I can’t believe anyone is still in favor of 4-20mA as the best approach to process automation, but there were some in the ISA LinkedIn group who favored it. This sends me into rant mode…
If you think 4-20mA is the right approach for process automation, you probably also think:
Three channels of television is enough… in black and white… and you have to get up to manually change the channel.
Cars should have rear wheel drive, carburetors, AM radios, 8-track tapes, and rust out in three years.
3-15psig was good enough for my plant; why did we change?
An eddy current clutch is a great way to control rotational speed. Almost as good as a variable-pitch sheave… with a canvas belt
with canvas-tipped wooden cleats. And a line shaft for speed coordination.
Temperature controllers should have a jeweled movement.
Relay ladder logic should be implemented with physical relays and drawn on D-size vellum with a parallel rule and Cutler-Hammer template. And hand lettered between guidelines drawn with a blue pencil. (Blue so they don’t show up on the blueprint.)
I may be showing my age since I have experienced all of the above, but I firmly believe that fieldbuses and Industrial Ethernets (especially PROFIBUS and PROFINET) must be the starting point for any project. If these are new to you, educate yourself; there are many, many resources. (Start here.) Or we’ll be stuck in the past where productivity was low and downtime was high, listening to 8-track tapes in our rusted-out Corvairs.