Ask Control Engineering
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control and embedded systems. Control Engineering answers questions from readers of Control Engineering's print and online magazines, newsletters and other publications. To comment on any blog posting, click on the post's highlighted question and scroll to the "Post a Comment" box at the bottom. Submit questions as comments to any existing post.
Pneumatic signaling still wheezes?
April 08, 2011
Dear Control Engineering: I was reading the article about using smart instrumentation, and it discusses some of the history of field devices, back when everything ran on pneumatic systems. Are pneumatic analog devices still around?
Pneumatic devices have been gone for the most part for quite a while, with the majority replaced by the 1980s. Of course their demise was not all in one fell swoop. The move to electronic analog and digital devices took some time and is probably still not entirely complete. Process instrumentation and control devices like valves, that operate on 3-15 psi are certainly still around, but you will have to hunt for them.
After a few minutes digging around the industrial listings on eBay, I managed to find an old Foxboro liquid level transmitter 13FA1-HK31A5 sold by a company called National Recycling. That should tell you something.
There are companies that still sell new 3-15 psi transmitters, but these are rare too. Here’s a company in Italy that still offers a small selection. There are certainly others if you care to look more thoroughly. More often than not, the pneumatic devices available now are converters that change pneumatic to electronic. Here’s one that changes a Foundation fieldbus signal to pneumatic for a control valve.
Suffice it to say, small delicate mechanisms with diaphragms and leak-prone tubing were not exactly maintenance friendly, so I doubt very many people were sad over the change to electronics. As the article points out, compressed air signals can’t convey much information. An air signal line is good for one signal and that’s it. No diagnostic data, no secondary variables, it’s pretty dumb.
Granted there are still folks out there that want to see trains pulled by steam locomotives, cars with carburetors, and their newspapers set on Linotypes, but the romance of such things gets eclipsed quickly when one has to deal with how maintenance intensive those older technologies are.
If you still use pneumatics and are proud of it, let us know.
Peter Welander, pwelander(at)cfemedia.com
Wednesday, 19-10-11 02:59
Dear Control Engineering: Would anyone advise control valve body material shall be in cast steel or forged steel? If you could explain the pros and cons it will be helpful. From my point of view it should be cast steel. Let me know your views also. Please advise.