At
5 psi the flow was 28 CFH.
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In
production shops
with higher current torches our measurements
find typical MIG systems
with pressures of 5 psi in the gas delivery hose
will flow 30 to 35 CFH. If it were not for the restrictions in the
solenoid, gas plumbing and torch the flow would be much higher.
To test flows with only a hose connected to the sys tem
a standard 20 foot long ¼ inch ID gas delivery hose with standard CGA 032
“B” fittings on both ends
was installed.
The outlet “B” fitting was inserted in a high flow rotameter (similar to but
not that shown in photo right). At 3 psi output pressure 135 CFH
gas flow was measured. At 5 psi the flow was 175 CFH.
Interesting question
frequently asked; "If only 5 psi is needed to flow shielding gas
in a MIG system,
why is the pressure so high in commercial flow systems?" The
answer is; higher pressures
are used so the flow setting does
not change when the torch bends and restricts the gas passage such as spatter
building in the torch nozzle or partially clogging the gas diffuser etc. A phenomenon called
"critical flow" or "choked flow" is used. If
the pressure (measured as absolute not gauge pressure; i.e. gauge pressure +
15 psi) upstream of the flow control orifice or needle valve is more
than twice the downstream pressure, regardless of the typical restrictions that occur
when welding, the flow will
not change from the preset value!
Click here for more details and to see why that pressure needs to be higher
than 25 psi! |
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Anecdote: It Can Happen To Experts!
An interesting situation shows what can happen if by error
a standard pressure
regulator is used instead of a flow setting device. An experienced field
Welding Engineer was testing a new model multiprocess power source in a fabricators
shop. It worked fine in the Stick and TIG mode. However when he switched
to MIG welding with a cylinder of 75% Argon 25% CO2 gas the arc was
harsh, the spatter was very high and the weld surface was poor. After checking grounding, polarity and
making settings changes he called the power supply manufactures Product Manager. After
discussing the problem, the Product Manager wondered about the gas flow rate
and asked to have it checked. The field Welding Engineer said it was set for
30 (he assumed CFH.) The
 Product
Manager had experience with poor performance when flow rates were set
excessively high. He asked the field Welding Engineer to put the torch
nozzle up to his cell phone and pull the trigger! The solution was
evident from the sound
this very experienced Product
Manager
heard - the flow was excessive! Sure enough the distributor salesman who
set-up the installation had used a standard inert gas regulator. It
was not a Regulator/Flowgauge with an outlet orifice and flow calibrated
pressure gauge. It was reading 30 psi not 30 CFH! What flow were they
getting? Probably over 150 CFH. (We have measured peak surge flows of
250 CFH when the gas delivery hose pressure is at 50 psi!) Even with a 5/8
inch torch nozzle, much more than 60 to 70 CFH flow will cause turbulent flow and mix air into the shielding gas stream! It takes
only a small amount of air to produce
a less stable arc with excess spatter and internal weld porosity-if
it is not visible! Perhaps more harmful than the visible defects is the
effect of Nitrogen on the weld deposit.
See What That
Can Cause.
Bottom
Line:
Even "experts" make mistakes! |
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Basic
Difference Between Pressure and Flow
 A
water analogy helps define the basic difference between
pressure and flow. In the sketch right a drip is coming out of a faucet.
In a typical house the water PRESSURE in the pipes is about 40 pounds per
square inch
(psi). That means if there were a square pipe 1 inch per side internal size
with an internal plug open to the atmosphere it would take 40 pounds of force to
keep the plug from
moving. See schematic A.
 If
there were a small leak past the plug, the pipe pressure would still be 40 psi but water will FLOW out. If the leaking water was caught in a 1
gallon bucket and it took 1 hour to fill to the top, a flow of
1 gallon per hour or 1 GPH would occur. See schematic B.
The same holds for gas flow. If a regulator is on a cylinder and the gas
output is regulated to 50 psi and a needle valve is used to allow one cubic foot of gas to leak out in an hour (now
the gas is at the atmospheric pressure at sea level of about 15 psi)
the flow is 1 cubic foot (at sea level pressure) per hour or 1 CFH. The flow rate is so low that
the pressure will not change significantly when gas flows.
Think of the volume of gas this way: a typical MIG shielding gas flow rate
is 27 CFH. 27 cubic feet is a cube 3 feet wide by 3 feet deep by 3 feet
high. It takes a 27 CFH flow rate 1 hour to fill that space up! Human
breathing rate is more than that! Shielding gas flow rates are set
this low for MIG welding not only to avoid waste but to prevent excessive
turbulence in the gas stream which causes
air
to be sucked in an create inferior welds. (Return to Top of
Page) |
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WHAT IS AN IDEAL GAS DELIVERY SYSTEM?
Saving Shielding Gas and Improving Weld Start Quality is Easy
"JUST REPLACE YOUR EXISTING GAS DELIVERY HOSE FROM SUPPLY TO FEEDER
WITH OUR GSS"
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What do you think of the TIP of the MONTH?
Please
Email.
Received this positive feedback regarding our first Tip of the Month about
the effects of leaks in a gas delivery system:
"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
PDF Download Available of Technical Paper Summarizing MIG Shielding Gas
Control and Optimization. Summarizes Key Points in Web Site.
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Hear a Message from the President of
WA Technology (Who was also 2007 President of the 52,000
Member American Welding Society)
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