Editing Instrumentation (section)

== History ==
{{see also|History of measurement|History of technology}}
[[Image:Steuerstand01.jpg|thumb|A local instrumentation panel on a steam turbine]]
The history of instrumentation can be divided into several phases.

===Pre-industrial===
Elements of industrial instrumentation have long histories. Scales for comparing weights and simple pointers to indicate position are ancient technologies. Some of the earliest measurements were of time. One of the oldest [[water clock]]s was found in the tomb of the [[ancient Egypt]]ian pharaoh [[Amenhotep I]], buried around 1500 BCE.<ref>{{cite journal | title = Early Clocks | journal = NIST | url = https://www.nist.gov/pml/general/time/early.cfm | access-date = 1 March 2012| date = 2009-08-12 }}</ref> Improvements were incorporated in the clocks. By 270 BCE they had the rudiments of an automatic control system device.<ref>{{cite web | title = Building automation history page | url = http://www.building-automation-consultants.com/building-automation-history.html | access-date = 1 March 2012 | url-status = dead | archive-url = https://web.archive.org/web/20110708104028/http://www.building-automation-consultants.com/building-automation-history.html | archive-date = 8 July 2011 }}</ref>

In 1663 [[Christopher Wren]] presented the Royal Society with a design for a "weather clock". A drawing shows meteorological sensors moving pens over paper driven by clockwork. Such devices did not become standard in meteorology for two centuries.<ref>
{{Citation
 | last = Multhauf
 | first = Robert P.
 | title = The Introduction of Self-Registering Meteorological Instruments
 | place = Washington, D.C. 
 | publisher = Smithsonian Institution
 | year = 1961
 | pages = 95–116
}} United States National Museum, Bulletin 228. Contributions from The Museum of History and Technology: Paper 23. 
Available from Project Gutenberg.</ref> The concept has remained virtually unchanged as evidenced by pneumatic chart recorders, where a pressurized bellows displaces a pen. Integrating sensors, displays, recorders, and controls was uncommon until the industrial revolution, limited by both need and practicality.

===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>

===Automatic process control===
[[File:Industrial control loop.jpg|thumb|Example of a single industrial control loop, showing continuously modulated control of process flow]]
In the early years of [[process control]], process indicators and control elements such as valves were monitored by an operator, that walked around the unit adjusting the valves to obtain the desired temperatures, pressures, and flows. As technology evolved pneumatic controllers were invented and mounted in the field that monitored the process and controlled the valves. This reduced the amount of time process operators needed to monitor the process. Latter years, the actual controllers were moved to a central room and signals were sent into the control room to monitor the process and outputs signals were sent to the final control element such as a valve to adjust the process as needed. These controllers and indicators were mounted on a wall called a control board. The operators stood in front of this board walking back and forth monitoring the process indicators. This again reduced the number and amount of time process operators were needed to walk around the units. The most standard pneumatic signal level used during these years was 3–15 psig.<ref name=Anderson>
{{cite book
 | last = Anderson
 | first = Norman A.
 | title = Instrumentation for Process Measurement and Control
 | publisher = CRC Press
 | edition = 3
 | pages = 8–10
 | year = 1998
 | isbn = 978-0-8493-9871-1
}}
</ref>

===Large integrated computer-based systems===
[[Image:Pneumatische regelaar.jpg|thumb|Pneumatic "three term" pneumatic [[PID controller]], widely used before electronics became reliable and cheaper and safe to use in hazardous areas (Siemens Telepneu Example)]]
[[File:Kontrollrom Tyssedal.jpg|thumb|A pre-DCS/SCADA era central control room. Whilst the controls are centralised in one place, they are still discrete and not integrated into one system.]] 
[[File:Leitstand 2.jpg|thumb|A DCS control room where plant information and controls are displayed on computer graphics screens. The operators are seated and can view and control any part of the process from their screens, whilst retaining a plant overview.]]
Process control of large industrial plants has evolved through many stages. Initially, control would be from panels local to the process plant. However, this required a large manpower resource to attend to these dispersed panels, and there was no overall view of the process. The next logical development was the transmission of all plant measurements to a permanently staffed central control room. Effectively this was the centralization of all the localized panels, with the advantages of lower manning levels and easy overview of the process. Often the controllers were behind the control room panels, and all automatic and manual control outputs were transmitted back to plant.

However, whilst providing a central control focus, this arrangement was inflexible as each control loop had its own controller hardware, and continual operator movement within the control room was required to view different parts of the process. With coming of electronic processors and graphic displays it became possible to replace these discrete controllers with computer-based algorithms, hosted on a network of input/output racks with their own control processors. These could be distributed around plant, and communicate with the graphic display in the control room or rooms. The distributed control concept was born.

The introduction of DCSs and [[SCADA]] allowed easy interconnection and re-configuration of plant controls such as cascaded loops and interlocks, and easy interfacing with other production computer systems. It enabled sophisticated alarm handling, introduced automatic event logging, removed the need for physical records such as chart recorders, allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs, and provided high level overviews of plant status and production levels.