[dynasty200sd]

So, I've been wondering how to figure out, how much preflow is necessary to purge the total distance my lines run from flowmeter to torch, without using more preflow than necessary. Then I got to thinking what does the math say? I used the equation for finding the volume of a cylinder: pi x r^2 x h

The rough total distance my lines run from flowmeter to torch is around 37' feet, and the inner diameter of the hose is 3/16" of an inch.

First, I needed to convert the hose i.d. from inches to feet.
0.1875 / 12 = 0.015625

Then, I had to find the radius of a 3/16" of an inch hose.
0.015625 x 0.5 = 0.0078125

Solving the equation, the cubic feet of a 3/16" of an inch hose 37' feet in length is 0.0071.
3.1415926535897932384626433832795 x 0.0078125^2 x 37 = 0.0070946611439710906874461551014

Since there are 3,600 seconds in a hour; at a flowrate of 15 cfh every second will flow 0.0042 cubic feet. This is not accounting for any friction or restrictions in the lines.
15 / 3,600 = 0.00416666666666666666666666666667

Two seconds of preflow will be sufficient enough to purge the total distance my lines run.
0.0042 cubic feet per second x 2 seconds = 0.0084 cubic feet

Is the math right? Am I even using the right equation in the first place to find what I would roughly need to purge my lines?

[weldin mike 27]

I think you are going too far with it, On such a small hose, The gas flow will be quick. Pre flow is kind of a non event, 1 second on the panel will be enough and that time will not waste much if you hit pedal or button to bring gas in to the torch before you start welding. Post flow is the most important one.

Sorry if I have trivialised your efforts.

Mick

[Captainbeaky]

I've never seen it worked out, but it looks right.

The only other thing to consider is that a 2 second preflow would be darn annoying in practice.

Also, it takes time for ALL of the shield gas in the pipe to exchange with air by brownian motion alone.
This is measurable In minutes.

With nothing actively purging the shield gas from the pipe, you don't have to purge the pipe every single time you use it. It's not like you have a source of air carrying out a 2 second purge of the shield gas every time you stop welding.

In practice, you only need to purge the pipe the first time you use the torch, and maybe if you have put it down for a while.
I "purge" mine by prodding the pedal just before I start, but this is more to make sure I hear the gas coming out ( that the gas is on and flowing, and I haven't been a numpty and forgotten to turn it on - again!).

In normal use, it's not really a factor.
Mine is set just off the zero on the scale - I can just about make out the click of the solenoid a fraction of a second before the HF start kicks in 0.1 to 0.2 second maybe?

[rick9345]

.1-.2 preflow,, a tap on the pedal when you are ready, will get postflow to clear hose before welding.

Preflow is there to establish a gas envelope to initiate the arc.

[Otto Nobedder]

rick9345 offers a great answer for gas conservation... I use no preflow at all, and, as he described, tap the pedal so that postflow establishes my envelope, then start welding almost immediately. Consider the gas surge when you initially hit the pedal... This clears the line much faster than your preset flow rate indicates in your math.

Steve S