using a flowmeter with a different gas

[metalmagpie]

I have long wondered how a particular gas will flow through a flowmeter calibrated for a different gas. There are tables of correction factors given for some welding gases. But what if I'm concerned with a gas not shown in such tables?

Here is a real world example. I have a Harris 355 flowmeter. Its glass tube has two scales. One is for CO2 and the other for argon. This flowmeter will attach to a helium cylinder. But there isn't a scale for helium, so how do I know how much helium is flowing?

Victor gives a table of correction factors for various welding gases:

argon.......0.85
CO2.........0.81
helium......2.69
nitrogen...1.02

If the argon scale reading is 20 cfh, to find the flow of helium through that flowmeter you multiply the argon flow by the ratio of correction factors.

Example: argon flow = 20 cfh; helium flow = 20 * (2.69/0.85) = about 63 cfh.

But where do the correction factors come from?

It all starts with the periodic table of the elements, which gives the atomic weight for a single atom. Of course, gases don't always come as single atoms. What really matters isn't the atomic weight, it's the molecular weight.

GAS....atomic weight....molecular weight
Ar..........39.95.............39.95
CO2.....C: 12.0 O: 16.....44
He..........4.00...............4.00
N2.......N: 14................28

The molecular weight of air is generally accepted to be 29. Now we can calculate the specific gravity of each gas: specific gravity = MW / 29

GAS........MW...SG
argon.......40...1.38
CO2.........44...1.52
helium......4.0..0.14
nitrogen...28...0.966

The correction factor can be calculated from the specific gravity: CF = SQRT(1/SG)

GAS........MW...SG.....CF
argon.......40...1.38...0.85
CO2.........44...1.52...0.81
helium......4.0..0.14...2.69
nitrogen...28...0.966..1.02

Comparing, the calculated correction factors correlate exactly with the table from Victor.

However, some welding gases are mixes, and the correction factor table from Victor doesn't contain gas mixes.

A very common gas is 75% argon and 25% carbon dioxide (C25). What if we have a flowmeter calibrated for C25 and we want to know how to set it for hydrogen (for example - I wouldn't ever really want to)?

The molecular weight of a known mix of gases is calculated from the molecular weights and percentages of the component gases.

C25 molecular weight: .75 * 40 + .25 * 44 = 41
C25 specific gravity: 41/29 = 1.414
C25 correction factor: SQRT(1/1.414) = 0.84

H2 molecular weight: 2 * 1.008 = 2.016
H2 specific gravity: 2.016/29 = 0.070
H2 correction factor: SQRT(1/.070) = 3.79

So the amount of hydrogen that flows through a C25 flowmeter is 3.79/.84 or about 4.5 times the scale reading.

Now I can calculate the flow of any known welding gas given the flow rate shown by a flowmeter calibrated for any other gas. Yay!

Some flowmeters do not have regulators - they are intended to be connected to a regulator. Such flowmeters are only accurate if the regulator is set to the pressure that the flowmeter is calibrated for. But what if such a flowmeter is used with a different input pressure? How to correct the reading?

Before I answer, a word about gas pressure. The pressure shown on the gauge is different from the absolute pressure. At standard temperature and pressure, to convert pressure shown on a gauge (psig) to absolute pressure (psia) you have to add 14.696 which is just atmospheric pressure.

The formula to correct for pressure is (indicated flowrate) * SQRT((14.696+operating pressure)/(14.696+calibrated pressure))

Example: a flowmeter calibrated at 50 psi is operated at 30 psi. How much is the flow reduced?
The correction factor = SQRT((30+14.696)/(50+14.696)) = SQRT(44.696/64.696) = 0.831
So the actual flow will be 83% of the scale reading.

metalmagpie

[docwelder]

too much math for me. i'd change the sight glass to one calibrated for helium and call it quits.

[mla2ofus]

too much math for me. i'd change the sight glass to one calibrated for helium and call it quits.

Same here!!

[Jawslandshark]

Great write up! I am lazy and just wing it based on how good my welds come out most of the time, i have made some marks with sharpies on some of my flowmeters based on calculations I have done in the past.

[Oscar]

As someone who has formal training/studies in mathematics and physics, I say bravo fortissimo.

[Oscar]

metalmagpie, this is dedicated to your hard work digging up all that info. I have incorporated everything you have found into a Google Spreadsheet that anyone can use on their mobile cell phone if they have/download the Google Slides app. Once you have the app, you have to save a copy of this spreadsheet for yourself so you can edit the cells.

It takes all the inputs from Steps 1-3 and finds the appropriate CF's and then does the math for you. Of course some Answers might be nonsensical due to the difficulty of trying to dial in super-low flowrates, but hey, I never said it would make it realistic in all situations!

Here are screen shots:

►Flowrate & Gas Mix Calculator◄








The 2nd tab is one that might be useful for those that don't have super expensive gas mixers and use Y's to tie together two tanks (it is for me, anyways). Enter your Desired flowrate for the specified gas mix, and it will spit out the actual individual flowrates:

Gas Mix Calculator tab








If anyone needs anything added to the Gas Mix tab for other weird gases, I can add them. Just ask.

[metalmagpie]

Wow, Oscar, that's AMAZING!

Only I'm never going to install any Google app. I value my privacy too much.

Really Excellent Idea!

metalmagpie

[Oscar]

Wow, Oscar, that's AMAZING!

Only I'm never going to install any Google app. I value my privacy too much.

Really Excellent Idea!

metalmagpie

Oh well, lol. We use Google for work so I have to have it anyways. If I can incorporate it into a regular web page later on, I will. I just have to teach myself web design as that is not a thing a currently dabble in.

Also, I can export my Google sheet an an Excel .xlxs and link it here for download. That would not require any app, but you would need Microsoft Excel on your computer or phone, or some other app/program that can open Excel .xlxs files.


Flowrate & Gas Mix Calculator Excel File

[Ingenuity]

Oscar:

May I ask you to check your spreadsheet for C25 gas with Ar (@75%) and CO₂ (25%)?

I think it is calculating C25 at 67% Ar and 33% CO₂.

Thank you.

[Popeye an old miner]

Dammit fellas...thats complicated stuff for a country boy.

My hats off to you both for figurin all that out. Im glad I dont have to deal with that type of stuff in the work I do.

That seems like a whole bunch of work there Oscar and Metalmagpie...all i can say is good luck with it...rather you than me

[Oscar]

Oscar:

May I ask you to check your spreadsheet for C25 gas with Ar (@75%) and CO₂ (25%)?

I think it is calculating C25 at 67% Ar and 33% CO₂.

Thank you.

Yup, you're right. Must have messed with it and not caught that glitch. Fixed now.

[Ingenuity]

Fixed now.

Thanks, Oscar.

[Welder Dave]

Linde used to have square flow-meters with 4 different scales for different gases that could be rotated for the one you were using.

http://www.bertolinoindustries.com/u...low-meter.html