How Much Gas Waste Can Be Stored in a 1/4 Inch Hose?

 HOW?

THE EFFECT OF PRESSURE ON GAS VOLUME

Consider an Argon gas cylinder.  One holding 310 Cubic Feet (CF) of Argon [measured at Standard Temperature and Pressure (STP); that's what you paying for]  has only 1.8 CF of physical internal volume.  How does it hold all that gas?  Because of the increased pressure over atmospheric. 

The volume of gas in the cylinder at STP is directly proportional to the absolute pressure.  At 2500 psi = 2514.7 psia (psia = absolute pressure. That is the gauge reading +14.7 psi at sea level.) Therefore the volume will be 2515 / 14.7 or 171 x 1.8 (physical volume) CF = 310 CF of gas at STP. 

Therefore when welding stops and the gas solenoid is closed  the shielding gas delivery hose is quickly pressurized to regulator or pipeline pressure.  It holds much more gas than the actual internal physical volume of the hose.  How much more depends on the pressure.  Typical flowmeter and flowgauge regulator pressures range from 25 to 80 psi and as do typical pipeline pressures.  To reach 80 psi there must be 6.4 times the amount of gas in the hose then when the gauge pressure is zero!  (The calculation is (80 psi+14.7 psi)/ 14.7 psi = 6.4.  That extra gas blasts out when welding starts or anytime the torch trigger is pulled even if only inching the wire to cut off the end!  NOTE: PRESSURE CAN NOT INCREASE UNLESS THE AMOUNT OF GAS IN THE HOSE INCREASES.

Interesting Facts:

1)  At full cylinder pressure Argon is still a gas but it's actually pretty heavy!  (310 CF weights about 31 pounds.)  When the gas is contained in the 1.8 CF cylinder it's about 30% the density of water! 

2)  Another fact to give perspective on how far apart gas atoms are at 75 F and atmospheric pressure; for Argon gas they are 845 times further apart then when in Liquid Argon!  Doesn't take much pressure to get them 6 times closer!

This schematic presents the volume of extra gas in the gas delivery hose.  The "bar graph" is shown as Red and Blue bars above the Green gas delivery hose.  Area in Blue represents the extra volume of gas in the hose over the physical volume due to the ~ 3 to 5 psi need to flow the required amount of gas when welding.  The flow control (needle valve or  orifice) creates the lower pressure when welding by causing the needed pressure drop.  The Area in Red shows the amount that can be excess due to the extra pressure in the gas delivery hose over the ~ 3 to 5 psi when welding stops.  This higher pressure is set by the regulator/flowmeter; regulator/flowgauge or pipeline pressure.  The Blue and Red areas are sized relative to the hose area shown in Green to provide proper perspective.

How Does The Pressure Get That High and Why it Must be Over 25 psi?

A restriction orifice or a needle valve is used to control shielding gas flow.  With Regulator/Flowmeters (Photo Below Left) regulator pressure ranges from 25 to 80 psi depending on the brand/model.  For CO2 shielding 80 psi is often used to help prevent ice formation in the very small orifice or needle valve used to control flow.  Flowmeters or orifices used on pipelines also allow gas to flow until it reaches pipeline pressure when welding stops.  A typical  pipeline pressure is 50 psi. 

Flowgauge/Regulators (Photo  Right) operate by setting a pressure above a critical orifice (a very small hole).  For typical MIG shielding gas flow rates the regulator pressure will range from 40 to 70 psi.

However the pressure needed at the wire feeder / welder to flow shielding gas though the solenoid, fittings and torch ranges from 3 to 8 psi depending on torch length and restrictions. The needle valve or orifice drops the pressure to this low level when flow settings are adjusted.  However when welding is stopped, gas continues to flow through the needle valve or critical orifice and  quickly increases the pressure in the gas delivery hose to that of the regulator output or pipeline. Therefore the pressure in the gas delivery hose will be up to (25/3) 8   to (80/3) 26 times the pressure needed to flow the desired amount of gas!  For Flowgauge/Regulators 13 to 23 times what may be needed! 

Note, a minimum of 25 psi is needed to maintain a very important feature called "Automatic Flow Compensation." This feature was understood by the engineers that developed the MIG and TIG gas control systems in the 1950's!  Unfortunately some companies have sold low pressure devices thinking they can reduce starting gas surge.  They may not even say they use low pressure and only indicate they eliminate or "guard" against surge. These systems  create bigger and less obvious problems which often frustrate welders!  Welders set higher gas flow rates in attempt to compensate - which only partially helps.  This wastes even more gas!!   CLICK TO SEE DETAILS.

HOSE EXPANSION

Hose expansion also has an effect on the extra volume of stored gas when welding stops.  Making measurements of a relatively heavy wall (0.093 inch wall, 1/4 inch ID) commercial gas delivery hose we obtained the following data:

When welding stopped, using a CO2 80 psi regulator/ flowmeter, the hose expanded 13% in volume.  Therefore in this case 87% of the excess gas in the hose is due to the increased pressure and the additional 13% due to the hose expanding.  In total the excess gas waste (most of which is wasted with each torch pull) = about 6 times the physical hose volume!  In addition to the gas waste the very high gas flow velocity that exists when this excess gas quickly exits the torch nozzle pulls moisture laden air into the shielding gas stream!  We have measured peak flow in excess of 200 CFH  with conventional gas delivery systems!

Want  more details on  pressure versus gas waste? 

See Welding Math or

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Learning Program For Welders

"Setting Shielding Gas Flow and Eliminating Waste"

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The amount of gas in a hose depends on the pressure.  For some regulator/flowmeters and many regulator flowgauges (those that have a pressure gauge reading cubic feet per hour, CFH) can cause a pressure of 80 psi in the hose when welding stops. Therefore the volume of gas in the hose is (80+14.7)/14.7 = 6.4 times the hose volume.  Accounting for the hose expansion that can be an extra 13% or 6.4 x 1.13 = 7.2 times the initial hose volume. Therefore the gas waste created every time the torch trigger is pulled is much more than the hose volume would indicate.   Our patented Gas Saver System reduces the amount of excess by 80%!

CAUTION!  The high pressure in the gas delivery system is designed to provide automatic flow compensation (along with other benefits) - lowering the pressure defeats this very important feature.  Higher pressure is needed to maintain the automatic control of gas flow when restrictions in the gas hose, torch cable and torch nozzle occur in production.  See this link for an explanation. 

That's why system pressures were designed that way since the invention of TIG and MIG welding!!  See high pressures are a "Good Thing."

Why is most MIG gas delivery hose 1/4 inch ID?  Hint; it's not for the very low pressure drop at the low flow rates used! CLICK to SEE WHY.

What Are Gas Costs as a Percent of Total Weld Cost

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