Question

what is the work of instrumentation engineers?

Answer 1

WORK OF INSTRUMENTATION ENGINEERS are :
i) Designing machines & equipment,
ii) Managing and maintaining equipment & machines which are used by engineers for various processes.

Answer 2

Instrumentation engineers do design, develop, install, maintain equipments that are used to monitor and control machinery (in big or small industries or at home or office).
These equipments measure and control process variables in a production or manufacturing area. So the engineers play very important role in automation of manufacturing etc. 
For example if in a thermal power plant, water is heated, vaporsized, used to rotate a big generator etc..  An instrument is needed to measure the temperature in the heating chamber and take action automatically to increase it or decrease it as needed.  Pressure of steam may be measured and controlled in order to maintain the rotational speed of generator.  so pressure monitoring equipment or instrument is needed. 
In an aero plane a number of instruments are used to measure and monitor its speed, direction, inside pressure, fuel level, engine status etc,  These quantities are parameters. When the values of these go above or below correct values, operators are informed and suitable control action is initiated automatically and manually.