Editing Instrumentation (section)

===Early industrial===
[[File:Analogue control loop evolution.png|thumb|The evolution of analogue control loop signalling from the pneumatic era to the electronic era]]
Early systems used direct process connections to local control panels for control and indication, which from the early 1930s saw the introduction of pneumatic [[transmitters]] and automatic [[Proportional–integral–derivative controller|3-term (PID) controllers]].

The ranges of pneumatic transmitters were defined by the need to control valves and actuators in the field. Typically, a signal ranged from 3 to 15 psi (20 to 100kPa or 0.2 to 1.0&nbsp;kg/cm2) as a standard, was standardized with 6 to 30 psi occasionally being used for larger valves. 
Transistor electronics enabled wiring to replace pipes, initially with a range of 20 to 100mA at up to 90V for loop powered devices, reducing to 4 to 20mA at 12 to 24V in more modern systems. A [[transmitter]] is a device that produces an output signal, often in the form of a 4–20&nbsp;[[Ampere|mA]] electrical [[current (electrical)|current]] signal, although many other options using [[voltage]], [[frequency]], [[pressure]], or [[ethernet]] are possible. The [[Transistor#History|transistor]] was commercialized by the mid-1950s.<ref>{{Cite journal | doi=10.1109/17.704244| title=The commercialization of the transistor radio in Japan: The functioning of an innovation community| year=1998| last1=Lynn| first1=L.H.| journal=IEEE Transactions on Engineering Management| volume=45| issue=3| pages=220–229}}</ref>

Instruments attached to a control system provided signals used to operate [[solenoid]]s, [[valve]]s, [[Regulator (automatic control)|regulators]], [[circuit breaker]]s, [[relay]]s and other devices. Such devices could control a desired output variable, and provide either remote monitoring or automated control capabilities.

Each instrument company introduced their own standard instrumentation signal, causing confusion until the 4–20&nbsp;mA range was used as the standard electronic instrument signal for transmitters and valves. This signal was eventually standardized as ANSI/ISA S50, "Compatibility of Analog Signals for Electronic Industrial Process Instruments", in the 1970s. The transformation of instrumentation from mechanical pneumatic transmitters, controllers, and valves to electronic instruments reduced maintenance costs as electronic instruments were more dependable than mechanical instruments. This also increased efficiency and production due to their increase in accuracy. Pneumatics enjoyed some advantages, being favored in corrosive and explosive atmospheres.<ref name=Anderson2>
{{cite book
 | last = Anderson
 | first = Norman A.
 | title = Instrumentation for Process Measurement and Control
 | publisher = CRC Press
 | edition = 3
 | pages = 254–255
 | year = 1998
 | isbn = 978-0-8493-9871-1
}}
</ref>