Bernoulli terrifies me

[vagulus]

Anything beyond integer mathematics is a struggle for me and the thought of chaining myself to Bernoulli's Equation for a month to figure this out gives me nightmares. Can someone please give me a quick, off the cuff, answer?

I have a gas flowmeter from an argon welder (MIG/TIG). You've no doubt seen them around. They work with a vertical tube with a tapered bore (small end down). In this tube is a ball. When infeed gas is supplied at the bottom of the tube the ball rises up the tube until such time as the gap between ball and tube is sufficient to allow the infeed quantity of gas to pass (given the pressure differential). The height of the ball in the tube gives the measurement of the gas flow rate.

Now, I cannot find any documentation which specifies an optimum infeed gas pressure for this gauge. Actually, I am blowing bubbles in a metre of water so the outlet pressure is stable at 0.1 Bar. What I am trying to figure out is whether infeed pressure has an effect on the accuracy of the flow reading.

I can set the flow rate to, say, 10 l/min using 3 Bar infeed pressure. If I then reduce the infeed pressure to 1 Bar the indicated flow rate drops. My question is, "If, keeping the lower infeed pressure, I reset the flow rate to 10 l/min will that reading be accurate?"



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Artificial Intelligence is no match for Natural Stupidity

 

[1503-44]

I'm not sure what you mean by resetting pressure and keeping the same flow rate.

Anyway...

I think it should be relatively independent of inlet pressure, other than when that causes large gas density changes. Flow meters function using the pressure difference. That would be the pressure at the meter's inlet minus the pressure at the meter's outlet. The pressure difference should not be much affected by inlet pressure. But the inlet pressure may have a significant effect on the gas' density. The pressure difference within the meter is created by the gas flow trying to get past the ball, hence the pressure drop within the meter is basically caused by the gas flow's drag on the ball, which is balanced by the ball's weight.

D = Cd * 0.5 * den * V^2 * A

https://www.grc.nasa.gov/

http://www/K-12/airplane/dragsphere.html

Note that Gas Density effects can be important. Also note that gas density is a function of Boyle's Law, PVT stuff, absolute pressures and temps. A small change in gas pressure at relatively low pressure ranges can easily cause significant relative density changes. I.e. when absolute pressure changes from 1 to 2 psia, relative density doubles, markedly affecting drag force.

Cd = 0.47 D for a ball in a free gas stream, but this ball is in a restricted pipe, which adds some additional friction, which we will conveniently ignore. Also this is for ideal gas, so no compressibility effects, etc. Cd will vary with Reynolds number, but we ignore that too and consider it constant.

So W = Cd x 0.5 x den x V^2 x A
Solve for V = (W/ Cd /0.5 /den /D /A)^0.5

Flow rate = V x A
You probably want to use the pipe's area, A, at wherever the ball is at the time you take the meter reading, if the pipe diameter changes with height of the ball. That may be the most complicated calculation you have to do.

 

[LittleInch]

Search rota meter.

The density of the gas affects the readings.

There's a LOT of difference in density between 3 bar and 1 barg.

You need to look at the certification and manual that came with the flow meter.

But inlet pressure will need to be matched to outlet pressure.

Most of these types of meter are only designed to work at near atmospheric pressure so be very careful working at higher pressures unless you are certain it is rated for that higher pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Agree. I was editing while you were writing. I finally realised the 1 to 3 bar change is the problem he was talking about. Isn't 3 barg pretty high for these gages.

 

[Compositepro]

Rotameters are variable area meters (the area of the annulus between ball and tube varies) and can operate at any pressure that they are designed for. The pressure drop across the meter is constant and equal to the ball weight. The metering orifice (needle valve) will almost always be at the inlet to the meter, so that it does not affect the pressure in the meter, and thus the calibration. When you increase the inlet pressure and see an increase in flow, that is because you are increasing the pressure drop across the metering orifice, and the flow through the orifice is, in fact, increasing. If the metering orifice were after the meter, increasing the inlet pressure would cause the meter to read lower flow due to lower gas velocity at the ball (due to higher gas density).

 

[georgeverghese]

Theoretically, feedgas pressure should affect pressure drop, and hence the ball position, in this rotameter. Install a backpressure regulator ( at rotameter exit) to maintain the rotameter at the required measurement pressure, in order to "insulate" the downstream pressure from the rotameter operating pressure.

 

[1503-44]

Yes. Increasing inlet pressure would not necessarily increase flow, if downstream pressure happened to increase as well. The regulator would remove the possible effect of an unknown downstream pressure.

 

[LittleInch]

vagulus,

You're going to have to sketch out your system because we can't understand what's going on.

where is the pressure regulation? U/s or D/S this meter?

If you do nothing else then yes flow at 1 bar will probably be less than at 3 bar, but we don't know where this pressure is.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

"I have a gas flowmeter from an argon welder (MIG/TIG). You've no doubt seen them around. They work with a vertical tube with a tapered bore (small end down). In this tube is a ball."

This pretty much describes his system, even though he may not fully understand how it works. It is measuring the flow of shielding gas to a gas lens at ambient pressure.

 

[sraesttam]

I'm not an expert at gas pressure regulation. Though, I have a MIG welder with a Radnor brand flowmeter regulator. I think the thing that you are missing here is that there is a single stage regulator built in to your piece of equipment. My flowmeter regulator is designed for up to 3000 PSI inlet and 0-50 psi outlet pressure. The regulator is dropping the inlet pressure to a reasonable level and then the manually operated valve is adjusting the flow rate. There must be a regulator built into the product because 1) it is called a flowmeter regulator and 2) if there was only an orifice, the entire upsteam bottle pressure would end up on the hose. I know for a fact this is not occurring because the tiny non-reinforced plastic hose inside of the welder is not holding back 3000 PSI - it would have exploded a long time ago.

I recommend that you do a quick google search and read about the differences between single and two stage pressure regulators. The advantage of the single stage (which is what I've always ASSUMED is inside of the flowmeter regulator combo product) is that it has low pressure loss due to friction while it is flowing and the disadvantage of the device is that the output pressure depends (a lot more) on the inlet pressure than a two stage regulator. (I'm sure that there are balanced regulators instead of atmospheric regulators, but we're talking about a sub $100 regulator to keep it simple here.) The single stage regulators are cheaper and used when periodic adjustments to flowrates / pressure are acceptable. In an inert gas welding application you are unlikely to drop the bottle pressure from 3000 psi to 30 psi in one hour. You are going to check it every so often and it's not a big deal to adjust the needle valve. I only change out the bottle once or twice a year for my hobby application.


The flowmeter is telling you the actual flowrate regardless of the input / output pressure differential. If you keep adjusting it to 10 L/min, you will keep getting 10 L/min out.