2/24/2020 4:18 AM | |
Joined: 9/27/2006 Last visit: 9/22/2024 Posts: 12282 Rating: (2685) |
Hello PaisPisco; Before current signals became a standard in control equipment, pneumatic networks were long used as input signals. Pneumatic signals were designated 3-15 psi levels, for analog ranges. The 3-15 psi range was selected so that if there was a break in a pneumatic tube and the system went to atmospheric pressure (0 psi) the system could produce an alarm, not a 0 signal. The same philosophy was used to design 4-20 mA electrical circuits. A wire break (open circuit) will generate a 0mA signal, which can be used for alarming. More information can be found in web articles such as this one; https://www.sensorsone.com/why-offset-4-20ma-3-15psi/ Hope this helps, |
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