Wikipedia defines Instrumentation as the art and science of measurement and/or control of process variables within a production or manufacturing area.
Instrumented Skid
The process variables measured in industry include, Pressure, Temperature, Level, Flow, Speed, Force/Weight, Vibration, Humidity, pH, Concentrations of various gases and substances in air or liquid, or any other physical property that someone has designed a device that can measure it, including analytical devices.
Instrumentation also includes Control Valves and sometimes other types of valves such as Pressure Relief Valves. Motors are typically not considered instruments. Solenoids and Relays also are generally not included in the instrumentation realm.
Pressure Gauges
Instruments can be simple indicators and gauges with no electronics or pneumatics attached. They include pneumatic and electronic devices and transmitters that bring the measured value (called the process variable) into the PLC, DCS, or other intelligent device that can display and/or control a process using the measured values from the instruments.
Instruments may also include panel mounted gauges, indicators, recording devices. However these have mainly been replaced with Computer based display devices.
Loop Diagram
Signaling from an instrument is typically through a 4-20mA current loop. The Instrument typically converts it’s internal measurement signal into a 4-20mA signal that corresponds to the measurement via a Transmitter. For example a pressure transmitter measures a pressure say of 0 to 100 psig and transmits a signal from 4 to 20 mA (milliamps) that corresponds to the 0 – 100 psig signal where the 4 mA signal corresponds to 0 psig and the 20 mA signal corresponds to 100 psig. A 0 mA signal indicates a broken wire or other wiring problem. A signal between 0 and 4 mA can indicate an out of range/under-range condition and a signal of >20mA indicates an over-range condition or other failure depending on the options available for the instrument. Loop diagrams are often created to show all of the signaling between the instrument and the PLC or DCS.
Signally between the instrument and the PLC, DCS, or intelligent device can also be done using voltage signals (such as 0-10VDC, or 1-5VDC, or even -10-10VDC for measurement of process variables that can just as likely go negative like speed), over various communications protocols such as Ethernet or wireless.
Control Valve
Instrument Engineers select appropriate instruments for a given process. Factors to consider include the desired measurement type, the measurement ranges, the process conditions that the instrument must withstand (temperature ranges, pressures, chemical resistance, environmental conditions, instrument reliability and history in prior installations under various conditions, etc.) Working out the signaling requirements and developing loop diagrams or schematic diagrams may also fall into the instrument engineers domain.
Process And Instrumentation Diagrams (P&IDs) often form the basis for the Instrument Engineers work. He usually assists in creating these drawings with the process engineer by adding all the instrument “bubbles”. Once completed then all of the instruments for a project are identified.
P&ID drawing
Working on projects instrument engineers may also assemble instrumentation lists, data/specification sheets used in specifying all the requirements
Lab Setup with Various Instruments
of the instrument and is used to get supplier quotations and as a record of the desired and provided instrument and even to track the history of the instrument. In this latter case the data sheet is often a report generated by a database program (such as Meridium).
Instrument Engineers may also collect data and history on instruments in their facilities, do cost benefit analysis, evaluate new instruments as they come to market, etc. Assisting the technicians by writing calibration and other maintenance procedures and assembling troubleshooting notes and tips may also fall in the Instrument Engineers domain.
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Created: 04/04/2015 10:00pm CST: Revised 05/02/2015
for Publishing on 04/13/2015 at 6:00 am CST
Harold On Controls 
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