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