The stainless steel solution

Testing a variety of laser gas delivery hoses reveals that stainless steel keeps gas mixtures intact

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The morning after a New Year’s Eve party, the previously helium-inflated balloons often reflect the current mood: deflated. There is a scientific explanation for this, of course, and, fortunately, understanding why will not induce a headache.

The molecules that comprise a typical party balloon are long and elastic, which accommodates the stretching necessary for inflation. The tradeoff is that as the material stretches, the walls of the balloon thin, rendering it permeable to gases.

The gas of choice for inflation is helium, the second element on the periodic table. Being a stable noble gas, helium floats around as single atoms. It is the helium atom’s tininess that allows balloons filled with it to float in air. This tininess, however, is the property that lets helium slip through the stretched balloon’s membrane.

So why don’t the shiny balloons in the supermarket droop down the aisles? It is the same eye-catching shininess (in this case metallized BoPET) that significantly reduces the loss of helium through the balloon membrane, allowing the birthday balloon to look as perky on Sunday as it did on Friday. 

Testing tubes

Laser users can face a similar predicament with helium – it can leech out of their gas delivery hoses. Not to be outdone by supermarket science, Larry Cherne, laser gas marketing manager at Praxair Inc., tested a variety of gas tubing used in CO2 laser applications.

“It’s common for laser owners to use plastic tubing,” Cherne says about the hoses required for the delivery of the gas in laser operations. “We continue to explain to the users, however, that [plastic] might not be the most prudent material to use.”

Taking note of clues suggesting a vulnerability in plastic tubing, Cherne did some background research. He submerged a piece of tubing typically used in laser applications in glycol for testing.

“We pressurized the plastic tubing with 50 psig of helium and within 24 hours it went down to 2 psig,” he explains. “Glycol is crystal clear, and the bubbles move very slowly within it, so they come to the surface very slowly. Because glycol is clear, the bubbles coming up are visible, making the pressure drop visible, as well.”

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Three separate gas streams are required in every CO2 laser.

The research revealed that traditional flexible tubing leeches helium, but the experiment wasn’t over. The question arose regarding the rate of diffusion across different materials using a typical gas mixture of helium, CO2 and nitrogen.

Cherne tested synthetic tubing made of high-density polyethylene (HDPE), polyethylene (PE) and Teflon. The tubing was filled with a gas mixture of 60 percent helium, 35 percent nitrogen and 5 percent CO2, a reasonable ratio and easy for the accompanying math. The starting pressure of 50 psig (pounds per square inch gauge) was matched with a temperature of 80° F. The duration was 72 hours.

“The reason we were looking at 72 hours is that it could represent a typical three-day holiday weekend,” Cherne says. “After a long weekend, it’s not uncommon for laser users to see a decline in cutting performance.”

Of the synthetics, HDPE performed best, only losing 10 psig over the duration of the test.

“On the poly, we lost 15 psig over 72 hours and with Teflon, we lost 30 psig over 72 hours,” Cherne explains. “ When we make synthetics, we’re using hydrocarbons in chain form. Because the chains are so long there’s room between the molecules, which means that gas can permeate through the tubing. Conventional molecules, like nitrogen, oxygen and CO2 are big enough that we don’t have an issue. Helium and hydrogen, however, are smaller, so they can sneak through the membrane.”

So what’s the material of choice for avoiding helium diffusion? It’s stainless steel – metal – the same thing often found on the receiving end of most industrial laser cutting equipment.

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Praxair tested the potential leak rate of synthetic tubing made of high-density polyethylene (HDPE), polyethylene (PE) and Teflon. HDPE performed best of the synthetics, but stainless performed best overall.

Ensuring consistency

“When we tested stainless steel, we saw a zero pressure change in the tubing over 72 hours,” Cherne says. “And these results will be consistent as long as users have stainless steel tubing in place.”

Using the 72-hour holiday weekend time frame, the stainless steel tubing maintained 50 psig and the recommended 60/35/5 percent gas mixture ratio, meaning the laser was ready to get back to work Monday morning. The specific pressure and gas mixture ratio matters because these variables directly affect the quality and performance of the laser’s cuts.

Cherne explains that “in order for a beam to cut, you need to have a point like on the edge of a knife. If the point is broken off, you have to produce a lot more energy to make the cut. When we have issues with bad gas or a dirty lens or mirror, it’s like trying to make a cut with a dull knife. Having those issues won’t produce a clean cut, and so instead, you’ll get a cut that looks like a dog chewed on it.”

In addition to cut quality, additional problems can also surface when the incorrect gas mixture is laying idle in the lines, including increased maintenance and the degradation of the resonator itself.

“Let’s say a laser operator is going to re-pressurize the Teflon line,” Cherne says of the leaky hose previously mentioned. “At this point, what’s left in the hose is 20 psig of 85 percent nitrogen and 15 percent CO2 – instead of the desired 50 psig with the 60/35/5 percent mixture. The new gas that he is going to add is a premix, so the mixture is going to be way off, causing him to have to purge the system.”

Depending upon the length and diameter, synthetic tubing may hold 0.050 to 0.2 cu. ft. of gas that is 5 percent to 15 percent off, compared to the original gas blend specifications, in the cylinder. When this happens, the best course of action is to purge the resonator gas line, which takes time and results in waste and lost productivity. 

If the operator doesn’t take the time to properly purge the line, they may avoid the immediate downtime. However, this ‘newly created out of specification resonator gas’ may cause the premature aging of resonator parts and/or mirrors. The resultant damage may not be seen immediately, but over time may contribute to higher repair and service costs.

This waste, lost productivity, and/or escalation in service and repair costs could all be avoided with a simple $125 investment in stainless steel tubing. In comparison to the waste and loss of purging or the wear and tear upon your laser from not purging, “this might be the most rewarding $125 you ever spent,” Cherne says.

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The pictured hose was tested to measure its susceptibility to leaks. Because helium is small enough to leak through a standard plastic hose, a resonator’s gas mix could potentially be compromised.

A stainless solution

To avoid the unnecessary time and labor of purging the system – and to avoid the risk of poor quality cuts – stainless steel tubing is clearly the best solution. Whether a fabricator or manufacturer is planning a new laser purchase or looking to retrofit an existing system, stainless steel tubing will be more cost-efficient in the lo ng run.

“And actually, plastic tubing is really only supposed to be temporary,” Cherne says. “OEMs include 50 ft. of tubing – like an extension cord – to use in the short term. Stainless steel tubing is the right material for a permanent installation.”

Seamless stainless steel tubing can be purchased from venders like Parker Swagelok as well as Praxair. The common sizes used are 1/4 in. ID and 8 mm ID, depending on the brand of the laser. Lengths vary from 3 ft. to 25 ft. When making the final hookup, if flexible stainless steel tubing is necessary, Praxair’s Prostar High Purity pigtail option is available for that application.

Overall, with a steady supply of the proper gas mixture to your laser, you can expect higher productivity – less time spent purging and refilling the lines. You can also expect consistent cut quality and edge accuracy. There is also the consideration of gas cost savings by relying on stainless steel’s impermeability to even the smallest helium molecules.

So, avoid the deflated balloon syndrome. Save yourself the headache and make it your New Year’s Resolution to upgrade to stainless steel tubing and protect your laser’s health and productivity.

Praxair Inc.

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