All environments are noisy.

Unless your field instruments are inside a Faraday cage, any and all environments are noisy, particularly nowadays, since there's just so much more RF (WiFi, Bluetooth, cellular, etc.) than every before. There are way more electrical appliances and equipment than before, and these are probably noisier than their predecessors, due to laxer enforcement of design rules, and just generally running at higher frequencies than every before. Just consider that a mere 30 years ago, the state of the art PC ran at 20 MHz(!!), while current PCs run 2 to 3 GHz. Power supplies have been forced to be smaller, to be more efficient, and to supply more power, which makes things noisier.

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If the site is mostly mechanical it will be quieter. Along with, if the sensors are sparse and no VFDs are on the premises and there are few solenoid valves about.

Most sensory equipment these days are designed to deal with 50/60Hz noise and so this type of interference and coupling does not greatly affect readings. But, short bursts from switching transients, EMI from variable speed motor systems, cell-phone transmissions, phase-controlled heating, and wireless control systems will all pile-on interference.

Keith Cress
kcress - http://www.flaminsystems.com

I think if you keep AC and all voltages greater than 50VDC far away from your control box you will be OK. Also don't use wireless and that kind of stuff - wire it straight. Stay away from 0-10V, 4-20mA in a shielded twisted pair is a good start. Keep distances short. Don't amplify mV outputs, just buy the 4-20 output. Relays and Solenoids can be a problem. Grounding...