[dynasty200sd]

I'm looking at different flowmeters used for inert gases Argon, Argon/CO2, and Helium made by Smith and also by Victor. Looking at their specifications, some flowmeters list a preset pressure of 25-30 psi, and other's having a preset pressure of 80 psi.

I'm not asking about the flowmeter's SCFH range.

What I'm trying to figure out is what is the purpose of the different preset pressures? Why are some using 25-30 psi, while other's are using 80 psi; why would one preset pressure be needed over the other?

I've looked for literature dealing with this matter, but have yet to come across any.

[UglyBlobs]

I am pretty new to all this stuff myself, and have run across several threads discussing this issue, so, until the smart guys weigh in, I will give this a shot. I know you mentioned you were not referring to the flowmeters SCFH ranges, but, in reality, that IS the issue.

For reference, look at this document:
http://victortechnologies.com/IM_Upload ... 0%2040.pdf

This document describes the HRF 2400 Series Medium Duty Regulator/Flowmeter for MIG/TIG applications.
This regulator/flow meter comes in two models: HRF 2425 (preset 25 PSI) and HRF 2480 (preset 80 PISI).

According to the literature:
2425 (Part No.0781-2731) is designed for Argon, Argon/CO2 Mix, and Helium (10-50 SCFH Argon, 20-150 SCFH Helium)
2480 (Part No. 0781-2727) is designed for CO2 (10-38 SCFH) – uses Flow Tube Ball 1015-0057
2480 (Part No. 0781-2728) is designed for CO2 Mix (7.5-37.5 SCFH Argon) – uses Flow Tube Ball 1015-0058

Although the literature doesn’t specify the reason for the 25 or 80 PSI presets, a little physics and inference points us in the right direction. For example, helium is a very small atom (2 protons and 2 electrons – with either 1 or 2 neutrons) compared to CO2 (a comparatively large molecule composed of 1 carbon atom and 2 oxygen molecules).

Note that with a 2425 (25 PSI preset), helium flow rates are between 20-150 SCFH. Imagine the flow rates if helium is hooked up to the 2480 (80 PSI preset). Even the minimum flow rate would exceed usable limits for MIG/TIG processes.
On the other hand, those big fat molecules in the CO2 and CO2/Argon mixes need 80 PSI presets to allow flows of 10-38 SCFH and 7.5-37.5 SCFH, respectively.

Now, I suspect many guys are working happily along with regulators and presets that are not optimized for the selected gas.
On the practical side, using a regulator with a higher PSI could amplify issues like pre-flow surge. For example, if you are using a TIG welder with a solenoid valve controlling gas flow, whenever the torch is not activated, pressure builds up in the hose between the regulator and the solenoid. When the torch is activated, a sudden surge of pressurized gas flows out. This may negatively affect arc start and/or cause other issues (as well as wasting a lot of gas).

While a number of factors can contribute to this condition, selecting a proper regulator/flowmeter will help reduce the negatives.

On the other hand, I have read several posts where users recommend a regulator with a higher preset pressure to compensate for restrictions in the shielding gas lines between the welder and torch/gun (long hose runs, numerous bends/kinks/restrictions etc.).

[hey_allen]

There are a number of people discussing reducing the pressure on TIG regulators to suppress the surge flow when the gas valve opens, among other surge reduction methods. (minimum diameter hose for the desired gas flow, moving the flowmeter/regulator as close to the valve as possible, reduced hose lengths, etc...)

The molecular size is an interesting theory as well, and I hadn't thought about it from that aspect, but I'm a newbie welder, so who knows what the "correct" answer is.

[Sandow]

My instinct is that the Ideal Gas Law is going to be a harsh mistress here and you are going to run into issues with actual volume of gas vs the way the gas flow rate is measured. I would guess on a floating ball type flowmeter, the mass of the gas changes the reading dramatically even at the same flow rates since the ball is lifted by the inertia of the gas hitting it and not the pressure differential.

-Sandow

[Braehill]

Any regulator that's made to be used in CO2 service needs to set at higher than 60 psi because that is the maximum pressure at which it can form dry ice. Above this pressure it will be a mixture of liquid and gaseous CO2. Below that it will be a mixture of dry ice and gas and will clog the flow meter in a high flow situation.

None of this is likely in a mixed gas use but they don't know what the end use will be while making them.

Len


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[Braehill]

Sandow,
Can you better explain how the Ideal Gas Law would be having an effect here? I'm not sure I follow.

Len

[Sandow]

Well, as far as molecule size goes, there should be no real difference in the behaviors between a super light and a super heavy gas. Now that is the ideal, not necessarily the reality. It is usually pretty close though and when you look at the flow rate constant between argon and helium, they are both about 1.4 despite an order of magnitude difference in mass.

Ideal gas law regards them all as the same because in any range of pressure and temperature in which things are gasses, the VAST majority of space is unoccupied and repulsive forces are such that everything is pretty much a ping pong ball (of the same size) bouncing around.

In the end the lesson is that the "Ideal" gas law is dominant most of the time but certainly not always. There are plenty of times where this doesn't quite apply and the fact that the flow rate constant for nitrogen is 1 despite having a mass close to that of argon and a similar size electron cloud. Intermolecular forces even in non-polar gasses can have large impacts and often in ways that we don't really understand.

If you need to take a side though, Ideal Gas Law dominates. As far as it goes for regulators, molecular size and weight are not at all predictors of gas behavior.

-Sandow

[Braehill]

Sandow,
Thanks for a very concise and well thought out answer, I appreciate it.

Len