Shielding gas flow rates and flow tube angle

[backpurge]

There has been a bit of discussion on other threads around the effect of flow tube tube angle on flow rates and also how the angle of the flow tube will effect the accuracy of the recorded flow rate so I thought I would conduct a simple exercise to illustrate this

I fitted my digital flowmeter to the end of the welding torch then set the flow at around 10 ltr/min using the flow tube and hit the purge button on the welder

As you will see the flow tube is registering 10ltr/min and the actual flow measured on the digital meter is showing 7ltr/min

I then tilted the flow tube over by 45 degrees

You will see the reading on the flow tube has increased to around 11 1/2 to 12ltr/min as would be expected due to the ball bearing requiring less force to lift it but the measured flow at the torch remains the same. So this is showing the angle of the tube has a small effect on the accuracy of the reading on the flow tube but doesn't effect what is actually coming out of the end of the torch

 

[Robotstar5]

What's worrying is even with the flow tube correctly orientated it's still under-reading by 40%!

 

[backpurge]

Do you mean over reading?
Have always stated they are not much more than a guide, they are also very dependant upon inlet pressure, think most rely on around 3.5 bar, to be honest I didn't pay a huge amount of attention to what I had set it at

 

[Parm]

Thats a very intersting point you raise there. The flow tube is called a rotameter and the float inside is (should be) a carefully balanced and weighed ball or float.

I use these every day at work and we spend hundereds of pounds on them every year. Its very important to remember that the markings on the glass tube are just printed. The better quality kit you buy the more accurate the display will be. I think the last lot we bought were plus or minus 5%.

To make these things more accurate we check readings against a primary standard calibrator (calibrated by an external UKAS accredited agency) and then plot a calibration graph to provide real flow against indicated flow. For soem of or work we also have to take into consideration air temperature and pressure as these can affect volume of air flow. Its a little extreme but when we are workign to strict calibration standards its required.

I cant imagine that a reasonably chepie roatamter is going to provide and accurate reading, at best its only indicationg flow of gas.

I set mine up at the torch by connecting a rotameter directly to the shroud

 

[zx9]

What's worrying is even with the flow tube correctly orientated it's still under-reading by 40%!

Assuming that you have had the digital flow meter calibrated at work, if not they both could be wrong!

 

[backpurge]

Assuming that you have had the digital flow meter calibrated at work, if not they both could be wrong!

Yep the digital flow meters are regularly calibrated by an independent company, not much point using them if they are not

 

[Robotstar5]

Never had any flow reading issues at one place, welders just turned them up 'til the ball wedged at the top!.

 

[backpurge]

Once saw a guy connect a flow tube up to the high pressure side of the reg, the ball bearing hit the roof then bounced around the shop like a bullet, why people mess around with stuff like this I will never understand

 

[Robotstar5]

Once saw a guy connect a flow tube up to the high pressure side of the reg, the ball bearing hit the roof then bounced around the shop like a bullet, why people mess around with stuff like this I will never understand

That's why we would always go out to a job rather than lend tools to a welder

 

[Jim Davey]

People forget the back pressure in the hose, machine and torch all have a bearing on flow rates and can make your readings at the flow meter appear high. It's quite normal on the average TIG set with a 3 metre 5/6mm gas hose and a 25ft torch with a gas lens to see 10-12 lpm on the flow meter at the bottle but more like 6-8 lpm at the ceramic measured with a tip flow meter (ok the tip flow meter is a cheap measuring device and I'm sure the accuracy isn't high by UKAS standards but they always read lower than the ones at the bottles) so I always use a tip flow meter for setting up the flow if I have something important to weld.