I'm sure engineers across disciplines and industries can appreciate your tales of water ingress protection. Lots of perspective advice, too, as to how to prevent or at least trouble shoot this issue. I'm sure water isn't the only environmental component that finds a way to wreck havoc on engineering efforts--what are some of the other major environmental influences that post common challenges for engineers?
One of the machines that I worked on had a pressurized hydraulic tank. About 4 psi from the plant compressed air supply was supposed to keep water from getting in. However, the design team had upgraded the reservoir to have a vented cap. When I advised them that a vented cap would not hold pressure, they told me " you don't understand". When I installed this version of machine I advised the customer not to try to pressurize the tank - to turn that regulator down to zero. On one of my service calls for a similar machine, the hydraulic oil looked milky. I found the plant had wet air. The air drop did not have a drip leg or drain valve. The machine's water trap was full of water and the automatic drain had been turned off, and the lubricator was also full of water, instead of oil. And the pressurization regulator was on. So effectively they had been adding water to the hydraulic tank through the pressurization system that was supposed to keep the water out.
During a recent contract, I was hired to design an enclosure intended to pass IP64 and temperature cycling extremes. Since I had experience designing both water-resistant, and water-proof (ambiguous terms invented by marketing,,,) enclosures in the past, I saw no real challenge to the task.
While laying out the preliminary product concept, I specified TPE (Thermoplastic Elastomer) as the gasket material between the housings. I had used that solution previously, and its sealing capability to prevent water -- even pressurized and/or blowing water -- was proven. TPEs are highly compressible and provide excellent dimensional compliance when gasketing between harder plastics.
Luckily, a peer who had been down a similar road looked over my shoulder and quickly pointed out a problem:remember, the product must pass not only IP64 but also temperature extremes. His past experience taught that a TPE, when compressed per design intent, would take a permanent compression-set at extreme cold.Later, as the product cycled back to higher temperature, the TPE would no longer be compliant as a gasket and allowed significant water intrusion after only one cold cycle.The solution was to use silicone rubber, which has true spring-back to original geometry, even after hundreds of temperature cycles.
While injection-molded Liquid Silicone Rubber (LSR) resolved that particular problem, it was not without a long list of other engineering challenges; but I'll save those for a subsequent post.The points to remember are, (1) TPEs take a "set" at extreme cold, and (2) don't be afraid to listen to your peers.
TJ, thanks for a clear explanation of what can go wrong with water ingress. Your mention of both the tugboat and industrial washdown environments made me wonder how much cross-over there might be from technology developed for naval environments to industrial contexts?
I would guess less than you might think. Take the junction box design, for instance. Any commercially available industrial J-box has a flange all the way around the door. It adds stiffness, and prevents directed jets from impinging directly on the door gasket.
Shipboard means of corrosion protection include layer after layer of heavy paint (assuredly not latex house paint, either). I don't see that sort of anticorrosion coating making a comeback in factories.
Thanks for the feedback. I can see where the corrosion protection would be different in the two environments. I guess I was thinking more about the water aspect of the environment, but it looks like the two are inseparable.
In my experience the scheduled cleaning of the service equipment was the biggest issue with corresion.I noticed that seals were not holdig with time and the quarterly cleaning let some moisture inside. This was the main cause for corrosion on power terminals and transformers, especially where the thermal cycling was evident.
Currently I am working on the characterization and performance evaluation of solar collectors, we are using thermocouples for temperatures measurment in a closed loop of solar collectors in order to read temperature elevation of a recirculation fluid. One of our headaches is water in the terminals of the sensors, it produces oxidation in the ferrite metal of the thermocouple and therefore lecture errors, if anyone can suggest an efective way to isolate thermocouple terminals from water it will be really appreciated.
Something to think about: If you make a sealed box with wires entering it and you daily heat this box to temperatures significantly above ambient, then at night cool it, you have made an efficient pump. If the box is vented, the vent needs to be in a location that draws in dry air. If it is not vented, the box will find a way to breathe expelling and replacing the air from somewhere.
Why do you use thermocouples? RTDs (Resistance Temperature Detectors) are more accurate, more reliable, cost less and are more corrosion resistant. Other than that there's no reason to use them.
Capillary action can draw water between the strands inside a cable from the connector outside the watertight box. a gastight gland is no protection against this.
It's worth remembering that a 1$ plastic bottle of soda has a perfect gastight screw-top seal with no o-rings or RTV.
I've had radar lines on a 90' sailboat mast dump water into a junction box that was sealed. Every fall when the mast was removed, the terminals in the box would be severely corroded and the wire needed to be stripped-back to find bare wire. The wire was sealed in the radar dome and sealed in the junciton box, however in winter the mast was stored horizontally with the dome removed and the wire exposed. Somehow water got into the cable and until it was replaced, caused corosion in the sealed box. Once you see this, you can find the same symptom in other areas. It makes for interesting trouble-shooting.
Watch out for the RTV. It outgasses acidic fumes when curing, and that can accelerate corrosion. If you seal warm damp air in a gas tught box, the humidity will condense when you cool it at night.
It is important, if you want to keep water out of enclosures, to consider that the water entering the enclosure may not be in liquid form. Some seals are great for keeping out liquid water, but may not prevent humid air from entering the enclosure. When the temperature drops, the moisture in the air condenses and ends up as liquid water inside the enclosure. Solutions include pressurizing the enclosure with dry air or an inert gas, making the enclosure air tight (as well as liquid-tight), or adding a condensate drain if high humidity inside the enclosure can be tolerated.
I have seen thermocouples hose water 50 feet, I have seen the shield braid on RG8/U cable hose wate farteher than that. BUt thermocouples are cheap and easy and adequate for many applications, that's why folks use them. I also had a situation on a house that we purchased where water ran out of the entry panel when it rained, because the cheap person who installed the entry cable did not use an extra foot for the drip loop.
Water Proof encloseures will fill with water brought in by cable and conduit problems, so the very best choice is to have a drain in the enclosure bottom. But be sure that water can't enter by the drain opening. Another method that will prevent water entry is to have a pressurized enclosure, with sufficient air pressure to prevent water entry. This has been used on communication cables for many years. It is an expensive option, but much cheaper than fixing the problems caused by moisture entry. Of course, you must use dry air or dry nitrogen for pressurizing. That might have worked on the tug boat problem.
For 3D printing to make the jump from rapid prototyping to manufacturing, engineers will need to find easier ways to move products from their CAD screens to their printers.
Gigabit and PoE are two networking technologies moving ahead in tandem as industrial users power remote Ethernet devices such as IP security cameras at 1,000 Mbps over existing CAT5 cable.
New versions of BASF's Ecovio line are both compostable and designed for either injection molding or thermoforming. These combinations are becoming more common for the single-use bioplastics used in food service and food packaging applications, but are still not widely available.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 5
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This radio show will show what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
2 
